particularly Eoin and Brendan Lawless, Alan Sarhan, Beth Edgell, David Hanlon, ...... Jeffries, D. S., Clair, T. A., Couture, S., Dillon, P. J., Dupont, J., Keller, W., ...
Lake sediment-based reconstructions of variations in levels of deposition of atmospheric pollutants from the industrial-scale combustion of fossil fuels and ecosystem response at three remote Irish lake sites. Barry O’Dwyer Thesis Submitted to the University of Dublin, Trinity College, for the Degree of Doctor in Philosophy
Dept. of Geography The University of Dublin Trinity College January 2009
Declaration I hereby declare that this thesis and has not been submitted in whole or in part for the purpose of obtaining a degree or any other qualification at this or any other University. Except where otherwise acknowledged, this thesis is entirely my own work. The Library of Trinity College may lend or copy this thesis on request
Signed: ___________________
Date:
_____________ ______
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Thesis summary Humans have greatly manipulated the environment over the last 200 years through processes of industrialisation, urbanisation and agricultural intensification.
As a result,
levels of anthropogenic air pollution have increased rapidly, and have been shown to have detrimental impacts on freshwater ecosystem health.
Both the direct (e.g. by altering
ecosystem structure) and indirect effects (e.g. pre-disposing ecosystem to further stressors such as climate change) of increased levels of emission, atmospheric concentration and deposition of atmospheric pollutants on freshwater ecosystems have long been recognised.
However, little is known about long-term variations in levels of atmospheric
contamination, and, as a result, it is difficult to determine the relative sensitivities of differing ecosystems to variations in levels of deposition of atmospheric pollutants.
Historical perspectives on levels of atmospheric contamination allow the identification of past levels of atmospheric pollutant availability and the establishment of how these levels have varied through time, while spatial analyses allow the identification of the areas that have experienced the highest levels of pollutant depositions. Combining temporal and spatial examinations allows for more reasoned debates on the sources and impacts of atmospheric contamination. Palaeolimnology is a multi-disciplinary science that uses the physical, chemical and biological information preserved in lake sediment profiles to reconstruct past environmental conditions in inland aquatic systems. Moreover, lakes act as sinks for pollutants deposited from the atmosphere either directly onto the lake surface or indirectly via the catchment.
Therefore, in addition to providing a record of in-lake
changes, lake sediments provide a reliable record of atmospheric deposition to the lake and its catchment. Employing palaeolimnological techniques, this thesis examines variations in levels of deposition of atmospheric pollutants from the industrial-scale combustion of fossil fuels and ecosystem response at three remote lake sites (Kelly’s Lough, Lough Maumwee, and Upper Killarney Lough), each located in a distinct region of atmospheric pollutant deposition in Ireland, all of which are oligotrophic, acid-sensitive and relatively remote from industrial and urban sources of pollution.
Short sediment cores were collected from the three study sites which, according to the 210
Pb and
137
Cs dating methods, account for time periods that are considerably longer than
existing historical and documentary records of atmospheric pollution and their effects in Ireland.
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Utilising sediment-based records of industrial-combustion by-products (SCPs,
trace elements and PAHs), both inter- and intra-site variations in levels of depositions of atmospheric pollutants were reconstructed, while variations in ecosystem response, particularly to acidification and toxicity, were assessed using sediment-based remains of diatoms and consensus-based SQG ratios, respectively.
SCP evidence indicates that atmospheric contamination by industrial-level fossil combustion by-products has varied both temporally and spatially. The highest levels of deposition were measured in the east of Ireland. In addition, inter-site differences in the timing of peaks in levels of SCP deposition appear to be due primarily to emissions originating from national sources, particularly power stations, and the prevailing wind direction, and this calls into question the general applicability of the SCP technique to the dating of recently deposited sediment-based records in Ireland. Temporal variations in levels of deposition of trace elements and PAHs were quantified at all three study sites, with the most likely origin of some trace element deposition in the west of Ireland being North American sources, as evidenced at Lough Maumwee and Upper Killarney Lough, while the highest contemporary levels of PAH deposition were measured at Lough Maumwee, highlighting the contribution of nationally-based sources to overall levels of atmospheric contamination at the site.
Variations over time in reconstructed diatom
assemblages and modelled DI-pH highlight differing ecosystem sensitivities to acidification pressures, particularly to changes in the severity of acidification pressures, which appear to result from differences in pollution load, water quality, hydromorphology, biotic interactions, catchment characteristics, precipitation and location. Consensus-based SQG ratios were calculated for measured levels of toxicants (trace elements and PAHs) in surface sediment samples from the three study sites and indicate that levels of toxicity are low and are not expected to have a detrimental effect on biota.
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Acknowledgements Firstly, I would like to thank Prof. David Taylor for his supervision and guidance throughout this research, as well as for all the time he has spent reading the various drafts of this thesis. His critical commentary on my work has played a major role in both the content and presentation of this thesis. I also express my gratitude to the EPA (Ireland) who provided financial support for this research. Many thanks to all my friends and colleagues in the Dept. of Geography at Trinity College, both past and present, particularly Francis Ludlow, Conor Quinlan, Julius Komolof, Arlene Crampsie, Niall McCrory, Laragh Larsen, Karen Sheeran, Anthony Brooks, Terry Dunne, Tara Nolan, Gayle McGlynn and Elaine Treacy, with whom it was a pleasure to work.
I am indebted to all those who helped me while on fieldwork, in sun, rain and snow, particularly Dr. Manel Leira, Conor Quinlan, Elaine Treacy, Dr. Catherine Dalton and all those from the University of Ulster, particularly Dr. Philip Jordan. I also express my utmost gratitude to Dr. Neil Rose, for his help and advice with SCP analysis, Dr. Manel Leira, for his advice on Irish lakes, Elaine Treacy and Tara Nolan for their help in the laboratory, and Conor Quinlan for guidance while working with ArcGIS.
To all my friends who gave so much support to me in ways that they may never even know, particularly Eoin and Brendan Lawless, Alan Sarhan, Beth Edgell, David Hanlon, Kevin Moroney, Eoin McNulty-Goodwin, Mark Doherty, Paul-Dylan Hyndes, Brian Murphy, and not to forget the McGetigans.
Finally and most importantly, I would like to thank my parents, Mary and JJ, who have encouraged and supported me throughout my life and in particular during the writing of this thesis, and all the rest of my family, Eoin, Brian, Eimear (particularly for the many lunches), Hazel, Claire, Darren, Grace, Mikey and Daniel who have provided me with a supportive and loving environment throughout the time it took to complete this research and in my life in general. I dedicate this thesis to you all.
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Table of contents Declaration...................................................................................................................i Summary ......................................................................................................................ii Acknowledgements.....................................................................................................iv Table of contents.........................................................................................................v List of figures...............................................................................................................x List of tables ................................................................................................................xvii List of plates ................................................................................................................xix List of equations..........................................................................................................xix List of abbreviations and acronyms..........................................................................xx
Chapter 1: Introduction..............................................................................................1 1.1 Atmospheric pollution ......................................................................................1 1.1.1 Industrialisation and atmospheric pollution .......................................1 1.1.2 Ecosystem health impacts ................................................................3 1.1.2.1 Acidification..........................................................................3 1.1.2.2 Toxicity.................................................................................4 1.2 Palaeolimnology ..............................................................................................4 1.3 Research rationale and thesis structure ..........................................................6 1.3.1 Research rationale............................................................................6 1.3.2 Thesis structure ................................................................................7
Chapter 2: Deposition of atmospheric pollutants in Ireland and selection of study sites .......................................................................................................................10 2.1 Contemporary spatial patterns in levels of atmospheric pollutant deposition in Ireland.......................................................................................................10 2.1.1 Location, prevailing wind direction and transboundary pollutant transport ......................................................................................13 2.1.2 National sources ...............................................................................17 2.2 Selection of study sites..................................................................................20 2.2.1 Kelly’s Lough, Co. Wicklow...............................................................23 2.2.2 Lough Maumwee, Co. Galway..........................................................26 2.2.3 Upper Killarney Lough, Co. Kerry .....................................................29
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Chapter 3: Palaeolimnological methods and proxies............................................. 32 3.1 Field based techniques ................................................................................. 34 3.1.1 Sediment coring .............................................................................. 34 3.2 Laboratory based techniques....................................................................... 36 3.2.1 Expressing palaeolimnological data................................................. 36 3.2.2 Palaeolimnological proxies confirming selection of study sites....... 37 3.2.2.1 LOI analysis ........................................................................ 38 3.2.2.2 Pollen analysis.................................................................... 39 3.2.3 Chronological control of lake sediment cores .................................. 41 3.2.3.1
210
Pb and 137Cs radiometric dating, and SCP dating .......... 41
3.2.4 Palaeolimnological proxies of depositions of atmospheric pollutants ................................................................................................... 47 3.2.4.1 SCP analysis ...................................................................... 47 3.2.4.2 Inorganic geochemical analysis........................................... 53 3.2.4.3 PAH analysis ...................................................................... 58 3.2.5 Palaeolimnological proxies of ecosystem response ........................ 61 3.2.5.1 Diatom analysis .................................................................. 62 3.2.5.2 SQG analyses .................................................................... 65 3.3 Numerical methods ...................................................................................... 66 3.3.1 Correlation analysis ......................................................................... 66 3.3.2 PCA ................................................................................................. 67 3.2.3 Transfer function.............................................................................. 68 3.4 Secondary sources ....................................................................................... 69 3.4.1 Inventories of SOX and SO2 emissions ............................................ 69 3.4.2 Power station capacity records and histories .................................. 71 3.4.3 Historical reconstructions of non-marine SO42- depositions............. 71 Chapter 4: Results and analysis............................................................................... 74 4.1 Kelly’s Lough ................................................................................................. 74 4.1.1 Palaeolimnological proxies confirming selection of study sites ........ 74 4.1.1.1 Pollen and LOI analysis......................................................... 74 4.1.2 Chronological control of sediment cores .......................................... 76 4.1.2.1 210Pb and 137Cs dating, and SCP dating ................................ 76 4.1.3 Palaeolimnological proxies of depositions of atmospheric pollutants .................................................................................................................. 78 4.1.3.1 SCP analysis ......................................................................... 78
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4.1.3.2 Inorganic geochemical analysis .............................................79 4.1.3.3 PAH analysis .........................................................................85 4.1.4 Palaeolimnological proxies of ecosystem response .........................88 4.1.4.1 Diatom analysis .....................................................................88 4.1.4.2 SQG analyses........................................................................90 4.2 Lough Maumwee ...........................................................................................91 4.2.1 Palaeolimnological proxies confirming selection of study sites ........91 4.2.1.1 Pollen and LOI analysis .........................................................91 4.2.2 Chronological control of sediment cores...........................................93 4.2.2.1 210Pb and SCP dating ............................................................93 4.2.3 Palaeolimnological proxies of depositions of atmospheric pollutants ...................................................................................................................95 4.2.3.1 SCP analysis .........................................................................95 4.2.3.2 Inorganic geochemical analysis .............................................96 4.2.3.3 PAH analysis ....................................................................... 103 4.2.4 Palaeolimnological proxies of ecosystem response ....................... 103 4.2.4.1 Diatom analysis ................................................................... 103 4.2.4.2 SQG analyses...................................................................... 107 4.3 Upper Killarney Lough ................................................................................. 108 4.3.1 Palaeolimnological proxies confirming selection of study sites ...... 108 4.3.1.1 Pollen and LOI analysis ....................................................... 108 4.3.2 Chronological control of sediment cores......................................... 110 4.3.2.1 210Pb and SCP dating .......................................................... 110 4.3.3 Palaeolimnological proxies of depositions of atmospheric pollutants ................................................................................................................. 112 4.3.3.1 SCP analysis ....................................................................... 112 4.3.3.2 Inorganic geochemical analysis ........................................... 113 4.3.3.3 PAH analysis ....................................................................... 120 4.3.4 Palaeolimnological proxies of ecosystem response ....................... 120 4.3.4.1 Diatom analysis ................................................................... 120 4.3.4.2 SQG analyses...................................................................... 123 4.4 Site summaries............................................................................................ 125 4.4.1 Kelly’s Lough .................................................................................. 125 4.4.2 Lough Maumwee ............................................................................ 126 4.4.3 Upper Killarney Lough .................................................................... 127
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Chapter 5: Discussion ............................................................................................... 130 5.1 Palaeolimnological evidence of temporal and spatial variations in levels of deposition of atmospheric pollutants ............................................................... 130 5.1.1 SCPs ................................................................................................ 132 5.1.1.1 Temporal variations ............................................................... 132 5.1.1.2 Spatial variations ................................................................... 141 5.1.2 Trace elements................................................................................. 144 5.1.2.1 Post depositional alteration of trace element profiles ............ 144 5.1.2.2 Detecting inputs of anthropogenically-derived trace elements ........................................................................................................... 145 5.1.2.3 Interpretation of the lake sediment record of anthropogenic inputs of trace elements .................................................................... 150 5.1.3 PAHs ................................................................................................ 154 5.1.3.1 Temporal variations ............................................................... 155 5.1.3.2 Spatial variations ................................................................... 157 5.2 Palaeolimnological proxy evidence of ecosystem response ......................... 158 5.2.1 Diatoms ............................................................................................ 158 5.2.2 SQGs................................................................................................ 167 5.3 Lake sediment-based reconstructions of temporal and spatial differences between the three APDRs in Ireland in levels of deposition of atmospheric contaminants from the industrial-scale combustion of fossil fuels over the last 50 – 150 years ............................................................................................................ 168 5.3.1 The relative contributions of national and transboundary sources to levels of deposition of atmospheric pollutants ........................................... 169 5.3.2 The effectiveness of recent legislation, mitigation strategies, and technologies aimed at reducing emissions of atmospheric pollutants....... 171 5.3.3 The applicability of the SCP dating technique in estimating sediment chronologies at lake sites in Ireland .......................................................... 172 5.4 Lake sediment-based reconstructions of ecosystem response to variations in levels of deposition of atmospheric contaminants from the industrial-scale combustion of fossil fuels .................................................................................... 174 5.4.1 Acidification and recovery ................................................................ 175
Chapter 6: Conclusions.............................................................................................. 178 6.1 Thesis summary ............................................................................................ 178 6.2 Research findings ......................................................................................... 178
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6.3 Limitations of the current research ................................................................181 6.4 Future directions............................................................................................182
References ...................................................................................................................185
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List of figures Figure 1. 1 Schematic diagram of the atmospheric pollutant lifecycle from emission to deposition.......................................................................................................................... 2
Figure 1. 2 A schematic diagram of the thesis framework adopted in this research. Both primary and secondary sources are detailed .................................................................... 9
Figure 2. 1 Modelled levels of SOX deposition for Ireland in 2005, as reported by Klein et al. (2007)........................................................................................................................... 13 Figure 2. 2 Maps detailing average levels of non-marine SO42- measured in precipitation at 10 sites throughout Ireland for the eight year period, 1991 – 1998, as reported by Aherne and Farrell (2002) (left), and levels of SCP concentration measured in surface sediment samples from 57 lakes throughout Ireland, as reported by Bowman and Harlock (1998) and Rose and Harlock (1998c) (right)...................................................................................... 15
Figure 2. 3 Map detailing the locations of fossil fuel combustion power stations > 50MW (BERR 2007) and major cities in Britain ........................................................................... 16
Figure 2. 4 Location map of power stations > 50 MW in Ireland. Percentage contributions to total levels of Irish emission of SOX as reported by EPER (2004) are detailed. The locations of the Dublin urban area and Shannon-based industries are also shown ......... 18
Figure 2. 5 Schematic map of Ireland showing the three identified APDRs. Wind roses and major industrial areas are also shown ....................................................................... 19
Figure 2. 6 Map of Ireland detailing locations of selected study sites. The three APDRs are also marked ................................................................................................................ 21
Figure 2. 7 Location map of Kelly’s Lough. Spot heights (in m) are marked on the map. Contours intervals are shown at 50 m intervals ................................................................ 25
Figure 2. 8 Bathymetry of Kelly’s Lough. The location of the coring site is marked with an X........................................................................................................................................ 25
x
Figure 2. 9 Location map of Lough Maumwee. Spot heights are marked on the map (in m). Contour intervals are shown at 100m ........................................................................ 28
Figure 2. 10 Bathymetry of Lough Maumwee. The location of the coring site is marked with an X ........................................................................................................................... 28
Figure 2. 11 Location map of Upper Killarney Lough. Spot heights are marked on the map (in m). Contour intervals are shown at 100m ................................................................... 31
Figure 2. 12 Bathymetry of Upper Killarney Lough The location of the coring site is marked with an X ........................................................................................................................... 31
Figure 3. 1 Schematic diagram of methodological approach adopted in this research. Note the prominence of palaeolimnological techniques ............................................................ 33
Figure 3. 2
238
U Decay Chain ........................................................................................... 42
Figure 3. 3 A schematic diagram of the 210Pb cycle ......................................................... 43 Figure 3. 4 Schematic diagram of a SCP (left) and an IAS (right). Scale 1 cm = 2µm..... 48
Figure 3. 5 A schematic SCP profile showing the main features referred to in the text. A the start of the SCP record; B - the rapid increase in SCP concentrations; and C - the subsurface peak in SCP concentrations ........................................................................... 50 Figure 3. 6 A schematic diagram of delivery pathways to lake sediment for both major and trace elements................................................................................................................... 54
Figure 3. 7 A schematic diagram of delivery pathways to lake sediment for PAHs ......... 59 Figure 3. 8 Temporal variation in reconstructed emission inventories of SO2 emission for Ireland as reported by Mylona (1993, 1996). Officially reported emissions data for SOX in Ireland are also shown ...................................................................................................... 70
xi
Figure 3. 9 Temporal variation in reconstructed emission inventories of SO2 emission for Europe as reported by Mylona (1993, 1996). Officially reported emissions data for SOX in Europe are also shown ..................................................................................................... 70 Figure 3. 10 Map detailing changes in the generating capacity and fuel type of power stations in Ireland, from the 1970s (left) until the 1980s (right). Symbol size and colour represent, respectively, the generating capacity and fuel type of individual power stations. The locations of study sites are also shown. .................................................................... 72 Figure 3. 11 Modelled levels of non-marine (denoted as ‘nm’ on the map) SO42- for Ireland and Britain in 1960, as reported by Mylona (1996) ........................................................... 73 Figure 4. 1 Kelly’s Lough: summary pollen diagram and results of LOI analysis. The pollen sum includes all identifiable pollen grains and spores, excluding those from aquatic plants. Percentage data calculated for abundances of aquatic plants were calculated on the basis of the total number of pollen and spores encountered, including aquatics. L. plants = lower plants ................................................................................................................................ 75
Figure 4. 2 Kelly’s Lough: (a) excess
210
Pb activity profile; (b) CRS calculated sediment
accumulation rate; (c) CRS calculated chronology,
137
Cs activity profile with the 1963 (1)
and 1986 (2) peaks marked, and the SCP concentration profile with the main identifiable dating features marked: the rapid increase in SCP concentrations (1) and the particle concentration peak (2) ...................................................................................................... 77 Figure 4. 3 Kelly’s Lough: up-core variations in concentration and accumulation rate data for SCPs............................................................................................................................ 78
Figure 4. 4 Kelly’s Lough: up-core variations in (a) concentration data for elements, LOI and sediment accumulation rate data; and (b) accumulation rate data for elements........ 80
Figure 4. 5 Kelly’s Lough: variation in levels of up-core loadings of components 1 and 2 .......................................................................................................................................... 83
Figure 4. 6 Kelly’s Lough: up-core variations in ratios calculated for Al-normalised (a) concentration data for trace elements; and (b) accumulation rate data for trace elements; and (c) EFs calculated for ratios of Al-normalised concentration data for trace elements .......................................................................................................................................... 84
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Figure 4. 7 Kelly’s Lough: up-core variations in concentration and accumulation rate data for total PAH. Up-core variations in accumulation rate data for individual PAH compounds are also shown .................................................................................................................. 86
Figure 4. 8 Kelly’s Lough: phenanthrene/anthracene and flouranthene/pyrene ratios..... 87
Figure 4. 9
Kelly’s Lough: summary diagram of up-core variations in abundances of
selected diatoms and DAZs. Up-core variations in levels of reconstructed DI-pH and their boundaries of uncertainty are also illustrated.................................................................... 89
Figure 4. 10 Lough Maumwee: summary pollen diagram and results of LOI analysis. The pollen sum includes all identifiable pollen grains and spores, excluding those from aquatic plants. Percentage data calculated for abundances of aquatic plants were calculated on the basis of the total number of pollen and spores encountered, including aquatics. L. plants = lower plants ......................................................................................................... 92 Figure 4. 11 Lough Maumwee: (a) excess
210
Pb activity profile; (b) CRS calculated
sediment accumulation rate; (c) CRS calculated chronology and SCP concentration profile with the main identifiable dating features marked: 1 – rapid increase in particle concentrations; 2 – the particle concentration peak .......................................................... 94 Figure 4. 12 Lough Maumwee: up-core variations in concentration and accumulation rate data for SCPs.................................................................................................................... 95
Figure 4. 13 Lough Maumwee: up-core variations in (a) concentration data for elements, LOI and sediment accumulation rate data; and (b) accumulation rate data for elements . .......................................................................................................................................... 97
Figure 4. 14 Lough Maumwee: variations in levels of up-core loading of components 1 and 2 ...................................................................................................................................... 100
Figure 4. 15 Lough Maumwee: up-core variations in ratios calculated for Al-normalised (a) concentration data for trace elements; and (b) accumulation rate data for trace elements; and (c) EFs calculated for ratios of Al-normalised concentration data for trace elements ........................................................................................................................................ 102
xiii
Figure 4. 16 Lough Maumwee: up-core variations in concentration and accumulation rate data for total PAH. Up-core variations in accumulation rate data for individual PAH compounds are also shown ............................................................................................ 104
Figure 4. 17 Lough Maumwee: phenanthrene/anthracene and flouranthene/pyrene ratios ........................................................................................................................................ 105
Figure 4. 18 Lough Maumwee: summary diagram of up-core variations in abundances of selected diatoms. Up-core variations in levels of reconstructed DI-pH and their boundaries of uncertainty are also illustrated .................................................................................... 106
Figure 4. 19 Upper Killarney Lough: summary pollen diagram and results of LOI analysis. The pollen sum includes all identifiable pollen grains and spores, excluding those from aquatic plants.
Percentage data calculated for abundances of aquatic plants were
calculated on the basis of the total number of pollen and spores encountered, including aquatics. L. plants = lower plants................................................................................... 109 Figure 4. 20 Upper Killarney Lough: (a) excess
210
Pb activity profile; (b) CRS and CIC
calculated chronologies; (c) accepted sediment accumulation rate; (d) accepted chronology and SCP concentration profile with the main identifiable dating features marked: 1 – the start of the SCP record; 2 – the rapid increase in SCP concentrations; 3 – the particle concentration peak and subsequent decline................................................................... 111 Figure 4. 21 Upper Killarney Lough: up-core variations in concentration and accumulation rate data for SCPs .......................................................................................................... 112
Figure 4. 22
Upper Killarney Lough: up-core variations in (a) concentration data for
elements, LOI and sediment accumulation rate data; and (b) accumulation rate data for elements ......................................................................................................................... 114
Figure 4. 23 Upper Killarney Lough: variation in levels of up-core loading of components 1,2 and 3. Note the reversal of the X axis for component 1 ........................................... 117
Figure 4. 24
Upper Killarney Lough:
up-core variations in ratios calculated for Al-
normalised (a) concentration data for trace elements; and (b) accumulation rate data for
xiv
trace elements; and (c) EFs calculated for ratios of Al-normalised concentration data for trace elements................................................................................................................. 119
Figure 4. 25 Lough Maumwee: up-core variations in concentration and accumulation rate data for total PAH. Up-core variations in accumulation rate data for individual PAH compounds are also shown ............................................................................................ 121
Figure 4. 26 Upper Killarney Lough: phenanthrene/anthracene and flouranthene/pyrene ratios ............................................................................................................................... 122
Figure 4. 27
Upper Killarney Lough: summary diagram of up-core variations in
abundances of selected diatoms and DAZs. Up-core variations in levels of reconstructed DI-pH and their boundaries of uncertainty are also illustrated ........................................ 124
Figure 5. 1 Temporal variations in accumulation rates of SCPs at all three sites examined, expressed as cumulative % data. The global trend in depositions of SCPs over time is also illustrated, as are levels of both officially reported and historically reconstructed emissions of SO2 for both European and Irish sources (Mylona, 1993; 1996; Klein et al. 2007). The three main SCP features are also marked: A – the start of the SCP records; B – the rapid increase in concentrations of SCPs; and C – the SCP concentration peak. In addition, the temporal range in which the three main SCP features have been reported to occur globally are delineated on the plot titled Global SCP trend (Rose, 2001) .................................... 134
Figure 5. 2 Comparative location maps of power stations in Ireland in 1960 (left) and 1990 (right). The capacities of power stations is indicated by symbol size, while fuel type is indicated by symbol colour. The locations of the three lakes forming the focus of the current study are also marked......................................................................................... 138
Figure 5. 3 Kelly’s Lough: up-core variations in SCP accumulation rate data and
210
Pb
estimated sediment accumulation rate data. Enhanced peaks in both SCP and sediment accumulation rate data are marked with a dashed line.
Officially reported data for
emissions of SOX from both Irish and European sources for the period 1980 - 2005 (Klein et al. 2007) are also detailed........................................................................................... 140 Figure 5. 4 Accumulation rates of SCPs measured for surface sediment samples from the three study sites. Accumulation rates of SCPs measured for surface sediment samples
xv
collected at lake sites throughout Europe (Rose et al. 1999b) are also shown.
The
accumulation rate of SCPs measured for surface sediment samples from Lake Arresjøen (Svalbard) is also included as an inset ........................................................................... 143
Figure 5. 5 Lough Maumwee: up-core variations in concentration data for SCPs, PCA loading, Al-normalised concentration data for trace elements, and EFs calculated for ratios of Al-normalised concentration data for total trace elements .......................................... 148
Figure 5. 6 Upper Killarney Lough: up-core variations in concentration data for SCPs, PCA loading, Al-normalised concentration data for trace elements, and EFs calculated for ratios of Al-normalised concentration data for total trace elements .......................................... 149 Figure 5. 7 Kelly’s Lough: up-core variations in concentration data for SCPs, PCA loading, Al-normalised concentration data for trace elements, and EFs calculated for ratios of Alnormalised concentration data for total trace elements .................................................. 149
Figure 5. 8 Accumulation rates of PAHs measured in surficial sediments from the three study sites. Published levels of PAH accumulation measured in samples of surficial lake sediments from a range of sites throughout Europe, as reported by Fernandez et al. (1999), are also shown. The accumulation rate of PAHs measured for surface sediment samples from Lake Arresjøen (Svalbard) is also included as an inset ........................... 158
Figure 5. 9 Up-core variations in concentrations of SCP, abundances of diatoms grouped according to the Hustedt classification system (1936-1939), and reconstructed levels of DIpH and their associated boundaries of uncertainty for the three study sites that form the focus of the current research. DAZs identified at Kelly’s Lough and Upper Killarney Lough are also marked .............................................................................................................. 166
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List of tables Table 2. 1 Summary locational, hydromorphological and water chemistry data for the three study sites ............................................................................................................ 22
Table 4. 1 Kelly’s Lough: results of correlation analysis. P values are displayed in the upper right diagonal while r values are displayed in the lower left diagonal. Significant correlations are marked in bold. Accum = sediment accumulation rate ....................... 81 Table 4. 2 Kelly’s Lough: PCA results .......................................................................... 83 Table 4. 3 Kelly’s Lough: variable loading scores for components 1 and 2 ................. 83 Table 4. 4 Kelly’s Lough: % contribution of individual PAH compounds to levels of total PAH ............................................................................................................................... 87 Table 4. 5 Kelly’s Lough: PEC ratio values calculated for individual trace elements and total PAH. PEC-Q MM (average concentration of trace elements) and PEC-Q MPP (average concentration of trace elements and total PAH) ratios are also shown.......... 90
Table 4. 6 Lough Maumwee: results of correlation analysis. P values are displayed in the upper right diagonal while r values are displayed in the lower left diagonal. Significant correlations are marked in bold. Accum = sediment accumulation rate ..... 98 Table 4. 7 Lough Maumwee: PCA results.................................................................. 100 Table 4. 8 Lough Maumwee: variable loading scores for components 1, 2, 3 and 4 . .................................................................................................................................... 100 Table 4. 9 Lough Maumwee: % contribution of individual PAH compounds to levels of total PAH ..................................................................................................................... 105
Table 4. 10 Lough Maumwee: PEC ratio values calculated for individual trace elements and total PAH. PEC-Q MM (average concentration of trace elements) and PEC-Q MPP (average concentration of trace elements and total PAH) ratios are also shown........ 107
xvii
Table 4. 11
Upper Killarney Lough: results of correlation analysis.
P values are
displayed in the upper right diagonal while r values are displayed in the lower left diagonal. Significant correlations are marked in bold. Accum = sediment accumulation rate ............................................................................................................................. 115
Table 4. 12 Upper Killarney Lough: PCA results....................................................... 117
Table 4. 13 Upper Killarney Lough: variable loading scores for components 1, 2 and 3. .................................................................................................................................... 117
Table 4. 14 Upper Killarney Lough: % contribution of individual PAH compounds to levels of total PAH....................................................................................................... 122
Table 4. 15 Upper Killarney Lough: PEC ratio values calculated for individual trace elements and total PAH. PEC-Q MM (average concentration of trace elements) and PEC-Q MPP (average concentration of trace elements and total PAH) ratios are also shown.......................................................................................................................... 125
Table 5. 1 Peak levels of accumulation rate for Cd measured in this study. Published peak levels of accumulation rate for Cd measured in cores of lake sediment from North America are also shown.............................................................................................. 152
Table 5. 2
Calculated enrichment factors (EFs) based on ratios of Al-normalised
concentration data for individual trace elements in sediments from both Lough Maumwee and Upper Killarney Lough. Ratios are a means of standardising surface sediment levels against concentration data for the lowermost sediment core sample analysed...................................................................................................................... 153
Table 5. 3 Concentration data for trace elements in surface sediments from Lough Maumwee and Upper Killarney Lough.
Estimates of levels of concentration for
uncontaminated (background) lake sediments are detailed.
