Dissolved Organic Matter (DOM) in Aquatic ... - Semantic Scholar

Dissolved Organic Matter (DOM) in Aquatic Ecosystems: A publication by the EU project DOMAINE (EVK3-CT-2000-00034)

A Study of European Catchments and Coastal Waters

Edited by Morten Søndergaard & David N. Thomas

(Blank page)

Dissolved Organic Matter (DOM) in Aquatic Ecosystems: A Study of European Catchments and Coastal Waters Edited by Morten Søndergaard & David N. Thomas

A publication by the EU project DOMAINE (EVK3-CT-2000-00034)

Data sheet

Title: Dissolved Organic Matter (DOM) in Aquatic Ecosystems: A Study of European Catchments and Coastal Waters Editors: M. Søndergaard & D.N.Thomas Publisher: The Domaine project Date of publication: June 2004 Editing complete: March 2004 Layout: Neri Graphic Work, Britta Munter Drawings: Neri Graphic Work, Britta Munter & Tinna Christensen Photos not described: from the Domaine project, CDanmark and high-lights ISBN: 87-89143-25-6 Printed by: Schultz Graphic. Certified under ISO 14001 and ISO 9002 Paper quality: Galerie Art Silk Internet-version: The book is also available at: http:/www.domaine.ku.dk

Dissolved organic matter (DOM). What is itand andacknowledgements why study it? Preface

B

y contract EVK3-CT-2000-00034 the European Commission initiated in January 2001 the 36 months research project “Dissolved organic matter (DOM) in coastal ecosystems: transport, dynamics and environmental impact” (www.domaine.ku.dk)*.

The overall aim of the project was to provide a better understanding of the terrestrial export of dissolved organic matter and its fate and impacts on coastal ecosystem functioning, i.e. the storage and cycling of carbon, nitrogen and phosphorus. The arguments for the project were that substantial amounts of nutrients are leaving terrestrial environments as dissolved organic matter and transported to coastal areas where the bound nutrients are made available for the biota. Neglecting this source of nutrients and the oxygen demand in DOM could lead to environmentally damaging management strategies. We therefore suggested that along with an increased control of the export of inorganic nutrients at the European level, there is a growing need to understand the production, fate and effects of DOM in coastal ecosystems. It is highly pertinent to increase our understanding, both quantitatively and qualitatively, and ultimately find out how we can manage such effects. The DOMAINE partners selected four very different European catchments/areas with respect to climate and land use for intensive seasonal studies on terrestrial DOM export. Additionally, a series of experiments were undertaken to expand our knowledge on DOM reactivity and the production of DOM within aquatic systems. In this booklet we summarise some of the major findings and advocate, why we find it so important to study DOM. Morten Søndergaard Coordinator Hillerød, March 2004

Partners Freshwater Biological Laboratory, University of Copenhagen, Denmark Finnish Environment Institute, Helsinki, Finland Department of Marine Ecology, The National Environmental Research Institute, Denmark School of Ocean Sciences, University of Wales, Bangor, United Kingdom Centre National de la Recherche, Laboratoire Arago, Banyuls-sur-Mer, France Institute of Marine Microbiology, University of Bergen, Norway Department of the Coastal Environment, Vejle County, Denmark Acknowledgements The partners would like to acknowledge the help by our scientific officer Dr. Christos Fragakis in Brussels. Many people gave us technical assistance during the practical execution of the project, although unfortunately they are too many to mention here. * DOMAINE is a constituent of the ELOISE Thematic Network and contributes to ELOISE concerning the human impact on the coastal zone and the development of modelling methods.

List of Contributors

Niels Henrik Borch, Freshwater Biological Laboratory, University of Copenhagen, Denmark David Bowers, School of Ocean Sciences, University of Wales, Bangor, United Kingdom Gustave Cauwet, Centre National de la Recherche, Laboratoire Arago, Banyuls-sur-Mer, France Pascal Conan, Centre National de la Recherche, Laboratoire Arago, Banyuls-sur-Mer, France Gaelle Deliat, Centre National de la Recherche, Laboratoire Arago, Banyuls-sur-Mer, France Dylan Evans, School of Ocean Sciences, University of Wales, Bangor, United Kingdom Pirkko Kortelainen, Finnish Environment Institute, Helsinki, Finland Theis Kragh, Freshwater Biological Laboratory, University of Copenhagen, Denmark Anker Laubel, Department of the Coastal Environment, Vejle County, Denmark Tuija Mattsson, Finnish Environment Institute, Helsinki, Finland Stiig Markager, Department of Marine Ecology, The National Environmental Research Institute, Denmark Mireille Pujo-Pay, Centre National de la Recherche, Laboratoire Arago, Banyuls-sur-Mer, France Antti Räike, Finnish Environment Institute, Helsinki, Finland Morten Søndergaard, Freshwater Biological Laboratory, University of Copenhagen, Denmark Colin Stedmon, Department of Marine Ecology, The National Environmental Research Institute, Denmark Frede Thingstad, Institute of Marine Microbiology, University of Bergen, Norway David Thomas, School of Ocean Sciences, University of Wales, Bangor, United Kingdom Torben Vang, Department of the Coastal Environment, Vejle County, Denmark Peter Williams, School of Ocean Sciences, University of Wales-Bangor, United Kingdom Anders Windelin, Department of the Coastal Environment, Vejle County, Denmark

Contents

Chapter

Chapter

Chapter

Chapter Chapter Chapter

Chapter

1 2 3 4 5 6 7

Dissolved organic matter (DOM): What is it and why study it?

Page

7

P. J. LeB Williams, M. Søndergaard and D. Evans

Sources of dissolved organic matter from land

Page 15

P. Kortelainen, T. Mattsson, A. Laubel, D. Evans, G. Cauwet and A. Räike

DOM sources and microbes in lakes and coastal waters

Page 23

M. Søndergaard, F. Thingstad, C. Stedmon, T. Kragh and G. Cauwet

Effects of DOM in coastal waters

Page 37

S. Markager, C. Stedmon and P. Conan

Fate of DOM in coastal waters

Page 43

N. H. Borch, G. Deliat, M. Pujo-Pay and C. Stedmon

Analysis of DOM at the catchment scale: Two European case studies

Page 51

A. Laubel, D. Evans, T. Vang, D.G. Bowers, C. Stedmon, N.H. Borch and M. Søndergaard

DOM and land use management

Page 63

A. Laubel, T. Vang, M. Søndergaard, P. Kortelainen and A. Windelin

Suggested further reading

Page 69

Glossary

Page 71

(Blank page)

Chapter

1

Dissolved Dissolved organic organicmatter matter(DOM). (DOM): What What is is itit and andwhy whystudy studyit? it?

Peter J. LeB Williams Morten Søndergaard Dylan Evans

The geochemical and biogeochemical cycles

T

he flux of minerals from eroded rocks via the rivers through the oceans to the marine sediments – the geochemical cycle – determines the composition of coastal and oceanic waters. The concentration of a particular element is controlled by a number of physical and chemical factors: The principal ones being residence time of the water and the solubility and volatility of the various forms of the compound the element may be part of. Generally, the shorter the residence time and the lower the solubility the lower the resultant con-

centration in the sea. Chlorine and sodium, which play minor roles in biological processes, have an exceedingly long residence time in the oceans, which is why the sea is salty. The fundamental physiological processes of photosynthesis (production) and respiration (decomposition) have a further effect upon the cycling of elements. Photosynthesis (Fig 1.1) involves the utilisation of simple inorganic compounds (carbon dioxide, nitrogen, phosphate, and other salts) and their conversion to organic material (Box 1.1), oxygen being essentially a by-product of the reaction. Respiration is basically the reverse overall process. Contrary to chlorine and so-

DISSOLVED ORGANIC MATTER (DOM) IN AQUATIC ECOSYSTEMS: A STUDY OF EUROPEAN CATCHMENTS AND COASTAL WATERS

7

Eutrophication – the potential for trouble

The growth of photosynthetic organisms in the sea – mostly algae (both phytoplankton and seaweeds), seagrasses and some bacteria species – requires a number of factors (Fig. 1.1). Water and carbon dioxide are in abundant supply resulting in the main chemical limitation to the growth of marine plants being the various salts of phosphorus (phosphate) and nitrogen (mainly nitrate and ammonium): collectively re-

ferred to as nutrients. The scale and yield of marine fisheries in a very broad way are controlled by the extent of algal growth. Thus, up to a point, the more extensive the growth of algae the greater quantity of fish an area can yield. However, if the growth of algae is excessive then deleterious effects can result. This commonly occurs when the concentration of the controlling compound (nitrogen and/or phosphorus) departs (most often increases) from the “natural” levels. This change in concentration is referred to as eutrophication. Although eutrophication is generally a detrimental process, it is important to stress that it is a reversible process. There are also instances when low levels of eutrophication can even be perceived as being a positive state for increasing the productivity of a specific water body.

LIG

HT n)

Organic material and oxygen

Re

sp

organisms. Respiration and decomposition release CO2 at the expense of

hesis (Pro yn t

t

s ho

pounds and oxygen by photosynthetic

Water, carbon dioxide nitrate, phosphate and salts

si

a complex mixture of organic com-

o

CO2, water and salts are converted to

Ph

The energy in light, inorganic nutrients,

n)

The Production/Decomposition Cycle.

n (Decompo

Figure 1.1.

tio

du

ira

ct

io

Strictly speaking eutrophication is a process by which the productivity of an aquatic system is increased, and can therefore be caused by factors other than nutrient input. These include reducing the suspended material in a water body and therefore increasing light for photosynthesis, or changing the residence time of water within a particular system. Coastal regions are immediate recipients of high dosages of nutrients both directly, via marine outfalls and discharges from estuaries. This, coupled with their relatively long residence time, makes coastal ecosystems especially vulnerable to eutrophication. This has resulted in the need for active management of these environments, which requires an understanding of the sources of the critical nutrients, intermediate treatments as well as the biological and physical dynamics of the ecosystem.

t io

dium, nitrate and phosphate are substantially involved in biological cycles and have much reduced residence times. For a great number of elements it is the biogeochemical cycle, which is the final determinant of its concentration. Thus, the biota creates a cycle within a cycle.

HE

AT

oxygen and recycle nutrients to the inorganic state. Prepared by Theis Kragh.

8


Chapter 1 Dissolved organic matter (DOM): What is it and why study it?

Dissolved organic matter (DOM). What is it and why study it?

Inorganic

Organic

Dissolved

Particulate

Dissolved organic carbon (DOC), nitrogen (DON) and phosphorus (DOP)

Phosphate, nitrate, ammonia and carbon dioxide

Living and dead particulate organic material (POC, PON, POP)

Sources of nitrogen and phosphorus

A broad quantification of the nutrient sources into coastal ecosystems is a requirement of the European Union environment monitoring policies, and since the late 1980s participating nations have reported annually on the release of specified chemical discharges. In the case of nitrogen and phosphorus, these elements may be present in a variety of organic and inorganic chemical forms. It is often neither practical nor desirable to quantify every nitrogen or phosphorus containing

Rarely analysed - no nitrogen forms

Figure 1.2. Analytical categories for carbon, nitrogen and phosphorus. Prepared by Theis Kragh.

Sources

Nitrogen

Phosphorus

Natural

Primary

Atmosphere

Rocks

Sources

Secondary

Soils, peat bogs, lakes & rivers

Soils, peat bogs, lakes & rivers

Primary

Inorganic fertilisers (NO3, NH4, urea)

Inorganic fertilisers (X-PO4)

Secondary

Animal wastes, silage, run off & ground water

Animal wastes, silage, run off & ground water

Table 1.1.

Sewerage Various Industrial wastes

Sewerage Processing of phosphate rock

waters. They may simply be separated

Anthropogenic Sources Agriculture

Domestic Industry

Summary of the major sources of nitrogen and phosphorus to coastal into natural and anthropogenic sources.


9

compound present, and a common practice is to separate them within four categories (Fig. 1.2), based on their chemical form (inorganic or organic) and physical state (particulate or dissolved), Box 1.1 and Box 1.2.

Control and management of discharges

As society and environmental control authorities became aware and concerned by the problem of eutrophication additional treatment of waste-water was incorporated to remove the inorganic products. The

different chemistries of nitrate and phosphate result in major differences in the ease with which they are removed. Whereas phosphate may be removed chemically by precipitation no suitable insoluble salts for nitrate exist, thus the removal is typically biological via denitrification where nitrate in an anoxic environment is transformed to gaseous nitrogen (N2); a difficult process to control and expensive. The consequence is that whereas the tertiary removal of phosphate is not uncommon, that for nitrate is rather rare and can be difficult to control. The

preferred procedure is to control the source i.e. the use of nitrate fertilisers and manure and to decrease the production of airborne nitrogen. Likewise, the non-point sources of phosphorus can only be controlled by land use management.

DOM the overlooked source of nutrients and oxygen demand

In the biogeochemical cycle inorganic nutrients are bound in organic compounds by photosynthesis and remineralised during decomposition, which is mostly microbial (Chapters

Box 1.1 Definition of organic Although definitions are rather mundane they are a pre-

pounds are held to be those containing carbon, hydrogen

requisite for good science. Despite the term organic being

and oxygen, but that would include sodium bicarbonate

in common usage, even chemistry textbooks rarely attempt

and exclude methane and many hydrocarbons. The unique

a rigorous definition. Historically it is defined as compounds

thing about organic material is that it contains a covalent

produced by living organisms; however, a glance at Fig 1.1

carbon-hydrogen bond – and it is this bond structure that

shows that this definition would include water and oxygen

best defines what is, and what is not, organic.

which are both biological products. Commonly organic com-

Box 1.2 Definition and physical separation of dissolved and particulate material

10

It is conventional to separate the organic components in wa-

tial food supply for these broad trophic categories. The

ter into dissolved and particulate. The separation has some

separation for analytical purposes is done by filtering the

biological validity. For the most part animals, from fish down

sample through fine glass mat filters (the effective pore size

to protozoa, feed on particulate material, whereas in the

being between 0.5 and 1 micrometer). This gives an opera-

case of the bacteria, which have neither mouths nor guts,

tional separation into particulate and dissolved, although in

their main immediate food source is organic material in solu-

reality there is no sharp boundary between the particulate

tion. Thus the separation gives some insight into the poten-

and dissolved states.


Chapter 1 (DOM). Dissolved organic matter (DOM): What is it and why study it?

3 and 5). However, many of the organics produced are not easily decomposed and can remain in an organic form for long periods. Leaching of rainwater through soils carries these compounds as dissolved organic matter (DOM) to watercourses and into coastal waters (Chapters 2, 4 and 6). Through the history of eutrophication studies the effects of inorganic nutrients have been the primary focus. However, both management and scientific communities have partly overlooked the large amounts of nitrogen and phosphorus embedded in terrestrial DOM (al-

lochthonous) transported to lakes and coastal waters and in the DOM produced within aquatic systems (autochthonous). Furthermore, as DOM also controls to a large extent, the light climate of aquatic systems, there are good reasons not to overlook the ecological effects of DOM (Chapter 4). It is pertinent to learn more about DOM, both with respect to quantities in transport from land to sea, how and at what time scales it releases nitrogen and phosphorus, how much oxygen is consumed in the degradation process, and how land use af-

fects the export (Chapter 6). Finally, the export and effects of DOM deserves consideration from management perspectives (Chapter 7). In the DOMAINE project we have tried to keep focus on how climate and land use can control the riverine flow of DOM to coastal waters and the ecological effects of DOM. In this booklet we summarise some of our primary findings and where possible view these at a European dimension and in a management context.

Photo 1.1 The Conwy Estuary, North Wales (photo by D. Thomas).


11

Analysis of organic matter in natural systems

There is no widely accepted principal to analyse a complex mixture of organic material in its entirety in aquatic samples. Contemporary approaches involve the measurement of the major (or ecologically important) constituent elements (carbon, nitrogen, phosphorus) or some physical property associated with the organic state. Chemical methods involve the oxidation of organic material (Box 1.3) and the measurement of one or more of the inorganic combustion products: CO2 in the case of carbon, NO2, N2 or NH3 in the case of nitrogen and H3PO4 in the case of phos-

phorus. The classical oxidation procedure at high temperatures can be used largely without modification for the particulate fraction using commercial elemental (CHN) analyzers. The chemical analysis of total organic material in natural waters in solution (DOM) is problematic for a number of reasons. First, unlike the particulate fraction, the elements in DOM occur alongside their inorganic counterparts, thus unless the inorganic forms can be removed prior to oxidation, the inorganic nutrient from oxidation of the organically-bound part of the element has to be determined by difference. The various strategies are outlined in Box 1.4. The second

problem is that it is not feasible to remove the organic material from solution or to evaporate the water sample prior to analysis, since in doing so the samples can become seriously contaminated. Thus in practice the analysis for DOM is carried out in solution. The physical analysis entails the measurement of one of two optical properties of the water sample – either its absorbance (typically at 355 and/or 440nm) or its fluorescence. Organic compounds absorb light at various wavelengths and to varying degrees. The pattern of absorption (and fluorescence) is not universal for organic compounds; lignins and phenolic compounds

Box 1.3 Oxidation procedures The chemical analysis of water for dissolved organic matter

approaches have their advantages and limitations. The

(DOM) falls into two steps – oxidation followed by determi-

HTCO approach may be assumed to effect complete oxida-

nation of the quantity of the oxidation product. Broadly the

tion, and the main problem with the method has been as-

oxidation procedures fall into two categories The first entails

sociated with assessing and minimising the analytical

the oxidation of the organic material in the sample in the

blanks. It is now successfully used for dissolved organic car-

liquid phase (wet oxidation), i.e. in water itself; the second

bon (DOC) analysis, where the end product, CO2, is typically

involves the oxidation of the sample at high temperatures in

measured by its infrared absorption. Its application to dis-

the gas phase in a stream of oxygen, characteristically in the

solved organic nitrogen (DON) has been less successful, but

presence of a catalyst (high temperature catalytic oxidation –

is improving.