Reported median
concentrations
(background) and
for
surficial
sediments
from
uncontaminated
contaminated North America lakes are also shown, while trace element concentrations reported for surficial sediments from a range of British sites, from relatively uncontaminated to highly contaminated, are also provided ........................................ 154
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Table 5. 4 Summary of PAH source ratios ................................................................ 155
Table 5. 5 Dates of occurrence of the three main SCP dating features identified at all three study sites. Previously reported estimates (Rose et al. 1995) for Irish lakes are also detailed ................................................................................................................ 174
List of plates Plate 2. 1 Kelly’s Lough................................................................................................ 24
Plate 2. 2 Lough Maumwee.......................................................................................... 27
Plate 2. 3 Upper Killarney Lough................................................................................... 30
Plate 3. 1 Examples of SCPs enumerated and photographed in sediment samples from the three study sites examined. In each photograph SCPs are demarcated by a red circle..............................................................................................................................52 Plate 3. 2
Examples of the main diatom types enumerated and photographed in
sediment samples collected from the three study sites examined: (a) Tabellaria flocculosa;
(b)
Eunotia
incisa;
(c)
Frustulia
rhomboides;
(d)
Achnanthidium
minutissimum; and (e) Brachysira vitrea ....................................................................... 64
List of equations
Equation 3. 1 Calculation of SCP concentration (Rose, 1994)...................................... 53
xix
List of abbreviations and acronyms Al
-
Aluminium
AMSL
-
Above mean sea level
ANC
-
Acid neutralising capacity
APDR
-
Atmospheric pollutant deposition region
C
-
Carbon
ca.
-
Current approximate
Cd
-
Cadmium
CIC
-
Constant initial concentration
cm
-
Centimetre
CMA
-
Centre for Microscopy and Analysis
Co
-
Cobalt
CO2
-
Carbon dioxide
COD
-
Chloro-octadene
Cr
-
Chromium
CRS
-
Constant rate of supply
Cs
-
Caesium
Cu
-
Copper
DAZ
-
Diatom assemblage zone
DI-pH
-
Diatom-inferred pH
DM
-
Dry matter
DOC
-
Dissolved organic carbon
EAPDR
-
Eastern atmospheric pollutant deposition region
EF
-
Enrichment factor
EMEP
-
European Monitoring and Evaluation Programme
EPA
-
Environmental Protection Agency
EPER
-
European Pollutant Emission Register
FAAS
-
Flame atomic absorption spectrometry
Fe
-
Iron
g
-
gram
GC-MS
-
Gas chromatography-mass spectrometry
GPS
-
Global positioning system
HCL
-
Hydrochloric acid
xx
HF
-
Hydro-fluoric acid
HFO
-
Heavy fuel oil
Hg
-
Mercury
HMN
-
Heptamethylnonane
HNO3
-
Nitric acid
H2O2
-
Hydrogen peroxide
IAS
-
Inorganic ash sphere
ICP-MS
-
Inductively coupled plasma mass spectrometry
LOI
-
Loss-on-ignition
mg
-
Milligram
Mg
-
Magnesium
ml
-
Millilitre
Mn
-
Manganese
MSC-W
-
Meteorological Synthesising Centre - West
MW
-
Megawatt
Na
-
Sodium
NaCl
-
Sodium chloride
ng
-
nanogram
NH3
-
Ammonia
Ni
-
Nickel
NO2
-
Nitrogen dioxide
NO3
-
Nitrate
PAH
-
Polycyclic aromatic hydrocarbon
Pb
-
Lead
210
Pb
-
Lead-210
PCA
-
Principle component analysis
PEC
-
Probable effect concentration
PEC-Q
-
Probable effect concentration-quotient
PEC-Q MM
-
Probable effect concentration-quotient of mean metals
PEC-Q MPP
-
Probable effect concentration-quotient of mean metals and total PAH
PM
-
Particulate matter
PM2.5
-
Particulate matter with a diameter of < 2.5 micrometers
PM10
-
Particulate matter with a diameter of < 10 micrometers
210
Po
-
Polonium-210
POP
-
Persistent organic pollutant
xxi
PTFE
-
Polytetrafluoroethylene
226
Ra
-
Radium-226
RMSE
-
Root mean square error
RMSEP
-
Root mean square error of prediction
SAC
-
Special Area of Conservation
SCP
-
Spheroidal carbonaceous particle
Se
-
Selenium
SO2
-
Sulphur dioxide
SO4
-
Sulphate
SO42-
-
Particulate sulphate
SOX
-
Sulphur oxides
SSWC
-
Steady-state water chemistry
Sq
-
Squalene
SQG
-
Sediment quality guideline
SWAPDR
-
Southwestern atmospheric pollutant deposition region
Ti
-
Titanium
238
-
Uranium-238
ug
-
Microgram
UNECE LRTAP
-
United Nations Economic Commission for Europe Long-
U
range Transboundary Air Pollution UNESCO
-
United Nations Educational, Scientific and Cultural Organisation
V
-
Vanadium
WA
-
Weighted averaging
WAPDR
-
Western atmospheric pollutant deposition region
WA-PLS
-
Weight averaging partial least squares
yr
-
Year
Zn
-
Zinc
xxii
Chapter 1: Introduction
1.1 Atmospheric pollution Humans have greatly modified the environment over the last 200 years through processes of industrialisation, urbanisation, and agricultural intensification. The depth and extent of this modification characterise what Oldfield (2005) terms the Anthropocene, which is the current part of the Holocene.
A characteristic feature of the Anthropocene is a rapid
increase in levels of anthropogenic air pollution (i.e. additional to the natural background of gases, aerosols and PM, originating from plants, radiological decomposition, forest fires and volcanic emissions). Enhanced levels of atmospheric pollutants have been shown to have widespread impacts, and to be particularly detrimental to the health of freshwater ecosystems.
1.1.1 Industrialisation and atmospheric pollution Widespread industrialisation beginning in the late 18th and 19th centuries, driven by the fossil fuel combustion process, resulted in a rapid increase in the levels of pollutant emission, loadings in the atmosphere, and subsequent deposition. The primary inorganic pollutants emitted during fossil fuel combustion are SO2 and NOX (Spengler et al. 1990).
Trace elements, organic compounds (e.g. polycyclic aromatic
hydrocarbons (PAHs)), and inorganic compounds (e.g. spheroidal carbonaceous particles (SCPs)) are also emitted (Rose 2001; Donahue et al. 2006).
Once emitted to the
atmosphere, the atmospheric residence time of a pollutant is determined by its chemical reactivity (potential to be oxidised to form a secondary pollutant) and how readily it can be removed from the atmosphere. The distance that a pollutant can travel in the atmosphere is determined by height and rate of emission, and losses through chemical reaction and through wet and dry depositions (Metcalfe and Derwent 2005). As exemplified in Fig. 1.1, diffusive transport of SO2 at the local scale is important as a large proportion of SO2 is efficiently dry deposited relatively close to source due to convective ground-based turbulent processes. However, if the pollutant is emitted above the height of ground-based processes, oxidation to SO42- may take place and the pollutant may then travel long distances before being wet deposited far from source (Mylona 1996).
1
During the 19th century, atmospheric pollution was primarily a local problem as depositions occurred relatively close to source owing to the generally low height of chimney stacks. However, during the first half of the 20th century, the spatial distribution of emission sources changed. Point source emitters moved from small local sources to larger power generation stations with taller chimneys, resulting in emissions being vented above groundbased and convective turbulent processes and allowing for the long-range atmospheric transport of pollutants (Rose 2001).
Distance Transformation to secondary pollutant
Altitude
e.g. oxidation of SO2 to SO42--
Zone of ground based and convective turbulence
Dry deposition
Wet deposition e.g. SO42-, SO2
e.g. SO2
Primary pollutant emission
Lake
Dry deposition e.g. SO2
e.g. SO2
Primary pollutant emission
Lake
e.g. SO2
Figure 1. 1 Schematic diagram of the atmospheric pollutant lifecycle from emission to deposition
By the 1960s, the detrimental impacts of increasing levels of atmospheric contamination on the health of freshwater ecosystems had been recognised. Oden (1968) highlighted the impacts of increasing levels of acidic deposition on levels of surface water acidity in Norway. In response to such concerns, stringent national pollution control programmes (e.g. the Clean Air Act (UK), 1956) and international protocols (e.g. the UNECE LRTAP, 1983) were adopted. Implementation of these measures has led to a reduction in levels of primary pollutant emissions (Wichmann 2004), particularly SO2, while emissions of NOX have decreased but less dramatically. For example, levels of SO2 and NOX emissions declined throughout Europe between 1980 and 2000, by 70% and 25 - 30%, respectively (Grennfelt and Ǿystein 2005). This difference in magnitude is primarily due to the differing sources of both pollutants. NOX emissions are sourced primarily from the transportation sector; even though measures (e.g. catalytic convertors) have been introduced to decrease emissions from transportation, increasing levels of vehicle ownership are serving to offset
2
any reductions (Erisman et al. 2003). In addition, emissions from other sectors of the transportation industry are of growing concern.
For example, international shipping
contributed 20% of total NOX emissions globally in 2000 (Grennfelt and Ǿystein 2005).
1.1.2 Ecosystem health impacts Increased levels of acidic depositions have resulted in the acidification of surface waters, causing alteration and deterioration of almost every aspect of aquatic ecosystems (Smol 2008). Furthermore, lake and river sediments have the potential to act as sources of toxic substances deposited from the atmosphere that have accumulated under different redox conditions (Rippey et al. 2008).
1.1.2.1 Acidification The acidification of precipitation by increasing levels of industrial pollutant emissions, particularly SO2 and NOX, and its potential for long-range transport and deposition as acid rain have long been widely known (Gorham 1998). The detrimental impacts of increasing levels of acidic depositions include declining fish populations (Hesthagen et al. 1995) and impoverishment of freshwater biota (Muniz 1991). However, the most emphatic evidence for surface-water acidification stems from lake water diatom-based pH reconstructions (DIpH) (Flower et al. 1987). Diatoms are uni-cellular aquatic organisms and water chemistry is a strong influence on the composition of diatom communities (Battarbee et al. 1999). Diatoms preserved in lake sediment cores permit the historical reconstruction of variations in lake water pH during the industrial period, and these reconstructions reveal reduced lake water pH for sites throughout Europe and North America (Battarbee and Charles 1986). The impacts of increased acidity of precipitation on lake ecosystems were evident as far back as the mid 1950s (Gorham 1955). Lake sites situated on thin soils underlain by relatively slow weathering hard rocks (e.g. granite) that release base cations very slowly were particularly vulnerable.
Under such conditions, acidic inputs are not neutralised
before entering the lake water, resulting in surface water acidification (Jenkins et al. 1998). Notably, decreasing levels of surface water pH were synchronous with increasing levels of deposition of particles and trace metals associated with the industrial-scale combustion of fossil fuels (Battarbee 1990).
3
The implementation of both national legislation and international protocols aimed at reducing emissions of acidic pollutants, e.g. the Helsinki Protocols (1985; 1994) and the Sofia Protocol (1988), have had some success leading to recent decreases in emissions of atmospheric pollutants. These reductions have been followed by evidence of chemical recovery of some surface waters.
However, there is little evidence for large scale
biological recovery of surface waters following decreased levels of acidic depositions (Skjelkvåle et al. 2003), although there are some signs of biotic change, particularly at the more contaminated sites (Monteith and Shilland 2007).
1.1.2.2 Toxicity Sediments are recognised as sinks and reservoirs for metals, metalloids and other toxic substances released to the atmosphere during fossil fuel combustion (Rippey et al. 2008). Toxic substances, particularly trace elements and PAHs, can accumulate in both contaminated lake waters and sediments to levels that may have adverse effects on biological communities.
For example, Crouteau et al. (2001) showed that Cd
contamination of sediments led to increased levels of Cd bio-accumulation, while Rippey et al. (2008) demonstrated that levels of toxic contamination measured in some UK lakes were detrimental to chironomid communities (Rippey et al. 2008).
Currently, little
integrated assessment of the biological effects of toxic chemicals in freshwater ecosystems has been undertaken.
The work that has been carried out is generally limited to the
application of sediment quality guidelines (SQGs), which provide an estimate of threshold contaminant levels above which detrimental biological effects are expected (Ingersoll et al. 2001). The Aarhus Protocol (1998), which provides a legislative framework for controlling emissions of toxic pollutants, also encourages relevant research, development and monitoring.
1.2 Palaeolimnology Historical perspectives on the atmospheric deposition of contaminants allow the identification of past levels of pollutant availability and the establishment of how these levels have varied through time, while spatial analyses allow the identification of areas that have experienced the highest levels of contamination. Such perspectives allow for more reasoned debates on impacts (e.g. surface water acidification) and the effectiveness of mitigation strategies, technologies and legislation. The monitoring of atmospheric pollution
4
in Europe is a relatively recent phenomenon, with most reliable monitoring networks being established in the 1980s. In the absence of long time-series data sets from monitoring, palaeolimnology offers a tool for reconstructing spatial and temporal variations in levels of deposition of atmospheric pollutants. Palaeolimnology is a multi-disciplinary science that uses the physical, chemical and biological information preserved in accumulations of sediment to reconstruct past environmental conditions in inland aquatic systems (Smol 2008). Lakes act as sinks for atmospheric pollutants deposited on the earth’s surface through both wet and dry deposition: as well as providing a record of within-lake changes, lake sediments provide a reliable record of atmospheric deposition on a lake and its catchment (Rose and Monteith 2005). Records of the deposition of atmospheric pollutants can be obtained directly from lake sediments, by analytical measurements of contaminants such as SCPs, PAHs, and trace elements (Rippey and Charles 1990; Von Gunten et al. 1997; Fernandez et al. 2002; Lima et al. 2003).
Alternatively, information on past levels of contaminants can be
obtained indirectly by determining changes in abundances of remains of organisms, such as diatoms, that are proxies of pollution-induced water quality changes (Davis 1987; Korhola et al. 1999). SCPs are a direct by-product of the industrial-scale combustion of fossil fuels.
Their
accumulation in sediment can therefore provide a direct indicator of past variations in the industrial-scale combustion of fossil fuels (Rose 2001). Thus, sediment-based records of variations in concentrations of SCPs show strong correlations with patterns of fossil fuel combustion, emissions of SO2, and power generation (Rose and Juggins 1994). PAHs are also formed during the fossil combustion process, emitted to the atmosphere and subsequently deposited onto lakes and their catchments, and their levels in sediments are therefore expected to correlate closely with those of SCPs. Indeed, PAHs have been accumulating in lake sediments in increasing quantities since the 19th century (Fernandez et al. 2003). Trace elements are also emitted during combustion of fossil fuels, and lake sediment records from sites located across the globe archive a pattern of increasing deposition of trace elements over the past 150 years as a result of the widespread effects of industrialisation and urbanisation globally (Renberg 1986; Renberg et al. 2001).
5
1.3 Research rationale and thesis structure 1.3.1 Research rationale Up until relatively recently little environmental monitoring has been carried out in Ireland. There is thus a paucity of monitoring data. This is particularly the case for depositions of atmospheric pollutants. For example, monitoring of atmospheric deposition in Ireland began in 1980 and, at this time, was limited to a single site: Valentia Observatory in Co. Kerry, in the southwest of Ireland. The monitoring network was not extended further until the 1990s (Bashir et al. 2006).
In addition, data relating to the effects of long-term
variations in levels of atmospheric deposition on freshwater ecosystems in Ireland are limited. Large areas of Ireland are sensitive to surface water acidification, particularly along the western seaboard and in the east of Ireland (Aherne et al. 2002). Moreover, levels of atmospheric deposition in Ireland are believed sufficient to acidify poorly buffered surface waters (Aherne and Farrell 2002). Currently, estimates of critical loads of acidity for freshwater ecosystems in Ireland are based on the steady state water chemistry (SSWC) model (e.g. Aherne and Curtis 2003). However, the applicability of such models is undermined by changes in chemistry due to inputs of sea-salt and dissolved organic carbon (DOC), both of which are common in acidification-prone parts of Ireland (Flower et al 1994; Aherne et al. 2002). Owing to this absence of long, time-series datasets from environmental monitoring, unequivocal evidence of the acidification of freshwater ecosystems by the atmospheric deposition of acidic pollutants is currently lacking (Aherne et al. 2002). This research seeks to address two main research questions. These research questions, and the means of answering them, follow: 1. What are the main spatial and temporal differences between the three atmospheric pollutant deposition regions (APDRs) in Ireland in the levels of deposition of atmospheric contaminants from the industrial-scale combustion of fossil fuels during the last 50 – 150 years? This research question is addressed by assessing:
6
−
the relative contributions of national and transboundary pollutant sources to levels of deposition of atmospheric pollutants;
−
the
effectiveness
of
recent
legislation,
mitigation
strategies,
and
technologies aimed at reducing emissions of atmospheric pollutants; and −
the applicability of the SCP dating technique in estimating sediment chronologies at lake sites in Ireland;
2. To what extent have these depositions impacted freshwater lake ecosystems? This research question is addressed by assessing: −
the relative sensitivities of the three study sites to variations in levels of deposition of acidic pollutants; and
−
the levels of toxicity in surface sediments at the three study sites and their potential for adverse impacts on aquatic biota.
1.3.2 Thesis structure The research framework underpinning this thesis is illustrated in Fig. 1.2.
Chapter 2 examines existing and relevant knowledge relating to contemporary spatial patterns in levels of deposition of atmospheric pollutants in Ireland. Three distinct regions of deposition, or APDRs, in Ireland are identified. The criteria for selecting the study sites, one from each of the three APDRs, are then described. Finally, based upon locational and palaeolimnological data, three suitable lake sites are identified for study. The characteristics of each of the three study sites and their respective catchments are then described.
Chapter 3 outlines the techniques, procedures and proxies employed in this study. The palaeolimnological methods and sediment-based proxies are first examined. The application of SQGs in determining levels of possible sediment toxicity are then described, while the numerical methods used in primary data analysis and for DIpH reconstruction are also outlined. The secondary sources of data (inventories of SO2 and SOX emission, power station capacity data and histories, and modelled
7
SO42- depositional data) utilised in the current research are then discussed and their relevance examined.
Chapter 4 details palaeolimnological data from the three lake sites, and the analyses of these data. Principal component analysis (PCA) and normalisation techniques are applied to geochemical data in order to distinguish between natural and anthropogenic inputs of trace elements and the details of PAH ratios are presented.
The transfer function approach is applied to diatom data, and the
accuracy of DI-pH reconstructions assessed while the details of SQG analyses are presented. Finally, site summaries are presented in order to confirm the suitability of the three study sites for the purposes of the current research.
Chapter 5 synthesises, discusses and interprets results presented in Chapter 4. Results of palaeolimnological proxies of depositions of atmospheric pollutants are discussed in relation to secondary sources of data, in order to test hypotheses about contaminant sources and also to assess the reliability of the lake sediment record in recording variations in levels of atmospheric contamination. Results of palaeolimnological proxies of ecosystem response are then discussed in relation to palaeolimnological
evidence
of
Palaeolimnological
reconstructions
depositions of
variations
of in
atmospheric levels
of
pollutants. atmospheric
contamination are then used to assess: the relative contributions of national and transboundary sources of pollutant emission to levels of deposition of atmospheric pollutants; the effectiveness of recent legislation, mitigation strategies and technologies aimed at reducing emissions of atmospheric pollutants; and the applicability of the SCP dating technique as a means for providing chronological control for sediments accumulating in lakes in Ireland. Finally, palaeolimnological reconstructions of ecosystem response to variations in levels of atmospheric pollutant deposition are discussed, particularly the underlying factors influencing the relative sensitivity / insensitivity of the three study sites to increased acidification pressures.
Chapter 6 summarises the current research, presents the main conclusions and addresses the limitations. Finally, suggestions for future research are provided.
8
Secondary sources
Secondary sources Palaeolimnological proxies confirming selection of lake sites
Identification of APDRs based upon multi-disciplinary research
−Loss-on-ignition analysis −Pollen analysis
Chronological control of sediment cores −210Pb dating −137 Cs dating −SCP dating
EAPDR WAPDR
SWAPDR
Palaeolimnological proxies of depositions of atmospheric pollutants −SCP analysis −Trace element analysis −PAH analysis
Palaeolimnological proxies assessing ecosystem response Core of lake sediment
–Diatom analysis −SQG analysis
Interpretation aided and reliability assessed by comparison to: •Officially reported and historically reconstructed estimate of SO2 emission •Power station capacity records and histories •Modelled levels of SO42- deposition
Reliability assessed by comparison with: Recently measured levels of surface water pH
Lake site selection based upon locational data, past palaeolimnological research and existing conditions
Primary sources Figure 1. 2 A schematic diagram of the research framework adopted in this thesis. Both primary and secondary data sources are detailed.
9
Chapter 2: Deposition of atmospheric pollutants in Ireland and selection of study sites This chapter examines existing and relevant knowledge relating to contemporary spatial patterns in levels of deposition of atmospheric pollutants in Ireland, and identifies three distinct APDRs. In the current research, ‘Ireland’ refers to the island of Ireland (both Northern Ireland and the Republic of Ireland) and the term ‘national’ is used to refer to the same geographic area. The criteria, based on the results from past palaeolimnological research, which lake sites must meet in order to be considered suitable repositories for information on temporal and spatial variations in atmospheric pollutant deposition are then described. Finally, based on palaeolimnological research undertaken in Ireland, three lake sites are identified as suitable, one from each of the previously identified APDRS. The characteristics of each site and their respective catchments are then detailed.
2.1 Contemporary spatial patterns in levels of depostion atmospheric pollutants in Ireland Ireland is a small island located off the northwestern shore of the European landmass (Mitchell and Ryan 1990).
Overall levels of deposition of atmospheric pollutants are
relatively low due to the predominance of unpolluted westerly air flows originating over the Atlantic Ocean (Bowman and Harlock 1998), and the favourable location of Ireland on the western fringe of the European continental shelf (Aherne and Farrell 2002). The prevailing wind direction is from the southwest, originating over the Atlantic Ocean (Sweeney 1997), and air masses and associated precipitation originating over the Atlantic Ocean are not normally anthropogenically acidified (Bailey et al. 1986), being dominated by sea-salts (Aherne and Farrell 2002). For example, in 2006, the average concentration of nonmarine SO42-, a direct indicator of pollutant inputs (Proctor and Maltby 1998), measured in precipitation samples at 4 European Monitoring and Evaluation Programme (EMEP) measurement sites in Ireland was 0.14 mg/l compared with an average of 0.37 mg/l measured across the entire EMEP measurement network (Hjellbrekke and Fjæraa 2008). However, a quantitative assessment of spatial patterns in total (wet and dry) levels of deposition of atmospheric pollutants in Ireland is difficult because of a lack of measurement data, particularly dry depositional data. Wet atmospheric depositions are
10
generally thought to reflect levels of pollutants from long-range transboundary sources, while dry depositions are generally thought to reflect levels of depositions from local or regional sources (Mylona 1996).
Owing to the insufficient geographic coverage of
measured depositions of pollutants, modelling of pollutant depositions has been undertaken in some parts of Europe, including Ireland. Atmospheric models describe the process of emission, transport, chemical transformation and removal of pollutant species from the atmosphere (Mylona 1993), and can be used to predict levels of pollutant concentration and deposition in regions deficient in empirical data.
The meteorological service in the Republic of Ireland (Met Eireann) has since 1958 operated a programme of monthly sampling and analysis of precipitation at several sites throughout Ireland. Data from this network have been used in a number of studies and deposition mapping assessments (Stevenson 1968; Fisher 1982; Jordan 1997). However, the network operated by Met Eireann was not specifically designed to provide accurate information on the deposition of pollutant species (Mathews et al. 1981). Consequently the monitoring sites are not ideally located, being situated in urban and coastal areas, and the frequency of sampling, being monthly, is too low.
More recently, a number of
measurement sites have been established throughout Ireland with daily and weekly sampling regimes.
Aherne and Farrell (2002) examined the chemical composition of
rainfall collected from 10 of these measurement sites during the eight year period, 1991 1998. All 10 sites were remote from sources of pollution and thus provide an accurate assessment of spatial patterns in levels of the wet deposition of atmospheric pollutants in Ireland. In the current research, of the three pollutant species measured by Aherne and Farrell (2002) in precipitation (non–marine SO42-, NO3- and NH4+), non-marine SO42- is considered to be the best indicator of atmospherically-derived industrial pollutant inputs. This is because the majority of non-marine SO42- depositions originate from industrial-level combustion sources, while the primary source of NO3- and NH4+ are, respectively, transport and agriculture (Erisman et al. 2003). In contrast, in Ireland, data that are used in the estimation of the dry deposition of pollutants are not available.
However, research
undertaken by Bowman and Harlock (1998) and Rose and Harlock (1998c) reported levels of SCP concentrations in surface lake sediments at 57 sites throughout Ireland, and spatial variations in concentrations of deposited SCPs are similar to those of deposited nonmarine SO42- (Rose and Juggins 1994; Rose and Monteith 2005).
Furthermore, SCP
deposition is thought to take place predominantly within ca. 100 km of the source (Fernandez et al. 2002). SCP data can therefore be used as a proxy for non-marine SO42deposition, and, thus, for levels of dry deposition of atmospheric pollutants from the
11
industrial-scale combustion of fossil fuels. In addition, EMEP have modelled levels of deposition of acidifiying pollutants in Ireland, including SOX, heavy metals, and PM for 2005. Generally, for acidifying pollutants, modelled results compare well with measured levels of pollutant deposition and concentration (e.g. Asman and van Jaarsveld 1991; Iverson 1993). Modelled levels of SOx deposition are considered to be a good indicator of depositions of atmospherically-derived pollutants originating from industrial-level sources (e.g. power stations) (Erisman 1993; Metcalfe and Derwent 1993). Using officially reported emissions data, the EMEP unified model was used to reconstruct variations in levels of deposition of SOX in Ireland for 2005 (Klein et al. 2007). The model uses meteorological and emissions data for 2005 and, as illustrated in Fig. 2.1, details levels of deposition of SOX on a national basis at a 50 x 50 km resolution (Klein et al. 2007). However, modelled depositions, as reported by Klein et al. (2007), take into account levels of deposition of marine-derived SOX and are limited in their spatial resolution. At a sub-grid scale, there can be significant variations in levels of pollutant depositions due to factors such as local sources of emission, and differences in vegetation cover and topography, amongst others. In addition, the validation of modelled depositional data for Ireland is limited to only one EMEP measurement station, Valentia observatory (Co. Kerry).
A high degree of
association was observed between measured and modelled levels of SO2 and SO42showing a bias of - 7% and- 31%, respectively, while SO4 showed a much lower degree of association, with a bias of - 84% (Klein et al. 2007).
As illustrated in Fig. 2.2, elevated levels of deposition of both wet deposited non-marine SO42- and SCPs are apparent in the east of Ireland and are thought to be due to depositions originating from a combination of transboundary and national emission sources (Bailey et al. 1986; Bowman and Harlock 1998; Aherne and Farrell 2002; Rose and Theophile 2004). In contrast, as indicated by the relatively low levels of SO42- measured in precipitation, the influence of transboundary pollution in the west is not marked (Bowman and Harlock 1998).
However, elevated levels of SCP deposition in the mid-west,
particularly in proximity to the Shannon estuary, are thought to be a result of emissions originating from sources located on the Shannon estuary (Bowman and Harlock 1998). The lowest levels of depositions of both wet deposited non-marine SO4-2 and SCPs are observed in the southwest, because this region is situated to the south (leeward) of the same emissions sources on the Shannon estuary (Bowman and Harlock 1998) and is bounded on three sides by the Atlantic Ocean.
12
Legend Units: mg(SOX)/m2 500-750 350-500 200-350
56 ˚N
55 ˚N
100-200
54 ˚N
53 ˚N
52 ˚N
0 11˚˚W
10 ˚W
09 ˚W
08 ˚W
50
100 Km 07 ˚W
51 ˚N
06 ˚W
Figure 2. 1 Modelled levels of SOX deposition for Ireland in 2005, as reported by Klein et al. (2007)
2.1.1 Location, prevailing wind direction and transport of transboundary pollutants Due to a combination of location and prevailing wind direction, Ireland’s exposure to transboundary air pollution is limited. However, occurrences of enhanced levels of deposition of transboundary pollutants, particularly in the east, are associated with easterly air streams, originating over mainland Europe and Britain (Bowman and Harlock 1998; Aherne and Farrell 2002). According to recent estimates, 67% of SOX depositions recorded in Ireland originate from transboundary sources, particularly from Britain, which is estimated to contribute 21% of SOX depositions (Klein and Benedictow 2006).
This is to be expected as there is a number of large point sources located in
the west of Britain and in close proximity to the east coast of Ireland, albeit just beyond the distance (approximately 300 km) over which the dry deposition of atmospheric pollutants generally occurs (Pickering 1997) (Fig. 2.3).
13
Ireland receives a small amount of polluted airflow from the west, as a result of the return flow of pollutants originating from the east (Nyberg 1977), from international and passenger cargo shipping in the Atlantic Ocean (Endresen et al. 2003), and episodes of trans-Atlantic pollutant transport from the United States (Nyberg 1977). As examples of the latter, both Coggins et al. (2006) and Schell et al. (1997) suggested that some trace elements deposited in Ireland originated in the northeastern United States, while Derwent et al (1998) maintained that episodes of trans-Atlantic transport of O3 to Ireland have also occurred.
In contrast, the occurrence of easterly air masses, originating over continental Europe and Britain, can lead to the deposition of relatively high levels of atmospheric pollutants, particularly in the east.
For example, Bowman and McGettigan (1994)
reported that levels of non-marine SO42- measured in precipitation at sites in the east and west of Ireland associated with easterly air masses were four to seven times (8 – 15 mg m-2) and two times (3 - 5 mg m-2) greater, respectively, than precipitation associated with westerly air masses (2 mg m-2 and 2 - 3 mg m-2, respectively). Moreover, an east to west gradient of declining depositions of atmospheric pollutants has been widely reported, with enhanced levels of depositions in the east being due to transboundary sources located in Britain and mainland Europe (Bowman and McGettigan 1994; Jordan 1997; Bowman and Harlock 1998; Aherne and Farrell 2002; Rippey and Douglas 2004).
14
Aherne Events Legend nss Non-marine SO42- (mg/l) 10
Legend
Legend FLAME Events -1 SCP SCP (gDM )
56
1,000
56
10,000
20 30
50,000
40
55
100,000
55
54
54
53
53
52
0
20
40
52
0
80 Kilometers
20
40
80 Kilometers
51 11
10
09
08
07
11
10
09
08
07
06
SO42-
Figure 2. 2 Maps detailing average levels of non-marine measured in precipitation at 10 sites throughout Ireland for the eight year period, 1991 – 1998, as reported by Aherne and Farrell (2002) (left), and levels of SCP concentration measured in surface sediment samples from 57 lakes throughout Ireland, as reported by Bowman and Harlock (1998) and Rose and Harlock (1998c) (right).
15
10°0'0"W
5°0'0"W
0°0'0"
65°0'0"N
Legend = Fossil fuel combustion power station > 50 MW
65°0'0"N
65˚N
= Major city
60°0'0"N
60°0'0"N
60˚N
Longannet
Glasgow
Cockenzie
Edinburgh Lynemouth
Newcastle
55°0'0"N
55°0'0"N
Teeside
55˚N Eggborough Ferrybridge
Manchester Liverpool Fiddlers Ferry Ironbridge
Drax Saltend
Sheffield South Humber Bank Ratcliffe Rugeley
Killingholme West Burton Cottarn
Birmingham Didcot Aberthaw
Bristol
Kingsnorth
London Grain
Cardiff Fawley Littlebrook
50°0'0"N
50°0'0"N
50˚N
0
10°0'0"W
10˚W
5°0'0"W
5˚W
100
200
400 Kilometers
0°0'0"
0˚W
Figure 2. 3 Map detailing the locations of fossil fuel combustion power stations > 50MW (BERR 2007) and major cities in Britain.
16
2.1.2 National sources Emission sources situated In Ireland are estimated to contribute to approximately 33 % of total SOX depositions (Klein and Benedictow 2006), and are dominated by a small number of large facilities, in particular power stations (Bowman and Harlock 1998). These power stations are mainly located in the east, in and around Dublin, in the west, notably on the Shannon estuary, and in the north-west. For example, the European Pollutant Emission Register (EPER) (2004) reported that total SOX emissions in Ireland amounted to 70,056 tonnes, of which 59,513 tonnes (85%) could be attributed to power stations > 50 megawatts (MW). As illustrated in Fig. 2.4, Moneypoint (coal-fired) and Tarbert (oil-fired), located on the Shannon estuary, account for 39% and 7% of total emissions, respectively.
By comparison, Kilroot (oil and coal-fired), situated in the
northeast, and Poolbeg (oil and gas-fired), situated in the east, account for 21% and 7% of total emissions, respectively.
Because of this distribution, the influence of
national emission sources on levels of deposition of atmospheric pollutants is evident primarily in the east and mid-west of Ireland, the latter particularly in the area surrounding the Shannon estuary.
Concentrations of SCPs in samples of sediment from lakes in Ireland provide a means of assessing the contribution of national sources of emission to total levels of deposition of atmospheric pollutants.
Accordingly, the highest levels of SCP
concentrations in surface sediments were measured in lakes situated in the east of Ireland (Bowman and Harlock 1998). For example, the highest recorded concentration of SCPs (78,629 gDM-1) was observed at Lough Bray, in the Wicklow mountains, approximately 20 km south of the Dublin urban area. Such a high concentration was believed to be the result of pollutant depositions sourced from the Dublin urban area being superimposed upon on a transboundary contribution (Bowman and Harlock 1998). The influence of transboundary sources was not found to be so marked in the west of Ireland, and elevated concentrations of SCPs (24,682 – 29,509 gDM-1) in surface sediments could be attributed to national sources, particularly Moneypoint power station (Bowman and Harlock 1998). In contrast, samples of surface sediments from lakes located in the southwest of Ireland showed the lowest concentrations of SCPs (2,922 – 16,473 gDM-1) (Bowman and Harlock 1998). Moreover, a five year annual mean concentration of non-marine SO42-, measured in precipitation at Valentia observatory, was the lowest recorded at any station throughout Ireland (Bashir et al. 2006).
17
As illustrated in Fig. 2.5, Ireland can be divided into three APDRs:
1. The Eastern Atmospheric Pollutant Depositional Region (EAPDR) which is in receipt of depositions of atmospheric pollutants originating from a combination of both national sources, particularly those located in the east of Ireland, and transboundary sources, located in Britain and mainland Europe; 2. The Western Atmospheric Pollutant Depositional Region (WAPDR), situated to the north (windward) of pollutant sources located on the Shannon estuary (e.g. Moneypoint power generation station), which is in receipt of depositions of atmospheric pollutants originating primarily from national sources; 3. The Southwestern Atmospheric Pollutant Depositional Region (SWAPDR), situated to the south (leeward) of pollutant sources located on the Shannon estuary, which is in receipt of relatively low levels of deposition of atmospheric pollutants from both national and transboundary sources.
497000
56
Ballylumford (1%) 397000
.000000
Coolkeragh (1.5%)
.000000
55
Kilroot (18%)
.000000
Bellacorrick (1%)
54
297000
Dublin urban Area Lanesborough (0.5%)
Edenderry Power (2%) (3%) Shannonbridge (1%)
Northwall (O%)
.000000
Poolbeg (7%)
197000
53
(39%) Moneypoint (33%)
Shannon-based industries
.000000
Tarbert (7%)
97000
Great Island (3%)
52
Aghada (0%) Marina (0%)
0
11
10
09
35
08
70
140 Kilometers
07
06
Figure 2. 4 Location map of power stations > 50 MW in Ireland. Percentage contributions to total levels of Irish emission of SOX as reported by EPER (2004) are detailed. The locations of the Dublin urban area and Shannon-based industries are also shown.
18
Wind direction frequency (Claremorris)
Wind direction frequency (Dublin Airport) North 15% Northwest
10%
55
Northeast
North 20%
5% West
0%
Northwest
East
Northeast 10%
Southwest
Belfast
Southeast
West
0%
East
South
Southwest
WAPDR
Southeast
54
South
EAPDR Dublin urban area Wind direction frequency (Valentia)
Shannon-based industries
53 Long-range transport of pollutants originating from Britain and mainland Europe
North Northwest
10%
Northeast
5% West
0%
Southwest
East
Southeast South
SWAPDR
0
10
09
08
85
07
170
06
52
51 340 Kilometers
05
Figure 2. 5 Schematic map of Ireland showing the three identified APDRs. Wind roses and major industrial areas are also shown.
19
2.2 Selection of study sites Lake sediment profiles widely record a pattern of increasing depositions of heavy metals (Rippey and Douglas 2004) and organic and inorganic combustion by-products (Rose et al. 2003; Quiroz et al. 2005) over the past 200 years. Much of this increase is attributable to the rapid expansion of heavy industry and the construction of power stations since the late 19th century.
In addition to these global patterns, more regional effects are evident,
reflecting local variations in processes of industrialisation (Cohen 2003). However, not all lakes provide equally useful records of past variations in the deposition of atmospheric pollutants, and remote lake ecosystems, such as on high mountains (Muri et al. 2006), are considered to be particularly accurate recorders of atmospheric pollution histories (Camarero et al. 1995). This is provided that the rate of sediment accumulation is sufficient to allow for the preservation of a finely resolved temporal record, and that there has been no post-depositional alterations of sediments, such as, for example, by re-working (Lima et al. 2003).
Remote lake ecosystems are characterised by not being influenced by direct forms of disturbance, such as waste water inputs or land-use change (Wathne et al. 1995), with atmospheric transport representing the main source of pollutant input (Fernandez et al. 2003). In addition, surface water ecosystems in mountainous areas are generally more vulnerable to pollutant inputs than lowland lakes because of climatic, edaphic, geological and topographic factors, and can be particularly sensitive to variations in atmospheric deposition (Mosello et al. 1992). As a result, remote lake ecosystems have been recognised as important indicators of global atmospheric problems such as acidification (Battarbee et al. 1989; Battarbee 1990), global warming (Hughen et al. 2000), and the long-range transboundary transport of pollutants, including heavy metals (Rose et al. 1998b), SCPs (Rose 1995) and persistent organic pollutants (POPs), such as PAHs (Fernandez et al. 2000; Barra et al. 2006).
For the purpose of the current research, three lake sites, one from each of the three previously identified APDRs, were selected for analysis. Initially, each site was required to meet two criteria: −
The lake and its catchment should not be in proximity to point sources of atmospheric pollutant emissions;
20
−
The lake and its catchment should be free from major changes in land cover during the period of interest and atmospheric transport should represent the main pathway of pollutant input.
Following a preliminary examination of sediments, each of the three potential study sites was required to meet two additional criteria: −
The lake should be sensitive to variations in levels of atmospheric deposition;
−
The lake should have a finely resolved temporal sedimentary record that has not been subject to post-depositional alteration.
On this basis, and as illustrated in Fig. 2.6, three lake sites were selected as suitable for study, based on palaeolimnological and locational data: Kelly’s Lough in the EAPDR; Lough Maumwee in the WAPDR; and Upper Killarney Lough in the SWAPDR. Summary locational, hydromorphological and lake water chemistry data are provided in Table 2.1.
56
WAPDR 55
EAPDR
#
54
Lough Maumwee
Kelly's Lough
#
53
Upper Killarney Lough
#
0
25
50
SWAPDR
52
100 Kilometers
51 10
09
08
Figure 2. 6 Map of Ireland detailing locations of selected study sites. The three APDRs are also marked
21
Lake name Kelly’s Lough
Location
County Wicklow 52° 57’ 17” N 6° 25’ 45” W Lough Maumwee County Galway 53° 28’ 30” N 9° 32’ 35” W Upper Killarney County Lough Kerry 51° 58’ 42” N 9° 35’ 38” W
Altitude (m amsl) 585
Lake area Catchment:surface Max. depth (ha) area ratio (m) 3 8.3 8.6
pH 5.1+
Conductivity Alkalinity (mg l-1CaCO3) (µS cm-1) 25+ 0.46+
oligotrophic+
48
27
15.9
7.9
5.9-6.7^
95*
0.58^
oligotrophic^^
18
169.9**
66.5
36.1**
6.41**
58**
2.8**
oligotrophic**
Table 2. 1 Summary locational, hydromorphological and water chemistry data for the three study sites. *Indicates data from Flower et al. (1994) **Indicates data from Leira et al. (2006) + Indicates data from Leira et al. (2007) ^Indicates data from Toner et al. (2005) ^^ Indicates data from De Eyto and Irvine (2007) ++ based on OECD (1982) classification
22
Trophic status++
2.2.1 Kelly’s Lough, Co. Wicklow Kelly’s Lough (52˚ 57’ 17’’ N, 6˚ 25’ 45” W) (Plate 2.1) is situated in the east of Ireland in the Wicklow mountains, within the Lugnaquillia complex, which is an extensive upland area. Lugnaquillia, located in the middle of the complex, is the highest mountain in the Wicklow mountains, at 925 m AMSL. Kelly’s Lough is a small dark water cirque lake located at 585 m AMSL, with surface and catchment areas of 0.03 km2 and 0.25 km2, respectively (Fig. 2.7), and a maximum water depth of 8.6 m (Fig. 2.8).
The bedrock of the Lugnaquilla complex is predominately granite surrounded by schist and slates (Leira et al. 2007). The main land-use of the area is sheep grazing and afforestation (Leira et al. 2007), although there is no afforestation above the altitude of the lake. The catchment area for Kelly’s Lough comprises blanket bog, with peat depths of up to 2 m, and bare rock surfaces, and there is evidence of considerable peat erosion in the area (Leira et al. 2007). Vegetation consists of a mix of heath, blanket bog and upland grass dominated by Calluna vulgaris, Nardus stricta and Eriophorum species (Leira et al. 2007).
Kelly’s Lough is situated approximately 45 km southeast and 164 km south of the large point source emitters located in the Dublin urban area (e.g. Poolbeg power station) and the northeast of Ireland (e.g. Ballylumford and Kilroot power stations), respectively. Furthermore, of the three study sites, Kelly’s Lough is the most proximate to transboundary sources of pollutant emission located in western Britain, particularly the urban centres of Liverpool and Manchester (situated approximately 233 km and 285 km east of the site, respectively), and Cardiff and Bristol (situated approximately 275 km and 310 km southeast of the site, respectively).
Although Aherne and Curtis (2003) maintain that lakes in Ireland are not under a significant threat from anthropogenic acidification, evidence exists of surface water acidification prior to the mid-1990s in several lakes and rivers in the Wicklow mountains, typically oligotrophic upland water bodies on acid substrata (Kelly-Quinn et al. 1996). Furthermore, Leira et al. (2007) examined Holocene-aged diatom and pollen records from Kelly’s Lough in order to determine the timing and extent of acidification in this upland lake. Acidic conditions were found to have prevailed in Kelly’s Lough throughout its entire history, although a recent phase of moderate acidification (pH 5.7 – 5.1) was recorded from the mid-twentieth century. Lowest DI-pH values were discovered to have occurred in the late 1970s (Leira et
23
al. 2007). Soil acidification and in-wash of organic acids from peatlands were not deemed to be a sufficiently effective mechanism to explain this decrease in pH, and it was suggested that
‘the effect of acid deposition on the waters of Kelly’s Lough is clear and it has probably caused these already naturally acid waters to acidify further’ (Leira et al. 2007: 49)
In addition, Leira et al. (2007) reported that the long-term sedimentation rate at Kelly’s Lough, estimated by 210Pb analysis, has remained relatively constant at 0.05 cm yr-1, while results of pollen analysis suggested that little change had occurred in catchment vegetation or conditions since approximately 1880.
Plate 2. 1 Kelly’s Lough
24
52˚ 58’ 00” = Forestry
52˚ 57’ 30”
52˚ 57’ 00” 06˚ 26’ 00”
06˚ 25’ 30”
06˚ 25’ 00”
06˚ 24’ 30”
06˚ 24’ 00”
Figure 2. 7 Location map of Kelly’s Lough. Spot heights (in m) are marked on the map. Contour intervals are shown at 50 m intervals
X
(metres)
Metres
Figure 2. 8 Bathymetry of Kelly’s Lough. The location of the coring site is marked with an X.
25
2.2.2 Lough Maumwee, Co. Galway Lough Maumwee (53˚ 28’ 30” N, 9˚ 32’ 35” W) (Plate 2.2) is an oligotrophic lake situated in a Special Area of Conservation (SAC), in a low-lying part of the Maamturk mountain complex of western Ireland. Lough Maumwee is an elliptical lake at an altitude of 48 m AMSL with a surface area of 0.27 km2. Maximum water depth in the lake was measured at 7.9 m, while the catchment area for the lake extends from 48 to 525 m AMSL and has a surface area of 4.3 km2 (figures 2.9 and 2.10).
The main basement rock types in the catchment are quartz and granite (Flower et al. 1994). Shallow stoney soils cover a relatively large proportion of the catchment (70%), while blanket peat (25%) and peatty podzols (5%) are less extensive. The catchment area is uninhabited and has sparse vegetation on its steep inclines; however, Molinia spp. and Calluna spp. are established in its lower lying areas (Flower et al. 1994). Low intensity grazing by cattle and sheep is the main land use, and peat-cutting for local use is on-going in the catchment.
Lough Maumwee is located approximately 98 km north of large emitters of pollutants on the Shannon estuary (e.g. Moneypoint power station) and approximately 41 km northwest and 109 km north of the cities of Galway and Limerick, respectively. The most proximate transboundary sources of pollutant emission are situated to the east in western Britain, approximately 455 km from the site, while large transboundary sources situated to the west of the site, in North America, are approximately 4,500 km distant.
Lough Maumwee is one of the three lakes in Ireland that, together with their inflowing waters, were selected by the EPA Ireland for long-term monitoring of acidification and recovery (Toner et al. 2005). This monitoring has been carried out since 2002 and involves the collection of water samples each December and April and their subsequent biological and chemical analyses. Previous palaeolimnological research at Lough Maumwee based on fieldwork in 1988 and published in Flower et al. (1994) reconstructed variations in levels of deposition of atmospheric pollutants, and consequent changes in aquatic biota and lake water quality.
The research utilised remains of diatoms, SCPs and trace elements
extracted from cores of lake sediment.
Flower et al. (1994) reported an average
sedimentation rate, estimated by 210Pb analysis, of approximately 0.3 cm yr-1. Furthermore, in-wash events were thought to have been insufficient to obscure the
210
Pb record, and
therefore that variations in catchment conditions as a result of anthropogenic activity had
26
not had a discernible effect on sedimentation at the site. Up-core variations in DI-pH indicated no consistent change in water acidity, remaining remarkably stable at DI-pH 5.7 – 6 throughout the period examined. The sediment-based record of SCPs commenced in approximately 1888, gradually increased to 1000 gDM-1 in 1973, before increasing strongly in the mid 1970s to a peak of 5000 gDM-1 in 1983. A subsequent slight decline to 1988 (the age of the surface sample of sediment at the time of fieldwork) was evident. Flower et al. (1994) opined that the 1980s peak may reflect national patterns in use of fossil fuels, in particular the commissioning of Moneypoint coal-fired power station.
Plate 2. 2 Lough Maumwee
27
53˚ 29’20” Forestry Regional road
53˚ 28’ 50”
53˚ 28’ 20”
53˚ 27’ 50” 09˚ 34’ 00”
09˚ 33’ 30”
09˚ 33’ 00”
09˚ 32’ 30”
09˚ 32’ 00”
09˚1 31’ 30”
Figure 2. 9 Location map of Lough Maumwee. Spot heights are marked on the map (in m). Contour intervals are shown at 100m
X
(metres)
Metres
Figure 2. 10 Bathymetry of Lough Maumwee. The location of the coring site is marked with an X.
28
2.2.3 Upper Killarney Lough, Co. Kerry Upper Killarney Lough (51˚ 58’ 42’’ N, 9˚ 35’ 38’’ W) (Plate 2.3) is an oligotrophic lake situated in the Killarney National Park in Co. Kerry, in the southwest of Ireland. In addition, the area has been designated a biosphere reserved by the United Nations Educational, Scientific and Cultural Organisation (UNESCO) in 1981. The lake is located at an altitude of 18 m AMSL and has a surface area of 1.7 km2, a maximum water depth of 36.1 m, and a catchment area of 113 km2 (Leira et al. 2006). Three rivers, the Crinnagh, the Owenreagh and the Gearhameen discharge into the lake, while the Longrange discharges from the lake (figures 2.11 and 2.12). The three rivers discharging into the lake measure 52.3 km in length, and their catchment includes part of the Macgillycuddy’s Reeks, the highest mountain range in Ireland (Kirk McClure Morton 2000).
The catchment for Upper Killarney Lough is underlain by Old Red sandstone, while the surface comprises peat bogs (83%), some forestry (6.3%), and small amounts of agricultural land (6.9%) (Kirk McClure Morton 2000; Leira et al. 2006). Vegetation on the extensive areas of blanket peat in the catchment is characterised by Calluna vulgaris and Poaceae (Kirk McClure Morton 2000) and there is no obvious evidence of recent anthropogenic activity within the catchment area other than the small extent of farmland. However, episodes of forest fires have been reported to have occurred in the woodlands surrounding the lake (Mitchell 1988, 1990; O’Sullivan 1990), and these could have effected levels of surface water acidity in the lake itself (Korhola et al. 1996). The lake is utilised primarily as a tourist amenity, being fished for Salmonidae, Salvelinus alpinus, and Alosa fallax killarnensis.
Upper Killarney Lough is not situated in close proximity to any large point sources of pollutant emissions: Killarney town is located approximately 15 km to the northeast of the lake; Cork city is situated 81 km to the southeast; and Shannon-based pollutant emission sources are situated approximately 71 km to the north. The most proximate transboundary sources of pollutant emission are situated to the east of the site in southwest Britain. For example, Aberthaw power station and the city of Cardiff are situated approximately 395 km and 410 km east of the site, respectively. Transboundary sources situated to the west, in North America, are approximately 4,950 km distant from the site.
29
Site selection in the SWAPDR was made difficult because of the relatively low rates of sediment accumulation previously identified for lakes in the southwest of Ireland (Leira et al. 2006). However, the sediment accumulation rate estimated for Upper Killarney Lough was known to be relatively high, at 0.25 cm yr-1 (Leira et al. 2006). Furthermore, Leira et al. (2006), based on a low resolution study of sediment-based proxies in a core of sediment from Upper Killarney Lough, maintain that conditions in the lake have remained oligotrophic for at least the last 150 years. This suggests largely stable ecological conditions have existed in the lake over a prolonged period, with lake water biota and water quality being relatively little affected by either nutrient enrichment or acidification. Finally, the existence of Salvelinus alpinus in the lake itself suggests that the lake remains relatively unimpacted by processes of eutrophication and acidification. Therefore, the fires in the catchment reported by Mitchell et al. (1988, 1990) do not appear to have had a significant impact on aquatic conditions, although they are likely to have been a source of combustion byproducts. Salvelinus alpinus is very sensitive to changes in water quality and were most likely the first freshwater fish to colonise Irish lakes after the ice age.
Since then,
populations throughout Europe have undergone a serious decline, and in some cases extinction, due to human-induced environmental pressures.
These declines have also
occurred in eastern Ireland, notably in Glendalough, Lough Tay and Lough Dan in the east of Ireland (Igoe et al. 2003).
Plate 2. 3 Upper Killarney Lough
30
51˚ 72’ 00”
51˚ 71’ 00”
gR on eL Th
ge an
51˚ 70’ 00”
51˚ 60’ 00”
Ow en r
ea gh
51˚ 59’ 00”
51˚ 58’ 00”
Minor road National road Forestry
51˚ 57’ 00” 09˚ 40’ 00”
09˚ 39’ 00”
09˚ 38’ 00”
09˚ 37’ 00”
09˚ 36’ 00”
09˚ 35’ 30”
09˚ 34’ 00”
09˚ 33’ 00”
09˚ 32’ 00”
Figure 2. 11 Location map of Upper Killarney Lough. Spot heights are marked on the map (in m). Contour intervals are shown at 100m.
X
(metres)
Metres
Fi gure 2. 12 Bathymetry of Upper Killarney Lough. The location of the coring site is marked with an X.
31
Chapter 3: Palaeolimnological methods and proxies The methodological framework in which the predominantly palaeolimnological field- and laboratory-based techniques are organised is illustrated in Fig. 3.1. Multiple cores were collected from each study site.
Pollen and LOI analyses were
employed to assess and confirm the suitability of lake sites for study. In addition, LOI analysis was used to align sediment stratigraphies between sediment cores collected from proximate locations at each lake site.
Once confirmed, chronological control of lake
sediment cores was provided through the use of the
210
Pb radiometric dating technique.
Subsequently, an examination of atmospheric pollutant deposition at each site was undertaken, employing a multi-proxy approach comprising SCPs, trace elements and PAHs. Such an approach allows for a more holistic examination and interpretation of variations in levels of deposition of atmospheric pollutants. SCPs provide a record of industrially-derived atmospherically deposited pollutants that, once deposited, are inert to changes in water and sediment chemistry. As a result, the SCP record can be considered both reliable and robust (Rose et al. 2004a), as SCPs unquestionably derive from industrial-sourced atmospheric pollution (Rose et al. 1994). In comparison, the interpretation of sediment-based trace element records is less straight forward owing to the influence of local site factors. In addition, post-depositional mobility of trace elements can occur, leading to further uncertainty (Boyle et al. 1999).
Similar
uncertainties apply to sedimentary records of PAHs. In order to assess the sources of atmospheric pollutants deposited at each lake site, a number of secondary sources of information are relied upon. In the absence of long-term atmospheric pollutant monitoring data, inventories of SO2 emissions, as reported by Mylona (1993, 1996) and EMEP, and information on power station capacity, as detailed by McCarthy (1957), Manning and McDowell (1984) and O’Riordan (2000), provide insights on possible levels of atmospheric pollution loads, and spatial and temporal changes in these. Moreover, modelled depositional data for SOX, as reported by Mylona (1993, 1996) and Klein et al. (2007), permit some examination of variability in levels of deposition of atmospheric pollutants.
Furthermore, secondary data sources (e.g. inventories of SO2
emission) can be used as a basis for assessing the reliability of the lake sedimentary record in reconstructing variations in levels of deposition of atmospheric pollutants.
32
In order to examine ecological responses to variations in levels of deposition of atmospheric pollutants at each study site, with particular reference to variations in surface water pH and possible levels of sediment toxicity, diatom and SQG analyses were employed, respectively. Diatoms are particularly sensitive to variations in levels of surface water pH. Recently reported surface water pH data can then be used to assess the reliability of DI-pH reconstructions. SQGs, established using matching sediment chemistry data and laboratory toxicity tests, define threshold concentrations of contaminants that determine whether detrimental biological effects can be expected. In this study, and in order to establish possible levels of sediment toxicity and their potential to affect biota, toxicity quotients for consensus-based probably effect concentration (PEC) and probable effect concentration-quotient (PEC-Q) values, as reported by McDonald et al. (2000), were calculated for measured concentrations of surficial sediment toxicants (trace elements and total PAH).
Primary sources
Interpretation aided by
Palaeolimnological proxies confirming selection of lake sites −Loss-on-ignition analysis −Pollen analysis
Secondary sources
Assessment of suitability of lake sites and alignment of sediment stratigraphies
Chronological control of sediment cores −210Pb dating −137 Cs dating −SCP dating
Palaeolimnological proxies of depositions of atmospheric pollutants −SCP analysis −Trace element analysis −PAH analysis
Core of lake sediment
Palaeolimnological proxies of ecosystem response –Diatom analysis −SQG analysis
Assessment of the reliability and accuracy of the lake sedimentary record. Assessment of levels of deposition of atmospheric pollutants at each lake site
Assessment of levels of surface water acidification
•Officially reported and historically reconstructed estimates of SO2 emissions •Power station capacity records and histories •EMEP modelled SOX deposition data
Recently measured levels of surface water pH
Assessment of possible levels of sediment toxicity and its potential for biological effects
Assesses reliability of
Figure 3. 1 Schematic diagram of the methodological approach adopted in this research. Note the prominence of palaeolimnological techniques.
33
3.1 Field based techniques
3.1.1 Sediment coring The collection of unmixed, ideally continuous sediment cores is the first and most critical step in the palaeolimnological process, as many of the errors or problems that are encountered during core collection cannot be corrected during subsequent analysis (Glew et al. 2001; Smol 2008). As most palaeolimnological analyses are time consuming, it is not generally possible to analyse more than a few sediment cores in detail. It is therefore critical that the sediments that are analysed are representative of overall limnological and other environmental changes (Smol 2008). Normally, this would be those sediments that are least disturbed and that have accumulated most rapidly. The choice of coring site can therefore be crucial (Allott et al. 1992), and often sediments accumulating in the deepest part of a lake are targeted, because of preferential deposition, or sediment focussing (Likens and Davis 1975).
Variability studies have shown that high reproducibility exists between multiple
cores of sediment retrieved from the deepest part of a lake basin (Anderson 1986; Charles et al. 1991; Rose et al. 1999a). Although some spatial heterogeneity exists, the stratigraphy and composition of sediments from proximate coring locations in the deepest part of a lake basin are often broadly similar (Rose et al. 1999a). The first requirement of sediment coring, particularly when relatively recent changes are the focus, is to recover an undisturbed sediment core sample, including the sediment-water interface (Glew et al. 2001). The basic requirements of an undisturbed sediment core are: first, there is no disturbance of structure; second, there is no change in water content; and third, there is no change in constituent or chemical composition (Hvorslev 1949). A wide range of coring devices are available to palaeolimnologists.
However, when studying
recent lake sediments, the gravity corer is probably the simplest, least expensive and most widely-used (Glew et al. 2001). Examples of gravity corers include the Hongve (Wright Jr. 1990), HON-Kajak / Renberg (Renberg and Hansson 2008), Glew (Glew 1991), and Limnos (Kansanen et al. 1991) corers.
Open barrel gravity corers rely on weight to
penetrate the sedimentary profile, and the sediment is then collected in an open barrel (usually a polycarbonate tube). After the coring drive is accomplished, the top of the tube
34
is closed forming a seal, and the corer, tube, cored sediments and sediment-water interface are recovered. Subsequent handling of the cored sediments depends upon the nature of the study, type of analyses required, and distance to the laboratory. In the case of multi-core, multi-proxy studies, considerable planning is necessary in order to ensure that the sediment collected is adequately preserved and archived (Glew et al. 2001). For example, contamination of samples for PAH analysis is a particular problem because of the very low concentrations of PAH in lake sediments (Grimalt et al. 2004; Quiroz et al. 2005).
Procedure In the current research, no contemporary limnological data were gathered during field work, as a wide range of published, palaeolimnological data already existed for the three study sites.
Selection of coring site In order to map lake basin topography and to identify the optimal coring locations, bathymetric surveys were undertaken at each of the three study sites. Geographic position was recorded, using a handheld GPS, and water depth was measured, using a handheld depth-sounder at ca. 10 m intervals along a series of parallel transects across each lake. On completion of the bathymetric survey, the deepest point of each lake basin was identified as the site for subsequent coring.
Sediment coring Three sediment cores were retrieved from the deepest point of each lake basin using a gravity corer (Renberg, HTH Teknik Varvagan 37, SE951 49 Lulea) (Renberg and Hansson 2008). Cores were collected from Kelly’s Lough in November 2005, Lough Maumwee in March 2006, and Upper Killarney Lough in May 2006. Cores were visually inspected in the field in order to ensure that the sediment-water interface had been sampled effectively, remaining relatively intact and undisturbed, and in order to identify any variations in sediment composition.
35
Sub-sampling All sediment cores were sub-sampled in the field immediately following collection using a vertical push rod-type extruder at 0.5 cm intervals for the first 10 cm and at 1 cm intervals thereafter. Sediment core samples obtained from two sediment cores (core numbers 1 and 2) were destined for SCP, geochemical, pollen, diatom, and LOI analyses, while sediment core samples from one sediment core (core number 3) were destined for PAH analysis. Sediment core samples obtained from core numbers 1 and 2 were stored in pre-labelled (name of coring site, core number, and depth in core) zip-lock bags and placed in a freeze box. Sediment core samples obtained from core number 3 were placed in tinfoil pre-rinsed with acetone, stored in pre-labelled (name of coring site, core number, and depth in core) zip-lock bags and placed in a freeze box. All sediment core samples were subsequently transported to Trinity College Dublin, and stored frozen until further analysis.