HTCO). In the former case the oxidation may be purely chem-

from the problem that they do not always completely oxi-

acid) or by photo-oxidation (using an intense ultraviolet light

dise all organic material. This results in a small, but signifi-

source), or a combination of photo and chemical oxidation.

cant underestimate of the organic material. Wet

In the case of the HTCO method, the sample, after the

12

The wet oxidations are simpler, however, they suffer

ical using a strong oxidising agent (typically persulphuric

oxidations are more general in their application having

removal of CO2 is injected into a quartz combustion tube

been used for DOC, DON and dissolved organic phospho-

(at 700°C), the sample being evaporated in the tube. Both

rus (DOP).


Chapter 1 1 Chapter Dissolved organic organic matter matter (DOM): (DOM). Dissolved What is is it it and and why why study study it? it? What

Box 1.4 Analytical strategies for DOC, DON and DOP The inorganic elements occur alongside their organically

DON and DOP analysis: as there are no practical pro-

bound counterparts; they are of course in many cases iden-

cedures to remove the inorganic forms prior to analysis,

tical to the combustion products. There are two solutions

the organic fraction is determined as the difference be-

to this problem and different strategies are used for DOC

tween the sums of the inorganic forms, before and subse-

and for DON and DOP.

quent to analysis. The determination by difference has to

DOC analysis: the inorganic forms of carbon, the car-

make the assumption that there is no loss of the inorganic

bonates, may be removed by acidifying the sample, con-

nutrient during analysis – this may not always be correct

verting the carbonates to CO2 and blowing off the CO2.

in the case of nitrogen (see Box 1.3).

There is the potential to lose volatile organics at this stage (e.g. methane) but this is regarded to be a minor problem in most aquatic systems.

absorb strongly, whereas other bioorganic compounds e.g. sugars and most amino acids absorb weakly at the above wavelengths. Thus, light absorption due to organic material is compound specific and because of this it is prone to be site specific. Organic material of terrestrial origin tends to be more coloured and

1800

they can be used to predict other parameters and whole river systems can be sampled quickly to identify areas deserving more detailed investigation. However, there are significant differences among different sites associated with the nature of their catchment area, and for example drainage waters from

DOC = 61.813x + 158.89 R2 = 0.734

1600 DOC (µmol l-1)

strongly absorbing than that of autochthonous origin. The high absorption of coloured compounds can be used as a proxy for the quantification of non-coloured compounds. None relationship is more striking than the relationship between absorption and DOC. Once relationships are developed

1400 1200 1000 800

Figure 1.3.

600

The River Conwy, North Wales, UK.

400

The relationship between dissolved

200

organic carbon (DOC) and coloured

0

0

5

10

15

20

CDOM absorption (m-1)

25

30

DOM (CDOM). The equation describes the linear relationship between the two measured parameters.


13

peaty soils tend to have higher DOCspecific absorbance coefficients than those from arable land. Thus, unlike the chemical methods, the optical methods have to be calibrated for particular sites or catchment types. The site specific difference in absorption and fluorescence can therefore be used to characterise DOM and to identify specific compounds or groups of compounds. The use of optical methods in DOM research is presented in more details in Chapter 3 (fluorescence) and in Chapter 4 (absorption). In summary, the scientific com-

munity has good control on how to measure DOC with high accuracy and precision. For DON it is very difficult to get accurate and high precision measurements when the concentration of nitrate is high. Such situations are often found in rivers draining agricultural areas. The same problem arises for trustworthy DOP measurements when the concentration of phosphate is high. Furthermore, DOP concentrations are generally very low, which at any circumstances make accurate and precise measurements difficult.

Photo 1.2 Sampling on Horsens Fjord, Niels Henrik Borch (photo by Stiig Markager).

14


Chapter

2

Dissolved organic matter (DOM). Sources of dissolved What is itmatter and why study it? organic from land

Pirkko Kortelainen Tuija Mattsson Anker Laubel Dylan Evans Gustave Cauwet Antti Räike

T

errestrial ecosystems are the primary source of freshwater dissolved organic matter (DOM), although decomposition products of aquatic organisms are important in eutrophic systems. The vast majority of aquatic ecosystems in the world have dissolved organic carbon (DOC) concentrations falling within the range of 40 to 4000 µmol l-1. Dissolved organic carbon varies in concentration from approximately 40 µmol l-1 for ground water and seawater to over 2500 µmol l-1 for coloured water from peatlands. Swamps, marshes and bogs have concentrations of DOC from 800 up to 5000 µmol l-1. In coastal marine waters typical surface water values

range from 100 µmol l-1 to 500 µmol l-1 in eutrophic lagoons and areas where water exchange is limited. The ranges of DON and DOP in natural waters are typically between 3 to 200 µmol l-1 and 0.05 to 2 µmol l-1, respectively. Generally, it is a combination of terrestrial and aquatic primary production and decomposition rates that control the amount of dissolved organic carbon (DOC). For example, Arctic, alpine and arid environments have low concentrations of DOC in rivers and lakes because of generally low primary productivity. In contrast aquatic production in warm temperate and tropical latitudes is much higher. However, decomposition of organic matter is also rapid


15

tending to lower DOC concentrations. The taiga has high production of organic matter and slower decomposition, resulting in higher DOC concentrations. At the global scale, there are clear patterns of DOM concentration and flux related to regional climate, either directly through hydrological effects or indirectly through vegetation. The principal sources of autochthonous DOM production vary across systems: Periphyton dominate in streams, macrophytes in lakes and phytoplankton in coastal waters, seas and oceans. It has been estimated that 95 % of freshwater lakes and wetlands receive > 97 % of their autochthonous DOM from macrophytes. In large lakes and oceanic systems, much of the DOM originates from phytoplankton. Wetlands are generally thought to be the main allochthonous DOC source into surface waters. High

amounts of DOC leach from wetlands (which retain a high water content throughout the year), and from soils where surface or subsurface runoff is a major feature. DOC can be strongly adsorbed to oxides and clay minerals in lower soil horizons in upland catchments, resulting in lower DOC concentrations being released. Changes in the export of DOC to aquatic ecosystems also influence the delivery and biogeochemical cycling of other components associated with DOC. However, studies including DON and DOP dynamics in addition to DOC are few. There are indications that the rate of release and fates of DOC, DON, and DOP in the soil may differ to a greater extent than previously assumed, and controls established for DOC might therefore not be valid for DON and DOP. Moreover, controls of DOM dynamics in soils have

mostly been focused on temperate regions and there is an urgent need for these to be extended to soils under various land uses and in other climate zones. DON concentrations have been found to correlate positively with percentage cover of forestry. Moreover, agricultural fields have been found to increase DON export and the use of organic fertilizers has been reported to increase the amount of water extractable organic matter. The major source of DOP is thought to be animal waste and sewage sludge, and consequently application of organic fertilisers to soils with a sandy texture is assumed to provide a high rate of infiltration to the groundwater.

Photo 2.1 The upper reaches of River Tech, France and Gustave Cauwet (photo by Stiig Markager).

16


Chapter 2 Sources of dissolved organic matter from land

Effect of climatic conditions on DOM transport

Vegetation litter and humus are the most important DOM sources in soils, and high microbial activity, high fungal abundance, and any conditions that enhance mineralisation all promote high DOM concentrations. However, in general hydrological control in soil horizons with high carbon contents may be more important than biotic control of DOM release. Hydrological control becomes significantly more important with increasing time, and on the time scale of several years water fluxes through the soil are considered to be the dominant factor controlling DOM fluxes in soil. In most rivers the organic matter concentrations vary with discharge and season and most of the DOM moving downstream is humic matter that has a turnover time exceeding the residence time of the systems through which it passes. DOC concentrations generally show a positive correlation with discharge. However, a strong positive relationship between discharge and DOC concentrations has been recorded at low discharges, whereas a negative relationship has been found at high discharges. Regional variation in annual export of DOC in North American rivers has been primarily attributed to differences in annual runoff, but DOC concentrations in single rivers were not strongly correlated to discharge alone, and an obvious rela-

tionship between leaching and different climatic regions is not easily defined. Seasonal variation in DOC concentrations often follow the pattern of increasing concentrations during spring and autumn high flow periods and decreasing concentrations during winter and summer low flow periods. Catchments with a significant wetland component may experience fewer fluctuations in stream DOC concentrations with changes in hydrologic flux. In upland catchments, the flow path of water through soil is an important determinant of DOC concentration. DOC concentrations are higher during periods when the dominant flow paths are near the surface through the organic-rich upper soil horizon rather than through the lower soil horizons that often have high DOC sorption capacity. In contrast, in a catchment containing a large wet-

land, DOC can decrease with discharge, since in wetlands, the water table remains close to the surface and additional water from precipitation does not greatly increase the contact with organic rich surface horizons. The strong relationship between DOC export and runoff suggests that climate change can have a great impact on DOC export. Increase in precipitation might increase DOC fluxes, whereas increasing evaporative demand under a warmer climate might offset the effect. Organic carbon export simulations based on climate change scenarios and neural network indicate increasing DOC fluxes in Canadian rivers and in Finnish headwater streams, while a 20 year data set from the Experimental Lake Area in Canada documents a significant decrease in DOC of lake water associated with a local climate warming.

Photo 2.2 Peat-land near Oulu, Finland (photo by Stiig Markager).


17

Controlling factors at the landscape scale

Large drainage basins are composed of numerous sub-basins, differing in character and arranged in complicated mosaic patterns (Chapter 6). The DOM concentrations in the outlets of large catchments give an average, integrated picture of the hydrology and the DOM dynamics in the sub-basins. The DOM concentrations are related to export from the catchments due to differences in climate, soil and vegetation type, but they are also influenced by internal processes in lakes and streams such as sedimentation, photo-oxidation, bacterial uptake and mineralisation. Upstream lakes are likely to increase the residence time of water, which is reflected as decreasing DOM concentrations and often also as reducing temporal and annual variations. Small headwater lakes and streams mainly have lake-less catchments, which is consistent with the observation that small lakes and streams have the highest DOC concentrations. In large and deep lakes, with long residence times, the degradation and sedimentation processes affecting organic matter are likely to be more complete, resulting in lower concentrations. High DOC concentrations are measured in peat and forest covered areas with few lakes, i.e. areas with large organic soil pools and short water retention times. Low concentrations are recorded in regions with sparse vegetation, poorly

18

developed organic soils and large areas covered by lakes. From western Norway, Scotland and Wales to eastern Finland, there is a gradient from high to low precipitation (3500 mm to 600 mm yr-1) and from mountain areas with thin and patchy soils to forested areas with thick soils. The pattern of total organic carbon shows a clear increasing gradient from west to east reflecting these changes, as well as a slightly increasing gradient from north towards south.

European river basins

The European continent covers about 10 million km2, stretching from the Atlantic Ocean and Iceland in the west to the Ural Mountains and the Caspian Sea in the east, and from the Barents Sea in the north to the Mediterranean and the Black Sea in the south. River catchments are numerous but relatively small, and rivers are short. The 31 largest rivers in Europe, all which have catchments exceeding 50000 km2, drain approximately two thirds of the continent. Approximately 42% of the total land area in Europe serves some agricultural purpose, 33% is covered by forest, 24% covered by mountains, tundra etc. and 1% by urban areas. These percentages, however, vary greatly among countries. Forest cover varies from 6% in Ireland up to 86% in Finland (classified as forestry land including forest land, scrub land and waste land). The proportion of land devoted to

agriculture varies from less than 10% in Finland, Sweden and Norway, up to 70% or more in Hungary, Ireland, Ukraine and the UK. However, there are large regional differences in the farming intensity, and the type of crops grown. For example, in Denmark agricultural land constitutes about 65% of the total land area and most of it is arable land. While in Ireland agricultural land constitutes 81% of the total land area, but only 18% is classified as arable land, most of the agricultural land being used for grazing. The highest peat-land proportion of the total land area in Europe is in Finland (32%), followed by Ireland (20%), Sweden (19%), Norway (9.2%), Great Britain (6.6%) and Poland (4.2%). The average annual runoff in Europe follows closely the pattern of average annual rainfall and topography. Precipitation is highest in the west and lowest in the east, while evaporation is highest in the south and east. Annual runoff may exceed 3000 mm in parts of Iceland, Norway, and the Alps, whereas it is below 25 mm in parts of Spain and southern parts of the Russian Federation. The greater variation of runoff in Western Europe, compared with Eastern Europe, reflects the greater variability in topography and rainfall. The pattern by which river flow varies during the year, i.e. the flow regime, is determined by the seasonal variation in climate, as well as the nature of the catchment i.e. soil and bedrock permeability,


Chapter 2 2 Chapter Sources of of dissolved dissolved matter Sources from land organic matter from land

land-management and vegetation. Because climatic and geological properties differ throughout Europe, the flow regimes of European rivers vary considerably. When extensive swamps, forests, and lakes are present in a river catchment they attenuate the natural fluctuation in discharge by storing the water and releasing it slowly.

Land use cover in the DOMAINE countries

The study catchments of the DOMAINE -project are situated in Denmark, Finland, France and Wales. The land use cover in these countries is variable: In Denmark forests and plantations cover 12%, agricultural land covers 65%, lakes cover 1%, and urban areas 4% of the land. Meadows, marshland, moor land, sand dunes and bogs cover 11% of the total land area in Denmark. In France forests and other wooded

land account for less than 28 % of the total land area and agricultural land covers 57 % of the total land area. In the UK 75% of the total land area is agricultural. The forest cover of Britain (11% of the land area) is unevenly distributed: 8% in England, 12% in Wales and 16% in Scotland. In Finland forestry land covers 86% of the land area, compared to 10% for agricultural use and 4% built up. Freshwater covers 10% of the total area of Finland, including approximately 56000 lakes larger than one hectare. In contrast the number of lakes larger than one hectare in Denmark and UK (England and Wales only), are only 690 and 1700, respectively, while France has 150 lakes larger than 10 ha.

Climate and land use cover in the DOMAINE catchments

The DOMAINE study catchments

differ significantly with respect to climatic conditions and land use cover. The mean air temperature and precipitation are highest in France and lowest in Finland (Table 2.1). The catchments in north Wales have nearly as high precipitation as the French catchments. Runoff data is not available from the Welsh catchments. However, due to colder climate and lower evaporation, the runoff in north Wales is presumably much higher compared to France. The Danish and Welsh catchments are dominated by agricultural land, while forests cover large parts of the French and Finnish catchments (Table 2.2). However, most of the agricultural land in Denmark is arable, while in the Welsh catchment studied most is permanent grassland. Wetland covers on average 27% of the Finnish catchments; in other countries the proportion of wetlands is minor.

Photo 2.3 The upper reaches of the River Conwy, North Wales (photo by D. Thomas).


19

Table 2.1.

Annual mean Total annual air temperature precipitation ºC mm yr-1

Average values for air temperature, precipitation and runoff in the study catchments of the DOMAINE project.

Finland (n=9) Denmark (n=10) Wales (n=10) France (n=5)

2.1 9.7 9.5 17

Table 2.2. Average values of land use cover in the DOMAINE study catchments.

Finland (n=9) Denmark (n=10) Wales (n=10) France (n=5)

Table 2.3. Average concentrations and average annual loads of DOC, DON and DOP, and average DOC/DON ratios in the DOMAINE catchments.