3.2 Laboratory based techniques 3.2.1 Expressing palaeolimnological proxy data Sediment-based proxy data used in palaeolimnological studies may be expressed as concentrations (e.g. mg g-1), relative frequencies (%), or as accumulation rate or influx values (e.g. mg cm-2 yr-1).
Relative frequencies and concentrations Relative frequency data are expressed as a % of a minimum count or sum, which is defined through a consideration of error (Bennett and Willis 2001) associated with the particular proxy.
Concentrations are measured as a mass fraction of sediment material,
normally as a function of dry weight (Cohen 2003). The interpretation of concentration and relative frequency data can be difficult because they are measured in relation to sample mass or a fixed sum, respectively. Consequently, concentrations or relative frequencies of constituents of a sedimentary component are not independent of each other; variation in the level of each constituent influences the concentration or relative frequency of all other constituents (Engstrom and Wright 1984).
36
Accumulation rate With the advent of absolute dating techniques (e.g.
210
Pb dating), concentrations can be
normalised to time and expressed as accumulation rates or fluxes (Norton and Kahl 1991). This eliminates the problem of co-variation among different constituents of a sediment component (Engstrom and Wright 1984) and, in addition, the rate of deposition of various elements may represent important palaeoenvironmental signals that cannot be deciphered from concentration data alone (Engstrom and Wright 1984).
However a number of
problems exist with accumulation rate data. First, sediment accumulation data are highly dependent on the quality of the dating. Most studies only obtain a few and interpolation and extrapolation are required. As a result,
210
Pb data points
210
Pb-derived accumulation
rate data can be considerably less precise than concentration data (Boyle 2001a). Second, the accumulation rate data from one particular core may differ from other cores from the same basin owing to, for example, sediment focusing (Rippey and Anderson 1996). Third, accumulation rates for individual components are the numerical product of their concentration in the sediment and the overall sediment accumulation rate. Thus, if variations in concentration of a component are small, relative to changes in sediment accumulation, they are likely to be difficult to discern (Engstrom and Wright 1984). Generally, least uncertainties are associated with interpretations that are based on palaeolimnological data that have been expressed in concentrations and accumulation rate form (Engstrom and Wright 1984). When considered in combination, concentration and accumulation rate data are to an extent complementary and therefore likely to enable a fairly reliable and detailed interpretation (Renberg 1986). However, concentration data are more precise and should be considered as the primary data type as they are less prone to error and are often easier to interpret (Boyle 2001a).
3.2.2 Palaeolimnological proxies confirming selection of study sites The current research assumes that the catchments for the three study sites have remained largely stable, and free from major disturbances, over the time period covered by the sediment records collected and analysed. In order to test this assumption, analyses of the organic and inorganic matter contents and of pollen and spore types and abundances preserved in samples of sediment from the cores were carried out. Both analyses should
37
record periods of catchment instability caused by, for example, changes in vegetation as a result of anthropogenic activity or climate change, amongst others.
3.2.2.1 LOI analysis Organic matter constitutes an important fraction of the palaeolimnological record preserved in lake sediments. The remains of aquatic plants together with parts of terrestrial taxa transported from the catchment comprise the primary sources of organic matter accumulating in lake deposits (Meyers and Ishwatari 1993). Accumulations of organic matter in lake sediments provide information that is important to interpretations of both natural and human induced changes in
ecosystems over local and regional scales
(Meyers and Teranes 2001). Moreover, variations in accumulations of organic matter can highlight the in-wash of catchment material, both minerogenic and organic. For example, increased supply of organic matter from allocthonous sources results from enhanced erosion of top-soil (Birks et al. 2004). Sequential LOI is a common and widely used method to estimate the organic content of lake sediments (Dean 1974). Dried sediment is placed in a muffle furnace and organic matter is oxidised at 500-550 ˚C to CO2 and ash. The weight loss during the reaction is measured by weighing the samples before and after heating and is closely correlated to the organic matter content. Variations in content of organic matter determined through LOI reflect changes in the balance of productivity, decomposition, and external inputs to a lake (Shuman 2003), and provide a basis for inferring variations in organic matter delivery rates and pathways to the lake. Furthermore, when carrying out a multi-proxy palaeolimnological study based on several sediment cores from the same lake basin, LOI can be used as a relatively simple and reliable method of aligning, stratigraphically, several cores of a similar length that have been taken from closely located coring locations (e.g. Carol et al. 1998 and Brancelj et al. 2002).
Procedure LOI analysis was undertaken using the method outlined by Dean (1974).
Sediment
samples for LOI analysis were taken contiguously at 0.5 cm intervals for the first 10 cms and at 1 cm intervals thereafter. Sediment samples were placed in pre-weighed nickel crucibles, weighed and dried overnight at 60˚C.
38
Samples were then re-weighed to
calculate % dry weight. Subsequently, the dry sediment samples were placed in a furnace at 550˚C for two hours. After ignition, samples were transferred to a desiccator to cool fully before re-weighing to establish % LOI.
3.2.2.2 Pollen analysis Palynology refers to the study of fossil pollen and spores. Pollen analysis is the principal technique available for determining vegetation response to past terrestrial environmental change (Bennett and Willis 2001).
Pollen sourced predominately from the catchment
surface and soils (Moore et al. 1991) and preserved in lake sediments, is frequently coupled with lake accumulation rate data to infer patterns of watershed disturbance (Cohen 2003). This enables variations in catchment conditions to be assessed over both long (millennial) and short (decadal) time scales, demonstrating the impacts of woodland clearance, grazing and crop cultivation (e.g. Huang and O’Connell 2000; Edwards and Whittington 2001; Watchorn et al 2008). Pollen from autochthonous sources (i.e. aquatic plants) is also useful to a palaeolimnologist as it can be a proxy of chemical and hydromorphological water quality. Pollen grains are produced by angiosperms, and gymnosperms, or seed-bearing plants (Birks and Birks 1980), as part of the plant reproductive process (Bennett and Willis 2001). Spores are produced by pteridophytes, bryophytes, algae and fungi. In order to fulfil their functions adequately, pollen grains need to be transported to the stigma of a plant of the same species and germinate there, while spores are required only to arrive at a site where they can germinate. Because the process is wasteful, pollen and spores are produced and dispersed in large numbers (Moore et al. 1991). Most pollen grains and spores are either wind-dispersed (anemophilous) or insectdispersed (entomophilous) (Bennett and Willis 2001). Due to mechanisms of transport and dispersal, pollen grains and spores share many similarities in their form. They are of similar size (generally 20 – 40 µm) and are surrounded by tough walls, composed of cellulose and sporopollenin, which are sculpted in distinctive ways (Moore et al. 1991), and that are resistant to decay and to the actions of strong chemical reagents. Once dispersed, most pollen grains and spores can be considered as excess to requirements, as they do not fertilise any ovules or arrive at a site where they can germinate (Birks and Birks 1980). This excess is deposited as pollen rain (Bennett and Willis 2001). The proportion of each
39
pollen and spore type in the pollen rain is a function of the abundance of the parent plant and other factors, such as dispersal mechanisms, topography etc. (Moore et al. 1991). Lake sediments, when in a saturated state, are excellent preservers of pollen grains and spores due to the prevailing anaerobic conditions. The sporopollenin in the exine or outer wall of a pollen grain or spore does not degrade readily. Sporopollenin is resistant to most forms of chemical and physical degradation. Consequently, concentration of pollen and spores in a sediment sample can rely on the use of strong chemicals to remove extraneous matter (Bennett and Willis 2001).
The distinctive sculpturing of pollen grains makes it
possible to identify grains to the parent taxon and allows for the conversion of a fossil pollen and spore assemblage to a parent plant community (Birks and Birks 1980). Furthermore, key pollen indicator types can be selected to represent the local vegetation communities present at a site, as opposed to plant communities some distance from the site of pollen and spore accumulation. For example, Sphagnum, Calluna and Potentillia can be used to assess variations in local peatland communities (Jones et al. 1989), while the appearance of Rumex and Pinus has been used as an indicator of increased anthropogenic activity (respectively, agriculture and aforestation) (Burga 1988).
Procedure In the current study, a temporal resolution of approximately 20 – 30 years was deemed adequate to detect any major anthropogenic-induced changes in catchment landcover evident in the pollen and spore data. Thus, pollen analysis was undertaken on 6 samples (0 – 1 cm, 3 – 4 cm, 9 – 10 cm, 15 – 16 cm, 21 – 22 cm, 27 – 28 cm) from the Kelly’s Lough core; 8 samples ( 0 – 1 cm, 5 – 6 cm, 10 – 11 cm, 15 – 16 cm, 20 – 21 cm, 25 – 26 cm, 30 – 31 cm, 35 – 36 cm) from the Lough Maumwee core; and 5 samples (0 – 1 cm, 10 – 11 cm, 20 – 21 cm, 30 – 31 cm, 40 – 41 cm) from the Upper Killarney Lough core.
Extraction Concentration of pollen and spores in sediment samples followed the standard laboratory protocol (Faegri and Iverson 1989). Prepared samples were suspended in silicone oil (2000 cs) and mounted on microscope slides and sealed beneath coverslips.
40
Enumeration and identification Pollen grains were identified and counted using a Meiji Techno ML5000 series light microscope.
The identification of pollen grains was aided by the use of the key and
illustrations of Moore et al. (1991), illustrations from Reille (1992) and reference material held in the Department of Geography, Trinity College, Dublin. A minimum number of 300 pollen grains and spores was counted in all samples analysed. In order to identify any major changes in catchment landcover at the three study sites, throughout the period covered by the sediment records, the pollen sum (Bennett and Willis 2001) included all identifiable pollen grains and spores, excluding those from aquatic plants.
In addition, and in order to assess any major changes in chemical and
hydromorphological conditions at each of the three study sites, abundances of pollen grains and spores from aquatic plants were calculated on the basis of the total number of pollen and spores encountered, including aquatics.
3.2.3 Chronological control of lake sediment cores The establishment of reliable and accurate sediment chronologies is crucial to the reconstruction and interpretation of palaeoenvironmental archives (Appleby 2001), as the timing of trends and events cannot be established without a reliable depth-age profile (Cohen 2003).
3.2.3.1
210
Pb and 137Cs radiometric dating, and SCP dating
Many of the most commonly used dating techniques in palaeolimnology involve the measurement of the decay of naturally occurring radioisotopes.
The use of unstable
radioactive isotopes for geo-chronometry relies on the fact that these isotopes are transformed into daughter isotopes, with the emission of various particles at known rates. For dating recent lake sediments (i.e. up to about the last ~150 years), the naturally occurring radioisotope of Pb (210Pb), part of the
238
U decay chain, is by far the most
commonly used method (Flower 1998; Fernandez et al. 2000; Leira et al. 2007; Smol 2008). Lake sediments are favourable for the use of the 210Pb method as they are an albeit temporal sink for
210
Pb, which decays at a known rate (half-life), whereby its activity is
halved every 22.3 years (Brush et al. 1982). In addition, validation of the
210
Pb chronology
41
can be achieved through the use of artificial radioisotopes, in particular
137
Cs, derived
through atmospheric fallout originating from the testing of nuclear weapons since the 1950s and also the Chernobyl nuclear accident of 1986 (Appleby 2001). Validation may also be possible through reference to other marker horizons, including distinctive features of variations in concentrations of SCPs (Rose et al. 1995). There is, however, a danger of circularity of reasoning if, subsequently, the
210
Pb chronology is used to date those same
distinctive features in the sediment record. 210
Pb dating 210
Pb method relies on radioactive decay along the
As illustrated in figures 3.2 and 3.3, the 238
U decay chain.
238
U is near-ubiquitous in the materials of the Earth’s crust (Gale et al.
1995) and it decays to an intermediate isotope,
Ra (half-life 1622 years), which exists in
226
Ra decays in turn and is transformed into
soils and exposed bedrock in trace amounts. an inert gas,
266
222
Rn (Cohen 2003). A fraction of the 222Rn atoms produced escapes into the
atmosphere by recoil on ejection of the alpha particle, and also by diffusion. Once in the atmosphere, isotopes to
222
Rn decays rapidly (half-life 3.8 days) through a series of short-lived
210
its pre-cursor,
Pb (Cohen 2003). As a result of the change in phase,
222
226
Ra, and its initial secular equilibrium is destroyed.
Rn is isolated from
222
products then reach a new secular equilibrium in the atmosphere, with 210
predominant radionuclide (Gale et al. 1995).
Rn and its decay
210
Pb becoming the
Pb becomes attached to aerosols, and is
deposited to the Earth’s surface through both wet and dry deposition.
210
Pb has a mean
atmospheric residence time of 5 - 10 days (Krishnaswamni and Lal 1978). Therefore, deposition takes place on a timescale that is much shorter than the half-life of the isotope. 210
Pb falling directly onto lakes is scavenged from the water column and incorporated to the
sediments. Evidence suggests that
210
Pb is quickly removed from the water column and
incorporated into the lake sediments (Benninger et al. 1975).
As a result, in recently
deposited lake sediments (the last 150 - 200 years), disequilibrium exists between and its parent isotope,
226
Ra (Appleby 2001). Therefore, in the absence of
210
Pb and 226Ra are expected to be in radioactive equilibrium.
→ 230Th → 226Ra→ 222Rn → 218Po→ 214Pb → 214Bi→ 214Po → 210Pb → 210Bi → 210Po → 206Pb Figure 3. 2
42
238
U decay chain
Pb
Pb fallout,
210
238U
210
The 210Pb method relies on the estimation of the residual activity of atmospherically derived (unsupported)
210
Pb over that of in-situ (supported)
by establishing the
210
Pb. Supported
210
Pb is determined
226
Ra content of the sample as the supported component will be in
radioactive equilibrium with
226
210
Ra (Appleby 2001). Unsupported
excess of the supported component. As long as the is isolated by burial, the excess
Pb is that measured in
210
Pb accumulating in lake sediments
210
Pb activity will decline with depth in accordance with the
radioactive decay law (Gale et al. 1995). To establish sedimentation rates, it is necessary to model unsupported
210
Pb activity in the sediment core. If the age of a point in a
sequence is known (e.g. surficial sediments represent the year of sample collection), it is possible to determine the chronology of sedimentation. Two models exist to facilitate this: the constant initial concentration (CIC) model, and the constant rate of supply (CRS) model. However, there are situations whereby neither model is fully applicable in isolation, such as when the sediments have been disturbed, for example by wind in a shallow lake, or where variations in the
210
Pb supply have occurred due to changes in sedimentation
(Appleby 2001).
Atmosphere: 222Rn
210Pb
(unsupported / atmospheric)
Wet and dry deposition 222Rn
Inwash of 226Ra in
In wash of unsupported 210Pb from catchment
eroded sediment 226Ra
Water Bedrock: 238U
226Ra
Sediment: Total 210Pb = unsupported 210Pb + 226Ra (supported / in-situ 210Pb)
Figure 3. 3 A schematic diagram of the 210Pb cycle
The CIC model According to Krishnaswamni et al. (1971), the original CIC model assumes that the rate of deposition of unsupported
210
Pb is constant, and that the
210
Pb in solution is quickly
scavenged by particles and deposited on the sediment surface. Furthermore, the model
43
assumes that the initial activity of unsupported 210Pb laid down on the lake bed remains unimpacted by post-depositional processes and decays exponentially with time in accordance with the radioactive decay law. For the CIC model to hold true, constant mass flux.
210
Pb deposition should be constant and give rise to a
Thus, there will be a constant rate of
sediment layer will have the same initial unsupported
210
210
Pb accumulation, each
Pb concentration and
210
Pb activity
will decline monotonically with depth. As a result, the CIC model has been shown to give good results at sites with uniform sediment accumulation rates (Appleby and Oldfield 1978).
The CRS model The CRS model for calculating sediment accumulation rates was originally developed by Appleby and Oldfield (1978) and Robbins (1978). It is particularly applicable to cores that have reached background levels of unsupported constant inputs of unsupported
210
Pb activity and have experienced
210
Pb. The CRS model has been shown to give good
results at sites with non-uniform sedimentation rates, as it is assumed that the absolute flux 210
Pb to the sediment water interface remains constant regardless of background
of
sedimentation (Cohen 2003). Thus changes in sedimentation rate through time will result in changes in the initial concentration of unsupported
210
Pb. In these circumstances, the
dates of older sediments are calculated not from their present concentration but from the distribution of
210
Pb throughout the sediment core (Appleby 2001). Therefore, unsupported
210
Pb activity below any given depth in the core can be compared with the total
unsupported
210
Pb in the core. A series of these calculations can then be used develop an
age-depth relationship.
Model choice Studies have suggested that the CRS model is reliable in the majority of cases as it allows for both temporal and local differences in sediment accumulation rate (Appleby 2001). The CRS model therefore should be valid as long as there has been no major hydrological changes in the lake that may impact sediment focussing, or cause hiatuses in the sediment record. In such cases, the CIC model may be more appropriate. The best solution is to calculate dates based on both models and where there is little difference, sediment accumulation rates can be assumed to have remained relatively constant. If differences
44
210
exist, then the dominant controlling mechanism of
Pb delivery to the lake site must be
identified (Appleby 2001). Validating the 210Pb chronologies 210
Pb profile, validation of
Regardless of which model is used to interpret the is important (Smith 2001). validation involve the
137
210
Pb profiles
As described above, the most commonly used means of
Cs isotope (McDonald and Urban 2007) and SCP concentration
profiles (Rose et al. 1995). 137
Cs dating
137
Cs originates from the atmospheric testing of nuclear weapons since the 1950s,
particularly between 1959 and 1963, and also from nuclear fallout originating from the 137
Cs deposited from the atmosphere to lakes is rapidly
Chernobyl accident in 1986.
scavenged by particles and settles to the lake bottom where it is preserved through burial. Therefore, sedimentation rates calculated from 137
137
Cs are based upon the occurrence of
Cs horizons (Robbins and Eddington 1978). Since fallout of
137
Cs occurred on a global
scale, peaks in concentrations associated with these events are often identifiable in cores of lake sediment (Appleby 2001).
137
Cs has a half life of 30.2 years and thus is appropriate
for testing 210Pb dates for the last 30-40 years (He et al. 1996). The largest peak identified in the sediment record is assumed to be as a result of nuclear weapons testing in 1963 (He et al. 1996), while a subsequent peak is assumed to be as a result of the Chernobyl accident (Appleby 2001). However, such two-peak profiles are indicative of extremely settled and undisturbed sediment (He et al. 1996). Deviations of the shape of the
137
Cs
profile from that expected from atmospheric deposition can be accounted for by postdepositional mobilisation either of the
137
Cs itself or of the sediment, through processes
such as diffusion or mixing, respectively (e.g. Schottler and Engstrom 2006). Deviations from the expected profile may also be caused by substantial sediment-associated Cs137 input from the catchment (He et al. 1996). Such inputs reflect the mobilisation by erosion processes of accumulated
137
Cs from the catchment. They may represent an important
addition to direct deposition from the atmosphere, in terms of both magnitude and timing, which may differ significantly from that of atmospheric fallout (He et al. 1996).
45
SCP dating The
210
Pb chronology may be compared with the three distinctive features of SCP profiles
from sediments accumulating at the same site for which dates have been previously established (see section 3.2.4.1: SCP analysis: temporal distributions for details). This technique assumes that there is no geographic variation in the ages of the distinctive features of SCP profiles. As this assumption is unlikely to be met (Rose and Appleby 2005), variations in concentrations of SCPs provide a guide to the accuracy of the chronology established through radiometric dating, rather than an absolute verification.
Procedure Sediment accumulation rates and geochronologies for cores of sediment from the three study sites were obtained through the Kelly’s Lough core.
210
Pb analysis, while
137
Cs analysis was undertaken on
Analysis of 39 sediment samples from Kelly’s Lough was
undertaken at the Department of Experimental Physics, University College Dublin, Ireland, by Luis Leon Vitro while analysis of 13 and 16 samples from, respectively, Lough Maumwee and Upper Killarney Lough was undertaken at Flett Research Laboratories, Winnipeg, Canada by Misuk Yun.
The choice of laboratories for
210
Pb analysis was
informed by costs and by the time constraint on the current research. Kelly’s Lough samples were analysed at 0.5 cm intervals for the first 10 cms and at 1 cm intervals thereafter. Samples for 210Pb analysis from the Lough Maumwee core were taken at sediment intervals of 0 – 0.5 cm, 2 - 2.5 cm, 4 - 4.5 cm, 6 - 6.5 cm, 8 - 8.5 cm, 10 – 11 cm, 14 – 15 cm, 18 – 19 cm, 22 – 23 cm, 26 – 27 cm, 30 – 31 cm, 34 – 35 cm, and 35 – 36 cm, while samples for
210
Pb analysis from the Upper Killarney Lough core were taken at
sediment intervals of 0 – 1 cm, 3.5 - 4 cm, 6 – 7 cm, 9 – 10 cm, 12 – 13 cm, 15 – 16 cm, 18 – 19 cm, 21 – 22 cm, 24 – 25 cm, 27 – 28 cm, 30 – 31 cm, 33 – 34 cm, 36 – 37 cm, 39 – 40 cm, 42 – 43 cm, and 45 – 46 cm.
Analysis In terms of the Kelly’s Lough samples, equal volume aliquots of dried sediment sample were placed in low background, polyethylene counting vials and the activities of
210
Pb and
226
Ra were determined by low-energy photon spectrometry, as outlined by Joshi (1987). In
addition, determinations of
46
137
Cs were made. With regard to the Lough Maumwee and
Upper Killarney samples, determination of 210
210
Pb was made indirectly through the
Po, a grand-daughter isotope of
determination of
210
Pb, as outlined by Eakins and
Morrison (1978); analysis was conducted using Ortec ‘Ortet’ alpha spectrometry. addition, determinations of
In
226
Ra were made for two samples from both the Lough
Maumwee (6 - 6.5 cm, 34 – 35 cm) and Upper Killarney Lough (30 – 31 cm, 39 – 40 cm) cores, using a bare photomultiplier tube and multi-channel analyser, in order to estimate supported 210Pb activity.
Interpretation Variations in sediment accumulation rate were immediately apparent from an initial examination of the excess
210
Pb activity profile in the Kelly’s Lough core. As a result, the
CRS model (Appleby and Oldfield 1978) was applied, while the examined to validate the
137
Cs activity profile was
210
Pb profile. Both the CRS (Appleby and Oldfield 1978) and CIC
(Robbins 1978) models were applied to the Lough Maumwee and Upper Killarney Lough samples. The
210
Pb-based chronologies for the three study sites were compared with up-
core variations in concentrations of SCPs determined from the sediments accumulating in the same cores, while the three dated SCP profile features were used to assess the accuracy of the chronologies.
3.2.4
Palaeolimnological proxies of depositions of atmospheric
pollutants 3.2.4.1 SCP analysis When fossil fuels (principally oil and coal) are combusted at industrial temperatures of up to 1750˚C (Environment 1981) and at a rate of heating approaching 104˚C s-1 (Lightman and Street 1983), the fuel-oil droplets or pulverised coal are burned efficiently, leaving only flyash (Raask 1984). Fly-ash comprises three components, each of which forms differently: non-combustible material (inorganic ash spheres (IASs)) from mineral inclusions in the fuel; combustible matter that was not burned (SCPs); and matter which was formed during the combustion process. The morphology and relative abundances of the different particles in fly-ash are determined during combustion (Rose 2001). The non-combustible material, when heated rapidly, undergoes some volatilisation, giving rise to sub-micron-sized particles termed fume. The non-volatile component coalesces forming hollow ash spheres
47
termed cenospheres (Raask 1984), with a variety of internal structures (Lightman and Street 1983). Less frequently, solid spheres and plerospheres (cenospheres containing encapsulated smaller spheres formed by variations in heating) are formed. Typically, 25% of coal is non-combustible (Goldstein and Siegmund 1985), and IASs can comprise 95% of coal fly-ash but less than 20% of oil fly-ash (Raask 1984; Marnane et al. 1986). On rapid heating, pulverised coal particles change from angular non-porous coal fragments to porous and frequently partitioned spheroids, with a molten appearance (Lightman and Street 1983).
These particles are delicate and will fragment if they remain in the
combustion chamber.
In oil-droplet combustion, as the droplet heats, volatiles are
produced and it is in this expelled cloud of hydrocarbons that ignition first occurs. As heat is given back to the droplet, burning of the volatiles on the particle surface takes place until the droplet collapses and becomes rigid and porous, forming the final SCP (Lightman and Street 1983). SCPs produced from the combustion of oil are often more porous than those from coal, due to the greater emission of hydrocarbons, and have a complex internal structure. Such particles exhibit a contour effect around the pores, and this contouring has been claimed to be characteristic of oil combustion-sourced SCPs (Griffin and Goldberg 1981; Lightman and Street 1983). However, this contouring has also been observed on coal particles (Rose 1991). As illustrated in Fig 3.4, the nature of their formation and the porosity of SCPs means that both oil and coal SCPs are never exactly spherical, but they do have some degree of sphericity to their morphology, and, as a result, have been termed spheroidal. In contrast, IASs are spherical and come in a variety of colours (colourless through yellow, red, brown, and black) depending on their elemental composition (Rose 2001).
Figure 3. 4 Schematic diagram of a SCP (left) and an IAS (right). Scale 1 cm = 2µm.
Lake sediments store deposited fly-ash particles and consequently provide a record of atmospheric deposition (Rose et al. 1999c). The sediment record of both SCPs and IASs act as indicators of anthropogenic emissions, both historically (using sediment cores) and spatially (using surface sediments from a number of sites).
48
However, most
palaeolimnological studies use SCPs in preference to IASs for two main reasons: first, SCPs are chemically more robust and are therefore easier to extract and enumerate than IASs, which have an aluminosilicate composition that makes them less able to withstand chemical attack (Rose et al. 1999c); and second, IASs are often morphologically and chemically similar to micro-spheres produced by some kinds of volcanic eruptions (Lefevre et al. 1986), whereas SCPs are not produced by any natural processes and can therefore be used as indicators of anthropogenic emissions (Del Monte et al. 1984).
The lake sediment SCP record Temporal and spatial variations in concentrations of deposited SCPs are remarkably consistent wherever they have been analysed (Rose 1995), and are also similar to those of other atmospheric pollutants, such as trace metals (Wik and Renberg 1991) and nonmarine SO42- (Rose and Juggins 1994; Rose and Monteith 2005). This is as a result of the major changes that have occurred in the development of fossil fuel combustion technologies and in the amounts of fossil fuels combusted over the last ca. 150 years, the effects of these changes on atmospheric composition globally, and the tendency for SCPs and other products of fossil fuel combustion to be deposited from the atmosphere and accumulate in lake sediments (Rose 2001).
Temporal distributions In the absence of evidence for sediment disturbance and hiatuses, temporal distributions of SCP concentrations preserved in lake sediments exhibit three main features, as illustrated in Fig. 3.5.
These features have been used as a means of dating sediment profiles
(Renberg and Wik 1984, 1985; and Rose et al. 1995; Leira et al. 2006). 1. The start of the SCP record, marked by the first appearance of SCPs, represents the onset of large-scale industrial fossil fuel combustion in an area. In almost every industrial country, this feature generally occurs within a few decades (1850-1900) (Rose 2001), and has been estimated to occur in Ireland in 1850 ± 25 years (Rose et al. 1995; Rose and Theophile 2004). However, SCPs have been identified in lake sediments prior to the first local or even regional emissions (Rose 1995), as a result of the long-range transport of airborne contaminants to areas that are relatively remote (Rose et al. 1998b).
49
2. The rapid increase in particle concentrations, which is a result of the massive expansion in consumption of fossil fuels at power stations, following the end of the Second World War, in order to meet higher demand for electricity (Laxen 1996). Throughout Europe, the rapid increase in particle concentrations generally occurs in the 1950s or 1960s (Rose 2001), and in Ireland, has been estimated to occur in 1950 ± 10 years (Rose et al. 1995; Rose and Theophile 2004). 3. The particle concentration peak and subsequent decline, which is a result of increasing combustion efficiencies, the move from traditional fuels such as coal and fuel oil to natural gas, and the introduction of emission and particle control technologies.
These changes led to a decrease in particle emissions in many
countries, despite a continued increase in fossil fuel combustion (Rose 2001). This feature is the most evident and least ambiguous. However, it is also the feature that is most open to local variability (Rose and Appleby 2005), due to regional and national differences in power generation and emission controls (Rose 2001). In Ireland, this feature has been estimated to occur in 1980 ± 3 years (Rose et al. 1995; Rose and Theophile 2004).
o
SCP concentration (gDM-1)
C
Depth / Age
B
A
Figure 3. 5 A schematic SCP profile showing the main features referred to in the text. A - the start of the SCP record; B - the rapid increase in SCP concentrations; and C - the subsurface peak in SCP concentrations
50
Spatial distributions Concentrations of SCPs have been shown to be correlated with distance to source in areas where single point sources provide the only significant origin within a region. In such cases, impacts from a single source can be identified up to 60 – 80 km away (Boyle et al. 1999). As a result, surface lake sediments from a number of lakes situated across a region can be used as spatially arrayed pollutant archives, identifying the regional patterns of deposition (e.g. Bowman and Harlock 1998).
SCP size distributions An important consideration in many studies of sediment-based SCPs is the distance from source to site of deposition (Vukic et al. 2006). For example, particles measuring < 5 µm in diameter are thought not to be deposited close to source (Ogren et al. 1984). Larsen (2000, 2003) examined SCP size distributions and suggested that counts dominated by SCPs > 10 µm in diameter were indicative of a local source.
However, SCP size
distributions are a product not only of distance from source, but also of combustion regimes (e.g. the quality of the fuel burnt and the operational procedures governing combustion) and other factors, such as topography and local meteorological conditions (Vukic et al. 2006).
Procedure Sediment samples for SCP analysis were taken at 0.5 cm intervals for the first 10 cm and at 1 cm intervals thereafter from all three study sites. As SCPs are composed of elemental C, strong chemical reagents can be used (Rose 2001), and there are several methods of concentrating SCPs in sediment samples. The two most frequently used methods are those described by Renberg and Wik (1985) and Rose (1994). In this study, the method described by Rose (1994) has been used, as it allows the identification, under a light microscope, of SCPs down to 2 µm. In contrast, the method described by Renberg and Wik (1985) allows the identification, under a binocular microscope, of SCPs down to 5 µm.
Concentration of SCPs HNO3, HF and HCL are used to remove organic, siliceous, and carbonate material, respectively. This digestion is carried out in PTFE tubes in a water bath and generally
51
results in a reduction in the mass of sediment from 1-2 g (the starting mass of sediment is labelled ‘M’) to less than 0.001 g. The reduced sediment is transferred to a labelled vial (the mass of the empty vial is labelled ‘VE’ and the mass of the vial + reduced sample is labelled ‘VS’).
A weighed sub-sample of carbonaceous material and any persistent
minerals are extracted from the vial (after extraction, the vial is reweighed and labelled ‘Vsub’) and evaporated onto a cover slip and mounted on a microscope slide using Naphrax.
Enumeration All SCPs on the microscope slide were counted at x400 magnification using a Meiji Techno ML5000 series light microscope (see plate 3.1 for examples of SCPs enumerated in sediment samples from the three study sites examined in the current study). Equation 3.1 is then applied to calculate SCP concentrations (number of SCPs per gram of dried sediment; gDM-1).
Plate 3. 1 Examples of SCPs enumerated and photographed in sediment samples from the three study sites examined. In each photograph SCPs are demarcated by a red circle.
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SCP concentration = 100N / E*M Where E is the percentage of the final solution evaporated onto the coverslip E = 100*(VS – Vsub)(VS – VE) Equation 3. 1 Calculation of SCP concentration (Rose 1994)
SCP size distribution analysis During enumeration, the diameter of all SCPs counted was measured using a reticule mounted in the focal plane of the microscope eyepiece.
Measured SCP diameters were
classified into three size distributions (Larsen 2000, 2003): 1. < 5 µm, significant in terms of long-range sources 2. 5 – 10 µm, significant in terms of both local and long-range sources 3. > 10 µm, significant in terms of local sources
3.2.4.2 Inorganic geochemical analysis As illustrated in Fig. 3.6, the inorganic geochemical record preserved in lake sediments is comprised of major and trace elements from several different sources. Major elements (e.g. Al, Mg, Na, Ca, Fe, Mn) comprise the vast majority of the preserved inorganic geochemical record, reflecting the compositional variability of the continental crust, and are transported to the lake through erosion and weathering of bedrock and soils in the catchment (Cohen 2003). Trace elements are found in small amounts in lake sediments (< 0.1%) (Hakanson and Jansson 1983) and enter lakes through multiple pathways: through bedrock erosion, as overland or ground-water discharges from mine tailings, or sewage, and also as a result of atmospheric depositions (Cohen 2003). The contribution of trace elements has typically been used as an indication of pollutant inputs through atmospheric deposition (Norton and Kahl 1991), and the overall trends in levels of trace element pollutants (e.g. Pb, As, Zn, Cu, Cd and Ni) are generally similar, showing substantial increases since the 1860s (Yang and Rose 2005), largely as a result of industrialisation. Once in the water column, elements are not normally found in solution but attached to suspended organic and inorganic particles, and in this form are transferred to the
53
sediments where they form oxides and sulfides that are very hard to dissolve (Hakanson and Jansson 1983).
Atmosphere Anthropogenic trace elements (e.g. Pb, Zn, Ni, Cd)
Atmospheric deposition
Weathering and erosion
Catchment soils Water
Catchment sourced major (e.g. Al, Na, Mg) and trace elements (e.g. Pb, Zn)
Sediment
Bedrock
Organic and inorganic carrier particles
Figure 3. 6 A schematic diagram of delivery pathways to lake sediment for both major and trace elements
Once deposited, bulk sediment disturbances such as slumping notwithstanding, a number of post depositional processes may occur that effect concentrations of trace elements in lake sediments: 1. Migration of trace elements, owing to mixing of sediments by physical or biological processes (Boyle 2001a); 2. Rapid exchange of trace elements between the surface organic sediment and the water column, due to the decay of freshly deposited organic debris, resulting in temporary enrichment of the sediment surface (Gobeil et al. 1987); 3. Diffusive migration of trace elements within the sediment via the pore-water (AlfaroDe La Torre and Tessier 2002). For example, Fe and Mn move from reduced to oxidised sediment layers, in response to strong redox controlled gradients in solubility.
Such processes have been found to influence other trace elements
(Boyle et al. 1998).
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Distinguishing between natural and anthropogenic inputs of trace elements Distinguishing inputs of trace elements from anthropogenic sources from those of natural sources can be difficult (Wu et al. 2007). A number of methods, however, are available that allow identification of trace elements in sediment that are from anthropogenic sources. For example, levels of trace elements for a particular time may be expressed as an EF relative to pre-anthropogenic/background concentrations.
The latter are presumed to
represent unpolluted geochemical concentrations, identified in a lower layer of a sediment core (Mackay et al. 1998; Ruiz-Fernandez et al. 2007).
Alternatively, concentration data
for trace elements may be normalised or expressed relative to a passive tracer element that is believed to be derived from natural mineral sources (Kemp and Thomas 1976). The principle is that a proportion of the trace elements are associated with natural sources. Therefore, the natural contributions will vary with the passive tracer element.
The
anthropogenic component of an element can then be identified by subtracting the natural/background concentration, estimated with reference to a passive tracer, from the total concentration. For example, past research has utilised Al (Bruland et al. 1974; Chen and Kandasamy 2008) and Ti (Norton and Kahl 1991) as suitable passive tracers for the background/unpolluted mineral component. Finally, the statistical PCA method has been used to distinguish the natural and anthropogenic inputs of trace elements (e.g Ruiz Fernandez et al. 2007; Wu et al. 2007). PCA enables a reduction in the dimensionality of a geochemical data set and allows description by a small number of new variables. The new variables can then be used to assess the source of trace elements, distinguishing natural from anthropogenic contributions (see Section 3.3.2: Numerical methods: PCA for details).
Procedure Analyses of elements preserved in sediments from Kelly’s Lough were undertaken at the Centre for Microscopy and Analysis (CMA), Trinity College, Dublin, by Leona Mulvey. Seven samples (0 -1 cm, 3 – 4 cm, 6 – 7 cm, 12 – 13 cm, 19 – 20 cm, 21 – 22 cm, and 30 – 31 cm) from a sediment core were analysed; sample choice was determined by the results of SCP analysis, with samples chosen to reflect periods of increasing or decreasing inputs of SCPs. Analyses of elements in samples of sediment from Lough Maumwee and Upper Killarney were undertaken at the Environmental Science Research Institute, University of Ulster, Coleraine, by Aine Gormley.
Samples from the cores from Lough
Maumwee and Upper Killarney Lough were analysed at 1 cm intervals for the first 10 cm and at 2 cm intervals thereafter, down to depths of 38 cm and 44 cm, respectively.
As a
55
result of the different methods of extraction and analysis employed at each laboratory, inter-laboratory comparisons of absolute values are more complicated than they would otherwise have been. The differences should not affect analyses of up-core variations in abundances at a particular site, however.
Extraction Analysis of the organic geochemical composition of lake sediments can be achieved through total, partial and sequential extraction techniques. Total extraction involves the removal of elemental material from a sediment sample, and allows measurement of the total concentration of elements in the sample (Boyle 2001a). Partial extraction aims to bring the readily available ions into solution, while leaving behind those that are inert in the lake system. Sequential extraction aims to extract ions from a number of fractions (e.g. the organic fraction, the biogenic silica fraction), through sequential chemical treatments (Boyle 2001a). Sediment core samples from Kelly’s Lough were partially extracted, in accordance with the standard laboratory protocol used by CMA, using microwave-assisted HNO3 and H2O2 extraction. Generally, this method principally extracts trace elements that are bound to organic material (Lottermoser et al. 1997). Lough Maumwee and Upper Killarney Lough samples were extracted sequentially, in accordance with the standard laboratory protocol used by the University of Ulster, using HF to break down silicates, HNO3 to oxidise the organic matter, and H2O2 to oxidise the resistant organics (Bock 1979). Analysis In this study, concentrations of trace elements in sediment core samples from Kelly’s Lough were determined using inductively coupled plasma mass spectrometry (ICP-MS), while those from Lough Maumwee and Upper Killarney Lough were determined by flame atomic absorption spectrometry (FAAS). For most elements, ICP-MS is the best method, being highly selective and having excellent detection limits. However, the technique is expensive. FAAS is a relatively inexpensive instrument for measuring a wide range of elements, but suffers from low precision when compared with ICP-MS (Boyle 2001a). The abundances of the elements Al, Mg, Na, Fe, Mn, Cd, Co, Cr, Cu and Pb were established for sediment core samples from all three study sites.
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In addition, V was
measured in material from Kelly’s Lough while levels of Ca and Zn were determined for material from Lough Maumwee and Upper Killarney Lough. The major elements Al, Ca, Mg, and Na are lithophilic and derived mainly from catchment rocks and soils. In contrast, abundances of the major elements Fe and Mn are determined by conditions both in the catchment and in the lake basin, and can be used as indicators of variations in lake sediment redox conditions, as outlined by Boyle et al. (1998). The trace elements Cd, Cu, Pb, Zn, Co, Cr, and V are all considered good indicators of atmospheric pollutant deposition (Cornett et al. 1992; Boyle and Birks 1999; Peters et al. 1999).
Correlation analysis Associations between elements and between elements and sediment characteristics were examined (see section 3.3.1: Numerical methods: Correlation analysis for more details). Inter-element correlations allow for the identification of characteristics (behaviour, origin) that may be common to different elements. Correlations between elements and organic matter were also calculated, as elements have been shown to display different degrees of affinity for organic matter (Renberg 1986). Furthermore, levels of correlation between elements and rates of sediment accumulation were determined because of the possible influence of varying sediment accumulation rates on concentrations of elements (Sigg et al. 1987; Boyle et al. 1998).
Distinguishing between natural and anthropogenic inputs of trace elements The choice of method for distinguishing between the anthropogenic and natural contributions of trace elements was determined primarily by the availability of data. The EF, or reference, approach has been employed where undisturbed pre-anthropogenic trace element concentrations have been identified.
Where they have not been identified
approaches based on PCA and normalisation have been employed. When utilising the geochemical normalisation method, the passive tracer element is selected through an examination of results from correlation analysis. Because of inter-site differences in geological and catchment characteristics, the identity of the passive tracer element used may differ between sites.
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3.2.4.3 PAH analysis PAHs are ubiquitous in the environment and are considered as part of the POP group. POPs may persist for many years in the environment, with environmental half-lives in the order of years to decades (Blais and Muir 2002).
PAHs are formed through both
anthropogenic and natural processes. In recent times, the anthropogenic contribution of PAHs has increased dramatically as a result of increased consumption of fossil fuels (Gevao et al. 1998). PAHs can be classified according to their source of origin (Mazeas and Budzinski 1999): 1. Pyrolytic PAHs which are products of the fossil fuel combustion process 2. Petrogenic PAHs which are products of oil spills and natural oil leakage 3. Diagenetic PAHs which are formed via the transformation of natural precursors As illustrated in Fig. 3.7, PAHs can enter lake sediments through atmospheric deposition, natural precursor transformation, and through surface runoff from the drainage basin (Venkatesan 1988). Once in a lake, most PAHs are associated with the particulate phase, due to their sorptive properties (Fernandez et al. 1999), and have a short residence time before being incorporated in sediments (Wong et al. 1995).
Once incorporated within
sediments, PAHs can be retained, although they are subject over time to a variety of loss processes, including biodegradation and transformation (Doik et al. 2005). Unfortunately little research has been carried out on the long-term environmental stability of PAHs, and there are problems in applying the results of laboratory-based studies on rates of degradation and transformation to situations in the field. Pyrolytic PAHs usually out-weigh the contribution from other sources in lake sediments, and historical increases of PAHs in lake sediments have been linked to combustion of fossil fuel (Gevao et al. 1998). Furthermore, due to the nature of their formation and similar physicochemical properties, groups of pyrolytic PAHs tend to co-occur in sediments (Stout et al. 2001), with mixtures of pyrolytic PAHs deposited at a site often remarkably constant in composition (Fernandez et al. 2000; Stout et al. 2001)
PAH ratios The sources of PAHs and the pathways through which they are transported to lake sediments can be approximated qualitatively by comparisons of ratios of individual PAH compounds to published values for fossil fuels (Geschwend and Hites 1981; Yunker et al.
58
2002).
The use of PAH ratios is based upon the understanding of the relative
thermodynamic stability of different parent PAH compounds, the characteristics of different sources of PAHs and differences in composition between mixtures of PAHs at source and those preserved in lake sediment (Yunker et al. 2002).
Atmosphere Pyrloytic PAHs
Atmospheric deposition
Catchment inwash: oil spill Catchment soils
Petrogenic PAHs
Water
Bedrock
Natural oil leakage
Sediment
Natural precursor transformations
Diagenetic PAHs e.g. Perylene
Figure 3. 7 A schematic diagram of delivery pathways to lake sediment for PAHs
Procedure Analysis of PAHs was undertaken at M-Scan Ltd, Berkshire, United Kingdom by Lindsay Maclean and Albert Rwarasika Ing. Because of the expense, only a limited number of samples could be analysed (nine in total). Surface and basal sediment core samples were analysed in order to determine, respectively, contemporary/recent and historical (possibly reference) levels of PAH contamination at each lake site. A third sample was selected in each core on the basis of measured levels of SCPs, which are also from the combustion of fossil fuels. The nine samples for analysis (Kelly’s Lough 0 - 2 cm; 14 - 15 cm; 30 – 32 cm: Lough Maumwee 0 – 1 cm; 10 – 11 cm; 31 – 32 cm: Upper Killarney Lough 0 – 3 cm; 15 – 16 cm; 30 – 31 cm) were despatched frozen and stored in a freeze box to the UK-based laboratory.
59
Extraction PAHs were extracted from sediments using ultra-sonication, in accordance with the standard laboratory protocol used by M-Scan Ltd (Zhou and Maskaoui 2003). Prior to extraction, 8 µg each of HMN, COD and Sq and 2 µg each of d8-naphthalene, d10anthracene and d10-pyrene were added as internal standards. 80 ml of isoproponal and 20 ml of hexane were added to the sample, which was then extracted using ultra-sonication (2 x 5 min, stirring in between) and then centrifuged at 1500 rpm for 10 min. The extract was then decanted, and partitioned between water and pentane (2:3 v/v 120 ml). The organic layer was collected in a pre-cleaned, 500 ml round-bottom flask.
A further 100 ml
isopropanol/hexane (4:1) was added to the sediment, and the extraction procedure was repeated, omitting addition of the internal standards. The organic layers were combined, and washed. The extract was reduced to 5 – 10 ml under a vacuum ( 1 are related to pyrolytic sources, particularly coal combustion, while ratio values < 1 are related to petrogenic sources. In order to provide a good estimate of sources of PAHs, both ratios were examined together.
3.2.5 Palaeolimnological proxies of ecosystem response In addition to direct evidence of atmospheric contamination, indirect evidence of pollution impacts, in the form of biological contamination, may also be preserved in lake sediments (Flower 1998). Recent acidification of surface waters is a response to increasing levels of acidic deposition, resulting primarily from anthropogenic emissions (particularly SO2 and NOx) linked to the combustion of fossil fuels and other industrial processes (Gorham 1998). Lake ecosystem response to acidification is rapid and results in virtually every aspect of the
61
ecosystem being altered (Smol 2008). Communities of diatoms are excellent indicators of variations in lake water pH, as pH has long been recognised as an important variable influencing the composition of diatom assemblages in freshwater systems (Battarbee et al. 2001). Lake sediments can also accumulate toxic substances deposited from the atmosphere, particularly trace elements and PAHs, and have the potential to act as sources of toxic substances under changed redox conditions (Pham et al. 2007).
Levels of toxic
contamination of lake sediments can be assessed at the most basic level, by an examination of increases in contaminant concentration with reference to background conditions (Chapman et al. 1999). However, if the potential for biological effects is to be assessed in the absence of biological results, SQGs can be employed. SQGs define threshold limits of concentration for contaminants that determine whether detrimental biological effects are expected to occur (MacDonald et al. 2000).
3.2.5.1 Diatom analysis Diatoms are classified as algae, division Bacillariophyta. They are unicellular organisms characterised by their siliceous cell walls and occur throughout the world in nearly all aquatic environments (Battarbee et al. 2001). Diatoms are often the most abundant algal group in freshwater systems (Smol 2008). In lakes, diatoms are found in both planktonic and benthic habitats (Battarbee et al. 2001). Silica is important to diatoms as it provides the cell walls with strength, constrains certain aspects of reproduction and facilitates the preservation of their cell walls (frustules) in lake sediments (Battarbee et al. 2001). There are thousands of species of diatoms. Their taxonomy is based primarily on the size, shape and sculpturing of their siliceous cell walls (Smol 2008).
The composition of the
communities of diatoms found in lakes depend on the range and extent of habitats available for growth, and on the combination of physical, chemical and biological conditions that prevail in the water column, and in particular in the photic zone, or epilimnion (Battarbee et al. 2001). Several systems of classifying diatoms exist, either according to morphology or as a reflection of ecological tolerances. The Hustedt (1937 - 1939) system classifies diatoms according to pH optima: acidobiontic (optimal pH < 5.5), indifferent (pH optimum around neutral), acidophilous (pH slightly under neutral), alkaliphilous (pH slightly greater than
62
neutral), or alkalibiontic (exclusively at pH > 7). The term indifferent has more recently been replaced by circumneutral (e.g. Van Dam et al. 1994).
In addition, the transfer
function approach enables the quantitative reconstruction of pH from the remains of diatoms. In this, the relationship between individual diatom taxa and pH are encapsulated in an ecological response function (Battarbee et al. 2001), and this function is then applied to fossil assemblages of diatoms preserved in sediments as a means of inferring pH (Birks et al. 1990). This approach has been widely applied in palaeolimnology (e.g. Dixit et al. 1993; Birks et al. 2004).
Procedure Diatom analysis was undertaken on 7 samples from the Kelly’s Lough core (0 – 1 cm, 6 – 7 cm, 10 – 11 cm, 14 – 15 cm, 20 – 21 cm, 24 – 25 cm, 28 – 29 cm), 8 samples from the Lough Maumwee core (0 -1 cm, 4 – 5 cm, 10 – 11 cm, 15 – 16 cm, 20 -21 cm, 26 -27 cm, 30 – 31 cm, 35 – 36 cm), and 9 samples from the Upper Killarney Lough core (0 – 1 cm, 4 – 5 cm, 10 – 11 cm, 15 – 16 cm, 20 – 21 cm, 25 – 26 cm, 30 – 31 cm, 35 – 36 cm, 40 - 41 cm).
Extraction A modification of the procedure described by Renberg (1990) was used in the preparation of diatom samples for analysis. This method allows the preparation of a large number of samples at once and involves digestion in H2O2 to eradicate organic material, followed by washing (with distilled water). Diatom samples were then mounted on microscopic slides using Naphrax.
Enumeration and identification Enumeration and identification were carried out using a Meiji Techno ML5000 series microscope at x 1000 magnification provided with x 100 oil immersion objective and a phase contrast condenser (see Plate 3.2 for examples of the main diatom taxa enumerated in sediment samples collected from the three study sites).
Diatom frustules were
enumerated on transects running horizontally across the microscope slide. At least 400 diatoms were counted per sample in order to ensure that a representative count was obtained (Battarbee et al. 2001). Diatom taxonomy followed standard floras (Krammer
63
and Lange-Bertalot 1986-1991), together with supplementary references (Foged 1977; Flower and Battarbee 1985; Camburn and Charles 2000).
(a)
(b)
(c)
(d)
(e)
Plate 3. 2 Examples of the main diatom types enumerated and photographed in sediment samples collected from the three study sites examined: (a) Tabellaria flocculosa; (b) Eunotia incisa; (c) Frustulia rhomboides; (d) Achnanthidium minutissimum; and (e) Brachysira vitrea.
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DI-pH reconstruction A series of DI-pH transfer functions have been developed using the Irish Ecoregion training set (Chen et al. 2008) as well as fossil diatom counts from all three lake sites (see section 3.3.3: Numerical methods: transfer function for more detail). In addition, the reliability and accuracy of DI-pH reconstructions has been assessed against the Hustedt classification system (Hustedt 1937 - 1939) and also recently measured surface water pH values as reported by Free et al. (2006) and Leira et al. (2006; 2007).
3.2.5.2 SQG analyses SQGs are established using matching sediment chemistry data and laboratory toxicity tests. They have been established, by regulatory agencies globally (e.g. US EPA), for a variety of purposes, including, amongst others, interpretation of historical data, identification of problem chemicals at a site, and the ranking of sites according to the level of contamination (Ingersoll et al. 2001). However, values for SQGs vary greatly across regions due to a number of reasons. For example: the receptors that are to be considered (e.g. benthic organisms, wildlife, or humans); the degree of protection that is to be afforded; the geographical area over which the SQGs are to be applied; and their intended use (e.g. screening tools or remediation objectives) (MacDonald et al. 2000). In an effort to focus on agreement apparent between individual SQGs, McDonald et al. (2000) compiled consensus-based PECs for both individual trace elements and PAHs. These were derived from existing effects-based SQGs for freshwater sediments. Utilising consensus-based PECs, the toxicity quotient method (Long et al. 1998) can then be used to assess whether the concentration of an individual chemical could possibly be toxic to biota in the sediment. The toxicity quotient of a chemical is concentration / PEC. The higher the toxicity quotient, the greater the possibility of the concentration of the chemical being toxic, and therefore of cause for concern (MacDonald et al. 2000). In addition to examining levels of possible sediment toxicity due to individual contaminants, levels of possible sediment toxicity owing to the combined action of chemical groups can also be assessed using the PEC-Q method, developed by Ingersol et al. (2001). The PEC-Q method employs consensus-based PECs (MacDonald et al. 2000), and is calculated using the average toxicity quotient of groups of chemicals, e.g. total levels of trace elements and PAHs.
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Procedure In this study, consensus-based PECs (MacDonald et al. 2000) are used to evaluate possible levels of toxicity due to individual sediment-based contaminants, while the PEC-Q method (Ingersoll et al. 2001) is used to evaluate the potential toxicity of combinations of sediment-based contaminants. Both consensus-based PEC ratios and PEC-Q ratios have been calculated for surficial sediment samples collected at all three study sites. PEC ratios were calculated based on concentration data for individual trace elements (Pb, Cd, Cu, Cr, and Zn) and total PAHs. Two PEC-Q ratios were calculated.
The PEC-Q MM ratio was calculated using the
average of the toxicity quotient for concentrations of individual trace elements (Pb, Cd, Cu, Cr, and Zn), while the second, the PEC-Q MPP ratio, was calculated using the average toxicity quotient for concentrations of individual trace elements and total PAHs.
3.3 Numerical methods 3.3.1 Correlation analysis The correlation between two variables reflects their degree of association, or the tendency of variables to increase or decrease together.
When applied to major element, trace
element, LOI and sediment accumulation rate data, correlation analysis allows for the identification of possible common characteristics between variables (such as abundance, origin etc.).
Procedure Pearson’s product moment correlation was performed on measured concentrations of major and trace elements.
It was also used to determine the degree of association
between two additional paired sets of data: elements and % organic matter; and elements and sediment accumulation rate. Prior to analysis, concentrations of both major and trace elements, % LOI, and sediment accumulation rate data were log-transformed, as some of the data had positively skewed distributions. The strength of the association was measured by the r value (-1 to 1); -1
66
indicting a perfect negative correlation, 0 indicating no correlation, and +1 indicting a perfect positive correlation. The significance of correlations was assessed using the p value; a p value of < 0.05 was considered significant, while a p value of < 0.01 was considered to be highly significant.
3.3.2 PCA In the current study, PCA was used to assess the relative contributions of natural and anthropogenic inputs of trace elements to levels of trace elements measured in sediment samples collected from all three study sites (see Section 3.2.4.2: Distinguishing between natural and anthropogenic inputs of trace elements). PCA is based on eigen (factor) analysis and can be used in the un-mixing of multivariate datasets (Boyle 2001a). This is achieved by reducing the dimensionality of a dataset by retaining the characteristics or dimensions that make the greatest contribution to its variance. The information is then expressed as a reduced number of PCA axes that are, by definition, orthogonal and uncorrelated (Birks and Birks 2006). PCA analysis produces a number of results (Kovach 1995): eigenvalues and the % of variance accounted for by each axis; eigenvector scores (composed of the variable loadings) for each axis; and the variable scores for each axis (computed by multiplying the variable loadings by the original data). Each PCA axis can then be interpretated in relation to their respective variable scores. Furthermore, PCA axes can also be treated individually and can be plotted in combination with sediment core data against depth.
Procedure In the current study, PCA was conducted on concentration data for trace elements, using the computer software programme PAST (Hammer et al. 2001). As many of the variables had skewed distributions and because of the presence of zero values in the dataset, data were log (x + 1) transformed prior to analysis.
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3.3.3 Transfer function The transfer function approach applied in this thesis involved the estimation of DI-pH levels from fossil diatom assemblages.
The fossil diatom assemblages were extracted from
sediment samples collected at all three study sites.
The transfer function utilised the
modern diatom-water chemistry data set produced specifically for Ireland (Chen et al. 2008). Transfer functions are most effective when calibration training sets are constructed containing modern diatom and water chemistry data from a number of selected lakes along the environmental gradient of interest (pH in this case), in the geographic region that contains the study site. The Irish eco-climatic and biogeographic region is considered a unique ecoregion under the auspices of the European Water Framework Directive (00/60/EC). Accordingly, Chen et al (2008) developed a 72 lake diatom training set for the island of Ireland (the Irish Ecoregion), in order to assess the response of diatoms to 17 different environmental variables, including pH. Within this training set, both pH and TP were identified as being the most significant environmental variables in determining the composition of diatom assemblages. Transfer functions were then developed for both lake water pH and TP in the Ireland. Birks et al. (1990) recommend WA calibration as the most effective and accurate method for pH reconstruction based on the transfer function approach.
WA is underpinned by the assumption that the best estimate of pH is the
weighted average of the pH optima of all the taxa present (Birks 1995).
Procedure Prior to DI-pH reconstruction, diatom species data were square root transformed to reduce the influence of extreme values. All models were calibrated using the Irish Ecoregion training set (Chen et al. 2008) and the program C2 (Juggins 2003). DI-pH was estimated using the WA method (Birks et al. 1990). In addition, the WA-PLS method (ter Braak and Juggins 1993; ter Braak et al. 1993) was applied. WA-PLS uses the residual correlation structure, after fitting the environmental variable of interest (through WA), in the data to improve the fit between the biological data and the modern training set (Birks 1995). The leave-one-out cross validation technique (Birks 1995) was applied to give an estimate of the overall accuracy of the reconstruction.
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The optimum, or most appropriate, model was deemed to be the one with the lowest RMSEP value (a measure of the overall predictiveness of the training set) and the highest r2 value (a measure of the strength of the relationship between the observed and inferred values) in cross-validation.
3.4 Secondary sources As a guide to potential sources of atmospheric pollutants accumulating in sediments at the three study sites, three main sources of secondary data have been utilised in the current study: 1. Officially reported and reconstructed inventories of, respectively, SOX and SO2 emission for Europe and Ireland; 2. Power station capacity records and histories; 3. Modelled depositional data for SO42-.
3.4.1 Inventories of SOX and SO2 emissions Measurements of depositions of atmospheric pollutants are ideally needed in order to correlate lake sedimentary data with long-term trends in levels of atmospheric contamination. However, in the absence of such monitoring data, emission inventories can play a crucial role. Inventories of emissions of pollutants provide a useful point of comparison, and also information on the nature and distribution of pollutant sources. However, as the dominant sources of emission differ for individual pollutants, estimates of individual emissions will vary in their uncertainty. Inventories of emissions of SO2 tend to be least uncertain, as the dominant sources are large static point entities (e.g. power stations and industrial plants) for which emission data are increasingly available (Metcalfe and Derwent 2005). Moreover, in the current research, sediment-based records of SCPs are used as a direct indicator of depositions of atmospheric pollutants from the industrial-scale combustion of fossil fuels, and, as already stated, levels of deposition of SCPs and non-marine SO42- are closely related (Rose and Juggins 1994; Rose and Monteith 2005).
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In the current research, inventories of both Irish and European emissions have been examined. Since the 1980s, emission data for SOX in Europe, submitted on a national basis, have been compiled and reported to EMEP, as part of the work under the UNECE LRTAP (1979). Furthermore, levels of emission of SO2 for the period 1880 – 1991, at a time step of every five years, have been reconstructed at a national level for European countries by Mylona (1993,1996). Calculation of emissions of SO2 from each European country was based on knowledge of the S content of fuels, and historical statistics relating to energy and industrial activity. Some verification of these hindcast data was possible, where the reconstructions overlapped with actual measurements from monitoring networks. As illustrated in Fig. 3.8 and Fig. 3.9, in the case of both Irish and European emission inventories, reconstructed emission inventories reproduce the basic trends evident in officially reported levels of SOX emissions.
250 200 150
Estimated total
100
Reported total
50 0 19 1 19 0 2 19 0 2 19 5 3 19 0 3 19 5 4 19 0 4 19 5 5 19 0 5 19 5 6 19 0 6 19 5 7 19 0 7 19 5 8 19 0 8 19 5 90 19 9 20 5 0 20 0 05
Thousand tonnes as SO2
Historical SO2 emissions in Ireland
Year
Figure 3. 8 Temporal variation in reconstructed inventories of SO2 emission for Ireland as reported by Mylona (1993, 1996). Officially reported emissions data for SOX in Ireland are also shown Historical SO2 emissions in Europe Million tonnes as SO2
60 50 40
Estimated, including USSR and Turkey Reported, including USSR and Turkey
30
Reported, excluding USSR and Turkey Reported, excluding USSR and Turkey
20 10
18 80 18 90 19 00 19 10 19 20 19 30 19 40 19 50 19 60 19 70 19 80 19 90 20 00
0
Year
Figure 3. 9 Temporal variation in reconstructed inventories of SO2 emission for Europe as reported by Mylona (1993, 1996). Officially reported emissions data for SOX in Europe are also shown
70
3.4.2 Power station capacity records and histories In addition to estimates of total emissions of SO2, data relating to the changing geographies and emission factors of national emission sources are important. Such data allow the assessment of variations in the relative impact of local sources upon levels of atmospheric contamination.