DOM concentrations and loads vs. climate and land use

DOM concentrations and loads vary greatly between geographical regions reflecting variations in climate, hydrology and land use cover. DOC concentrations and loads were highest in Finland and lowest in France (Table 2.3). In the DOMAINE catchments a high proportion of wetland is associated with elevated DOC concentrations as well as a high proportion of managed forests in a catchment (Fig. 2.1). The average percentage of wetlands and forests in the catchments is highest in

20

DOC Finland (n=9) 1100 Denmark (n=10) 600 Wales (n=10) 460 France (n=5) 150

590 810 970 1100

Forest

Agricultural %

48 15 6 43

12 66 56 14

DON DOP µmol l-1 36 78 25 8.4

Annual runoff mm yr-1

0.25 1.1 0.59 0.46

Finland resulting in the highest DOC concentrations and loads. The precipitation and runoff values are lowest in Finland, and highest in France (Table 2.1), which results in a negative relationship between DOC concentrations and precipitation and/or runoff (Fig. 2.2). In many other studies a positive relationship between DOC and discharge has been found, although no clear relationship between DOC concentration and mean annual runoff in different climate zones has been recorded. When the relationship between DOC and runoff is considered within one catchment or biome the

DOC:DON 33 11 19 18

310 380 560

Lakes 5 0.7 4 0

Wetlands 27 3 0.8 0

DOC DON DOP mol m-2 yr -1 0.34 0.21

0.011 0.026

0.000076 0.00034

0.072

0.0042

0.00010

correlation can be different. A southnorth gradient with highest DOC concentrations in the northernmost DOMAINE catchments was apparent (Fig. 2.3). These relationships indicate that several factors play a role in controlling DOC, including wetland and forest cover, precipitation, hydrological processes, and possibly also temperature and sunlight. DON concentrations and loads were highest in the Danish catchments and lowest in the French catchments (Table 2.3). In the Finnish and French catchments, DON concentrations correlated positively with DOC (Fig. 2.4). Terrestrially de-


Chapter 2 2 Chapter Sources of of dissolved dissolved matter Sources from land organic matter from land

land in the catchment (Fig. 2.5), while in the data from Denmark and France no such relationship was found. DOP concentrations and loads were low compared to DOC and DON (Table 2.3). The lowest values were recorded in Finland and the highest in Denmark, and DOP concentrations were positively related to the percentage of agricultural land in the catchment (Fig. 2.6). The use of organic fertilizers has been reported to increase the amount of water extractable organic matter, probably contributing to the posi-

2000

2000

1600

1600

DOC (µmol l-1)

DOC (µmol l-1)

rived organic matter often has high DOC:DON ratios compared with DOM produced by phytoplankton and aquatic plants. In the Danish catchments the average DOC:DON ratio was low (11) indicating a large contribution of aquatic sources of DOM. In contrast the Finnish catchments had a ratio three times higher and riverine DOM mostly originates from terrestrial sources. In the DOMAINE catchments in Wales and Finland DON concentrations increased significantly with the increasing proportion of agricultural

1200 800

Figure 2.1. 800 400

0

0

2000

Average, minimum and maximum DOC concentrations in the DOMAINE catchments with wetland cover ranging from 0 to 10%, and from 20 to 50%, and with managed forest cover ranging

0-20 30-50 Managed forest (%)

2000

DOC = -0.9x + 1385 R2 = 0.27

from 0 to 20%, and from 30 to 50%.

DOC = -2.9x + 1828 R2 = 0.55

1500

DOC (µmol l-1)

1500

DOC (µmol l-1)

1200

400

0-10 20-50 Wetland (%)

tive correlation between agricultural land in the DOMAINE catchments and DON or DOP concentrations. DOC, DON and DOP concentrations were on average larger during the warm period (April-September) compared to the colder one (October-March). However, in the Danish catchments DOC and DON concentrations were somewhat higher during the cold period probably due to larger runoff compared to the warmer period. There was also a seasonal variation in the DOC:

1000

500

1000

500

Figure 2.2. 0

0

500 1000 1500 Precipitation (mm yr-1)

0

Relationships between DOC concentra0

200 400 600 Runoff (mm yr-1)

800

tion vs. precipitation and runoff in the DOMAINE catchments.


21

DON ratio, especially in the Danish and the Finnish catchments, DOC:

2000

DON ratio being lower during the warm period.

2000

DOC = 41x + 1639 R2 = 0.54

Figure 2.3.

1500

DOC (µmol l-1)

DOC (µmol l-1)

1500

DOC = -63x + 1172 R2 = 0.49

1000

500

1000

500

Relationships between DOC versus lati0

in the DOMAINE catchments.

40

12

50

Wales. Figure 2.6. Relationship between DOP and the proportion of agricultural land in the DOMAINE catchments. One outlier has been omitted.

22

0

40 30 20 10

France 100 200 DOC (µmol l-1)

0

300

Finland 0

2.5 Finland Wales DON = 1.0x + 24 R2 = 0.86

40 30 20

DON = 0.21x + 13 R2 = 0.79

10 0

DON = 0.031x + 2.1 R2 = 0.49

60

DON (µmol l-1)

DON (µmol l-1)

proportion of agricultural land in the DON (µmol l-1)

Relationships between DON and the DOMAINE catchments in Finland and

70

4

60

-5 0 5 10 15 20 25 Annual mean air temperature (˚C)

50

70

Figure 2.5.

0

80

6

0

in stream water in France and Finland.

70

8

2

Relationships between DOC and DON

60 Latitude

DON = 0.053x + 0.47 R2 = 0.97

10

Figure 2.4.

50

20 40 60 80 Agricultural land (%)

100

2000

1.5 1.0 0.5 0

0

500 1000 1500 DOC (µmol l-1) DOP = 0.2124e0,0176x R2 = 0.57

2.0

DOP (µmol l-1)

tude and annual mean air temperature

0

20 40 60 80 Agricultural land (%)

100


Chapter

3

Dissolved organic matter (DOM). DOM sources and microbes What is itand andcoastal why study it? in lakes waters

Morten Søndergaard Frede Thingstad Colin Stedmon Theis Kragh Gustave Cauwet

Both allochthonous and autochthonous DOM are complex mixtures of many different organic compounds, which can influence aquatic ecosystems via their diverse physical and chemical properties (Chapter 4). The bulk of terrestrial DOM and some of the “freshly” produced autochthonous DOM degrade slowly so DOM can be transported long distances from its original sources. Thus, the oxygen demand and nutrients bound in the DOM constituents DOC, DON and DOP can be released uncoupled in time and space from its production. Bacterial utilisation and photochemical reactions are the two most important processes removing and

transforming DOM (Chapter 5). In this chapter some chemical properties of DOM and the interactions of DOM and microbes in lakes and coastal waters will be summarized.

Microbial dominance: Bacteria and DOM turnover

It is estimated that the global bacterioplankton carbon demand (organic carbon needed to fuel bacterial growth and respiration) averages some 40-60 % of phytoplankton primary production. Bacteria can only utilise small dissolved molecules so high bacterial activity can only take place together with a high production of readily available DOM.


23

UV

PAR

Fish

BDOM

Zooplankton

RDOM Figure 3.1 A simplified and conceptual water column food web model. The “classical”

Virus

Phytoplankton

particulate food chain with phytoplankton, zooplankton and fish is pro-

CO2 DIN DIP

ducing DOM and dead particles for the microbial loop with bacteria exploiting dissolved compounds and recycling DOM to the particulate phase. Also inserted are the photochemical action of solar radiation, the interchange between recalcitrant (RDOM) and biodegradable DOM (BDOM) and the remineralisation of DOM to CO2, inor-

Flagellates/ ciliates

Bacteria Aggregates

ganic nitrogen (DIN) and orthophosphate (DIP). Original by Theis Kragh.

24


Chapter 3 DOM sources and microbes in lakes and coastal waters

The “classicalc (before 1974) view of the turnover of organic matter in a water column ignored an active DOM compartment, since DOM was deemed a large but inert part of the ecosystem. The conceptual picture of dominant pelagic processes was a linear particulate food chain, where phytoplankton were grazed by small zooplankton, which in turn were eaten by small fish and again eaten by larger fish at the top of the food chain. The classical particulate food chain is on the right side of the food web cartoon in Figure 3.1. This “classical” view totally changed in the late 1970s and early 1980s. New improved techniques to measure bacteria showed high abundance and activity of free-living bacterioplankton in lakes and marine waters. The microbial loop with bacteria at the base of a DOM “food web” was born (Fig. 3.1, middle and left side)

Turnover of organic matter via bacteria is generally important; however, there are large seasonal and spatial variations among different aquatic systems. Oligotrophic systems – whether lakes or oceanshave very high bacterial carbon demand amounting to approximately 80 % of phytoplankton primary production, while lower values are found in more eutrophic and shallow systems, where much of the produced organic material is decomposed in the sediment. In humic lakes and other water bodies with high terrestrial DOM import, the bacterial carbon demand can surpass primary production due to the ability of bacteria also to utilise allochthonous DOM, aided by the photochemical production of small and easily assimilated substrates from large complex DOM fractions. Although bacteria easily degrade only a small fraction of ter-

restrial DOM the total import is large and can sustain high bacterial production. Most water columns in lakes are net heterotrophic, i.e. produce more CO2 than O2 due to respiration of allochthonous DOM. The conclusion that came out of this new “era” was that in many aquatic ecosystems bacterial utilisation of DOM is THE major route of organic carbon turnover. Furthermore, due to the high nitrogen and phosphorus requirement of bacteria compared with other organism in the plankton, the cycling of N and P are also tightly linked to microbial activity. Modelling such interactions is one key to understand the factors shaping biological structure in coastal waters and to understand how food webs can control DOM. A newly developed “minimum” model with interactions of bacteria, phytoplankton and dissolved inorganic and organic nutrients is presented in Box 3.1.

Photo 3.1 Bacterioplankton stained with SYTO 13 (green particles). The red particles (white arrows) are small algal cells flourescing red (photo by Anne Jacobsen).


25

Box 3.1 Analysis of food web effects using idealised models: Is bacterial consumption of DOC influenced by the trophic interactions of the microbial food web? Food webs consist of a network of balancing processes;

tures. Somewhere in-between is the arena for a “conceptual

growth and death, remineralisation and nutrient uptake,

minimum model”. This is the idealized version of the food

competition and predation. Focusing on one of these as-

web that has just enough components to allow an under-

pects only, inevitably gives a biased discussion that easily

standing of its central features, but no more. As one sugges-

miss essential aspects of how these processes work together

tion for such a “minimum model”, it seems that many

as a system. Combining ALL processes and organisms in a

important properties of the food web in Fig. 3.1 (which al-

natural ecosystem into the discussion would on the other

ready is an idealisation) can be summarized in the following

hand create a monstrous network from which it would be

(even more idealised) network.

impossible to distinguish important from unimportant fea-

Heterotrophic Flagellates

LDOC

Heterotrophic Bacteria

Ciliates

Autotrophic Flagellates

Copepods

Diatoms Si

PO4

DOM – a dominant constituent and complex chemical pool

Chemical measurements of the major organic constituents show in most aquatic systems an overwhelming dominance of the dissolved fraction compared with the particulate fraction. DOC is typically found in concentrations 10 to 100 times

26

higher than particulate organic carbon (POC). The ratios between dissolved and particulate organic nitrogen and phosphorus can be lower and very variable as nitrogen and phosphorus during the growth season are efficiently harvested by organisms. All chemical measurements of DOM show an enrichment of carbon

relative to nitrogen and phosphorus when compared with the molar C:N:P Redfield ratio of 106:16:1 found in many plankton organisms and organic particles in the water column. The C:N and C:P ratios of DOM are indicators for the nutritional value of DOM and deviations from Redfield (C:N at 6.6 and C:P at 106) toward carbon enrichment generally indi-


Chapter 3 3 Chapter DOM sources sources and and microbes microbes DOM in lakes lakes and and coastal coastal waters waters in

The blue arrows and boxes illustrate the flows and pools

occurs regularly both in freshwater and in some marine en-

of phosphorus through the food web. The red boxes and ar-

vironments, additional mechanisms are needed to prevent

rows illustrate how a lack of available organic-C substrates for

such an inevitable transition to carbon-limitation.

bacteria (LDOC), or a lack of silicate (Si) needed by diatoms,

Predation from heterotrophic flagellates prevents the

may restrict the phosphorus flow through the left “microbial”

build-up of a large bacterial biomass that would immobilise

or the right “classical” part of the food web, respectively.

the available phosphorus, and some phosphorus will thus be

A steady state of such a network is one where the oppos-

left for the competing phytoplankton. With bacteria sand-

ing processes balance each other and there is no change over

wiched between predatory control of biomass and competi-

time although material continuously circulates through the

tion control of growth rate, bacterial production (=growth

food web. In our context, one important feature of the sug-

rate x biomass) will remain low. Bacteria use organic carbon

gested minimum model is that it has sufficient elements to al-

for two purposes, for production of new biomass, and for

low steady states both with bacterial growth limited by

respiration. With production now controlled by our compe-

mineral nutrients (in this case phosphate) and with organic

tition-predation mechanism, and respiration constrained

carbon limitation.

within limits given by bacterial physiology, bacterial con-

To illustrate the mechanism, assume that bacterial growth is

sumption of organic carbon will, in our minimum model, be

phosphorus limited. Since bacteria, with their high surface-to-

restricted to some low level. If degradable organic matter

volume ratio, are assumed to be the best competitors for phos-

now is supplied by allochthonous and/or autochthonous

phate, one could argue that they should out-compete the

sources at a rate exceeding this restricted rate of bacterial

phytoplankton until primary production is reduced to a level

consumption, the excess of otherwise perfectly degradable

where bacterial growth becomes carbon-limited. If this was

DOC will simply accumulate. This is what we believe we

true, one should only observe steady states in nature with car-

have seen when, in some systems, there is no bacterial re-

bon-limited bacteria. Since this is not in accordance with obser-

sponse to addition of an easily accessible carbon source like

vations, which strongly suggest bacterial phosphorus limitation

glucose, and most of the added glucose just accumulates.

cate poor substrate quality. High carbon enrichment would be expected for terrestrial DOM with its origin from terrestrial vegetation and soils, however, enrichment is also influenced by land use (Chapter 2). The consequence of high carbon enrichment is that microbes often have to extract the carbon (and energy) with the use of inorganic nutrients from

the environment and ultimately mineralise DOM with a high oxygen demand compared with the release of inorganic nitrogen and phosphorus. The concentrations of DOC, DON and DOP and the C:N:P stoichiometry of DOM at the land-sea interface can be exemplified by a study of Hansted Stream draining an agricultural dominated Danish catch-

ment with Horsens Fjord as the coastal end-member (see Fig 3.2 and Chapter 6). The DOM in Horsens fjord, as is the case for most coastal waters, is carbon rich and poor in nitrogen and phosphorus relative to the Redfield ratio. The N:P ratio of DOM in the fjord is 42 and about 10-fold higher than in a bacterial cell. From


27

Box 3.2 Fluorescence of DOM Fluorescence spectroscopy is a sensitive technique for

lying sub-fractions. The dynamics of the different frac-

tracing quantitative and qualitative changes in fractions

tions in different aquatic environments can then be

of DOM. When irradiated with ultra violet and blue light,

traced and used as a proxy for the changing characteris-

a sub-fraction of the DOM pool fluoresces. The concentra-

tics of the DOM pool as a whole. This example show the

tion and chemical composition of the DOM pool deter-

eight fractions identified in DOM from the Horsens catch-

mine the intensity and shape of the fluorescence spectra

ment and estuary in Denmark. Components 1 to 6 exhibit

measured. The exact location of the fluorescence peaks

fluorescence characteristics similar to that of humic mate-

varies with the composition of the fluorescent DOM.

rial (both of allochthonous and autochthonous origin),

Parallel factor analysis (PARAFAC) allows the decompo-

and components 7 and 8 represent a protein-like fluores-

sition of the measured spectra into the different under-

cence (autochthonous DOM).

0.2

0.1

0

Measured DOM fluorescence.

Emission wavelength (nm)

600

300

1

1 2

3

4

5

6

7

8

300 400 Excitation wavelength

The model is the sum of 8 fluorescent subfractions.

PARAFAC model of DOM fluorescence. 0.02

0

-0.02

Emission wavelength (nm)

600

Residuals difference between measured and modelled. 300

28

300 400 Excitation wavelength



600

400

40

200

0

20

Stream

Lake outlet

Estuary

25

80

60

are due to autochthonous DOM production in the lake and the fjord diluting the terrestrial DOM signatures. Down stream dilution of the total bulk organic pool is described by DOC, which decreases in concentrations from 1110 to 230 µmol l-1. It is the interactive combination of selective microbial degradation and photochemical transformations that makes freshwaters, estuaries and coastal waters function as a sieve removing terrestrial signals from DOM on its way to the oceans where terrestrial signals are absent.