For example, in the absence of actual data on levels of
emissions, records of power station capacity can be used as a proxy (e.g. Rose et al, 1998b; 2003), although it is important to recognise that records of capacity of power stations do not take into account the introduction of cleaning technologies, e.g. flue-gas desulphurisation, or changes in fuel type. Therefore the relationship between emission and capacity is likely to vary over time. In Ireland, the major sources of emissions are predominantly power stations (as detailed in Section 2.1.2: National sources).
The estimation of emissions originating from power
stations in Ireland therefore is particularly important.
McCarthy (1957), Manning and
McDowell (1984), and O’Riordan (2000) examined the historical development of the power generation network in Ireland. In doing so, they reported data relating to the variations in generating capacities of power stations. Furthermore, data relating to the commissioning, de-commissioning, and fuel type changes of individual power generation stations were also detailed. In combination these data allow an estimation of the variation in the contribution of individual power stations to total levels of pollutant emissions. For example, growth in individual power station capacity and changing fuel type in Ireland from the 1970s to 1980s can be clearly seen from Fig. 3.10
3.4.3 Historical reconstruction of non-marine SO42- depositions In addition to EMEP modelled deposition data for SOX (Klein et al. 2007) (see section 2.1), Mylona (1996) modelled levels of deposition of non-marine SO42-, a direct indicator of atmospherically-derived industrial pollutant inputs, for Ireland for the years 1900, 1930, 1960, and 1991. Historical reconstruction of SO42- deposition, as reported by Mylona (1996), was achieved using the EMEP MSC-W Lagrangian acid deposition model. The model uses historical
71
Legend Power Station Capacity (MW)
Year: 1970
Fuel Type
100
Peat
250
Coal
500
Oil
Power Station Capacity (MW)
56
100
Peat
250
Coal
Coal / Oil
Gweedore
Ballylumford
Coolkeeragh
Gweedore
750
Gas
55
Kilroot
Gas / Oil
56
Oil
500
Gas
1,000
Year: 1980
Fuel Type
Coal / Oil 750
Legend
55
Ballylumford
Gas / Oil
Kilroot
1,000
Bellacorrick
Bellacorrick
Arigna
Arigna
54
Lanesboro
54
Lough Maumwee
Lough Maumwee
Lanesboro
#
Screeb
Rhode Shannonbridge Ferbane
Allenwood Portarlington
Ringsend Northwall
Shannonbridge Ferbane
Allenwood
53
Portarlington
53
# Kelly's Lough
Poolbeg
Rhode
Screeb
Poolbeg Ringsend Northwall
Malbay
Malbay
Kelly’s Lough Moneypoint
Tarbert
Tarbert
Great Island
Cahirsiveen Upper Killarney Lough
#
Great Island
Cahirsiveen
Upper Killarney Lough
52
52
Marina
Aghada Marina 0
11
10
09
25
08
50
100 Kilometers
07
06
0
11
10
09
25
08
50
100 Kilometers
07
06
Figure 3. 10 Map detailing changes in the generating capacity and fuel type of power stations in Ireland, from the 1970s (left) until the 1980s (right). Symbol size and colour represent, respectively, the generating capacity and fuel type of individual power stations. The locations of study sites are also shown.
72
estimates of SO2 emissions(see section 3.4.1) and meteorological data from 1991 and, as illustrated in Fig. 3.11, details levels of deposition of non-marine SO42- at a spatial resolution of 150km X 150km. As a result, modelled levels of deposition do not account for inter-annual variability in meteorological data, which in the short-term has been shown to have a larger affect on depositions than of changes in emissions (Mylona 1991; Iversen 1993).
However, on the longer time scale, the stochastic nature of meteorological
variability will be less important in determining levels of deposition of non-marine SO42than systematic emission changes (Mylona 1996). Furthermore, and similar to EMEP modelled deposition data for SOX, modelled data for historical reconstructions of levels of deposition of non-marine SO42- are limited in their spatial resolution. At a sub-grid scale, there can be significant variations in levels of pollutant depositions due to factors such as local sources of emission, vegetation and topography, amongst others (Dore et al. 2006).
Legend Unit: mg (nm SO42-)/m2 2000 - 5000
65˚N
1000-2000 500-1000
60˚N
55˚N
50˚N 0
100
10˚W
200
400 Kilometers
5˚W
Year: 1960
0˚W
Figure 3. 11 Modelled levels of deposition of non-marine (denoted as ‘nm’ on the map) SO42- for Ireland and Britain in 1960, as reported by Mylona (1996).
73
Chapter 4: Results and analysis This chapter details palaeolimnological proxy data from the three study sites that form the focus of the current research. The PCA and normalisation approaches are applied to geochemical data to distinguish between the relative contributions of naturally and anthropogenically sourced trace elements, and results of PAH ratios are detailed. The transfer function approach is applied to diatom data, and the accuracy of DI-pH reconstructions assessed against recently measured and published surface water pH values for each of the three study sites, while the details of SQG analysis are also presented.
Summaries of palaeolimnological proxy data for each study site are then
presented in order to confirm the suitability of the three study sites for the purposes of the current research.
4.1 Kelly’s Lough 4.1.1 Palaeolimnological proxies confirming selection of study sites 4.1.1.1 Pollen and LOI analysis A total number of 50 pollen and spore types were encountered in samples of sediment from core number 1. Maximum and minimum counts of 346 and 317, respectively, were achieved, while damaged grains accounted for less than 8% of the total pollen sum in all samples counted, indicating good preservation of pollen and spores at the site.
Up-core variations in pollen and spore abundances are illustrated in Fig. 4.1, and are minimal.
Pollen from small trees, shrubs and herbs, particularly Calluna, Corylus,
Sphagnum, and Poaceae, all of which are generally found growing in abundance on peatlands and on mountain slopes (Webb et al. 1996), characterise the counts.
Sediments in the cores were generally homogenous and largely organic. Variations in organic matter content with depth for core number 1 are also shown in Fig. 4.1. A gradual decrease in levels is evident from the base (81%) to the surface (47%), with average values of 69%. In addition, variations in organic matter with depth for core numbers 2 and 3 are also shown in Fig. 4.1, and both show similar levels of up-core variation in organic matter content as was observed for core number 1. A decline in organic matter content is
74
Depth (cm)
Aquatics
o. 3
o. 2
Aq
N LO IC or e
LO IC or e
ua tic LO s IC or e
an ts pl L.
H
Sh
er bs
ru bs
s
N
N
to n ed am ag
D
Tr ee
m yll u
ag a ta m
te s
nu m
yr io ph
Po
M
Is oe
ha g
o. 1
L. plants
Sp
Sa
an ta g
o xif ra ga Pt ce er ae id iu m
Herbs
Pl
Po
ac e
al lu na
ae
Shrubs
C
C
or yl u
tu la
nu s Pi
Be
Al
nu s
s
Trees
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 0 510 0 510 0 510 0 %
%
%
10 20 30 0 %
10 20 30 0 %
10 20 30 0 510 0 510 0 510 %
%
%
%
0
10 20 %
0 510
0 510
0 510
%
%
%
0 510 0 %
10 20 30 0 %
10 20 30 40 50 60 0 %
10 20 30 40 50 0 %
20
0
10 0 %
20
40
60 %
80
100 0
20
40
60 %
80 100 0
20
40
60
80 100
%
Figure 4. 1 Kelly’s Lough: summary pollen diagram and results of LOI analysis. The pollen sum includes all identifiable pollen grains and spores, excluding those from aquatic plants. Percentage data calculated for abundances of aquatic plants were calculated on the basis of the total number of pollen and spores encountered, including aquatics. L. plants = lower plants.
75
apparent from the lowermost core samples to the uppermost sediment samples, with average values of 69% and 67% measured for samples from cores number 2 and 3, respectively.
4.1.2 Chronological control of sediment cores 4.1.2.1
210
Pb and 137Cs radiometric dating, and SCP dating
Excess 210Pb profile
As illustrated in Fig. 4.2a, the excess
210
Pb profile shows a steady decline with depth, with
levels of activity decreasing from 91.2 DPM g-1 in the surface sample to 15.6 DPM g-1 in the basal core sample. However, a flattening of the 210Pb profile is evident in the upper section of the core (0 – 5.25 cm), and the level of activity measured in the basal core sample suggests that background levels of 210Pb activity were not reached. Core chronology
As excess
210
Pb activity does not decline monotonically with depth, the CRS model was
used. Conditions were not ideal for the application of the CRS model, however, because background levels of 210Pb were not reached.
According to the results of the CRS model, sediment accumulation rate averaged 0.046 g cm-2 yr-1 (0.63 cm yr-1) (Fig. 4.2b), with the basal core sample dating to the early 1950s (Fig. 4.2c). The
137
Cs profile is generally in good agreement with results from the CRS
model. Two well defined peaks are apparent; the first at 12.5 cm, identified as originating from fallout as a result of the Chernobyl accident (1986), while the lower peak (24.5 cm) can be attributed to weapons testing (1963) (Fig. 4.2c). On this basis, a mean annual sediment accumulation rate of 0.046 g cm-2 y-1 was inferred. Furthermore, concentrations of SCPs generally agree with dates obtained from the CRS model. SCPs were identified in the basal core samples, indicating that the basal sample dates to post 1850, while a rapid increase in SCP concentrations is apparent in the 1960s, and an SCP concentration peak and subsequent decline is evident in the late 1970s.
However, a second SCP
concentration peak is also apparent in Kelly’s Lough sediments, occurring in the 1990s (Fig. 4.2c).
76
(b) 2005
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
2000 1995 1990 1985
Year
Depth (cm)
(a)
1980 1975 1970 1965 1960 1955 1950 0.00
0
20
40
60
80
0.02
0.04
100
0.06
0.08
0.10
g cm-2 yr-1
DPM g-1
Depth (cm)
SC P
C
13 7
RS
Cs
(c)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
2 2 1 1
1945 1955 1965 1975 1985 1995 2005 CRS Year
0
100
200
300
Bq kg-1
400
0
20000
40000
gDM-1
Figure 4.2 Kelly’s Lough: (a) excess 210Pb activity profile; (b) CRS calculated sediment accumulation rate; (c) CRS calculated chronology, 137Cs activity profile with the 1963 (1) and 1986 (2) peaks marked, and the SCP concentration profile with the main identifiable dating features marked: the rapid increase in SCP concentrations (1) and the particle concentration peak (2).
77
4.1.3 Palaeolimnological proxies of depositions of atmospheric pollutants 4.1.3.1 SCP analysis SCPs were identified in all samples analysed and, with the exception of the upper part of the sediment core, up-core variations in concentration and accumulation rate data for SCPs were broadly similar (Fig. 4.3). A double concentration peak is conspicuous in upcore variations of total SCP.
The lowermost core sample exhibits relatively low
concentrations, and the lowermost peak in concentrations (45,190 gDM-1) is observed at 15.5 cm (ca. 1978), while a second concentration peak (49,790 gDM-1) is observed at 4.75 cm (ca. 1999). A gradual decline in concentrations occurs above the second concentration peak to 1.25 cm (ca. 2005), followed by a minor peak at 0.75 cm.
In contrast to
concentration data, accumulation rate data show a high degree of variation over the depth range 10 to 1 cm, characterised by high levels at 9.5 cm (ca. 1991) (1680 cm-2 yr-1) , 6.75 cm (ca. 1995) (1540 cm-2 yr-1), 4.25 cm (ca. 1999) (2770 cm-2 yr-1), and 0.75 cm (ca. 2005) (2500 cm-2 yr-1). Fig. 4.3 also illustrates SCP concentration and accumulation rate data according to three different particle sizes classes. All three particle size classes are well represented throughout the profile, although SCPs < 5 µm predominate.
SCP concentrations according to particle size
1 2 3 4 5 6 7 8 9 10
5
10 .µ m >
-1 0.
m
µm
n io at ul
5.µ
10 >
5
CP
m cu ac
2%) observed in sediment samples analysed were accounted for by the training set. Up-core variations in DI-pH were within the estimated boundaries of uncertainty for the model. However, the decline in levels of DI-pH (0.3) that characterises DAZ KL2 is on the margins of significance.
4.1.4.2 SQG analyses
Results of SQG analyses for surface sediment samples collected at Kelly’s Lough are summarised in Table 4.5. The possibility for sediment toxicity, as estimated by PEC-Q MM and PEC-Q MPP ratios, indicate that the concentration of mean trace elements measured in the surface sediment sample show the highest contribution to levels of possible sediment toxicity. However, and on the basis of PEC ratios, levels of total PAH measured in the surface sediment sample show the highest contribution of all the contaminants examined to levels of possible sediment toxicity
Pb/PEC
0.11
Cd/PEC
0.03
Cu/PEC
0.01
Cr/PEC
0.01
Zn/PEC
N/A
∑PAH/PEC
0.13
PEC-Q
PEC-Q
MM
MPP
0.19
0.18
Table 4. 5 Kelly’s Lough: PEC ratio values calculated for individual trace elements and total PAH. PEC-Q MM (average concentration of trace elements) and PEC-Q MPP (average concentration of trace elements and total PAH) ratios are also shown.
90
4.2 Lough Maumwee 4.2.1 Palaeolimnological proxies confirming selection of study sites 4.2.1.1 Pollen and LOI analysis A total number of 59 pollen and spore types were encountered in samples from core number 1.
Maximum and minimum pollen and spore counts of 446 and 323 were
achieved, respectively, while damaged grains accounted for less than 8% of the total pollen sum in all samples counted.
This indicates that pollen and spores have been well
preserved at the site.
Little up-core variation in the relative abundances of individual pollen and spore types is apparent, as illustrated in Fig. 4.10. Similar to sediments from Kelly’s Lough, pollen from small trees and shrubs, particularly Corylus, Calluna and Poaceae characterised the counts. In addition, pollen from Isoetes, presumably the aquatic plant I. lacustris, by far the more common of the two species found in Ireland (Webb et al. 1977), was relatively abundant.
Cored sediments were generally organic and showed no evidence of stratification. Variations in organic matter with depth for core number 1 are also shown in Fig. 4.10 and, with the exception of a sharp peak (41%) at 5 – 5.5 cm, are minimal. A small decline in organic matter content is apparent from the base (32%) to the surface (27%), with an average value of 29%. In addition, Fig. 4.10 illustrates variations in levels of organic matter with depth measured for core numbers 2 and 3. With the exception of the sharp peak at 5 – 5.5 cm, both core numbers 2 and 3 show similar levels of up-core variation in organic matter content as was observed for core number 1. A minimal decrease is apparent from the lowermost sediment sample to the uppermost sediment sample, showing average values of 32% and 33% for core numbers 2 and 3, respectively.
91
IC
no .3 or e
or e
or e
IC
LO
LO
ic s at
LO
L.
Aq u
pl an ts
bs H er
Sh r
Tr ee
s
ub s
d
to ag a
D am
Po ta m
ag Sp h
ag e
nu Is m oe te s
iu m rid Pt e
a
Er ic ac Po eae ac ea e
C al lu n
s
yl us C or
Pi nu Depth (cm)
no .2
Aquatics
n
L. plants
IC
Herbs
Shrubs
no .1
Trees
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 0
10 0 %
10 20 0
10 20 0
%
%
10 0 %
10 20 %
0
10 %
0
10 0 %
10 20 30 40 50 60 %
0
10 %
0
10 %
0
10 20 30 40 0 %
10 20 30 40 50 60 70 0 %
10 20 30 40 50 0 %
20 0
20
40
60 0
20
40
60 %
80 100 0
20
40
60 %
80 100 0
20
40
60
80 100
%
Figure 4. 10 Lough Maumwee: summary pollen diagram and results of LOI analysis. The pollen sum includes all identifiable pollen grains and spores, excluding those from aquatic plants. Percentage data calculated for abundances of aquatic plants were calculated on the basis of the total number of pollen and spores encountered, including aquatics. L. plants = lower plants.
92
4.2.2 Chronological control of sediment cores 4.2.2.1
210
Pb and SCP dating
Excess 210Pb profile
As illustrated in Fig. 4.11a, overall the core shows an exponential decline in levels of excess
210
excess
210
Pb activity, albeit with some variation over the 0 – 4.5 cm interval. Measured Pb activities decrease from 70.34 DPM g-1 in the surface sediment sample to
6.42 DPM g-1 in the lowermost core samples. activities of excess
The latter suggests that background
210
Pb have not been reached.
Core chronology
As excess
210
Pb activity does not decline monotonically with depth, the CRS model was
used, although once again conditions were not ideal, because background levels of
210
Pb
activity were not reached. According to the CRS model, an average sediment accumulation rate of 0.0241 g cm-2 yr-1 (0.32 cm yr-1) was estimated (Fig. 4.11b).
This suggests that the basal core sample
corresponds to 1892 (Fig. 4.11c). The three identifiable SCP dating features are in good agreement with the CRS model results (Fig. 4.11c). SCPs were identified in the basal sample of the core, indicating that the basal sample dates to post 1850.
The rapid
increase in SCP concentrations is observed from the 1940s, while a peak in SCP concentrations dating to the mid 1980s is evident, although a subsequent decline in concentrations, which is generally observed at lake sites globally (Rose 2001), is not apparent in the uppermost sediments.
93
(b) 2005
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 34 36 37
2000 1995 1990 1985 1980 1975 1970 1965 1960 1955
Year
Depth (cm)
(a)
1950 1945 1940 1935 1930 1925 1920 1915 1910 1905 1900 1895 1890
0
20
40
60
0.00
80
0.02
0.04
0.06
g cm-2 yr-1
DPM g-1
Depth (cm)
CR
SC P
S
(c) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
2
1
1860 1880 1900 1920 1940 1960 1980 2000 Year
0
3000 6000 9000 12000
g DM-1
Figure 4. 11 Lough Maumwee: (a) excess 210Pb activity profile; (b) CRS calculated sediment accumulation rate; (c) CRS calculated chronology and SCP concentration profile with the main identifiable dating features marked: 1 – rapid increase in particle concentrations; 2 – the particle concentration peak.
94
4.2.3 Palaeolimnological proxies of depositions of atmospheric pollutants 4.2.3.1 SCP analysis As illustrated in Fig. 4.12, levels of up-core variation in concentration and accumulation rate data for SCPs are broadly similar.
Levels of SCPs were relatively low in samples in the
lower part of the core (35.5 – 14.5 cm), showing a mean concentration of 1,350 gDM-1. A gradual increase in levels of SCPs is then apparent from 14.5 cm (ca. 1943) to 4.75 cm (ca. late 1980s), followed by relatively high levels to the uppermost core sample (mean concentration of 7,324 gDM-1). Fig. 4.12 also illustrates concentration and accumulation rate data for SCPs, according to particle size. SCPs < 5 µm were the main contributor to total levels of SCPs below 16 cm (ca. 1940).
Thereafter, larger size SCPs made an
increasing contribution, particularly SCPs > 10 µm from 9.25 cm (ca. 1970).
t io tra
cu m
ce n
0. µ m
0.µ m
>1
51
.µm
ta lS C
To
10 µ
m
µm
-1 0
µm
5
1, and are therefore likely to be of pyrogenic origin.
4.2.4 Palaeolimnological proxies of ecosystem response 4.2.4.1 Diatom analysis In total, 107 diatom types were identified in core samples from Lough Maumwee, while maximum and minimum diatom counts of 444 and 414 were achieved, respectively. As illustrated in Fig. 4.18, acidophilous diatoms, particularly Frustulia rhoimboides (Van Dam et al. 1994), were the most abundant diatoms identified and little variation in the relative abundances of individual diatoms is apparent up-core, with the exception of a slight increase
103
en e
Accumulation rate data for individual PAH compounds
D
(g nz o Be
ib e
hi )p
nz o( a, h) an
er yl
th
en
e
ra c
re n o( 1, 2, en In d
Be
Be
nz o
nz o
(a
(k )fl u
)p y
re n
3cd
e
)p y
ne or an
or an )fl u
C
Be
hy rs
nz o
en
(b
e
)a n (a nz o
e re n Py
Be
e ra n
An
Fl uo
Fl uo
re n
th ra c
e
en
e
th en
e re n th en a Ph
m ul at cu Ac
th e
en th ra c
te ra io n
n tio tra on ce n C
th e
e
ne
e
Total PAH
0-1cm (2002)
10-11cm (1977)
31-32cm (pre 1900)
0
1500
ng g-1
3000
0 10 20 30 40 50
ng cm-2 yr-1
0.0
0.3
0.6
ng cm-2 yr-1
0.000
0.030
0.060
ng cm-2 yr-1
0.00
0.03
0.06
ng cm-2 yr-1
0.0
2.0
4.0
ng cm-2 yr-1
0.0
2.0
ng cm-2 yr-1
4.0
0.0
0.4
0.8
ng cm-2 yr-1
0.0
2.0
4.0
ng cm-2 yr-1
6.0
0
4
8
12
16
ng cm-2 yr-1
0.0
1.0
2.0
3.0
ng cm-2 yr-1
0.0
0.5
1.0
1.5
ng cm-2 yr-1
0.0
4.0
8.0 0.00
ng cm-2 yr-1
0.40
ng cm-2 yr-1
0.0
2.0
4.0
6.0
ng cm-2 yr-1
Figure 4. 16 Lough Maumwee: up-core variations in concentration and accumulation rate data for total PAH. Up-core variations in accumulation rate data for individual PAH compounds are also shown.
104
0-1cm
10-11cm
30-31cm
(ca. 2002)
(ca. 1980)
(ca. pre 1900)
Phenathrene
1.6
2.8
1.7
Fluorene
0.13
0.3
0
Anthracene
0.15
0.2
0.5
Fluoranthene
6.7
9.8
5.3
Pyrene
6.4
7.3
7.1
Benzo(a)anthracene
1.6
1.8
1.2
Chyrsene
12.4
11.7
17.2
Benzo(b)fluoranthene
32
32
38.8
Benzo(k)fluoranthene
4.7
3.5
4.1
Benzo(a)pyrene
2.9
3.7
2.8
Indeno(1,2,3-cd)pyrene
15.8
14.3
10.7
Dibenzo(a,h)anthracene
1.3
1.1
1.2
Benzo(ghi)perylene
11.4
10.9
8.6
Table 4. 9 Lough Maumwee: % contribution of individual PAH compounds to levels of total PAH
16 10-11cm (1977)
Phenanthrene/anthracene Phenanthrecene/Anthracene
14 12
0-1cm (2002)
10 8 6 4
31-32cm (pre 1900)
2 0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Flouranthene/pyrene Fluoranthene/Pyrene Figure 4. 17 Lough Maumwee: phenanthrene/anthracene and flouranthene/pyrene ratios
105
Depth (cm)
106 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 0 % 20 0 % 0 % 0 % 20 0 %
Circumneutral
0 % 0 % 0 % 20 0 % 0 %
Acidophilous
0 % 0 % 0 % 0 % 0 % 20 0 % 20 0 % 0 % 0 % 0 % 0 %
20 0 % 20 0 20 %
Ac id bi on DI tic -p H
v it re a En cy on op Ni sis tz sc m hi ic a Ac r hn p er oc e m p an i nu ha l th a ta id iu En m m cy in on ut is s Ps o p im am sis um ce m sa ot Ps tii am hidi um m o St ob a u thid lo iu ng ro m fo rm ps ellu m eu a Ca ex do ig vin s w ui ul az fo a rm i Br is ac c oc co hy En sira n ei fo cy br on eb rmi e i ss s En m o a cy n e n ii on Eu e m ogr a a no p e cile t ia r p h Eu us i no emi c y llum t ia c lu in Fr s cis us a tu lia rh om Na bo vic id ul es a le pt Ch os tri am at ae a Pe pi nn ro ul ni ar a ia fib Ta m ul be ed a lla io r ia cr Ko is flo ba c ya cu si l Al o el sa ka lip l a s ub hi lo us tilis si m a Ci rc um ne ut ra Ac l id op hi lo us
Br ac hy sir a
Ye ar
Alkaliphilous Acidobiontic Acidobiontic
2006 2003 1998
1988
1979
1971
1959
1943
1932
1922
1908
1896
1892 40 0 % 5.40 5.80 pH Units
6.20
Figure 4. 18 Lough Maumwee: summary diagram of up-core variations in abundances of selected diatoms. Up-core variations in levels of reconstructed DI-pH and their boundaries of uncertainty are also illustrated.
in the abundances of acidophilous diatoms, particularly Frustulia rhoimboides and Tabellaria flocculosa, from levels measured in the lowermost sediment sample to those measured at 20.5 cm (ca. late 1920s). This may be indicative of a slight increase in levels of surface water acidity at the site.
In the reconstruction of DI-pH, and after leave-one-out cross-validation, the WA with classical de-shrinking model performed the best, achieving the lowest RMSEP value (0.33 pH units) and the highest r2 value (0.88). As illustrated in Fig. 4.18, up-core variations in levels of DI-pH were within the boundaries of uncertainty for the model, and, based on the diatom content of the surface sediment sample, generated estimated levels of current pH (5.83 ± 0.33 pH units) that are close to measured (5.9 – 6.7 pH units) (Toner et al. 2005). In addition, 90% of diatom taxa (abundances > 2%) identified in Lough Maumwee sediments were accounted for by the training set.
4.2.4.2 SQG analyses Results of SQG analyses for surface sediment samples are detailed in Table 4.10. On the basis of PEC-Q MM and PEC- MPP ratios, concentrations of mean trace elements make the highest contribution to levels of possible sediment toxicity. However, on the basis of PEC ratios, of all the contaminants analysed, concentrations of Pb show the highest contribution to levels of possible sediment toxicity at the site. Pb/PEC
0.45
Cd/PEC
0.06
Cu/PEC
0.07
Cr/PEC
0.19
Zn/PEC
0.25
Total
PEC-Q
PEC-Q
PAH/PEC
MM
MPP
0.12
0.29
0.39
Table 4. 10 Lough Maumwee: PEC ratio values calculated for individual trace elements and total PAH. PEC-Q MM (average concentration of trace elements) and PEC-Q MPP (average concentration of trace elements and total PAH) ratios are also shown.
107
4.3 Upper Killarney Lough 4.3.1 Palaeolimnological proxies confirming selection of study sites 4.3.1.1 Pollen and LOI analysis In samples from core number 1, a total number of 54 pollen and spore types were encountered, while maximum and minimum counts of, respectively, 360 and 308 were achieved. Of all the samples counted, damaged grains accounted for < 9% of the total pollen sum, indicating good preservation of pollen and spores at the site.
As illustrated in Fig. 4.19, little up-core variation in the relative abundances of pollen and spores was apparent. Similar to sediments from Kelly’s Lough and Lough Maumwee, pollen from small trees, shrubs and herbs, particularly Corylus and Poaceae, characterise the samples. In addition, and in contrast to sediments from Kelly’s Lough and Lough Maumwee, pollen from large trees, particularly Pinus, was encountered in relatively high abundances.
A visual inspection of cored sediments showed no evidence of stratification, and cored sediments were, for the most part, homogenous and largely organic. Variations in levels of organic matter with depth for core number 1 are also illustrated in Fig. 4.19 and, for the most part, are minimal. A slight overall up-core increase in levels of organic matter content is apparent, from values of 26% measured in the basal core sample to values of 32% in the surface sediment sample. Measurements of organic matter content in sediment samples from core number 1 showed an average value of 28%. In addition, variations in organic matter content with depth for core numbers 2 and 3 are also illustrated in Fig. 4.19, and both exhibit similar levels of up-core variation in organic matter content as was observed for core number 1. A distinctive decrease in LOI values at 22.5 cm is apparent in core numbers 2 and 3, and average values of 40% and 32% were measured for core numbers 2 and 3, respectively.
108
no .3 LO Ic or e
or e LO IC
or e
ics ua t Aq
L.
er b
LO IC
ts pl an
s
s ru b
no
no
.2
.1
Aquatics Aquatics
H
L. plants
Sh
s
Herbs
Be
Al nu Depth (cm)
Shrubs
tu la Pi nu s Q ue rc C us al lu n C a or ylu s Er ic ac C ea on e va Pl lla an ria ta go Po ac ea e R an u Sa ncu xif las r P o aga lyp ce ae o Pt dium er id iu m Sp ha gn um Is oe te s D am a T r ge ee d s
Trees
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 0
10 0 %
10 0 %
10 0 %
10 0 %
10 0 %
10 20 0 %
10 0 %
10 0 %
10 %
0
10 20 0 10 0 10 0 10 %
%
%
%
0
10 %
0
10 %
0
10 %
0
10 0
20 %
40 0
20
40 %
60 0
20
40 %
0
20
0
20 0 %
20
40 %
60 0
20
40 %
60 0
20
40
60
%
Figure 4. 19 Upper Killarney Lough: summary pollen diagram and results of LOI analysis. The pollen sum includes all identifiable pollen grains and spores, excluding those from aquatic plants. Percentage data calculated for abundances of aquatic plants were calculated on the basis of the total number of pollen and spores encountered, including aquatics. L. plants = lower plants.
109
4.3.2 Chronological control of lake sediment cores 210
4.3.2.1
Pb and SCP dating
Excess 210Pb profile
As shown in Fig. 4.20a, overall levels of excess
210
Pb activity declined with depth, albeit
with some variation over 0 - 13 cm, while measured activities remain more or less constant between 33 cm and 46 cm. Maximum and minimum activities are observed at 0.25 cm and 36.5 cm (0.0 DPM g-1), respectively. Values for 210Pb activity slightly above zero are found at sample depths below 36.5 cm and may represent disturbance of the sedimentary record, such as through reworking.
Overall, however, background levels of
210
Pb activity appear to have been
reached towards the base of the core.
Core chronology
The nature of the sediment record obtained from Upper Killarney Lough meant that conditions were not ideal for the application of either the CIC (fluctuations in activities of excess
210
Pb toward the top of the core) or the CRS (disturbance of sediments towards the
base of the core) models. However, as illustrated in Fig. 4.20b, over the depth range 0 cm to 13 cm, both the CIC and CRS models estimated a similar chronology, and as a result, the CRS model was applied to this section.
Thereafter
210
Pb activities decline
exponentially with depth and estimates from both models become increasingly divergent. As a result, the CIC model was applied over the depth range 13 to 46 cm. Thus, in the case of the core from Upper Killarney Lough, chronological control was provided by a combination of both the CIC and CRS models applied separately to different parts of the same core. Such a method has been used previously by Appleby et al. (1998) when dating cores of recently accumulated sediment from Lake Baikal (Siberia). An average sediment accumulation rate of 0.0435 g cm-2 yr-1 (0.3 cm yr-1) was estimated using a combination of both the CIC and CRS models (Fig. 4.20c). This suggests that the basal core sample corresponds to 1864 (Fig. 4.20d). As illustrated in Fig. 4.20d, the three identifiable SCP dating features generally agree with the
110
210
Pb chronology: The start of the
(b) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 34 36 37 38 39 40 41 42 43 44 45 46
Depth (cm)
Depth (cm)
(a)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
- CIC
1820
0
20
40
60
80
(c)
(d)
Depth (cm)
2005 2000 1995 1990 1985 1980 1975 1970 1965 1960 1955 1950 1945 1940 1935 1930 1925 1920 1915 1910 1905 1900 1895 1890 1885 1880 1875 1870 1865 1860 0.00
0.02
0.04
g cm-2 yr-1
0.06
1860
1900
1940
1980
2020
Year
DPM g-1
Year
- CRS
Acceptedchronology Chronology Accepted
SCP SCPConcentration concentration
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
3
2
1
1840 1870 1900 1930 1960 1990 2020 Year
0
2000 4000 6000 g DM-1
Figure 4. 20 Upper Killarney Lough: (a) excess 210Pb activity profile; (b) CRS and CIC calculated chronologies; (c) accepted sediment accumulation rate; (d) accepted chronology and SCP concentration profile with the main identifiable dating features marked: 1 – the start of the SCP record; 2 – the rapid increase in SCP concentrations; 3 – the particle concentration peak and subsequent decline.
111
SCP record dates to about 1900, while a rapid increase in SCP concentrations is apparent in the mid 1950s, rising to a peak in concentrations of SCPs in 1980.
4.3.3 Palaeolimnological proxies of depositions of atmospheric pollutants 4.3.3.1 SCP analysis For Upper Killarney Lough samples, up-core variations in levels of SCPs when expressed in both concentration and accumulation rate form are similar (Fig. 4.21).
SCPs were
absent in the basal core sample (40 cm, ca. 1890s) and were first encountered at 35.5 cm (ca. late 1890s). Up-core variations in levels of SCPs show two peaks, the first at 15.5 cm (ca. 1970) (5720 gDM-1; 250 cm-2 yr-1), and the second at 10.5 cm (ca. early 1980s) (6,100 gDM-1; 240 cm-2 yr-1), and then decline towards the surface sediment sample (ca. 2006). A minor peak in SCP concentrations is also evident at 23 – 25 cm (ca. late 1940s). Fig. 4.21 also illustrates concentration and accumulation rate data for SCPs according to particle size. SCPs < 5 µm were the main contributor from 35.5 cm (ca. 1890s) to 33.5 cm (ca. 1910), while SCPs 5 -10 µm were identified from 32.5 cm (ca. 1912) and SCPs > 10 µm were identified from 24.5 cm (ca. 1935).
SCP accumulation according to particle size
10 .µ
m
µm
>
5
5
10 was observed at 0 – 3 cm, which is indicative of a petrogenic origin. Values for the fluoranthene / pyrene ratio increase from the basal core sample to the 15 - 16 cm sample, before decreasing slightly in the surface sample, all values indicating a pyrogenic source of origin.
4.3.4 Palaeolimnological proxies of ecosystem response 4.3.4.1 Diatom analysis
A total number of 109 diatom types were encountered in core samples from Upper Killarney Lough and maximum and minimum diatom counts of, respectively, 420 and 408 were achieved.
120
ne
Accumulation rate data for individual PAH compounds
D
Be
nz o( gh i)p
o( a, h) an nz ib e
er yl
th
en e
ra ce
re n -c d o( 1, 2, 3 In de n
Be
nz o( a) py
re n
e
)p y
ne or an th e )fl u Be
Be
C
Be
Py
nz o( k
ne se
e re n
hy r
he ne Fl uo ra nt
An
Ph
th ra c
Fl uo re ne
en
e
hr en e en at
m ul at cu Ac
nz o( b) flu
en nz o( a) an th ra c
te ra io n
ra tio n nt on ce C
or an th e
e
ne
e
Total PAH
0-3cm (2006)
15-16cm (1969)
30-31cm (1920)
0
500 1000 1500 2000 0
ng g-1
25
50
ng cm-2 yr-1
75 0.0
1.0
2.0
ng cm-2 yr-1
3.0 0.0
0.1
0.2
ng cm-2 yr-1
0.3 0.0
0.1
0.2
ng cm-2 yr-1
0.3 2.0
4.0
6.0
ng cm-2 yr-1
8.01.0
3.0
5.0
ng cm-2 yr-1
7.0 0.0
1.0
2.0
3.0
4.0 1.0
ng cm-2 yr-1
3.0
5.0
ng cm-2 yr-1
7.0 4.5
7.5
10.5 13.5 16.50.0
ng cm-2 yr-1
1.0
2.0
3.0
ng cm-2 yr-1
4.0 0.0 1.0 2.0 3.0 4.0 5.0 1.0 3.0 5.0 7.0 9.0 11.00.2 0.4 0.6 0.8 1.0 1.2 1.0
ng cm-2 yr-1
ng cm-2 yr-1
ng cm-2 yr-1
3.0
5.0
7.0
9.0
ng cm-2 yr-1
Figure 4. 25 Upper Killarney Lough: up-core variations in concentration and accumulation rate data for total PAH. Up-core variations in accumulation rate data for individual PAH compounds are also shown
121
0-3cm
15-16cm
30-31cm
(ca. 2006)
(ca. 1969)
(ca. 1920)
Phenathrene
4
2
3.1
Fluorene
1.3
0.19
0.11
Anthracene
0.3
0.22
0.34
Fluoranthene
12.4
9.5
9.5
Pyrene
10
6.4
8.5
Benzo(a)anthracene
2.7
2.4
4
Chyrsene
8.4
11.9
8.5
Benzo(b)fluoranthene
27
29.4
19
Benzo(k)fluoranthene
4.6
5.8
4.6
Benzo(a)pyrene
4.6
3.9
5.7
Indeno(1,2,3-cd)pyrene
10.8
14.3
12.3
Dibenzo(a,h)anthracene
1.27
1.2
1.3
Benzo(ghi)perylene
9.2
10.3
9.9
Table 4. 14 Upper Killarney Lough: % contribution of individual PAH compounds to levels of total PAH.
16
Phenanthrene / anthracene Phenanthrecene/Anthracene
14 12 0-3cm (2006)
10
30-31cm (1920) 15-16cm (1969)
8 6 4 2 0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Flouranthene / Pyrene Fluoranthene/Pyrene
Figure 4. 26 Upper Killarney Lough: phenanthrene/anthracene and flouranthene/pyrene ratios
122
Up-core variations in abundances of diatom types are illustrated in Fig. 4.27. Relatively high abundances of acidophilous and circumneutral diatoms, particularly Achnanthidium minutissimum (Kütz.) Czarnecki (Van Dam et al. 1994), characterise the samples. With the exception of the lowermost core sample which shows a diatom assemblage that is distinct from samples further up the core, little up-core variation in the relative abundances of individual diatoms is apparent. Thus, the diatom profile has been divided into two DAZs (UK1 and UK2) to aid description.
DAZ UK 1 encompasses the lowermost sediment sample analysed only and is characterised by relatively high abundances of diatoms associated with low acidity waters, particularly acidophilous and circumneutral diatoms. In addition, peak, albeit relatively low, abundances of alkaliphilous diatoms were measured in DAZ UK1 (Van Dam et al. 1994). This zone is superseded by DAZ UK2 which extends to the uppermost sediment sample and is characterised by diatoms of more strongly acidic conditions. Abundances of circumneutral diatoms, particularly Achnanthidium minutissimum (Van Dam et al. 1994), increase while abundances of the acidophilous diatoms increase slightly and abundances of the alkaliphilous diatoms decrease.
Up-core variations in levels of reconstructed DI-pH are also illustrated in Fig. 4.27. In the reconstruction of DI-pH, and after leave-one-out cross-validation, the WA with classical deshrinking model performed the best, achieving the lowest RMSEP value (0.33 pH units) and the highest r2 value (0.85). In addition, the training set accounted for 85% of diatom taxa (abundances >2%) observed in the samples counted. On this basis, estimated levels of current pH (6.14 ± 0.33), based on the diatom content of surface sediment, were close to measured (6.41 pH units) (Leira et al. 2006). Similar to Kelly’s Lough, up-core variations in levels of DI-pH were within the estimated boundaries of uncertainty associated with the model. However, the decrease in magnitude in levels of DI-pH (0.3) that characterises DAZ UK2 is on the margins of significance.
4.3.4.2 SQG analysis Results of SQG analysis are summarised in Table 4.15. On the basis of PEC-Q MM and PEC-Q MPP ratios, concentrations of mean trace elements and total PAH make a similar contribution to levels of possible sediment toxicity. In contrast, according to PEC ratios, concentrations of Cr make by far the highest contribution to these levels.
123
Depth (cm) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
124 ar
% % % % % % % % % % % % % % % %
0 10 0 10 0 10 0 10 0 10 0 10 0 10 20 30 40 50 0 10 0 10 0 10 0 10 0 10 0 10 0 10 0 10 0 10 0 10 20 0 10 0 10 0 10 0 10 20 0 10 0 10 0 10 20 0 10 0 % % % % % % % % % % 20 0 20 % 40 60 0 20 % 40 0 %
D
ilo
us
id ob I-p iont H ic
Ac
ph
Acidophilous
id o
Circumneutral
Ac
C
la co yc seir lo a C tell gra yc a lo a nu C tell ff . p lat a yc a au lo d c G tell istin ipu om a st g u e n ct a p N ho r iat nd ta itz ne a a s va m r. Ac chia a di h n p a cu st an erm m in gu i th na i n id en u t u iu ta m da m m in ut is Ps sim am um E n mo cy t hi d o En no ium cy p s ob i Fr one s ce lon ag m s a gel a i lu St laria mi tii m n au u ro gra t um G f o ci om r m lis a G pho ex om ne i g m u p Pi ho a if or nn ne pa m is u m r C lar a vulu ha ia gr m a m v Br ae irid cile ac pin is hy nu sir la r a Br ga i a m ac rre ed hy ns i o E n si is cri cy r a s br o n E u e eb no m a iss ne on Eu tia no im p ogr ii t ia lic ac in ata ile ci Eu sa no t Fr ia us pe c t Ta ulia t ina be rh lis l l a o m va r ia b r. Eu flo oide und cc s no ul at ul Al t ia os a ka bil a lip un h i ar lo C us is v irc ar .b um ilu ne na ut ris ra l
Au
Ye
Alkaliphilous Acidobiontic Acidobiontic
DAZ
2006
2001
1993
1985
1975
1970
1962
1955
UK2
1938
1931
1921
1910
1891
1881
UK1
5.5 5.8 6.1 6.4 6.7 7.0 pH Units
Figure 4. 27 Upper Killarney Lough: summary diagram of up-core variations in abundances of selected diatoms and DAZs. Up-core variations in reconstructed levels of DI-pH and their boundaries of uncertainty are also illustrated.
Pb/PEC
0.23
Cd/PEC Cu/PEC Cr/PEC
0.15
0.11
0.98
Zn/PEC
0.3
Total
PEC-Q
PEC-Q
PAH/PEC MM
MPP
0.02
0.41
0.42
Table 4. 15 Upper Killarney Lough: PEC ratio values calculated for individual trace elements and total PAH. PEC-Q MM (average concentration of trace elements) and PEC-Q MPP (average concentration of trace elements and total PAH) ratios are also shown.
4.4 Site summaries On the basis of locational and palaeolimnological data, three study sites were selected for examination as suitable repositories for information on temporal and spatial variations in levels of deposition of industrial-derived atmospheric pollutants. Palaeolimnological proxy data presented in this chapter appear to confirm their suitability and are summarised below.
4.4.1 Kelly’s Lough Little up-core variation in pollen and LOI data is apparent for Kelly’s Lough samples (Fig. 4.1), and the dominant pollen and spore types observed in the samples, particularly Calluna, Corylus and Poaceae, are currently found in abundance in the Kelly’s Lough catchment. This indicates that there has been no major change in catchment land cover throughout the period covered by the sedimentary record collected from the site. Furthermore, of the three study sites examined, Kelly’s Lough sediments show the highest levels of organic matter content (Fig. 4.1), reflecting the predominantly peaty nature of the catchment. Evidence from LOI analysis also indicates that levels of up-core variation in organic matter content were similar for all three cores collected from the site, and sediment stratigraphies were closely comparable. The extrapolation of
210
Pb-based estimates of
sediment core chronology and accumulation rates to the two undated cores collected from proximate locations was thus possible.
With the exception of the uppermost sediment samples, activities of excess
210
Pb show a
steady decline with depth (Fig. 4.2a), indicating that the sediment record collected from the site has not been subject to post-depositional alteration.
On this basis, however, the
125
average rate of sedimentation estimated for the site (0.63 cm yr-1) was far higher than a previously reported estimate of 0.27 cm yr-1 (Leira et al. 2007), and actually greater than published for any other lake site in Ireland (Taylor et al. 2006). In addition, a hiatus in sediment accumulation is apparent in the uppermost sediment samples. A possible reason for the high rate of sediment accumulation established in the current research may be that a site of preferential sediment accumulation was cored, especially as up-core variations in activities of
137
Cs and concentrations of SCP also support a very high rate of sediment
accumulation (Fig. 4.2c). Peat erosion in the catchment, which possibly dates to as early as ca. 1450 cal. years BP (Leira et al. 2007), may have contributed to these high levels of sediment input onto the coring location. Bowler and Bradshaw (1985) have shown that significant peat growth has occurred in the Wicklow Mountains over the last 150 years, alongside active erosion. Peat erosion may have been initially as a result of natural factors (e.g. climate change). More recent human and domestic animal disturbance, along with atmospheric pollution, however are thought to have intensified rates of erosion (Bradshaw and McGee 1988). The flattening of the excess 210Pb profile in the upper part of the Kelly’s Lough core may be indicative of either an increased rate of sediment accumulation (Appleby 2001), or of physically- or biologically-mediated vertical mixing of sediments (Robbins et al. 1977; Garcin et al. 2007). In this case, the general agreement observed between the CRS estimated chronology, the
137
Cs profile, and the SCP dating features
indicates that the former is most likely the case.
In sediment samples from Kelly’s Lough, measurable levels of excess 210Pb were detected, indicating that atmospheric inputs to the site are being effectively recorded by the lake sedimentary record.
Moreover, evidence of contamination by industrial-derived
atmospheric pollutants is apparent in sediment samples in the form of measurable concentrations of SCPs (Fig. 4.3), a direct indicator of industrial-level combustion of fossil fuels (Rose 2001).
4.4.2 Lough Maumwee Little up-core variation is apparent in pollen and LOI data for samples collected at Lough Maumwee (Fig. 4.10), indicating that there have been no major changes in catchment land cover over the period covered by the sedimentary record. The main pollen and spore types identified in the samples analysed, particularly Calluna and Corylus, are, at present, observed in abundance in the lake’s catchment. In addition, the relatively high abundances
126
of pollen from the aquatic plant Isoetes (presumably I. lacustris) would appear to indicate a high level of water transparency (Webb et al. 1977), suggesting that levels of in-wash of eroding soil and organic matter from the catchment have been minimal. Compared to Kelly’s Lough, the relatively low level of organic matter content measured in the samples analysed (Fig. 4.10) appears to reflect the composition of land cover in the lake’s catchment, being covered predominantly by stoney shallow soils (Flower et al. 1994). Furthermore, all three cores collected from Lough Maumwee show similar levels of up-core variation in organic matter content and allowed for the extrapolation of
210
Pb estimates of
sediment accumulation rate and chronology to the other two undated cores collected from proximate locations at the site. According to 210Pb analysis, the basal core sample collected from Lough Maumwee dates to ca. 1890 and the average sedimentation rate estimated for the site (0.0241 g cm-2 yr-1) is in close agreement with a previously reported estimate of 0.027 g cm-2 yr-1 (Flower et al. 1994).
Measurable levels of excess
210
Pb were evident in Lough Maumwee samples indicating
that sediments collected from the site contain a record of atmospheric inputs. Furthermore, evidence of contamination of sediments by SCPs (Fig. 4.12) confirms that the site is in receipt of depositions of industrial-derived atmospheric pollutants (Rose 2001). Results of SCP analysis reported in the current study are in close agreement with those previously reported for the site (Flower et al. 1994). Based on fieldwork undertaken in 1988, Flower et al. (1994) reported relatively low concentrations of SCPs in sediments dated to the early 20th century, followed by a strong increase from the mid-1970s to a maximum of 5000 gDM1
in sediments dated to 1984. Moreover, results presented in the current study extends the
sedimentary record of SCP contamination for the site by approximately 20 years, showing that peak concentrations observed by Flower et al. (1994) in the surface sediment sample, dated to 1988, have been, for the most part, sustained to ca. 2006.
Furthermore, up-core
variations in levels of reconstructed DI-pH reported in the current study (Fig. 4.18) are also in close agreement with those published by Flower et al. (1994). Flower et al. (1994) reported that levels of DI-pH at the site remained remarkably stable (5.7 – 6 DI-pH) from 1864 to 1988.
4.4.3 Upper Killarney Lough Minimal up-core variation in the relative abundances of individual pollen and spore types is apparent in sediment samples collected at Upper Killarney Lough (Fig. 4.19).
This
127
indicates that catchment landcover has undergone no major changes throughout the period covered by the sedimentary record. The main pollen types, particularly Calluna, Corylus, Poaceae and Pinus, are all, currently, well represented by their parent plant taxa in the catchment for the lake. LOI results for Upper Killarney Lough indicate no major changes in levels of organic matter inputs to the site (Fig. 4.19). Furthermore, the LOI data suggest that sediment inputs as a result of peat erosion in the catchment are minimal.
This
suggestion receives support from the presence, albeit in relatively low abundances, of the aquatic pollen type Isoetes. In addition, LOI results show that all three cores collected from the site show similar levels of up-core variation in organic matter content, allowing for the 210
Pb-based estimates of sediment accumulation and chronology to the
extrapolation of
undated sediment cores collected from proximate locations.
According to
210
Pb analysis results, the basal core sample from Upper Killarney Lough
dates to ca. 1880, and the average sediment accumulation rate estimated for the site (0.3 cm yr-1) is in close agreement with a previously reported estimate of 0.27 cm yr-1 (Leira et al. 2007).
However, a hiatus in sedimentation is apparent in the lowermost sediment
samples as similar levels of excess
210
Pb activity are observed (Fig. 4.20a).
This is
possibly due to a sedimentary disturbance / slump, resulting in a period of rapid sedimentation onto the coring location (Appleby 2000). Results of geochemical analyses appear to confirm this, as relatively high concentrations of both the major and trace elements were measured (Fig. 4.22a), coincident with
210
Pb evidence of a sedimentary
disturbance. This is indicative of an increased supply of trace elements due to increased rates of sedimentation rather than atmospheric deposition (Lotter et al. 2002).
For
example, the Upper Killarney Lough catchment area is known to have been subjected to episodes of forest fires in the past (Mitchell 1988; 1990), and such episodes can lead to temporary increases in catchment erosion (Wilmshorst and McGlone 2005). However, there appears to be no reported evidence of forest fires occurring in the catchment during the period in question (ca. the late 19th century). Nonetheless, the effects of forest fires on the sediment record can become apparent in the period after the fire. This is because the area which has been affected by fire may lack an organic surface layer. Thus runoff from the area may cause increased inputs of elements to the lake (Bayley et al. 1992). Alternatively, the sediment disturbance may be due to the re-suspension of sediments due to increased levels of lake water turbidity (Bloesch 1994). Such an effect would not be unexpected at a site such as Upper Killarney Lough, where several relatively large rivers discharge to and from the lake (Fig. 2.10).
128
Measurable levels of excess
210
Pb were detected in the sediment samples analysed,
indicating that the sediment samples contain a record of atmospheric inputs. The level of excess
210
Pb measured in the surface sediment sample from Upper Killarney Lough (60.1
-1
DPM g ) is of a similar magnitude to the level measured for the surface sediment sample from Lough Maumwee (70.34 DPM g-1).
This suggests that sediment inputs from the
relatively large catchment area are not diluting or obscuring the sedimentary record of atmospheric inputs, as high rates of sediment inputs from the catchment can lead to dating problems, as radionuclide concentrations are diluted too close to background (Schotbolt et al. 2006). Moreover, measurable levels of SCPs were determined in sediment samples from 35.5 cm (ca. the late 1890s) in depth to the sediment surface (Fig. 4.21), confirming that the sediment record contains a record of industrial-derived atmospheric inputs throughout this period (Rose 2001).
129
Chapter 5: Discussion Palaeolimnological proxy data presented in Chapter 4 indicates that the three study sites selected for analysis provide a suitable basis for addressing the research questions stated in the introductory chapter. This chapter will first examine, discuss and interpret the data in terms of (a) evidence of temporal and spatial variations in levels of
deposition of
atmospheric pollutants; and (b) evidence of ecosystem response to contaminant-induced pressures. This chapter will then go on to address the key research questions as stated in the introductory chapter.
5.1 Palaeolimnological evidence of temporal and spatial variations in levels of deposition of atmospheric pollutants A discernible record of depositions of atmospheric pollutants from the industrial-level combustion of fossil fuels is apparent in the sediments collected at the three study sites, and these are discussed below:
SCP SCPs are an unambiguous indicator of atmospheric contamination from the industrial-level combustion of fossil fuels. Unlike trace elements and PAHs, SCPs, once deposited, do not suffer from post-depositional alteration (Rose et al. 1998b). In the current study, SCPs have been utilised as direct evidence of atmospheric deposition of pollutants from the industrial-level combustion of fossil fuels. Furthermore, SCP size class data have been used to provide an estimate of the location of the source of emission of SCPs (see Larsen 2003). SCP evidence, from all three sites, is discussed below in terms of:
Temporal variations, with reference to the three generally most prominent SCP concentration profile features (see section 3.2.4.1):
Spatial variations.
Trace elements Trace elements are also emitted during the fossil fuel combustion process, and lake sediment records from across the globe archive a pattern of increasing deposition of trace elements over the past 150 years as a result of the rapid expansion of industry (Renberg
130
1986; Renberg et al. 2001). However, in contrast to SCPs, interpretation of sedimentbased records of trace elements is more complex due to the influence of local site factors. In addition, post-depositional mobility of trace elements can occur, leading to further uncertainty (Boyle 2001a). As a result, trace element evidence obtained from the three study sites is discussed below in terms of:
Post-depositional alteration of trace element profiles, particularly Fe and Mn remobilisation;
Distinguishing inputs of trace elements from anthropogenic sources from those of natural sources employing, in the case of this research, the PCA and geochemical normalisation methods;
Interpretation of the lake sediment record of atmospheric trace element contamination, in terms of both temporal and spatial variation, with particular reference to SCP data.
PAHs Sediment-based records of PAHs from the three study sites are discussed below in terms of:
Sources of PAHs and the ability of sediments to record the atmospheric deposition of PAHs;
Temporal and spatial variations in levels of atmospheric PAH contamination, with particular reference to SCP data.
In addition, three main sources of secondary data are discussed to test hypotheses regarding the sources and levels of deposition of pollutants: both reconstructed and officially reported inventories of, respectively, SO2 and SOX emissions (Mylona 1993, 1996; Klein and Benedictow 2006; Klein et al. 2007); EMEP modelled depositional data for SOX (Klein et al. 2007); and power plant capacity data and histories (Manning and McDowell 1984; O'Riordan 2000).
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5.1.1 SCPs 5.1.1.1 Temporal variations As shown in Fig. 5.1, inter-site temporal variations in accumulations of SCPs are apparent, and indicate that global and national depositional patterns are playing a role in determining SCP inputs to each site. Temporal variations in levels of SCP input throughout Europe (Rose et al. 1999b), Africa (Rose et al. 2003), China (Boyle et al. 1999), Siberia (Rose et al. 1998b), Australia (Cameron et al. 1993), and the USA (Charles et al. 1990) are similar to those preserved in the sediments extracted from the three sites studied in the current research. As mentioned previously, three main SCP features are identified globally: the start of the SCP record (A on Fig. 5.1); the rapid increase in particle concentration (B on Fig. 5.1); and the particle concentration peak (C on Fig 5.1). This is due to the widespread, almost co-occurrence, of major changes in the industrial-level combustion of fossil fuel. Superimposed upon the general patterns are local variations in depositions of SCPs, which are relatively minor (Rose 2001).
The start of the SCP record Of the three sites examined, the start of the SCP record is apparent only at Upper Killarney Lough, where it occurs relatively late, compared to a previously reported estimate for Irish lake sites (1850 ± 25 years) (Rose et al. 1995; Rose and Theophile 2004), and dated to the late 1890s. Although the start of the SCP record was not observed in the core from Lough Maumwee, SCPs were observed in the lowermost core samples, dating to ca. 1890. This indicates that atmospheric contamination of Lough Maumwee commenced earlier than at Upper Killarney Lough. However, the limits of
detection associated with the SCP
technique ( 7)
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Figure 5. 9 Up-core variations in concentrations of SCP, abundances of diatoms grouped according to the Hustedt classification system (1936-1939), and reconstructed levels of DI-pH and their associated boundaries of uncertainty for the three study sites that form the focus of the current research. DAZs identified at Kelly’s Lough and Upper Killarney Lough are also marked.
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5.2.2 SQGs Due to the relatively low amounts of deposition of atmospheric toxicants (PAH and trace elements), levels of toxicity in samples of surficial samples from the three study sites are expected to be low. This expectation receives some support from results of SQG analysis.
In the current study, PEC-Q ratios were used to identify levels of possible sediment toxicity due to contamination by groups of trace elements (PEC-Q MM), and by groups of trace elements in combination with total levels of PAHs (PEC-Q MPP). In examining possible biological effects of sediment toxicity, both Ingersol at al. (2001) and Rippey et al. (2008) assumed trace elements to be the main sediment-based toxicant, and concluded that biological effects are expected to occur with PEQ-MM ratios > 2. In addition, trace element / PEC and total PAH / PEC ratios were employed in the current study to assess the degree to which individual contaminants contributed to levels of possible sediment toxicity. PEC-Q MM and PEC-Q MPP values indicate that Upper Killarney Lough (Table 4.15) shows the highest levels of possible sediment toxicity, while Lough Maumwee sediments (Table 4.10) show slightly lower levels and material from Kelly’s Lough shows the lowest levels (Table 4.5). However, levels of possible toxicity are relatively low at all three lakes, and in general are at the lower end of the range of ratio values reported for sediments from lakes in Britain (0.11 – 1.08 and 0.06 – 0.65 for PEC-Q MM and PEC-Q MPP ratios, respectively) (Rippey et al. 2008). Metals make the greatest contribution to levels of sediment toxicity at all three study sites. Moreover, the relatively high PEC-Q ratio values at Upper Killarney Lough are due primarily to high concentrations of Cr measured in the surface sediment sample.
As
already discussed, evidence indicates that enrichment of Cr in the surface sediment sample at Upper Killarney Lough may be due to changes in sedimentary redox conditions. The post-depositional re-mobilisation of metals under changed redox conditions has long been recognised (Boyle 2001a, 2001b), and, as documented in this study and elsewhere (e.g. Miao et al. 2006; Bibi et al. 2007), such effects can increase levels of toxicity in surface sediments.
Therefore, increases in the intensity and frequency of anoxia in
sediments, which may occur as a result of, for example, eutrophication or climate change, can lead to the increased transfer of sedimentary trace metals to the surface sediment, and thus to enhanced levels of surficial sediment toxicity (Pham et al. 2007).
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Significant biological effects at the three study sites are unlikely given the relatively low levels of contamination, although there are no biological or benthic data to verify this suggestion (and see Chapman et al. 1999). The likelihood of few if any biological effects is confirmed by PEC-Q MM ratio values, all of which are far below the threshold limit (ratio value of 2). Further confirmation of a paucity of biological impacts is provided by the presence of Salvelinus alpinus at Upper Killarney Lough. Populations of Salvelinus alpinus are particularly sensitive to levels of ecosystem toxicity, and local extinctions of this species in European waters have been associated with increased levels of deposition of atmospheric pollutants (Igoe et al. 2003).
However, the level of toxicity in freshwater
sediments increases by 25% over the PEC-Q MM range of 0.1 to 1.0 (Ingersoll et al. 2001). Thus, although levels of sediment toxicity may have not reached a point whereby ecosystem health has been affected detrimentally to a significant degree, levels at all three study sites have been enhanced relative to baseline conditions.
Moreover, sediment-
based PEC-Q MM ratios do not consider the bio-available fraction of toxic chemicals (Rippey et al. 2008), and the potential for bio-accumulation of toxic chemicals within the species that consume them, including humans (MacDonald et al. 2000), and therefore may underestimate the actual contaminant risks to biota.
5.3 Lake sediment-based reconstructions of temporal and spatial differences between the three APDRs in Ireland in levels of deposition of atmospheric contaminants from the industrial-scale combustion of fossil fuels over the last 50 - 150 years The first key research question that this thesis sought to answer was, as stated in Chapter 1, ‘What are the main differences between the three APDRs in Ireland in the levels of deposition of atmospheric contaminants from the industrial-scale combustion of fossil fuels over the last 50 – 150 years?’. In Chapter 2, on the basis of previously published research, the three APDRs in Ireland were identified, and then, on the basis of locational and palaeolimnological data, three study sites, one from each of the three APDRs, were selected for examination.
The suitability of each study site for addressing the main
research questions was then confirmed by palaeolimnological proxy data presented in Chapter 4.
Palaeolimnological proxy evidence of variations in levels of deposition of
atmospheric pollutants (presented in Chapter 4) was then used in the current chapter to reconstruct the temporal and spatial differences in levels of deposition of atmospheric
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pollutants at the three study sites. With this in mind, and with reference to previously cited data and discussions in this thesis, existing and relevant records of industrialisation, particularly the changing capacity of emissions from power stations, appear to confirm the reliability of lake sediment-based reconstructions of spatial and temporal differences in levels of deposition of atmospheric pollutants at the three study sites. However, inter-site differences in rates of sediment accumulation meant that time periods of various lengths were accommodated in the sediment cores collected and analysed. The cores collected from Lough Maumwee and Upper Killarney span the last 100 – 150 years, while the lowermost samples from Kelly’s Lough dated to ca. 1950. In addition, and as a means of answering this primary research question, this thesis aimed to assess the relative contributions of national and transboundary sources to levels of deposition of atmospheric pollutants, and also to assess the effectiveness of recent legislation, mitigation strategies, and technologies aimed at reducing emissions of atmospheric pollutants. Such information is of particular interest to policy makers and environmental regulators in Ireland.