0

C:N C:P

20

1000 800

15

600

10

400

5

200

0

Stream

Lake outlet

Estuary

Molar C:P ratios

DOC DON DOP

DON and DOP (µmol l-1)

DOC (µmol l-1)

800

compounds traveling in a stream into a lake and further into an estuary. Additionally, the mixing of allochthonous with autochthonous DOM can be followed using a newly developed method identifying specific optical signals linked to the origin of DOM (Box 3.2). Hansted Stream once more can be used as an example to follow DOM from land to sea. In the upper reaches of the stream the terrestrial and humic compound groups 1 to 6, as identified by the PARAFAC modelling, dominate as expected (Fig. 3.3), but the relative distribution and concentrations change after passing a eutrophic lake and more so in the fjord. The changes in composition

Molar C:N ratios

these measurements of the DOM nutrient stoichiometry we learn that microbial exploitation of DOM in most cases requires bacterial uptake of inorganic nitrogen and phosphorus or an ability to exploit DON and DOP specifically. The consequence of the chemical composition of DOM is a high oxygen demand during degradation and a bacterial community competing with phytoplankton for limiting nutrients DOM from terrestrial environments has a high content of humic material and a very complex chemistry. The coloured humic material provides specific optical markers for the terrestrial origin, which can be used to trace the fate of terrestrial

0

Figure 3.2 Seasonal averages of DOC, DON and DOP concentrations

creased and the C:P and N:P ratios almost doubled. In the

and molar C:N and C:P ratios of DOM in the Danish stream

fjord the ratios resembled DOM at the lake outlet. The C:N:P

Hansted, in the outlet from a eutrophic lake, and in the re-

ratio of the fjord end-member is 740:42:1. In comparison, the

ceiving estuarine end-member Horsens Fjord. The average

C:N:P ratio of DOM in surface waters of oceans totally domi-

C:N and C:P ratios for DOM in the stream are well above Red-

nated by autochthonous DOM is typically about 430:36:1, i.e.

field ratios, while the N:P ratio is close to Redfield. In the out-

much less carbon enriched than in this system dominated by

let from the lake, DOM is less carbon rich relative to N due to

terrestrial DOM.

an increase in DON; however, the concentration of DOP de-


29

Photo 3.2 Station 13 in the Hansted catchment (photo by Stiig Markager). 35

1 2 3 4

30

5 6 7 8

Figure 3.3 Fluorometric analysis of DOM sampled in the upper reaches of Hansted Stream, at the outlet of a eutrophic lake and in the estuarine end-member Horsens Fjord. Eight compound groups were identified and it is shown how there is a shift in the relative distribution moving from the steam to the fjord. The methodology of the compound modelling is outlined in Box 3.2.

30

Fluorescence (% Raman units)

25 20 15 10 5 0

Forest stream (1112 µmol l-1 DOC)

Hansted outflow (486 µmol l-1 DOC)

Outer estuary (233 µmol l-1 DOC)


Chapter 3 DOM sources and microbes in lakes and coastal waters

Two types of DOM and its biodegradation

gradable DOM such as glucose and amino acids can only occur if the activity of the bacterial community is suppressed by high grazing keeping the active bacterial biomass low or if bacterial growth rates are limited by nutrients (see Box 3.1). At steady state between autochthonous DOM production, terrestrial import and DOM removal by microbes and photochemical processes, i.e. when the rates of input and removal are balanced, the concentration of degradable compounds will be constant and inversely proportional to their biological lability (how easy they are utilized). This is why the DOM pool in most situations is dominated by compounds that can only be degraded slowly (semi-labile and recalcitrant) or not at all within time scales of decades or

DOC DOC DOC

DON DON DON

The allochthonous DOM and some of the autochthonous DOM production can be measured chemically because it has not yet been removed by microbial and photochemical processes. In essence, most of the measured DOM must be somewhat resistant to degradation. The other part of the DOM production is the fraction that is removed as fast as it is produced, and this fraction can only be measured indirectly as bacterial production and respiration. This latter fraction typically consists of amino acids, small carbohydrates, sugar alcohols, and small-chained fatty acids among others. Their concentrations in natural waters are extremely low (nanomol l-1) and vary little in time and space. High concentrations of very biode-

Semi-labile Semi-labile Semi-labile

DOP DOP DOP

Labile Labile Labile


Recalcitrant Recalcitrant Recalcitrant

hundreds, even thousands of years (refractory compounds). Experiments on biodegradation of terrestrial DOM show that most DOC is recalcitrant with degradation rates of less than 0.2% per day, although a slow degradation rate is not unique to terrestrial DOC. A variable but quantitatively significant part of the primary production in lakes, coastal waters and oceans is routed to a carbon rich DOM pool. Biodegradation of a DOM pool not influenced by terrestrial DOM is exemplified by results from surface waters collected in the Atlantic Ocean off the US northeast coast (Fig. 3.4). It is of interest to notice that nitrogen and specifically phosphorus are “utilised” selectively compared with carbon. After biodegradation the remaining DOM is even more carbon

Recalcitrant Recalcitrant Recalcitrant

Semi-labile Semi-labile Semi-labile


Recalcitrant Recalcitrant Semi-labile Recalcitrant Semi-labile Semi-labile

Figure 3.4 The relative distribution of biodegradable and recalcitrant

polls had half-lives below 113 days. Most DOC (75%) was

fractions of DOC, DON and DOP in surface water samples

not degraded at all. The biodegradability of DON and DOP

from the continental shelf area off the US northeast cost.

was higher at 40 and 80%, respectively. Data compiled from

Degradation experiments were run for 180 days. The labile

Hopkinson et al. (2002).

pools had half-lives below 12 days, while the semi-labile


31

rich than at the beginning of the experiment. DON and DOP were transformed to inorganic nutrients, while DOC was mineralised to CO2. Why should a manager of coastal waters and lakes care about the production, accumulation and degradation of autochthonous DOM? The answer is because nutrients and the

related oxygen demand are “hidden“ inside the organic pool and not necessarily “released“ at the site of production. DOM has a slow but variable reactivity leading to export from and import to different regions. The result can be transport of oxygen demand and nutrients from productive to less productive areas. Ne-

glecting this feature of DOM may lead to misinterpretation of causes and effects with respect to anoxia events and nutrient load (Chapter 4). In the following sections we focus on the processes producing autochthonous DOM in the water column, how it can accumulate and how it is subsequently decomposed.

Box. 3.3 Production of autochthonous DOC and DON in coastal waters Large (11m3) mesocosms were used to control phytoplank-

ing the last 5 days with high nitrate dosing (DON is scaled

ton by the addition of different concentrations of inor-

on the secondary y-axis). Thus, the molar C:N ratio of the

ganic nutrients leading to either nitrogen deficient or

newly produced and accumulating autochthonous DOM

replete growth. The three conditions were; Phase I: the

varied from very high (∞) in the beginning of the experi-

phytoplankton bloom was created; Phase II: a nitrogen de-

ment to between 11 and 20 in phase II and III. The latter

ficient community, and Phase III: a nitrogen replete and

ratio is comparable to the values measured in Horsens

blooming community.

Fjord and in oceans (Fig. 3.2). The accumulating DOM was

The addition of nutrients in Phase I increased the production of algae (here measured as particulate organic

From the chemical and biological measurements it is

carbon = POC), which was immediately followed by an in-

possible to calculate the total production of DOC and the

creased bacterial production (BP). After a few days DOC

sequestration into DOC immediately removed by bacteria

started to accumulate at the same speed as bacterial pro-

and the amount accumulating. Bacterial production did

duction. During the nutrient deficient Phase II, DOC con-

not result in a higher biomass so grazers effectively re-

tinued to accumulate despite that the production of

moved the produced bacteria.

particles almost ceased. Bacterial production also contin-

During nutrient replete growth (Phase I and III) about

ued to increase. The addition of a surplus of nutrients in

half the total carbon production was routed via DOC and

Phase III resulted in a major diatom bloom where the pro-

half of this was accumulating and could be measured

duction of new DOC closely followed the POC production.

chemically. During nutrient deficiency in Phase II the pro-

DON also accumulated, but lagged behind the increase

32

carbon rich, as expected.

duction was totally dominated by DOC (90%) and about

in POC and DOC. Production of new DON was detected

half accumulated.

Day 12 and an enhanced accumulation was detected dur-

BR = bacterial respiration.



Autochthonous DOM sources

Phytoplankton photosynthesis is the ultimate DOM source in systems without large littoral zones and extensive import of terrestrial DOM. However, autochthonous DOM production has multiple sources. During each step in the complex plank-

ton food web production of DOM takes place (see Fig. 3.1). DOM is released during photosynthesis, grazing, when organisms die, during lysis of cells, from viral attack, and when bacterial enzymes solubilise particles and aggregates. Extracellular release by phytoplankton. Phytoplankton cells inevitably

loose a variable but significant amount of newly produced organic matter directly to the environment. Phytoplankton exudation (loss) is a normal process and an important source of organic carbon readily available for bacteria. Most exudates are considered very labile. On average the loss is about 13 % of the

Photo 3.3. Pontoon bridge with experimental mesocosms positioned in the inner Raunefjord at the former EU Large Scale Facility at Espeland, near Bergen in Norway (photo by Stiig Markager).

150

Phase II N+P

30

Phase III 5N+P POC DOC

25

120

20 BP

90

15 10

60 DON

30 0

0

5

10 15 Time (days)

20

5

25

0

50 40 (µmol l-1 d-1)

Phase I N+P

Net DON production (µmol l-1)

Net carbon production (µmol l-1)

180

POC DOC BP BR

30 20 10 0

Phase I

Phase II

Phase III

Data from Søndergaard et al. (2000).


33

total water column primary production. However, there are large variations both within and among systems (from 5 to 50%). Grazing. When particle-eating organisms eat, the food particles are not utilised with 100% efficiency. Partly digested food and dissolved organic matter are defecated or excreted. Particles may also be broken up by the mouth parts and cell sap and blood released into the water. This process is called “sloppy feeding”. The DOM production during grazing is not linked to any specific size of organism and occurs at all levels of the food web. Grazing is considered one of the most important DOM producing processes and the DOM is of high nutritious value. Cell lysis and particle solubilisation. Planktonic organisms can die of “natural” causes other than grazing and predation. Physiological stress due to nutrient depletion and age can result in attack by saprophytic fungi and bacteria followed by death. Before and after death the organism is leaching DOM into the surrounding water (cell lysis) and attracting motile bacteria sensing a good meal. Motile bacteria also colonise dead organic particles and aggregates and utilise the organic matter not protected by cell walls and membranes. The utilisation of polymeric material is facilitated by the action of hydrolytic ectoenzymes either excreted to the surroundings or connected to a cell. The hydrolytic activity produ-

34

ces dissolved organic substrates not only available for the bacteria at the degradation site but a surplus of low molecular DOM diffuses away from the site sustaining the growth of free-living bacteria. Not all autochthonous DOC, DON and DOP are immediately used in the microbial loop. Seasonal and episodic accumulations of DOM fractions occur in oceans, coastal seas and lakes revealing that a significant part of the produced DOM is utilized slowly and even enters the refractory DOM pool with turnover times of tens to hundred of years. With the current knowledge, it is not possible to make any general and global conclusions concerning the quantitative sequestration of primary production into DOM. However, measurements of bacterial production and respiration have unambiguously proven that the DOM route is one highway for the processing of autochthonous organic production in plankton dominated aquatic ecosystems. A few empirical examples may enlighten current knowledge.

DOM production in a coastal plankton community

One example to “illuminate” the complexity of DOM production and organic carbon sequestration in a coastal plankton community is taken from an experiment carried out at the former EU large-scale facility in Bergen. The purpose of the

study was to find out how nutrient replete and deplete conditions might affect the production and accumulation of DOC and DON (Box 3.3). Although the Bergen experiment cannot be used for global generalisations, it clearly shows that a high proportion of plankton production can be stored in the DOM pool and not immediately converted to its inorganic constituents in the microbial loop. Accumulation of DOC and DON shows that DOM produced in natural waters persists long enough to be exported to other locations. Transport can for example be from productive surface layers to bottom waters or from lakes and estuaries to coastal waters. An analysis of the ecological consequences of DOM production and export needs to include knowledge about its degradation. How much of the autochthonous DOM is biodegradable, what is the time scale of degradation and how important is the photochemical reactivity? The latter subject is treated in Chapter 5.

Bacterial degradation and mineralisation of autochthonous DOM

Degradation experiments with DOM from the continental shelf of the Atlantic Ocean showed a distinct difference in the distribution of recalcitrant and biodegradable compounds among DOC, DON and DOP (Fig. 3.4). The DOC and DON pools were characterised by large recalcitrant background values, which may not



300

Degradation

200 150 100 50 0

0

50

100

150

200

250

about 12% of the new DOC was recalcitrant or refractory. Organic nitrogen and phosphorus were mineralised; however, the “leftover” DOM was enriched with respect to nitrogen and phosphorus and is most probably of bacterial origin. We have now learned that a large part of freshly produced DOC and also autochthonous DON and DOP are mineralised rather slowly with turnover times of several weeks. This is a time scale longer than the

15

4 NO3 + NH4

12

3

9 2

PO4

6

1

3 0

0

50

100

Time (days)

150

200

250

Phosphate (µmol l-1)

250 DOC (µmol l-1)

fractory compounds can only be studied experimentally. Once more we used a plankton bloom to produce new autochthonous DOM and then measured how bacteria degraded DOC and mineralised DON and DOP (Fig. 3.5). Bacterial degradation of new DOC has two phases. About half of the degradable compounds were utilized within a few weeks, while the other half was more slowly degraded over a couple of months. Furthermore,

Inorganic nitrogen (µmol l-1)

be representative for newly produced DOM. However, the results convincingly demonstrate how a mixture of old and new autochthonous DOC behave with respect to biodegradation and how the different degradability of DOC, DON and DOP controls the C:N:P ratio of the chemically measurable DOM pool. Biodegradation of newly produced autochthonous DOC without interference from old and re-

0

Time (days)

Figure 3.5 The increase in DOC during an experimental freshwater plank-

is characterised as recalcitrant. The term refractory is saved

ton bloom and its decrease during bacterial degradation and

for organic pools with turnover times of years.

the concentrations of inorganic N and P during degradation.

DON and DOP were not measured, but measurements of

Net mineralisation of DON and DOP is identified as inorganic

inorganic nitrogen and phosphate allowed the net minerali-

concentrations higher than when the degradation experiment

zation of DON and DOP to be followed. Initially, the growing

was initiated.

bacteria used inorganic N and P concomitant with the fast

Over 22 days the bloom of freshwater phytoplankton pro-

degradation of DOC. With respect to inorganic N, a net pro-

new DOC. The DOM was isolated,

duction occurred after about 50 days and in total 4 µmol l-1 N

added bacteria and allowed to degrade over 230 days. Initi-

was mineralised. A net production of inorganic N can only be

ally, the degradation of DOC was fast and after 25 days half

explained by mineralisation of DON. For P a net mineralisation

the DOC was removed. The degradation then slowed down

was found after about 75 days and inorganic P was slowly in-

and almost came to a halt after 230 days. At that time about

creasing over the next 100 days. Data from Kragh & Sønder-

12 % of the new DOC was not degraded. This “leftover” DOC

gaard (Aquatic Microbial Ecology, in press).

duced about 275 µmol

l-1


35

water renewal in many estuaries, coastal waters and shallow lakes. Thus, autochthonous DOM can be exported and its oxygen demand and nutrients “released” gradually over time and possible far from the site of its production.

Conclusions

The load of inorganic nutrients to aquatic environments is still the major threat to the quality of most

36

surface waters at the European scale. However, our message is that we should not neglect how nutrients and oxygen demand are transported in DOM and how microbes via DOM dominate the nutrient recycling in coastal areas. Microbial processing of DOM can explain a major proportion of the turnover of organic matter in many aquatic ecosystems. With the increasing regulation of the release of inorganic nu-

trients, the relative importance of the DOM export from land to sea will increase. It therefore becomes crucial to include DOM in coastal management strategies and to understand the quantities exported from land and the environmental effects. Furthermore, it is important to increase our knowledge on how autochthonous DOM production influences water quality.


Chapter

Photo: Helene Munk Sørensen.

Stiig Markager Colin Stedmon Pascal Conan

4

Dissolved Effects oforganic DOM matter (DOM). What is it and why study it? in marine ecosystems

D

OM affects marine ecosystems in at least three ways: DOM absorbs light thereby reducing the light level. This occurs primarily in the blue and ultraviolet (UV) part of the light spectrum and the consequence is that there is less light available for plant and algal growth. At the same time this absorption means that aquatic organisms, to some extent, are protected from damaging UVradiation. Both nitrogen and phosphorus are bound to DOM and these nutrients are carried into the system as part of the DOM pool and may stimulate plant and algal growth, potentially contributing to eutrophication and associated problems. The nutrients

bound in DOM are not directly available for plant growth, but after degradation, either photochemically or by microbes, the nutrients are released and enter the bioavailable nutrient pool. Carbon is an important component in DOM and the carbon in DOM can serve as substrate for bacteria resulting in a demand for oxygen.