In
examining such effects, research carried out as part of this thesis attempted to assess the applicability of the SCP dating technique as a means of dating sediments accumulating in sedimentary environments in Ireland.
5.3.1 The relative contributions of national and transboundary sources to levels of deposition of atmospheric pollutants Atmospheric pollutants generally do not respect state boundaries (Akimoto 2003): pollutant emissions from sources within a state may affect air quality in that state as well as in other countries located downwind (Holloway et al. 2003). The contributions of transboundary sources of emissions to levels of atmospheric contamination are relatively high in Ireland, particularly in the eastern part of the country (Aherne and Farrell 2002). For example, 67 % of SOX depositions in 2004 were thought to have originated from transboundary sources (Klein and Benedictow 2006).
As a result, national abatement measures aimed at
decreasing levels of atmospheric contamination are unlikely to be entirely effective in Ireland, as in many other countries. The determination of past variations in the deposition of atmospheric pollutants evident in the sedimentary records preserved at three carefully selected lakes in Ireland was an important aim of the current research. Moreover, defining the likely source area- either national or long-distance – of pollutants associated with the combustion of fossil fuels was of particular interest, especially because of the insights provided on the extent to which ambient levels of atmospheric pollution in Ireland are a
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result of industrial activities taking place in neighbouring and more distantly located countries, and on any changes in the relative contributions from these non-native sources that may have occurred over the last 50 – 150 years. Sediment-based evidence from Upper Killarney Lough indicates that the main sources of atmospheric pollutants deposited at the site are national and regionally-based, particularly Killbarry and Marina power stations. Depositions of atmospheric pollutants from national sources, notably Moneypoint power station on the lower Shannon, are also evident in parts of the sedimentary record from Lough Maumwee, dating to the period post ca. the late 1980s. At Kelly’s Lough, the situation is more complex for the period post 1990, due in part to a more variable rate of sediment accumulation, possibly as a result of periodic sediment focussing.
However depositions of atmospheric pollutants at Kelly’s Lough appear
primarily to be a function of local (i.e. Dublin-based) sources. Evidence of the deposition of transboundary pollutants is present in the sedimentary records from both Kelly’s Lough and Lough Maumwee. In contrast, no such evidence is apparent at Upper Killarney Lough. This is most likely due to the predominant contribution of national sources to levels of atmospheric contamination at the site, which serve to mask the transboundary contribution. However, levels of atmospheric contamination measured at Upper Killarney Lough are relatively low, and, therefore, the contribution from transboundary sources is likely to be minor. The relatively high levels of atmospheric contamination measured at Kelly’s Lough throughout the period covered by the sediment record are most likely a reflection of contributions from a combination of sources of atmospheric pollutants, situated in Ireland, Britain and mainland Europe. Moreover, at Kelly’s Lough, it is thought that the contribution from British sources, in particular, must be playing a prominent role in determining the relatively high levels of atmospheric pollutant deposition reported for the site. This is due to the close proximity of emission sources situated in Britain, particularly those situated in west Britain, to the east of Ireland when compared with those located on the European mainland. However, from ca. the early 1980s until ca. 2005, evidence, particularly levels of deposition of PAHs, appears to indicate the decreasing contribution of transboundary sources to levels of atmospheric contamination at the site.
The low levels of atmospheric contaminants in sediments
accumulating at Lough Maumwee appear to have originated from transboundary sources: it is thought that SCP contamination of the site pre-1940 originates primarily from sources situated in Britain and mainland Europe, while trace elements, particularly Cd, originate from North America.
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A decrease in the contribution of trans-Atlantic sources is evident
from the late 1970s to early 1980s in sediments from Lough Maumwee. However, it is more difficult to distinguish the more recent contribution from transboundary sources situated in Britain and mainland Europe. This is because of the relatively high levels of nationally sourced contributions of SCPs, which serve to mask the weaker signal from transboundary sources.
5.3.2 The effectiveness of recent legislation, mitigation strategies, and technologies aimed at reducing emissions of atmospheric pollutants The detrimental impacts of increasing levels of atmospheric pollution on freshwater ecosystem (e.g. acidification) health have long been recognised (Gorham 1998).
In
response to such concerns, stringent national (e.g. the Clean Air Act implemented in the UK in 1956) and international pollution control programmes (e.g. UNECE LRTAP, 1983) have been implemented, aimed at reducing atmospheric pollutant emissions through abatement strategies, such as the introduction of new technologies (e.g. flue-gas desulphurisation) and the switching over to less polluting fuels (e.g. gas) (Wichmann 2004). For example, Ireland ratified the UNECE LRTAP (1983) in 1985, and since has ratified both the Sofia Protocol (1988) in 1994, and the Oslo Protocol (1994) in 1998. In addition, Ireland has signed, but not yet ratified, the Aarhus Protocol (1998) and the Gothenberg Protocol (1999). Evidence from lake sediments has previously been shown to reflect, to a reasonable degree of accuracy, reduced levels of deposition of atmospheric pollutants as a result of such measures being implemented (e.g. Flower et al. 2006; Rose and Monteith 2005). The effects on atmospheric pollutant loadings of changes in fuel-type at power stations in Ireland are particularly evident in the sediments examined in the current research, while the impacts of international measures aimed at reducing levels of transboundary pollutants are also apparent. Fuel-type changes have been previously shown to have a measurable effect on levels of sediment-based contaminants (Rose and Monteith, 2005). Sedimentary evidence from Upper Killarney Lough indicates that changes in the fuel used at Marina power station, which converted from coal and oil dual-fired combustion processes to gas-fired combustion in 1980, may have led to decreased deposition of atmospheric pollutants at the site. For example, contemporary levels of deposition of both SCPs and trace elements were less than half the levels measured in a sample dated ca. 1980, and were similar to levels measured in the early part of the 20th century. Similarly, at Kelly’s Lough, decreasing
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depositions of atmospheric pollutants from the late 1970s / early 1980s until ca. 1990 appear to be the result of fuel-type changes undertaken at both Poolbeg and Northwall power stations, which converted from coal- and oil-fired combustion processes, respectively, to dual-fired oil and gas combustion processes from the early- to mid-1980s (O'Riordan 2000). Finally, at Lough Maumwee, declining trace element contamination of the atmosphere from ca. 1980 can be, in part, attributed to decreasing pollutant emissions originating from North American sources due to the implementation of the US Clean Air Act (1970). This suggestion receives support from the composition of sediment-based records, both in North America (e.g. Norton et al, 2007) and in the west of Ireland (Coggins et al. 2006), which show decreased levels of trace element contamination from the mid to late 1970s.
5.3.3 The applicability of the SCP dating technique in estimating sediment chronologies at lake sites in Ireland SCPs, once deposited, do not easily degrade in waterlogged lake sediments (Rose 2001). Provided the sediment record has not been disturbed or altered, measurements of the concentrations of SCPs are generally seen as reliable, robust, and replicable representations of depositions of atmospheric pollutants over relatively broad geographic areas, such that the three main features of SCP concentration profiles – the start of the particle record, the rapid increase in particle concentrations, and the SCP concentration peak – can be used for dating recently accumulated lake sediments (e.g Yang and Rose 2003; Yanhong et al. 2005; Leira et al. 2006; Guhren et al. 2007).
However, SCP
concentration profiles have been found to be regionally specific, particularly in areas where sources are many and over-lapping. As a result, relatively fine-scale sampling of multiple sites (one lake per 100 x 100 km) may be needed in order to establish the full extent of regional variability (Rose and Appleby 2005). However, in areas where single or clusters of closely located sources dominate, such fine-scale sampling may not be required (Rose and Appleby 2005). Uncertainty is attached to the precise dating of the three SCP dating features in Ireland; as there have been few studies on variations in depositions of SCP over time and at different locations. The exception is the study by Rose et al. (1995), who examined three lakes in the northwest of Ireland and one in the west of Ireland. On this basis, Rose et al. (1995) estimated that, in Ireland, SCPs first appeared in ca. 1850 ± 25 years, increased rapidly from ca. 1950 ± 10 years, and peaked in ca. 1980 ± 3 years. However, the cores of sediment examined by Rose et al. (1995) were limited
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geographically to the west and northwest of Ireland and the results are therefore unlikely to be representative of the whole of Ireland. That the conclusions of Rose et al. (1995) may not have been representative for the whole of Ireland appears to receive support from the current research. Results from the current research highlighted inter-site differences in the estimated age of the various characteristic features of the SCP profiles.
Regional differences within Ireland in the history of
depositions of SCPs, and presumably other products of the industrial-level combustion of fossil fuels, are therefore evident. This regional variation within Ireland is likely to be a reflection of changes in the power generation industry in Ireland and farther afield, upon which are superimposed the effects of local and regional factors, such as topography and altitude. The inter-site differences in dates for the main features of the profile of varying concentrations of SCPs are shown in Table 5.5. Not only do the dates differ between the study sites; they are also at variance with the estimated ages published in Rose et al. (1995). The following briefly summarises these differences. The start of the deposition of SCPs is only evident in sediments from Upper Killarney Lough (the sediment records at the other two sites are too short, temporally, to include material that pre-dates contamination by SCPs), although the date for onset of accumulation of SCPs is relatively late when compared with previously published work for Ireland (e.g. Rose et al. 1995). There is some consistency with the dating of the second characteristic of the SCP profile – the rapid increase in concentrations. This feature is evident in sediments from all three study sites, although the timing is later at Kelly’s Lough than at Lough Maumwee and Upper Killarney Lough and later than the Ireland-wide dates suggested by Rose et al. (1995).
This
consistency breaks-down with the third characteristic feature, however – the peak in levels of SCPs, which is the most ambiguous feature of the dated SCP profiles. At two of the study sites (Kelly’s Lough and Upper Killarney Lough) the peak is actually two peaks, and only one of these at each site dates to ca. 1980 (the date suggested by Rose et al. (1995) as applicable Ireland-wide).
At Upper Killarney Lough, the peak dated ca. 1980 is
preceded by an earlier peak, while the opposite is the case at Kelly’s Lough.
By
comparison, only a single peak in concentrations of SCPs is evident from samples from Lough Maumwee, where it is dated far later than the ca. 1980 date suggested by Rose et al. (1995).
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Inter-site differences in the timing of the three main SCP dating features provide an indication of the extent of within-Ireland variability in depositions of SCPs. Such regional variability has also been reported for Britain, where eight regions with characteristic patterns of deposition of SCPs have recently been identified (Rose and Appleby 2005). Regional variation in SCP depositions are likely to be due to a combination of factors, including the direction of the prevailing wind (and therefore the airshed and pollution sources within it), topography and the history of atmospheric polluting activities locally. As a result of this regional variation, SCP based chronologies should only be applied to broad geographic areas with a high level of caution: as with pollen zones, their potential as a dating tool would appear to be geographically restricted. Determining the full extent of regional variability should not necessarily involve detailed analyses of cores of sediment from a large number of sites, however.
According to Rose and Appleby (2005), the
influence of single sources or clusters of closely located sources on the general pattern of depositions of SCPs is often clear and can therefore be established on the basis of relatively few studies (Rose and Appleby 2005). Thus, the three main dated SCP features identified at the three sites examined in the current study may be applied more broadly, and on a regional basis, possibly to each of the three APDRs in Ireland. However, these results provide an indication of regional variability only at this stage: further research is required, in order to establish the reliability of these estimates and the full geographical extent of their potential application.
Kelly’s Lough Lough Maumwee Upper Killarney Lough Dates ascribed to Irish lake sites (Rose et al. 1995)
Start of the SCP record N/A N/A Late 1890s 1850 ± 25 years
Rapid increase in concentrations ca. the early 1970s ca. the late 1940s ca. 1955 1950 ± 10 years
SCP concentration peak ca. 1980 / ca. 2000 ca. the 1990s ca 1970 / ca. 1985 1980 ±- 3 years
Table 5. 5 Dates of occurrence of the three main SCP dating features identified at all three study sites. Previously reported estimates (Rose et al. 1995) for Irish lakes are also detailed.
5.4 Lake sediment-based reconstructions of ecosystem response to variations in levels of deposition of atmospheric contaminants from the industrial-scale combustion of fossil fuels The second key research question that this thesis sought to answer was ‘To what extent have depositions of atmospheric contaminants from the industrial-scale combustion of
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fossil fuels impacted freshwater lake ecosystems?’. Sediment-based remains of diatom analysed from the three study sites examined in the current study indicate inter-site differences in ecosystem responses to variations in levels of atmospheric deposition of acidic contaminants.
Such differing responses are thought to be due to inter-site
differences in levels of deposition of atmospheric pollutants, hydromorphology, biotic interactions, water quality and catchment characteristics, such as extent of various types of vegetation, underlying geology and residence time. In addition, this thesis aimed to assess levels of ecosystem toxicity due to the deposition of atmospheric pollutants.
This
assessment was achieved through an examination of levels of toxicity measured in surface samples of lake sediments. These were deemed to be particularly important to aquatic ecosystems, as sediments play a critical role in food webs and can also act as reservoirs of toxic contaminants for bio-accumulation and trophic transfer (Burton Jr 2002). Results presented in this thesis attest that levels of toxicity measured in surficial lake sediments at the three study sites are relatively low. Such low levels of toxicity are unlikely to have a detrimental effect on biological communities. Thus the following discussion focuses on inter-site differences in ecosystem responses to acidification pressures.
5.4.1 Acidification and recovery Increasing levels of acidic depositions associated with industrialisation and increased power generation based on fossil fuels during the period post-World War II resulted in the acidification of surface waters, causing alteration and deterioration of almost every aspect of aquatic ecosystems (Battarbee 1990). Subsequently, changes in power generation and the introduction of cleaner technologies have led to decreases in levels of acidic depositions, and eventually to biological recovery at some lake sites (Monteith and Shilland 2007). This study aimed to examine inter-site differences in the response of three acid sensitive aquatic ecosystems in Ireland to variations in levels of acidic depositions. Moreover, identifying the underlying factors influencing the relative sensitivity / insensitivity of the three study sites was of particular interest. Evidence of acidification and recent recovery, seemingly associated with changing depositions of the products of the industrial-scale combustion of fossil fuels, is clearly apparent in data obtained from sediments accumulating at Kelly’s Lough and dated to the late 1990s. The highest levels of atmospheric deposition of material commonly associated with acidification pressures on freshwater ecosystems were measured at Kelly’s Lough.
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Acid-sensitive lakes elsewhere in Europe have acidified as a result of the deposition of similar levels of acidifying pollutants (e.g. Jones et al. 1993). Furthermore, Kelly’s Lough is a humic lake and has been shown to be naturally acidic throughout its post-glacial history (Leira et al. 2007). As such, Kelly’s Lough is expected to be sensitive, chemically, to increased levels of acid deposition (Brakke et al. 1987; Ginn et al. 2007a). In addition to chemical sensitivity, however, sedimentary evidence also reveals a relatively rapid biological response to changing acidification pressures. Such biological sensitivity has previously not been widely reported (Skjelkvåle et al. 2003).
In particular, biological
recovery at humic lake sites is not generally expected in part because levels of DOC inputs from the catchment are thought to have increased, as levels of acidic depositions decrease, and soil chemistry returns to its more natural state. Increased levels of DOC inputs, and associated humic and fulvic acids, can provide a buffer against biological recovery (Jeffries et al. 2003; Evans et al. 2005; Monteith et al. 2007).
However, the
actual severity of contamination may be crucial in determining biological recovery, as some highly contaminated lake sites in the UK appear to be showing recent evidence of recovery (Monteith and Shilland 2007): evidence from Kelly’s Lough, which is a relatively contaminated site, appears to agree with this finding. In contrast, and aside from a reaction to a possible early (pre 20th century) phase of acidification at Upper Killarney Lough, there is little evidence of ecosystem responses to variations in levels of atmospheric deposition apparent in sediments from Lough Maumwee and Upper Killarney Lough.
However, levels of deposition of acidic pollutants have
historically been relatively low at these two sites, and under such conditions the response to changes in acidification pressures have tended to be much more variable (Flower et al. 1994; Cameron et al. 2002). In the case of Lough Maumwee and Upper Killarney Lough, the retention time of acidic inputs in the lake and in the catchment, respectively, are thought to be effectively buffering the low levels of acidic inputs. The relative base-poor nature of the bedrock and soils and the relatively short residence time that characterise the catchment for Lough Maumwee provide for only negligible buffering capacity (Hornung et al. 1995; Jeffries et al. 2003). However, due to the relatively rapid delivery of acidic inputs to the lake and the relatively long lake water residence time, in-lake generation of alkalinity may be buffering acidic inputs, as has been reported elsewhere (Schindler 1986; Schindler et al. 1986; Giblin et al. 1990; Silver et al. 1999). The transparency of the water may also be a factor, particularly at Lough Maumwee. As discussed previously, clear water lakes are often relatively insensitive to acidification and alkalinisation, because of the low levels of CO2 (Arts 2002). The situation is quite different at Upper Killarney Lough, where acidic
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inputs are seemingly being retained and neutralised within the large catchment, and before they reach the lake (Hornung et al. 1995; Kopacek et al. 2004; Rose et al. 2004b). Such catchments can eventually switch from being a sink of acidifying substances to being a source (Prechtel et al. 2001; Kopacek et al. 2004; Rose et al. 2004b), however, as catchment-retained acidity is released through mineralisation, reduction and weathering (Prechtel et al. 2001), although the effects of such a transition remain a subject for debate (Weyhenmeyer 2008).
The abrupt release of acidifying ions may explain evidence for
early acidification (i.e. pre 20th century) at Upper Killarney Lough, and could have been triggered by unusually heavy rainfall, disturbances in the catchment that are not otherwise detected in the sedimentary record, or a sudden shift in levels of atmospheric inputs of marine-derived SO4.
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Chapter 6: Conclusions 6.1 Thesis summary As described in Chapter 1, in the absence of long-term monitoring records of variations in levels of deposition of atmospheric pollutants in Ireland, one of the key research questions that this thesis sought to answer was ‘What are the main spatial and temporal differences between the three APDRs in Ireland in the levels of deposition of atmospheric contaminants from the industrial-scale combustion of fossil fuels during the last 50 – 150 years?’. Furthermore, in the absence of long-term monitoring data of ecosystem response to these variations, the second key research question that this thesis sought to answer was ‘To what extent have these depositions impacted freshwater lake ecosystems?’.
A
palaeolimnological approach was adopted in order to answer these key research questions. The main research work and results have been introduced and discussed in previous chapters: Chapter 4 presents results of palaeolimnological proxy analyses, while Chapter 5 examines, discusses and interprets these results, and then addresses the key research questions.
However, in answering these research questions, a number of
limitations became apparent in the methodologies adopted in the current research. This chapter presents the main research findings, before describing the main limitations of the study and then going on to make suggestions for future research.
6.2 Research findings As a means of answering each of the two key research questions that formed the focus of the current research, a number of key assessments were made; the findings of these are detailed below: Research question 1: ‘What are the main spatial and temporal differences between the three APDRs in Ireland in the levels of deposition of atmospheric contaminants from the industrial-scale combustion of fossil fuels during the last 50 – 150 years?’ Adopting a palaeolimnological approach, the current research has reconstructed variations in levels of deposition of atmospheric contaminants from the industrial-scale combustion of fossil fuels at three remote Irish lake sites, one from each of the three APDRs. Due to inter-site differences in rates of sediment accumulation, the time periods accommodated by
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the sediment cores collected from the three study sites differ. Sediment cores collected from Lough Maumwee and Upper Killarney Lough span the last 100 – 150 years, while the lowermost core samples collected at Kelly’s Lough dated to 1950. Existing and relevant records of industrialisation, particularly those relating to the changing generating capacities of power stations, appear to confirm the reliability of the sediment-based reconstructions presented in the current research. Moreover, as a means of answering this research question, the current research sought to:
Assess the relative contributions of national and transboundary pollutant sources to levels of deposition of atmospheric pollutants
Levels of atmospheric contamination in the east of Ireland, as represented by depositions in sediments accumulating at Kelly’s Lough, are determined by national (particularly those based in the east of Ireland) and transboundary sources of pollutant emission. Atmospheric depositions in the past in the west of Ireland, recorded in sediments from Lough Maumwee, were predominately due to the long-range transport of atmospheric contaminants from sources located in North America, Britain and continental Europe, while national sources, particularly Moneypoint power station, have become predominant since ca. the late 1980s. In contrast, national sources of pollutant emissions are the primary determinant of levels of atmospheric contamination in the southwest, as reflected in depositions at Upper Killarney Lough.
Assess the effectiveness of recent legislation, mitigation strategies, and technologies aimed at reducing emissions of atmospheric pollutants
The effectiveness of recent fuel-type changes undertaken at power stations in Ireland has had the greatest influence on decreasing levels of pollutant deposition in the east and in the southwest of Ireland.
In the west of the country, it is thought that international
legislation, e.g. the US Clean Air Act (1970), has resulted in decreased levels of deposition of atmospheric-borne trace elements since ca. 1980.
Assess the applicability of the SCP dating technique in estimating sediment chronologies at Irish lake sites
Temporal variations in levels of deposition of SCPs can potentially be used to date sediment profiles from lakes in Ireland, although, as shown in this thesis, depositions vary
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spatially also. As a result of distinct geographic differences in the dating of characteristic features of up-core variations in SCP concentrations, further studies are required in order to establish the number and extent of regions in Ireland with characteristic patterns of deposition of SCPs and, the accurate dating of variations in concentrations of SCPs within each region. Research question 2: ‘To what extent have these depositions impacted freshwater lake ecosystems?’ Palaeolimnological results presented in Chapter 4, and subsequently synthesised, discussed and interpreted in Chapter 5 attempt to examine and assess ecosystem response to variations in levels of deposition of atmospheric pollutants from the industrialscale combustion of fossil fuels.
For the most part, it appears that sediment-based
reconstructions presented in the current study provide reliable evidence of past ecosystem responses to these variations.
Furthermore, as a means of answering this research
question, the current research sought to: Assess the relative sensitivities of the three study sites to variations in levels of
deposition of acidic pollutants Kelly’s Lough shows evidence of biological recovery since the late 1990s due to decreased levels of acidic depositions. Such a biological response is perhaps surprising given the lake’s humic status. However, the relatively high levels of contamination may, in part, explain biological recovery at the site.
In addition, acid-sensitive sites in receipt of
relatively low levels of acidic depositions may be insensitive to
reduced acidification
pressures due to processes such as in-lake generation of alkalinity, reduced levels of CO2 and catchment retention and neutralisation of acidic inputs, as appears to be the case at Lough Maumwee and Upper Killarney Lough. Assess surficial sediment levels of toxicity at the three study sites and their potential for
adverse impacts on aquatic biota According to SQG analysis results, levels of toxicity in surface sediments at all three study sites were deemed to be relatively low, most likely because of the relatively low levels of deposition of toxic contaminants. In addition, levels of sediment toxicity were far below the threshold limit at which biological effects would be expected to occur. However, of the
180
three sites examined, the highest level of possible sediment toxicity was measured at Upper Killarney Lough, and was primarily due to peak levels of Cr measured in the surficial sediment sample.
These relatively enhanced levels appear to be caused by redox-
mediated changes in the ability of the sediment to bind the contaminant rather than any increases in atmospheric deposition.
6.3 Limitations of the current research This study sought to examine both temporal and spatial variations in levels of deposition of atmospheric contaminants from the industrial-scale combustion of fossil fuels in the three APDRs in Ireland.
In doing so, three lake sites, one from each of the APDRs, were
selected for analysis, as has been described in Chapter 2. Owing to the often site-specific nature of lake-sedimentary records, however, caution must be exercised when inferring regional trends, or in this case the overall trends for each the APDRs, from sedimentarybased evidence from a single site (Stoddard et al. 1998). In order to establish whether or not sediment-based evidence collected from each lake is reflecting the broader trends in levels of deposition of atmospheric pollutants for each of the APDRs, it would be necessary to examine a wider range of lake sites from each of the APDRs. For example, the choice of Kelly’s Lough as representative of the EAPDR may be problematic due to site-specific factors, such as the high rate of sediment accumulation measured at the site, and also because pollutant inputs may have been enhanced by the seeder-feeder effect, which may serve to mask or distort the overall regional pattern. The current research examined variations in levels of deposition of atmospheric contaminants from industrial-level fossil fuel combustion processes. In doing so, a range of palaeolimnological proxies (PAHs, trace elements and SCPs) were employed. However, of the three proxies examined, SCPs, in particular, were deemed to provide unequivocal evidence of atmospheric contamination from the industrial-level combustion of fossil fuels, as levels of SCP deposition show a good relationship with levels of deposition of SO42- (Rose and Juggins 1994; Rose and Monteith 2005).
This research has not,
however, provided much insight into NOX, the levels of which have recently become a source of concern. Thus, emissions of NOX, derived principally from transportation, a nonindustrial-level combustion source, have become a problem, both in Ireland (Lehane and Sheils 2008) and in Europe (Erisman et al. 2003). Legislation (e.g. the UNECE LRTAP, 1983) aimed at decreasing levels of emissions of atmospheric pollutants has achieved
181
much larger reductions for emissions of SOX than for NOX. This difference is primarily due to differing sources of both pollutants. For example, in Ireland, emissions from transport have more than doubled since 1990 (Lehane and Sheils 2008), while in Europe, transportation currently accounts for 42% of total NOX emissions (Kousoulidou et al. 2008). Moreover, emissions of NOX can act as pre-cursor to O3, which contributes to global warming (Erisman et al. 2003). Due to the financial and time constraints of this research, the temporal resolution of some of the sediment-based proxies was limited. This is particularly the case for PAHs, as only three samples per study site were analysed.
The low resolution of some analyses
constrained interpretation, and again this was particularly the case for PAHs. A more detailed examination of sediment-based proxies would allow a more holistic view of variations in levels of deposition of atmospheric pollutants. Furthermore, due to variations in transport mechanisms of PAHs and SCPs, such an examination would allow for a better understanding of variations in the relative contributions of the different sources of pollutants to lake sediment-based records.
6.4 Future directions The extent to which regional trends in levels of atmospheric deposition can be inferred from sediment-based evidence from a single site is problematic. As a result, in order to establish the degree to which lake sediment-based reconstructions reported in the current study reflect trends more broadly, further examination of sediment-based records from a number of lake sites situated in each of the three APDRs is required.
If such an
examination was to provide confirmation of lake sediment-based evidence reported in the current study, these same records could be used for the validation of atmospheric deposition models and sediment dating. Lake sedimentary records from the three study sites examined in the current study appear to provide a reliable long-term record of variations in levels of deposition of atmospheric pollutants.
As such, in the absence of monitoring data, as is the case in Ireland,
sedimentary records collected and analysed in the current study could be used to validate results obtained from atmospheric deposition models. For example, modelled depositional data for SOX in Ireland for 2005, reported by Klein et al. (2007), is validated by only one air quality monitoring station: Valentia observatory. However, atmospheric deposition models
182
tend to be averaged over relatively large areas (e.g. 50 x 50 km) and, as already discussed, sediment records from individual lake sites may not be representative of regional patterns in levels of atmospheric deposition, as individual watersheds may have distinct meteorological regimes and hence patterns of atmospheric deposition. The use of variations in concentrations of SCPs to date sedimentary records is now widespread (e.g. Rose et al. 2005; Leira et al. 2006; Bindler et al. 2007). However, as shown in the current study, sediment-based SCP concentration profiles in Ireland vary geographically, and as a result, a single, SCP-based chronology cannot be applied on an all Ireland-basis. Evidence from this research highlights three distinct SCP concentration profiles, all of which are calibrated to
210
Pb chronologies. The current research may not
have captured all of the regional variation in depositions of SCPs, however. Indeed, SCP dating profiles have been shown, in some cases, to be not only regionally- but locallyspecific (e.g. Vukic and Appleby, 2003). Therefore, additional research on depositions of SCPs at other Irish lake sites should allow a much clearer understanding of the actual regional pattern of variations in depositions of SCPs to emerge than is currently the case. Large areas of Ireland are sensitive to freshwater acidification, particularly along the western seaboard and in the east of Ireland.
Moreover, levels of atmospheric deposition
of acidic pollutants in Ireland are deemed sufficient to acidify poorly buffered surface waters (Aherne and Farrell 2002). The determination of critical loads of acidity for freshwaters in Ireland is currently provided by the SSWC model (e.g. Aherne and Curtis 2003). However, the application of such models can be problematic, due to the changes in chemistry that occur at acidification-prone sites in Ireland. These changes in chemistry are caused by variations in inputs of DOC and sea-salt (Aherne et al. 2002). Results reported in the current study provide long-term dynamic data, supplying unequivocal evidence of acidification of freshwater ecosystems by atmospheric depositions of acidic contaminants. The data also show that the effects of variations in levels of acidic depositions on each of the three study sites are tempered by individual lake characteristics, such as watershed vegetation and hydrology, and both catchment and in-lake water retention time. The data could therefore potentially be used to validate or constrain model-based estimates of critical loads of acidity.
In addition, evidence of ecosystem response to long-term
variations in levels of atmospheric deposition could be used to calibrate dynamic acidification models through hindcasting, where model output is compared with estimates of up-core variation in levels of DI-pH. For example, the MAGIC model of catchment soil and surface water acidity simulates past and future development of chemistry of lake
183
ecosystems on an annual basis, and is used for dynamic assessments of acidification and to produce target load functions (Sverdrup et al. 2005).
Modelled results of past
ecosystem response can then be compared with palaeolimnological data, in order to assess the reliability of future predictions (e.g. Stuchlίk et al. 2002). Evidence of biological recovery at acidified lake sites following decreased levels of acidic depositions is limited (Skeljvåle et al. 2003), although there are signs of biotic change, particularly at some of the heavily contaminated sites (Monteith and Shilland 2007). This preliminary finding is supported by evidence from Kelly’s Lough and discussed in this thesis. However, in order to fully understand the patterns and extent of biological recovery following reduced acidification pressures, further evidence relating to the relative differences between acid sensitive sites is required. Moreover, the factors that enable or retard biological recovery at acid sensitive sites need to be better understood.
184
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