DOM, light and marine systems

DOM colour varies from yellow to even dark brown, because light absorption by DOM increases exponentially with decreasing wavelength. Very little red and yellow light is absorbed whereas absorp-


37

Box 4.1 Light absorption by DOM

Ultra violet light

The absorption of light by organic compounds is depend-

30

ent on their chemical nature. The presence of different chemical groups and different atomic bonds will lead to electromagnetic spectrum. As the DOM pool in natural waters consists of a complex mixture of compounds, its absorption spectrum represents the sum of the different overlapping absorption peaks. CDOM absorption spectra can be used to trace relative changes in the composition and size of the DOM pool by modelling the characteristics of the absorption curve in the UV and visible regions (slope and intensity). DOM originating from differing sources differs in chemical composition, causing the light absorption properties to vary which in turn can control the underwater light environment for plants and animals.

Stream draining agricultural land Peatland drainage (x10-1) Estuarine CDOM Autochthonous CDOM Mountain river

25

CDOM absorption (m-1)

the absorption of light (energy) in different regions of the

Visible light

20 15 10 5 0

300

400 500 Wavelength (nm)

600

Photo 4.1 Water from a creak near Oulu, Finland. The brown colour is due to DOM. The DOC concentration was 1143 µmol l-1 and absorption at 320 nm was 109 m-1 (photo by Stiig Markager).

38


Chapter 4 Effect of DOM in marine ecosystems

5 4

Kd spectra and DOC concentration (µmol l-1)

~450

~300

3 2 1 0

~180 ~100

300

400


700

6 Irradiance (µmol nm-1 m-2 s-1)

Diffuse attenuation coefficient Kd (m-1)

6

Horsens Fjord, inner station

5

Surface

4

0.74 m

3 2 1 0

3.49 m

300

400


700

Figure 4.1

Figure 4.2

The effect of DOM concentration on the light attenuation

Variation in irradiance with depth in Horsens Fjord, Denmark.

(Kd ) at different wavelengths in water from Horsens Fjord, Jutland (DOC > 200 µmol l-1) and from Skagerrak (DOC < 200 µmol l-1). It is clear how the attenuation at low wavelengths increases with increasing DOC concentrations whereas the effect is small in the red part of the spectrum.

Photo 4.2 At very high concentrations DOC makes the water yellow-brown as seen in the Bothnian Bay outside Kiiminginjoki River, Oulu, Finland (photo by Stiig Markager).


39

tion is high in the blue and UV part of the spectrum (Box 4.1). The effect of DOM on the underwater light climate is a marked increase of light attenuation when DOM concentrations increase. In the UV part of the spectrum, below 400 nm, absorption is almost entirely due to DOM and therefore proportional to light absorption by DOM. Therefore it is closely linked to the actual concentration of DOM in the water (Fig. 4.1). In the blue part of the spectrum (between 400 and 500 nm) DOM absorption also dominates, although absorption by phytoplankton can also be important in more eutrophic systems (Fig 4.1). Above 500 nm light attenuation is mainly due to scattering and absorption by water it self. The rather high concentrations of DOM (> 200 µmol l-1) in many coastal waters mean that they appear yellowgreenish and not deep blue like oceanic water where DOM concentrations are typically lower than 100 µmol l-1. When DOM concentrations exceed about 500 µmol l-1, the water will appear brown which is commonly the case in estuaries receiving freshwater from wetlands or peat-lands. Figure 4.2 shows the light spectrum with depth in an estuary on the East Coast of Jutland. At the surface, irradiance is approximately constant with wavelengths down to 480 nm, but rapidly declines in the blue and UV-region. In water, the red and particularly blue and UV

40

wavelengths are attenuated so the peak in irradiance at a depth of 3.49 m occurs at 582 nm. With depth and with increasing concentrations of DOM the peak irradiance will move toward higher wavelengths, and so will the peak irradiance for light escaping the water, which is responsible for the colour seen when looking at the water.

DOM and the distribution of macrovegetation

Seagrasses and macroalgae are important components in coastal ecosystems. They contribute to the overall primary production but they also act as structuring components forming habitats for other organisms such as crustaceans and fish. These larger plants and algae generally have lower growth rates than phytoplankton and a slower turnover rate of the nutrients bound in their biomass. Hence, a large population of macrovegetation is generally considered as positive for the environmental state of coastal ecosystem. The depth limit for occurrence of macrovegetation is set by the availability of light. It has been shown that a seagrass such as Zostera marina requires about 13% of the surface irradiance and that large brown macroalgae (seaweeds) can grow at depths where only 1% of the surface light is left. Since DOM is an important component of the light attenuation, the DOM concentration will effectively set the depth limit for macrovegetation. The effect of DOM

on the distribution is controlled by the concentration, the attenuation by particles and the bathymetry of the area. The largest effect will occur in systems where a large area of the bottom, otherwise suitable for macrovegetation, is at a depth similar to the depth for the light limit of the dominating plant type. If the banks are steep or light attenuation by suspended particles in the water is high due to re-suspension of sediment particles or due to high concentrations of phytoplankton in the water, the effects of DOM will be small.

DOM and primary production by phytoplankton

Phytoplankton is the other algal component in coastal ecosystems. Their production is regulated by complex interactions between physical factors such as temperature, circulation in the water column, surface light, nutrients, grazing and light attenuation. Since DOM is important for light attenuation, a high DOM concentration will have a negative effect on primary production. In most coastal systems less than 20%, and often much less, of the surface irradiance is absorbed by phytoplankton pigments (dominated by chlorophyll) for photosynthesis. The rest is absorbed by other components, mainly water it self and DOM, resulting in a competition for photons between the light absorbing components. It can be shown that the fraction of surface light absorbed by one


Chapter 4 4 Chapter Effect of of DOM DOM Effect in marine marine ecosystems ecosystems in

component is the ratio between the absorption coefficient for that component and the total absorption coefficient for the water. The consequence is that if the light absorption by one component increases, the fraction of surface light absorbed by other components will decrease. Since the absorption by water is basically constant, the concentration of DOM and detritus are the regulating factors that determine the fraction of surface light absorbed by phytoplankton between systems with equal chlorophyll concentrations (Fig. 4.3). Since primary production of phytoplankton is based on light and nutrients, and DOM reduces the amount of light available for phytoplankton, it is proposed that at high DOM concentrations a higher nutrient supply is required to maintain the same production per unit area compared to systems with a low DOM concentrations.

DOM as a carrier of nutrients to marine systems

Nutrients are supplied to marine systems by atmospheric deposition and freshwater runoff, either directly via rivers and streams or with groundwater. Only a small fraction of the nitrogen in atmospheric deposition is DON (generally less than 15%). From a practical and quantitative point of view, DOM in atmospheric deposition can be neglected. There is meager information available about the importance of groundwater to the total nitrogen loading to marine systems, and even less about the forms of nitrogen in it. Nitrogen from ground water can be important in some estuaries where runoff from rivers is low. However, nitrogen in groundwater is most likely in the form of nitrate, and DON contributions are probably low.

The fraction of the total nitrogen load contained within riverine DOM varies greatly between systems. In areas with intense agriculture activities it is a minor, though not a negligible, fraction. In DOMAINE we found that DON contributed 12% of the annual load to the temperate estuary Horsens Fjord. In natural catchments, particularly if dominated by wetlands, peatland or forest, DON will be the dominating fraction. For the Baltic Sea, 60% of the total nitrogen loading is due to DON. The values above are for the land/sea interface. Once the nitrogen reaches the sea, there is a rapid transformation of inorganic nitrogen to organic fractions due to uptake by bacteria and phytoplankton. For Horsens Fjord it was found that although only 12% of the loading came as DON, 83% of the nitrogen leaving the estuary was as DON. Thus, estu-

Figure 4.3 0.3

Simulation of the fraction of surface light absorbed by phytoplankton at

0.2 4 µg Chl l-1

100% 75%

0.1

2 µg Chl l-1

1 µg Chl l-1 0

different chlorophyll and DOC concentrations in Danish marine waters. Left

125%

0

100

200 300 400 DOC concentration (µmol l-1)

500

600

50% 25% 0%

Percent change

Fraction absorbed

150%

axis is the fraction of surface light absorbed by phytoplankton. Right axis is the percent change from a situation with 2 µg Chl l-1 and DOC at100 µmol l-1. Photosynthesis is fuelled by the amount of light absorbed and primary production is determined by carbon fixation and the efficiency of converting fixed carbon to growth. The latter is mainly controlled by the availability of nutrients.


41

aries and lakes act as reactors where DIN is incorporated into DON (see Fig. 6.5). The form of nitrogen export is important because it influences the potential of nitrogen loading in eutrophication processes. Inorganic forms and some organic forms like urea and free amino acids are directly available for phytoplankton and bacteria and therefore have the full potential to contribute to eutrophication. In contrast, nitrogen bound in complex structures such as humic material, does not contribute to eutrophication until they are mineralised to inorganic forms. The results from DOMAINE indicate that within 5 months a considerable fraction of terrestrial DOM exported to the sea is degraded whereas the remaining part is relative recalcitrant. We estimate the degradable fraction to average 40% but to be variable depending on the land use which affects the quality of DOM. The freshly formed DOM in estuaries

42

is more reactive, but with time, parts of it are transformed to more recalcitrant DOM compounds where the nutrients are bound for longer time, probably many years. In summary, nutrients bound in DOM can represent a considerable fraction of the total loading, but is not likely to cause eutrophication in the zone near to the outfall. However, on a longer time scale, when residence time exceeds 1-2 months, there is probably little difference between the effects of the different form of nitrogen with respect to eutrophication.

Oxygen demand from DOM

DOM is organic material with an oxygen demand. When it is degraded the process will consume oxygen. Since hypoxia (reduced oxygen conditions) and anoxia (oxygen free conditions) are common problems in marine areas this could be of concern. However, these problems are

usually related to oxygen consuming processes in the deeper part of the water column or associated with the sediment. Since DOM is carried to estuaries in freshwater it will be layered into the upper part of the water column, where the close contact of surface waters with atmospheric oxygen and phytoplankton produced oxygen normally prevents hypoxia. One of the pathways for the degradation of DOM, photodegradation, only takes place in light and will therefore always occur together with oxygen production from photosynthesis. The situation where oxygen consumption related to DOM is likely to be a problem is when there is intense flocculation of DOM in the transition zone between freshwater and seawater (Chapter 5). Here a fraction of the DOM can settle out causing an increased oxygen demand in the underlying sediment leading to low oxygen levels.


Chapter

5

Dissolved organic matter (DOM). Fate of DOM What is it and why study it? in estuaries

Niels Henrik Borch Gaelle Deliat Mireille Pujo-Pay Colin Stedmon

L

arge amounts of organic and inorganic material are carried to the oceans each year, and the riverine transport of water and different compounds is an essential part of the global geochemical cycling of the elements. Most of the organic matter exported to coastal waters is removed in estuaries and near-shore environments, and thus compounds of terrestrial origin are hardly traceable in the open ocean. The lack of terrestrial signals from organic compounds in the oceans cannot be exclusively explained by dilution in the vast amounts of ocean water. The annual runoff from land can cover the entire ocean with a layer of only 10 cm, and the ocean

is on average 3800 m deep. The continuous supply of terrestrial matter, and the long survival time of the most inert and hard to degrade compounds should eventually lead to some accumulation in the open ocean. Considering the large amounts of organic matter transported through the rivers, this removal in the coastal waters must be very effective. Terrestrially derived organic matter therefore seems to be highly modified or remineralised inside rivers, estuaries and coastal waters before a very small fraction is further exported to the open sea. Except for extraordinary cases with flooding, most organic matter transported by rivers to


43

Respiration

Export DOC

Photochemical Degradation Microbial Degradation

Grazing

Grazing

Aggregation

Denitrification

Export DON

Photochemical Microbial Activity

Grazing

Grazing

Aggregation Algae

Remineralisation

Figure 5.1 Schematic diagram of the fate of organically bound carbon, nitrogen, and phosphorus in coastal waters. Red arrows indicate loss processes where the compound is transformed to gaseous matter and red dashed arrows loss processes by sedimentation. Green arrows indicate processes where organically bound nitrogen or phosphorus is made available for the phytoplankton by either photochemical or microbial degradation.

44

Export DOP

Photochemical Microbial Activity

Grazing

Grazing

Aggregation Algae Remineralisation


Chapter 5 Fate of DOM in estuaries

the ocean is found in the dissolved phase as dissolved organic matter (DOM). In the context used here, DOM obviously includes organic carbon, but DOM also contains large albeit variable amounts of organically bound nitrogen and phosphorus (DON and DOP, respectively). In general there are three major pathways for dissolved organic matter removal: i) Physical particle formation (aggregation) and subsequent sedimentation ii) Photochemical degradation iii) Microbial degradation. In terms of the fate of DOM in coastal waters, it is important to distinguish between the fate of DOC and the cycles of DON and DOP. Figure 5.1 illustrates the major differences between carbon cycling and organic N and P cycling. For carbon, the main pathways are removal through respi-

ration by organisms (thereby transforming organic carbon into its inorganic form CO2), burial in the sediments in sinking organisms or following aggregation, or export out to open waters. Since respiration and export probably are the two most important loss terms, both the absolute amount and the rate by which DOC is degraded by microbes are important factors for studying the fate of DOC in coastal areas. Organically bound N is not lost from the system by respiration but can be incorporated in the microbial food webs, and subsequently be either remineralised to their inorganic compounds (primarily nitrate and ammonium), buried in the sediment in sinking particles or transferred to higher trophic levels through grazing food chains. For the nitrogen buried in the sediment there is a loss term as nitrogen under anaerobic conditions (without the presence of oxygen) can be denitrified by bacte-

ria to N2, which is subsequently lost as a gas. In general it is assumed that 10 to 15% of all the nitrogen (i.e. both inorganic and organic forms) entering an estuary is lost from the system by denitrification. For phosphorus the cycling is simpler. Organically bound phosphorus is predominantly remineralised by bacteria to its inorganic form PO43-, and then incorporated by phytoplankton. Some of the phosphorus incorporated into phytoplankton cells sinks to the sediment, is remineralised and eventually mixed into the water column again. There is no respiratory loss term for phosphorus, and the fate of terrestrially derived phosphorus is either burial in the sediments, continuous recycling or export to the open waters.

Aggregation

The term aggregation is used in this context to represent the transformation of dissolved material to particles

1600 1400

Figure 5.2

DOC (µmol l-1)

1200

Concentration of DOC in a salinity gra-

1000

dient experiment with water from dif-

800

ferent Danish freshwater systems. A

600

humic lake (open squares), a stream influenced by municipal waste (open tri-

400

angles), wetland (filled squares), an

200 0

agricultural dominated stream (open circles), forest stream (filled circles) and 0

5

10

15 Salinity

20

25

an algal culture (open diamonds). Data from Søndergaard et al. 2003.


45

as a result of a change in the physicochemical environment. Such changes can occur during the mixing of freshwater with salt water during estuarine mixing, where considerable changes in salinity and pH occur. Once in a particulate form, organic material can then be removed from the water through sedimentation (sinking) or by filter feeding organisms. There are two general mechanisms by which DOM can aggregate: Adsorption or flocculation. Adsorption is the attraction of DOM onto the surface of inorganic particles (e.g. clay particles or sand grains) resulting in a film of organic material enveloping the particle. Flocculation is the process where large dissolved organic molecules combine to eventually form particulate material. Both processes occur during estuarine mixing and result in an area of turbid water (the turbidity maximum) where flocculates form due to changes in salinity and pH, and where bottom sediments are re-suspended into the water as a result of turbulence. The importance of flocculation and subsequent removal of organic matter as a result of salinity changes is controversial. It must be noted here that almost all studies have exclusively dealt with carbon removal, and ignored organically bound nitrogen and phosphorus. Earlier studies showed that some DOC (generally below 10%) is removed when salinity increases above 5 ppt, but since then

46

only few studies have investigated this under controlled conditions. In the DOMAINE project, we have tried to quantify the amount of DOC that can be removed from water from different land-use areas. We have used different sources for the water recognising the fact that different land-use patterns have an influence on the chemical characteristics of the DOM in the water. Only with water with a very high content of humic matter did we measure a slight effect of increased salinity (Fig. 5.2). In general, for the Danish waters studied, it appears that aggregation is only a minor removal process for DOM, and can be more or less ignored in terms of mass transport of organic matter to the sea.

Photodegradation of DOM

The structural changes of the compounds in the DOM pool, initiated by photochemical reactions, lead to alterations in its biological, chemical and physical properties and therefore have the potential to influence its role in aquatic ecosystems. The degradation of DOM by photochemical processes is thought to control the removal of a considerable proportion of DOC in surface waters, substantially decreasing its lifetime. Not only do these processes represent a removal for organic carbon in natural waters but they also can have an extensive influence on aquatic ecosystems resulting in:

i) A loss in the light absorbing properties and hence increased exposure of the water column to ultraviolet (UV) and visible light. ii) Remineralisation of organic carbon (e.g. CO and CO2 production) iii) Oxygen consumption iv) Production of labile organic compounds and compounds rich in nitrogen and phosphorus. v) Production of biologically resistant DOM vi) Destruction of organic ligandbinding capacity, causing the release of trace metals, micronutrients and even toxins (e.g. Cu, Fe, Mn). Absorption of light (energy) by a molecule results in a transition into an excited state. Immediately after this transition, relaxation to the ground state then occurs through loss of the absorbed energy via internal conversion (heat and molecular motion) and fluorescence or phosphorescence. During a primary photochemical reaction, a chemical alteration occurs whilst the molecule is in the excited state. The products of the reaction can be a new stable molecule(s) and/or new reactive species, which then go on to initiate secondary reactions. Whereas primary reactions only concern the organic compounds that can absorb light (e.g. coloured CDOM), secondary reactions can affect all compounds present (i.e. the whole DOM pool), depending on the lifetimes, concentrations and reactivity of the



reactive species. Primary photoreactions can result in molecular cleavage and/or rearrangement of the molecules. As a result of the complex nature of DOM in aquatic environments, there are many possible secondary reactions, however, the reactions of organic and inorganic free radicals produced as a consequence of primary reactions, are major degradation pathways for DOM. Radicals are highly reactive and influence both

the chemistry and biology of sunlit waters. For example, light absorption by CDOM is the principal source of a variety of reactive oxygen species (ROS) in surface waters, which play a pivotal role in the degradation of DOM. Inorganic chromophores (e.g. nitrate and nitrite) are also a significant source of ROS, if they are present in high enough concentrations. The underwater light environment can be divided into three wavebands; UV-B (280-315 nm), UV-A

450 400

N-H C-O C-C

350 Energy (Kj mol-1)

A

Energy of photons Bond dissociation energy

C-H

(315-400 nm) and visible light (400800 nm). The energy associated with a photon of light decreases with increasing wavelength (Fig. 5.3A). For example, a photon at 300nm has 33% more energy than a photon at 400 nm. UV-B light is rapidly attenuated in natural waters due to CDOM absorption. Figure 5.3B shows the depth at which there is 1% of surface irradiance left for different sampling sites in the DOMAINE project. UV-A light and visible blue light (400 nm) pene-

C-N

300

C-P

C-S

250 200

UVB

UVA

Visible

150 100

300

400

500 Wavelength (nm)

600

700

Depth of 1% surface intensity (m)

0

B

2

Figure 5.3 A: Variation in the energy of a photon

4

at different wavelengths. Also plotted are the bond dissociation energies

6

(i.e. the energy needed to break/cleave UVB (300nm) UVA (350nm) Visible (400nm)

8 10

Mesocosm

River

Stream Location

Estuary

Peatland

a chemical bond in a molecule) for a selection of organic bonds. B: The depth of 1% of surface light for UV-B, UV-A and visible light in five localities from the DOMAINE project.


47

trates much further into the water column than UV-B and therefore has the potential to influence a greater volume of water. Therefore, although the energy per photon in the UV-B is high, it appears that UV-A and visible wavelengths of light are the ones that probably dominate photodegradation processes. In addition, it is clear that photons in the UV-A and the blue light regions have enough energy to initiate various photochemical reactions in sunlit waters (Fig. 5.3A). So when investigating light induced degradation of DOM in natural waters it is important to consider the trade-off between photon energy, in situ light intensity and CDOM light-absorption. Photodegradation reactions both compete with bacteria to degrade a fraction of DOM and also aid bacterial degradation via the cleavage of non-bioavailable components of DOM. Bacteria use DOM for growth and as an energy source (see below). As they can only exploit energy from bond cleavages taking place inside their cell, DOM must first be taken up i.e. enter through their cell membranes. The fragmentation of high molecular weight DOM via extracellular enzymes (enzymes attached to the outside of the cell membrane) and photodegradation processes therefore control the availability of DOM to the microbial community. Research over the last decade has shown that photodegradation of terrestrially derived DOM (allochthonous) has a positive effect on its

48

bioavailability, reducing its average molecular weight and producing a suite of labile organic compounds, which are readily available to the microbial food web. On the contrary, results have shown that photodegradation of DOM produced within the system (autochthonous) can render the pool more resistant to microbial degradation, suggesting that the two processes compete for a degradable fraction.

Microbial degradation

Aggregation and photodegradation of DOM only occurs at specific interfaces where the right conditions exist, for example in surface waters or at the turbidity maximum. A more ubiquitous removal process for DOM is degradation by microbes. Bacteria are the main consumers of dissolved compounds in all aquatic environments, and bacterial processes occur under all circumstances. Since it is not light dependent, bacterial DOM degradation is not restricted to surface waters and therefore takes place in the entire water column. In most natural waters the concentration of bacteria range from 1 to 10 million cells per millilitre. The ecological role of bacteria is to consume dissolved organic compounds, transforming them into bacterial biomass, and, during that process, also respire some of the carbon to CO2. Unicellular protozoa, flagellates and ciliates, graze upon the bacteria, and are in turn prey for larger grazers. In this way a portion of the DOM

which is transformed into bacterial biomass, is passed up the food chain (Chapter 3). Bacterial processes have time frames of minutes to hours. They can respond quickly to inputs of food (DOM), and have very efficient uptake systems for readily available substances. Under the right conditions they can double their biomass within less than a day and therefore have a large potential to degrade organic carbon (DOM). However, in nearly all aquatic systems there is a substantial amount of DOM, suggesting that bacterial uptake of DOM is limited in some way. Three main factors influence the capacity of bacteria for DOM degradation: i) The DOM is not degradable by bacterial enzymes. ii) The DOM is originally degradable but is transformed into nondegradable substances. iii) The bacteria are limited by other factors than the food available. DOM degradation is directly linked to the intrinsic nature of DOM. In aquatic environments, DOM utilisable by bacteria is principally present as large polymeric molecules. To be used by bacteria, an organic molecule must first get into the bacterial cell. Specific enzymes, called permeases, located in the bacterial cell wall, are responsible for this process. But only substrates of low molecular weight (small physical size) can be taken up. Prior to bacterial uptake, macromol-



ecules therefore have to be broken down into smaller molecules by ectoenzymes present on the outside of the bacteria cells or released by the bacteria. Ectoenzymes are highly specific, and each enzyme can only break certain specific chemical bonds. For example, proteases can only break the peptide-bond between two adjacent amino acids in a protein, and even this process is limited to common amino acids. No single bacteria species can produce all the enzymes to break down all types of organic matter. However, aquatic bacterial communities are generally made up of many different populations and species, each with different capabilities for breaking down DOM. Bacterial communities can cope with most organics. Dissolved organic matter consists of a myriad of different compounds all produced by biological processes. Even though most biological production is well characterised i.e. we know what and how much can be produced by plants; current chemical analyses can only identify a small part of DOM at the molecular level as belonging to the three major classes of organic compounds: Proteins, carbohydrates, and lipids. Organic material originating from different sources differ in chemical composition and therefore in degradability (Table 5.1). The most degradable material is sewage and algal derived organic matter, which presumably is of most recent biological

origin. It is noteworthy that water that has passed a modern sewage treatment plan still is rather degradable, reflecting the fact that sewage treatment is primarily focused on removal of inorganic nutrients. In some treatment plants organic material is even added to the water to improve nutrient removal. The least degradable land-derived material is found in drainage from forests, and reflects the low degradability of the structural elements in trees (lignin and its derivatives). It is also interesting that the degradability of DOM in rivers and marine waters is rather uniform despite the differences in geographical regimes and terrestrial systems they represent. It is our hypothesis that the explanation is that most (all?) DOM that we measure chemically has been through a process of

Source

Degradability (%)

Sewage

40

Algal produced DOM

52

Forest, Denmark

6

Stream, Denmark

18

River, Finland

20

River, France

9

Marine, Denmark

3–15

Marine, Finland

4–17

Marine, France

13–17

microbial and photochemical degradation and transformation. In addition to the chemical nature of DOM other factors can limit the capability of bacterial communities to consume DOM in natural environments. The two major causes are inorganic nutrient limitation, and control of bacterial biomass by either grazing and/or viral infections (Chapter 3). Aquatic bacteria have to satisfy their need for nutrients (primarily nitrogen and phosphorus) in parallel to carbon acquisition. This can be achieved either directly by uptake of inorganic compounds (principally nitrate, ammonium and phosphate) or indirectly by assimilation of the organically bound nitrogen (e.g. protein and amino acids) and phosphorus (basically in DNA). Bacteria

Table 5.1 Degradability of dissolved organic carbon from different locations sampled during the DOMAINE project. The relative amount of carbon that could be degraded by microbes in 5 months is shown.


49

compete directly with phytoplankton for the available nutrients. Therefore bacteria and phytoplankton can use all the inorganic nutrients available, and this has been proposed to explain the seasonal DOM accumulation occurring in almost all aquatic systems. Much of the inorganic nutrients are incorporated into algal cells, which sink to the sediments. Nutrients are temporarily removed from the water phase. However, the nutrients are returned to the water column during subsequent mixing of the water. It should be stressed that water exported from agricultural dominated catchments, is rarely deficient in nutrients compared to carbon. Predation by grazers, mostly unicellular ciliates and flagellates (see

50

Fig. 3.1), contributes to the control of the bacterial biomass and to decrease the total activity of a bacterial community. Recent results have reported a selective grazing on active bacterial cells and consequently the possible regulation of bacterial productivity within natural communities. The increase in bacterial mortality by virus attack might also modify the capability of bacterial communities to use DOM. But grazing and viral lysis only act on short time scales, and it has been shown that when there is heavy grazing on bacteria, the remaining cells increase their activity so the overall result is that the degradation of DOM is as fast as without grazing pressure. The degradation of DOM is a process acting continually during its

transport from the terrestrial environment to coastal waters. The fraction of DOM left upon arrival to the coastal zone is then a function of both the transport time through the river system and the removal rate by bacteria and by abiotic processes. Most biologically labile compounds are removed from the water before it is exported. They simply do not exist long enough in the environment to be of practical concern, but because they are also produced during this transport, they constitute a measurable but minor part (less than 5%) of total DOM. It is the semi-labile and refractory components that will be exported to coastal waters, and these will have the maximum effects on the microbial dynamics and nutrient recycling in these waters.


Chapter

6

Dissolved organic (DOM). Analysis of DOM matter at the catchment What it and why study it?studies scale: is Two European case

Anker Laubel Dylan Evans Torben Vang David Bowers Colin Stedmon Niels Henrik Borch Morten Søndergaard

T

errestrial production of DOM is at large scales regulated by fundamental climatic parameters with linkage to site specific attributes like elevation, topography, soil type and land use (forest, wetlands, lakes, agriculture etc). Large scale terrestrial DOM sources and the regulation of DOM export are discussed in Chapter 2, where it is shown that the four catchments selected for study in the DOMAINE project are different and have unique export patterns with respect to quantities of DOC, DON and DOP and their relative distribution (see Table 2.3). These values are the integrated outcome of specific DOM loss patterns from many subcatchments, each with their own ef-

fect on the whole. In order to understand the role of each sub-catchment it is necessary to have a detailed knowledge of land use and discharge and a sampling programme that can reveal how specific attributes, e.g. agriculture, forest cover and lake area may influence the DOM export and its composition. Such knowledge is a prerequisite for a more comprehensive understanding of the factors and processes that regulate DOM export and, perhaps, more importantly, to provide an informed basis to ask and answer “what if?” questions in future management scenarios (Chapter 7). In 2001 and 2002 DOMAINE established seasonal sampling and


51

Skagerrak

13 14

Kattegat

North Sea

27

Hanstad Å system Søvind Å

Bygholm Å system

8

Lake

Lake

6

11

10

9 7 4

16

3

12

2

1

Kokkedal Å

1 km Sampling stations Figure 6.1 The Danish DOMAINE catchment Horsens Fjord showing the position of 14 stream and estuarine sampling stations

•), two major lakes and the Fjord.

(

52

measuring programmes to provide empirical data for DOM and inorganic nutrients for the selected catchments in Denmark, Wales and France. In this chapter we show the distribution of DOM export in a Danish catchment dominated by agriculture. Furthermore, we use the data from Wales to illustrate an analytical strategy for the handling of a complex dataset based on cluster analysis.

Horsens Fjord, Denmark The catchment

The Horsens Fjord study area is situated on the east coast of Jutland, Denmark (Fig. 6.1). The catchment is about 500 km2 and dominated by intensive farming. The climatic and land use information can be found in Tables 2.1 and 2.2. The landscape is mainly gently undulating moraine with elevations of less than


Chapter 6 Analysis of DOM at the catchment scale: Two European case studies

170 m. The soil composition ranges from sand to sandy clay loam, with peat soils constituting less than 3% of the total catchment area. Horsens Fjord is a micro tidal estuary with a residence time from 12 to 18 days and with end-member salinity about 29 ppt. With respect to nutrient load the estuary is highly influenced by terrestrial runoff, es-

pecially the inner estuary. DOM and inorganic nutrients were measured with a frequency of 14 days at 10 stream stations and 4 estuarine stations (Fig. 6.1). The 10 sampled sub-catchments were 385 km2, equivalent to about 77% of the total area. They could be placed in three major land use categories:

thereby closer to a natural reference condition (Fig. 6.2). There is only one urban sub-catchment in the study area. A large sewage treatment plant is situated in the town Horsens (the only major town in the catchment). It is upstream of sampling Station 16, and very close to the outlet to the estuary. Land use and DOM

100%

Percent

75% 50% 25% 0%

Agriculture (N=6)

Urban Other nature

Mixed rural (N=3)

Lakes Wetland

Urban (N=1) Forest Agriculture

Figure 6.2 Distribution of land use for sub-catch-

1. Agricultural: >75% agriculture; mean human population density of 51 per km2 2. Mixed rural: < 75% agriculture; mean human population density of 14 per km2. 3. Urban: 100% urban area; mean human population density of 1800 per km2. When compared with the agricultural sub-catchments, the mixed rural sub-catchments contain more forest, lakes and wetlands and are

The data made it possible to estimate the seasonal averages of DOC, DON and DOP concentrations exported from each type of subcatchment (Fig. 6.3). Combined with measured discharge in all streams the total load to the estuary can therefore be calculated. The average DOC concentration for agricultural sub-catchments (530 µmol l-1) was lower than for the mixed rural and urban sub-catchments (730 µmol l-1). One plausible explanation for the lower concen-

900

180

600

120

Agriculture

Mixed rural

Urban

DOP (µmol l-l)

300

0

2.0 1.5

DON (µmol l-l)

DOC (µmol l-l)

ment categories

60

0

1.0 0.5 0

Agriculture

Mixed rural

Urban

Agriculture

Mixed rural

Urban

Figure 6.3 Seasonal averages of DOC, DON and DOP in the sub-catchment categories.


53

trations of DOC leaving agricultural areas is low accumulation of organic matter in the soils due to the removal of crop. Furthermore, the agricultural sub-catchments have very low areas of forest and wetlands. As explained in Chapter 2, forest and wetlands are major sources of DOC and the highest DOC concentrations in our Horsens study were those measured in forest streams (see Fig. 3.3). The high DOC concentrations in the mixed rural sub-catchments are in accordance with this general picture. The rather effective sewage treatment in Horsens town, our only urban area, reduced the DOC concentrations to the same level as in the mixed rural sub-catchments; however, the high biodegradability at about 40% of the total (see Table 5.1) is a distinct sewerage signature.

The mixed rural areas with their forests, wetlands and shallow lakes delivered the highest concentrations of DON with a mean concentration of 155 µmol l-1. Despite that the lowest DON concentrations were found in drainage from agricultural areas, these sub-catchments are the main DON load to the fjord due to their high total area. The urban catchment with a mean of 70 µmol DON l-1 takes a position between the two other catchment types. The export of DOP showed a pattern opposite to DON. The “natural” areas with low population density (mixed rural) exported the lowest mean concentrations of DOP and contribute a rather small proportion of the total export to the estuary. Agricultural (1.5 µmol l-1) and urban subcatchments (1.7 µmol l-1) had concen-

trations about 3-fold higher than more natural areas (Fig. 6.3). Agriculture, lakes and DOM

The land use in the Horsens Fjord catchment is dominated by very intensive agriculture (about 66% of the area and near the percent for Denmark) using both chemical fertilisers and animal manure, although with a regulated management practice including loading permissions and seasonal timing. Fertilising with manure from pigs with its very high content of organic phosphorus could lead to the reasonable hypothesis, that the export of DOP and possibly also DON should be positively related with the known manure loading in the sub-catchments (the manure application is about 24 kg P ha-1 yr-1). This was not the case. Positive

Figure 6.4 Average seasonal concentrations of DOC, DON and DOP at the in- and outlet of Lake Bygholm and Lake Nørrestrand. 1.5 Inlet Outlet

Inlet Outlet

250

DOP (µmol l-l)

75

500

0

54

100

Inlet Outlet

DON (µmol l-l)

DOC (µmol l-l)

750

50 25

Bygholm

Nørrestrand

0

Bygholm

Nørrestrand

1.0

0.5

0

Bygholm

Nørrestrand



statistical relationships were neither found for DOP, DON nor DOC versus manure load. With respect to the export of nitrate and phosphate the relationships were positive, strong and significant, so manure loading is creating an export of inorganic nutrients. Mineralisation, uptake by crop and soil retention are active and keep organic leaching -not low- but unrelated statistically to the manure load. Still, relatively high DOP concentrations are leaving the agricultural areas (Fig. 6.3). In our detailed study of this catchment we also found that shallow lakes did affect the export of DOM at the catchment scale, despite low residence time (5-6 days). The two lakes are positioned in the lower reaches of the two most important streams (Fig. 6.1). The lakes Bygholm

and Nørrestrand are 0.52 and 1.2 km2, respectively and draining 310 km2, about 60% of the entire catchment. Both lakes are highly eutrophic, as expected. The lakes are neutral with respect to DOC, which did not change in concentration between in- and outlet (Fig. 6.4). However, both the concentrations of DON and DOP were affected by the lake passage. The DON concentrations increased at the expense of nitrate and the DOP levels decreased. However, at the catchment scale the allochthonous DOM sources are more important than within lake processes. The role of the two lakes may very well be beneficial for the environmental condition of the estuary, as the load of inorganic N and P and DOP to the estuary is reduced by passage through

the lakes. The loss of phosphorus must be due to sedimentation. The effect of the transformation of nitrate to DON is more difficult to interpret. Ultimately, all the nitrogen aside losses due to denitrification will become available for primary producers, but most of the DON bound nitrogen may not be released at a time scale shorter than the residence time of the estuary (about 2 week) thus affecting systems outside the estuary. The production of DON in the lakes may well be an export of the problem to somewhere else. DOM and nutrients from land to sea

We can now make a summary for the behaviour of DOM and inorganic nutrients at the largest scale of our Danish study, the entire

Figure 6.5 Annual average concentrations of DOC, total dissolved nitrogen (DON and nitrate) and total dissolved phosphorus (DOP and phosphate) from all streams and in a horizontal transect through the Fjord.

Dissolved nitrogen (µmol l-l)

DOC (µmol l-l)

500 400 300 200 100 0

300 200 100 0

Streams

Inner Outer Estuary Estuary

DIN DON

Streams

2.0 Dissolved phosphorus (µmol l-l)

400

600



DIP DOP

1.5 1.0 0.5 0

Streams


55

Horsens Fjord catchment and in the estuary. The overall picture shows a dilution of all variables moving from land to sea; not an unexpected result (Fig. 6.5). The most dynamic element is nitrate. From a very high concentration in the steams (about 310 µmol l-1) it decreases due to plant uptake, denitrification and dilution in the estuary. Likewise, phosphate decreases, but not so dramatically. The obvious thing to notice is that in the estuary most of the dissolved nutrients are bound in DOM, which now becomes a prime player in the ecosystem. The ecological effects of DOM are treated in Chapter 4. For the entire catchment we can now calculate the export to be: DOC: 200 mmol m-2 year-1; DON: 18 mmol m-2 year-1 and DOP: 0.4 mmol m-2 year-1. However, nitrate export of 150 mmol m-2 year-1 is clearly the major nutrient problem for the estuary and phosphate with 0.3 mmol m-2 year-1 is only of slightly less importance than DOP. An evaluation of export is not complete without considering nutrients bound in particles. For nitrogen the particulate fraction was low at about 6% of the total, however, for phosphorus the freshwater sources carried a high particulate load amounting to about 21% of the total. Water leaving the estuary had a high relative contribution from particulate phosphorus (54%), while PON was almost undetectable. In summary: With respect to the dissolved organic export our catch-

56

ment scale study revealed the agricultural sub-catchments to export higher DOP concentrations than mixed rural areas with a higher contribution of forest, wetlands and lakes. With respect to DOC and DON the higher concentrations were exported from the mixed rural areas. However, it is to be noted that the absolute DOM concentrations exported from land to the estuary are high when viewed at the European scale (Chapter 2, Table 2.3), but most probably representative for catchments dominated by intensive agriculture. The DOM loading to this estuary is important for phosphorus more than for nitrogen. The inorganic nitrogen and phosphorus species and the particulate phosphorus export from this catchment should be kept in managerial focus, but the DOM loading is not unimportant. The function of the estuary is to transform a nutrient load with high concentrations of inorganic species to DOM species leaving the estuary.

The River Conwy, North Wales, U.K. The Catchment

The Conwy is the third largest river to discharge into the Irish Sea from the North Wales coast. It drains a catchment of some 670 km2, the main drainage channel covering a distance of approximately 56 km (Fig. 6.6). The Conwy rises in the Snowdonia national park at around 460m above

sea level. The upper Conwy flows across upland peat moors through to grazing land, falling some 450m to emerge as the lower Conwy, which flows through extensive flood plains to meet the effective tidal limit at the town of Llanrwst. As with many North Wales rivers the Conwy can be described as “flashy”; there is near immediate runoff, with only a small proportion of rainfall being retained by the predominantly thin peaty soils of the upper catchment. Sampling Strategy

When confronted by a disparate array of ecosystems and habitats, both aquatic and terrestrial, choosing a representative sample is not a trivial matter. It is necessary to be sure that carefully collected and analysed samples both do represent the system that one wishes to describe, and that they do so objectively. The use of proxy measurements for quantification of the total DOM pool are well established, in particular, the linear relationship between DOC and coloured DOM (CDOM) is well explored (Chapter 1). As DOC is known to be the major contributor to the total dissolved organic matter pool, at least in terms of absolute amounts, its relationship to CDOM is considered a good indicator of the whole DOM pool. Such relationships permit the rapid and inexpensive assessment of DOM variability on catchment, or other large scales. Initially, 45 locations within the catchment, along the whole of the



M

5 km

E

L K J I H G F D

A

M

C

B

Llandudno Junction

Conwy Glan Conwy

L Eglwysbach

K Dolgarrog

Trefriw

J Capel Curig

Llanrwst

Figure 6.6

I

The River Conwy Catchment, North

Betws-y-Coed

Wales, UK. The letters in red circles

H

show sample sites. The brown circles

Dolwyddelan

G

Penmachno

A

Sampling sites 5 km

B

C

represent human settlements with size proportional to inhabitant density.

E D Ysbytu Ifan

Llyn Conwy Settlements

Pentrefoelas

F

Insert ‘1’ is a scaled equivalent of the whole. Insert ‘2’ is a schematic description of the sampled locations, specifically it demonstrates the relative distance from the lake at the head of the catchment, and the position of the confluence of the rivers Conwy and Eidda.


57

river system were repeatedly sampled for CDOM concentration using standard methods. If any of these locations were frequently found not to harbour statistically significantly different CDOM concentrations to their nearest neighbours, they were simply discarded. In effect, these locations were not providing any “new” information, and were therefore likely to reduce the overall efficiency of the sampling programme. In this way, 13 sites were eventually chosen to represent the whole catchment. By surveying these 13 locations approximately every 21 days over the

course of two years, a detailed picture was constructed of any inputs to, and exports from, the Conwy system. Amongst 16 analysed water quality determinants were chromophoric dissolved organic matter (CDOM), dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and dissolved organic phosphorus (DOP). Sub-Catchments

Once the 13 sites had been selected, sub-catchment boundaries for each were calculated using the Arc View Geographical Information Systems (GIS) software package (version 3.1).

By definition, the location from which a sample is taken must be the lowest point in the sub-catchment area, which that sample represents. Rain drop tracing software, written for the Arc View GIS package, enables the calculation of sub-catchment extent based upon this principle (Fig. 6.7). Essentially it calculates the area from which a drop of rain could theoretically make its way to the sample point, while taking into account any topographical features which may serve to impede flow. This procedure, by extension calculates the area drained by the water

M L

Figure 6.7 The River Conwy Catchment and Sub-catchments, North Wales, UK.

K

A – Llyn Conwy

I

B – Road Bridge C – Flood Plain

J

D – Ysbytu Ifan

I

E – The River Eidda F – Pont Padog (Conwy) H – Fairy Glen I

A B C

– Betws y Coed

J – Llanrwst K – Dolgarrog L – Tal y Cafn M – Pier

58

F&G

H

G – Pont Padog (Mixed)

E D Km

10

0

10

20



body flowing past the point from which the sample was taken. In all but one case, the 13 selected sites come from nested sub-catchments, that is, they wholly incorporate one another as one moves downstream along the main drainage channel of the river. The exception being samples representing the Afon Eidda (E, from Figs. 6.6 and 6.7). This tributary drains a topographically isolated and very small (8.75km2) sub-catchment. Table 6.1 describes the major features of the Conwy catchment and the 12 subcatchments considered by this study.

Land Use

The Conwy catchment is rural with sheep farming as the main agricultural land use both to the east and west of the main drainage channel. The lowland flood plain, downstream of Betws y Coed (I), does have some arable farming, which requires an enhanced level of drainage. To the west of Betws y Coed there is a large afforested area, Gwydyr Forest, maintained and managed independently. The forest hides many abandoned mines, which themselves leach waters laden with metals into several tributaries of the

Name

ID

Sample Alt (m)

Str Len (km)

Pop n

Llyn Conwy

A

523

1.69

0

Road Bridge

B

432

3.69

0

Flood Plain

C

390

4.01

2

Ysbytu Ifan

D

217

14.97

270

The Eidda

E

169

6.57

280

Pont Padog (Conwy)

F

170

20.33

2835

Pont Padog (Mixed)

G

169

20.45

2835

Fairy Glen

H

20

26.76

3000

Betws y Coed

I

18

28.83

3455

Llanrwst

J

5

36.88

7460

Dolgarrog

K

2

40.41

13610

Tal y Cafn

L

1

44.53

13800

Pier

M

0

55.88

28002

main river. At the seaward end of the catchment, the Conwy estuary contains several extensive salt marshes important to migratory wading birds. One of the most pervasive influences on water quality is known to be the concert effects of land use (and hence land type) and climate. Given this, the 13 chosen sites fall into six broad categories (Table 6.2). Understanding the distribution of DOM

When the information from the surveys described above was analysed, it quickly became apparent that the

Table 6.1 The River Conwy Catchment and SubCatchments, North Wales, UK. Sampling location, altitude relative to sea level (m), drainage channel length (km), and human population density (n total).


59

Sample site

Predominant influence on water quality

L, M

Estuarine

J, K

Marine influenced

F, G, H, I

Agricultural

E

The River Eidda, a distinctsubcatchment

B, C, D

Upland peat

A

The source lake, Llyn Conwy

Table 6.2 The River Conwy catchment and subcatchments, North Wales, UK. Broad descriptive categories of the predominant aquatic and terrestrial variables affecting DOM dynamics.

data described a dilution phenomenon where the majority of the DOM, particularly DOC, is exported from a single area, the high catchment peat moors, via the river, to the coast (Fig. 6.8). In order to understand the distribution, both through space, and time (spatial and temporal), of DOM at catchment or other large scales it is often instructive to use a technique that enables the incorporation of all the measured parameters concurrently. These methods allow the best possible descriptive resolution from collected data. The output from such multivariate techniques, however, must be intuitive. Meaning that it should be both easy to generally understand, and if it is to be used for the purposes of longer time scale monitoring, the output must candidly reflect change. Such a technique was developed by implementing a slight, but very instructive, alteration to a firmly established

method called cluster analysis (Box 6.1). From the outcome of the cluster analysis and the developed dendrogram (Fig. 6.9) we can now see that the six final clusters represent accurately, the whole of the river and its catchment, in a single, easily comprehensible, two-dimensional form. Sites M and L are given their own grouping, principally due to the extent to which they are influenced by the sea. Sites K and J are still influenced by the marine environment, but to a far lesser extent, and as such have formed their own cluster. Sites I, H, G and F represent areas where the land use adjacent to the river is mostly agricultural, and have thus been classified together. Site E denotes the River Eidda, a tributary of the Conwy, which carries much lower concentrations of DOM. Its own catchment is geographically isolated from those upstream of it along the main drainage channel

800

Year One Year Two

700

DOC (µmol l-1)

600

Figure 6.8

400

200

relationship between DOC and distance

100

the negative linear relationships between the two variables.

R2 = 0.903

300

The River Conwy, North Wales, UK. The from the source (km). The lines represent

60

500

0

R2 = 0.904

0

10

20 30 40 Distance from source (km)

50

60



and as such has been awarded its own cluster. Site A is the source lake, and again represents a biogeochemically distinct system. Finally the upland peat system, represented by sites D, C, and B is grouped separately. It is noteworthy that this is the grouping, which is most distinct from any other. This is an example of how apparently complex data sets involving several multi-parameter relationships can be reduced to simple diagrammatic representations. Here, for demonstrative purposes we have highlighted only the spatial distribution of DOM, however the method is also easily applicable to seasonal studies. Within systems where the relationship between export and deliv-

ery to the coast by rivers is not clear, or changes seasonally, the developed method may be employed. Now the data could, for example, be split into seasons, or rainfall could become one of the parameters used in the model. Thus, simply by observing the change in the shape of the dendrogram, subtle multivariate changes in the biogeochemical characteristics of the system can easily be tracked. This method is also useful for relating any changes to differing land use practices, and by extension, in linking cause and effect. As the technique involves only the collection and subsequent analysis of water samples, it is entirely objective. In fact, by the very nature of the analysis it cannot be influenced by the an-

alyst as ultimately all that the model does is to group together collections of numbers, which change at similar rates relative to one another. Perhaps the biggest advantage that the technique has over purely statistical methods is that it does not require expert knowledge. It only requires, in the first instance at least, the ability to recognise changes in the shape of the dendrogram produced. In the event of such change further work can then be initiated in a directed and efficient manner. In summary: Relatively simple methods can be very valuable in the analysis of large scale distributions of DOM. They contribute positively both to the efficiency of sample collection, and subsequent statistical

25.93

% Similarity

50.62

Figure 6.9 The River Conwy catchment and sub75.31

catchments, North Wales, UK. The dendrogram from the Cluster analysis, produced by normalised correlation matrices, represents the six broad land

100.00

use categories, and therefore the six

M

L

K

J

I

H G F E Sample locations

A

D

C

B

major controls on DOM concentration in all of the sub-catchments.


61

treatment of the gathered data. It must be remembered, however, that although the cluster analysis enable the bigger picture to always remain

in focus, more conventional statistical methods, such as multiple linear regression should not be discounted. Rather the technique described here

should serve to efficiently direct the analysis of large data sets such as those collected by workers in the field of water quality management.

Box 6.1 Cluster analysis of DOM distribution Cluster analysis is based on the premise that the ratios of

distinct land use types throughout the catchment (Table

the concentration of chemical elements of interest in two

6.2), and therefore the task of specifying the final number

or more samples are better indicators of the common ori-

of clusters was simple, namely six.

gin of the samples than the concentrations themselves.

Covariate and linear relationships between parameters

The method involves a hierarchical process that begins

are often found within carefully collected biogeochemical

with the assumption that all observations are separate,

data sets, a good example being the linear relationship

and therefore each forms its own cluster.

between CDOM and DOC (Chapter 1). When using multi-

In the first step, the two locations that are most similar

62

variate statistical methods, which concurrently analyse the

in terms of the relative concentrations of their dissolved

differences between many parameters, these relation-

constituents are joined. In the next step, either a third lo-

ships, but more particularly their effect on the analysis, is

cation joins the first two or two other locations join to-

often ignored. While analysts take great care to avoid in-

gether to form a different cluster. This process will

troducing bias due to, for example, differences in scale,

continue until eventually all clusters are joined into one.

very few consider the skewing effects of correlations

This is of no value and it is therefore necessary to either,

within the data set. This can be removed by the use of

specify the final number of clusters, or, to specify the level

correlation matrices, which effectively negate the effect

of similarity at which clusters are considered comparable

of this bias on the analysis; the clusters from the cluster

enough to be joined. There are no mathematical rules,

analysis then become truly meaningful (Fig. 6.9). In order

which guide this process. By extension is it therefore nec-

to generate the dendrogram shown in Fig. 6.9, the graph-

essary to have some knowledge of the system under con-

ical representation of the clustering process, concentra-

sideration. It must be stated here that this method has no

tion values relating to the following determinants were

basis in statistical proof, but rather guides the analysis in

used; CDOM, DOC, DON, DOP, ammonium, nitrite, nitrate,

an unbiased way, such that locations deserving of further

phosphate, silicate, and Chlorophyll. All of which are im-

data mining can be easily identified, as can broad yet spe-

portant, dynamic and commonly assessed water quality

cific patterns within the data. There are six obvious and

parameters.


Chapter

7

Dissolved matter (DOM). DOM andorganic land use What is it and why study it? management

Anker Laubel Torben Vang Morten Søndergaard Pirkko Kortelainen Anders Windelin

E

xcessive loads of nutrients whether inorganic or bound in DOM have the potential to harm aquatic ecosystems (Chapter 1). Inorganic nutrients are readily available for plant and algal uptake while nutrients in DOM have to be mineralised by microbes and/or light before they become a major water quality problem. Thus, it is necessary to include a time scale for mineralisation before evaluating the effects of nutrients in DOM. Water quality management of coastal waters and lakes strives to control nutrients from point sources (e.g. sewerage) and the terrestrial export via diffuse sources. Control at point sources is relatively easy, although not without

problems for nitrogen removal. The control of terrestrial export from diffuse sources has the potential for conflict between intensive agricultural production and water quality. The EU Water Framework Directive has the wellbeing of ecosystems as its main quality criterion. Therefore it is necessary from now on, not only to focus on export quantities, but also to analyse how nutrients (both inorganic and organic) and other substances such as CDOM and oxygen demand in DOC may affect coastal water quality. Land use management and management practise at large geographical scales are the only options for such an enterprise. The need for future environmental


63

management to operate at catchment scales and to include all (most) environmental impacts is one reason why we in the DOMAINE project have combined studies of DOM and inorganic nutrient export from land to sea (Chapters 2 and 6) with studies on the fate (Chapters 3 and 5) and effects (Chapter 4) of DOM. By detailed studies at catchment scales and knowledge of land use and DOM export we collected empirical data for use in predicting how changes in land use and management practices would affect DOM export and ultimately the ecological quality of receiving coastal waters. As explained in Chapter 4 both allochthonous and autochthonous DOM have multiple effects in aquatic ecosystems. Nutrients, oxygen demand and light attenuation are among the most important, however, management regulations only with an eye on DOM is too narrow a view. Changes in land use have im-

plications for all nutrient species whether inorganic or organic. It might be beneficial for one system to have a change from an inorganic to an organic load, but this is not necessarily the case for every system. Here we shall use the data presented in the previous chapters and specifically in Chapter 6 to explore how a change in land use can affect the DOM and nutrient export from a catchment dominated by intensive agriculture.

Management, water quality and ecological conditions

When introducing new land use or management practise it is important that all effects are considered. This seems an obvious statement, however, we still do not understand all the consequences on freshwater quality from a number of land use and management conditions. The focus is often restricted to

one substance (e.g. nitrate) that is assumed to impose the main negative environmental influence. In some cases, the implementation of new management changes may in fact turn out to have unwanted side effects. One lesson came from the introduction of “winter green fields” in Denmark. Since the late 1980s it has been allowed and encouraged to start a new crop in autumn. The purpose was to decrease the leaching of nitrate during winter and early spring. However, it soon became evident that soil erosion and phosphorus export increased and posed a threat to phosphorus limited lakes and coastal waters. Thus, it is important to understand and quantify the overall impact before implementing major land use and management changes.

Land use, DOM and inorganic nutrients

A detailed study of the sub-

Box 7.1 Link between management, freshwater input, and environmental conditions of water bodies Land use and management practise influence the quan-

ticulate matter, DOM etc. The fundamental issue is to de-

tity, quality, and timing of water supplied to surface wa-

fine the wanted ecological quality (The Water Framework

ter bodies. Freshwater input criteria can include concen-

Directive) and then work backwards to establish a land use

trations and loads of nitrate, phosphate, suspended par-

and management practice leading to the goal.

Land use and management practice

64

Freshwater input: quality, quantity and timing

Environmental condition of a water body


Chapter 7 DOM and land use management

catchments supplying freshwater to Horsens Fjord showed that different combinations of land use resulted in different export concentrations of DOC and nutrients as well as in the relative distribution between dissolved organic and dissolved inorganic nutrients (Chapter 6). In summary: mixed rural catchments with forests and wetlands deliver high concentrations of DOC and DON but low DOP and inorganic nutrients, while intensive agricultural activity delivers very high nitrate and DOP and somewhat lower concentrations of DOC and DON. It was also shown that lakes

could reduce the delivery of phosphorus from land to sea. This picture is for an agricultural landscape and is a somewhat biased view, at the larger European scale, due to the fact that none of the investigated subcatchments had agricultural land cover lower than 40% of the area. In the DOMAINE project we have extended the analysis of land use and export to 34 catchments and subcatchments positioned in a northsouth and a west-east gradient covering North Wales, most of southern Finland, a mountainous catchment in southern France and a low land agricultural area in Denmark. The

relationship between dominant categories of land use and basic natural variables and the concentrations of DOC and dissolved nutrients can be analysed by a Rank Correlation Analysis (Box 7.2). The quantitative data are presented in Chapter 2. The relationships presented in Box 7.2 must be seen as a generalised picture and only used as a first step to understand how land use affects the DOM export in different areas. A detailed and proper evaluation has to include very specific catchment features to be valid. One example is the function of lakes where residence time and exposure time of

Box 7.2 Relationships between land use and the concentrations of dissolved organic and inorganic species in 34 European catchments In the Rank correlation analysis the

Independent variable

strength of the relationships is given by a value from 0 to 1 and a positive or negative relationship is indicated by sign. Single + or – values indicate a weak relationship. Both land use and basic natural conditions affected DOM and inorganic nutrient concentrations. Wetlands, lakes and forests had the most important positive relationship with DOC and negative for DOP,

DOC

Agricultural areas

-0.37

+

+0.67

+0.46

Managed forest

+0.51

+

-0.52

+

Wetland

+0.67

+

-0.37

Lakes

+0.66

Dry natural areas with no forest

-0.37

Urban areas Population density

density showed high positive relationships

Basic natural conditions

for most nutrient species.

Sandy soils Runoff

-0.78

descriptors of basic conditions resulting in

Temperature

-0.77

both positive and negative relationships.

Days with intense rain

and temperature (annual averages) are

DIP

Land use

while agriculture and human population

Soil type, runoff, precipitation events

Dissolved species DON DOP DIN


+0.37

–

-0.41

–

-0.69

–

-0.91

+

+0.38

+0.71

+0.54

+0.40

+0.61

+0.84

+0.52

+0.65

+0.79

–

– +0.46

+0.57

+0.53

-0.44

+ +0.56

65

DOM to light are important specific features. In Finland many lakes are actually sinks for DOC and humic matter and not producers. Agriculture is a major land use activity in large parts of Europe and the overall negative ecological effects of chemical fertilisers and high loads of manure with eutrophication of coastal systems as a consequence are well known. Regulation of nitrogen fertilisation is a common tool to reduce DIN export to surface waters. The concern regarding phosphorus fertilisation has grown stronger over the last one or two decades, and it has been documented that not only an increase in particulate but also in dissolved phosphorus is often associated with agricultural activity. This is also obvious from the empirical data presented in Figure 6.3. The data from DOMAINE present an opportunity to create scenarios of changing land use and to predict the quantitative effects on the quality of freshwater export. As high nutrient export from arable land is a major problem for coastal waters at the European scale, we shall here present an example on how a change in land use can work.

Changing land use: The Horsens Fjord catchment

The Horsens Fjord catchment is an agricultural landscape with low coverage of forest, wetlands and lakes and can be used as a model catchment for Denmark and other European areas with intensive agriculture. Current trends in manage-

66

ment of land use – partly regulated by EU- are to reduce the area of arable land and replace with new forests, wetlands and occasionally reconstruction of lakes and meandering streams. One realistic scenario would be to change the agricultural landscape toward a mixed rural landscape by reducing the area of arable land and urban settlements by 25 and 3% respectively, and replace with 25% forest, 2% wetland and 1% lake area. From the results presented in Chapter 6 (see Fig. 6.2) the quantitative outcome of such a change can be evaluated (Box 7.3). It is pertinent to address the question: What happens now to the export of DOM and inorganic nutrients and is the change only beneficial for the water quality in the estuary? The two most important features in this land use change scenario are the reduction in phosphorus export and a 25% decrease in the export of nitrate, but now exported as DON. With respect to phosphorus the reduction would be further enhanced by a reduction in the particulate phosphorus load mainly caused by high erosion when farmers in the autumn are preparing for “winter green fields“. This reduction is not included in the scenario. The terrestrial load of nitrate is the main reason for the eutrophic status of the Horsens estuary (Vejle County environmental assessments). Thus, a reduction in nitrate by 25% could lead to a considerable improvement in the status of the estuarine

ecosystem due to a reduction in phytoplankton biomass, less sedimentation, better light climate and a lower probability of oxygen depletion in the bottom waters. The export of total nitrogen would not decline, but nitrate would be exchanged with relatively recalcitrant (even refractory) terrestrial DON. Microbial and photochemical degradation processes would gradually release nitrogen to the ecosystem during its passage through the estuary and coastal waters and ultimately become available for biological production. However, the released nitrogen would be continuously diluted during coastal mixing and more importantly, with an increasing fraction lost by denitrification over time. The scenario for land use change predicts, with respect to nitrogen and phosphorus, an improvement in estuarine ecosystem quality. Changing agricultural areas with forest and wetlands will increase the export of DOC and therefore coloured compounds (CDOM) absorbing light in the water column (Chapters 1 and 4). An increase in terrestrial DOC of about 15% would cause a 20% increase in CDOM. Two consequences are apparent: 1) DOC consumes oxygen during degradation and thus would result in an increased oxygen demand in the estuary. 2) DOC absorbs light and would reduce the depth at which primary production can take place.


Chapter 7 DOM and land use management

These are both negative environmental effects. For Horsens Fjord we evaluate the positive effects to be more important than the negative, but cannot at this stage provide quantitative arguments. The oxygen consumption may be delayed and exported out of the estuary. Furthermore, in a stratified estuary the input of freshwater floats at the surface and the effect of an imported terrestrial oxygen demand is probably minor (Chapter 4). Photochemical bleaching of CDOM over time may counteract the negative effects of light absorption. It can be argued that the role of DOM exported from the terrestrial

environment in this estuary is rather limited due to the massive impact of intensive agricultural activity in the catchment. However, we want to emphasize that land use and managerial practice have major impact on the export of both DOM and inorganic nutrients and that a receiving estuary filters away most of the inorganic nutrients and export these as DOM. This is just one more argument that it is necessary to gain an even better understanding of the dynamics and characteristics of DOM moving from land to sea.

Conclusions

port of nutrients to coastal waters. Land use has a major impact on both the amount and the relative distribution of the nutrient species. A change from intensive agriculture toward more forest and wetlands generally changes the export from inorganic to organic species and reduces the phosphorus export. At the same time more DOC and coloured DOM are exported. Such a change will be beneficial for coastal waters with catchments dominated by intensive agriculture. In the future we need a better handle on the fate of DOM to reach an ability to quantify effects at appropriate time scales.

DOM is an integrated part of the ex-

Box 7.3 Changing agriculture to forest and wetlands in the Horsens Fjord catchment The increase in forested areas by 25%, wetlands by 2%

the distribution between DON and DIN changed dramati-

and lakes by 1% at the expense of agriculture (25%) and

cally. About 25% of the very high nitrate export was re-

minor urban settlements (3%) and without a change in

placed by DON. The land use change resulted in a major

the present regulation of farming practise will have a

reduction in dissolved phosphorus and a minor change in

measurable impact on the export of all dissolved sub-

the relative distribution between DIP and DOP. Sedimen-

stances. Dissolved organic carbon (DOC) increased, while

tation of phosphorus in the lakes makes a significant

the total quantity of nitrogen did not change. However,

(20%) contribution to the effect on phosphorus export.

200 N export (mmol m-2 yr-1)

DOC export (mmol m-2 yr-1)

300 250 200 150 100 50 0

Agriculture Mixed rural

160 120 80 40 0

0.8

DIN export DON export

P export (mmol m-2 yr-1)

350

0.6

DIP export DOP export

0.4 0.2


0


Figure showing export of DOC, DON and DIN, and DOP and DIP.


67

Photo 7.1 Agricultural fields – the Horsens catchment (photo by Stiig Markager).

Box 7.4 Converting mol to weight In limnology there is a tradition to use concentrations in weight units; e.g. phosphate is presented in µg or mg P l-1. However, organisms respond to the concentrations of molecules and not to their weight. Thus, we have used molar units. It is simple to convert from one to the other using the atomic weight of the element. The area unit for land use and export analyses are often presented in hectares (1 ha = 10,000 m2) and not in m2 as used in aquatic ecology. The conversions are shown in the following table. Dissolved organic carbon, nitrogen and phosphorus = DOC/DON/DOP. Dissolved inorganic nitrogen and phosphorus = DIN/DIP

Concentration units mmol l-1 mg l-1 (element)

68

Export units mol m-2 yr-1 kg ha-1 yr-1 (element)

DOC

1

=

0.012

1

=

120

DON/DIN

1

=

0.014

1

=

140

DOP/DIP

1

=

0.031

1

=

310


Suggested further reading

Books & websites Anonymous. 2004. Multivariate Statistics – An Introduction. http:// trochim.human.cornell.edu/tutorial/ flynn/multivar.htm European Environment Agency. 2003.

Dobris Assessment. European Environment Agency, Copenhagen. Thurman, E.M. 1985. Organic geochem-

1994. A review of the export of

hoff, Dordrecht.

carbon in river water – fluxes and

Wetzel, R.G. 2001. Limnology – Lake and River Ecosystems. 3rd edition,

assessment. Environmental assess-

Academic Press. Williams, P.J leB., Thomas, D.N. and

Office for Official Publications of the

Reynolds, C.S. (eds.). 2002. Phyto-

European Communities.

plankton Productivity. Blackwell

Findlay, S.E.G and Sinsabaugh, R.L (eds.) 2003. Aquatic ecosystems: Interactiv-

Publishing, Oxford.

ity of dissolved organic matter. Aca-

Articles & book chapters

demic Press, New York.

Alber, M. 2002. A conceptual model of

Hansell, D.A. and Carlson, C. A. (eds.) 2003. Biogeochemistry of marine dissolved organic matter. Academic Press, Amsterdam.

Progress Series 43: 1-10. Hope, D., Billett, M.F. and Cresser, M.S.

istry of natural waters. Martinus Nij-

Europe’s’ Environment: the third ment report No 10. Luxembourg:

system overview. Marine Ecology

estuarine freshwater inflow management. Estuaries, 25, 1246-1261. Azam F., Fenchel T., Field, J.G., Gray,

processes. Environmental Pollution 84, 301-324. Hopkinson, Jr C.S., Vallino, J.J. and Nolin, A. 2002. Decomposition of dissolved organic matter from the continental margin. Deep-Sea Research II, 49, 4461-4478. Kalbitz, K., Solinger, S., Park, J.-H., Michalzik, B. and Matzner, E. 2000. Controls on the dynamics of dissolved organic matter in soils: a review. Soil Science, 165, 277-304. Kortelainen, P., Mattsson, T., Rantakari,

J.S., Meyer-Reil, L.A. and Thingstad,

M and Räike A. 2004. Organic carbon

F. 1983. The ecological role of water-

concentrations in Finnish lakes and

1994. European Rivers and Lakes: As-

column microbes in the sea. Marine

rivers. In Eloranta P (ed.). Finnish

sessment of their environmental

Ecology Progress Series, 10, 257-263.

inland and coastal waters. Palmenia

state. EEA Environmental Mono-

Blough, N.V. and Zepp, R.G. 1995. Reac-

Kristensen, P. and Hansen, H.O. (eds.)

graphs 1, European Environment

tive Oxygen Species in Natural Wa-

Agency, Copenhagen.

ters. In Active Oxygen Chemistry (C.S.

Publishing, University of Helsinki, Saarijärvi, Finland (In press). Kortelainen, P. and Saukkonen, S. 1998.

Kirk, J.T.O. 1994. Light and photosyn-

Foote, J.S. Pacentine, A. Greenberg

Leaching of nutrients, organic carbon

thesis in aquatic ecosystems. Cam-

and J.F. Liebman Eds.), pp. 280-333.

and iron from Finnish forestry land.

bridge University Press.

Chapman and Hall, New York.

Water, Air, and Soil Pollution, 105,

Pelkonen, P., Pitkänen, A., Schmidt, P.

Bowers, D.G., Evans, D.W., Thomas,

239-250.

Oesten, G., Piussi, P. and Rojas, E.

D.N., Ellis, K. and Williams, P. J leB.

(eds.) 1999. Forestry in Changing So-

2004. Interpreting the colour of an

Production and bioavailability of au-

cieties in Europe, Part II. Silva Net-

estuary. Estuarine and Coastal Shelf

tochthonous DOC: Effects of meso-

work.

Science, 59, 13-20.

zooplankton. Aquatic Microbial Eco-

Perdue, E.M. and Gjessing, E.T. (eds.)

Chantigny, M. H. 2003. Dissolved and

Kragh, T. & Søndergaard, M. 2004.

logy (In press).

1990. Organic acids in aquatic ecosys-

water-extractable organic matter in

tems. John Wiley & Sons, Chichester

soils: a review on the influence of

Light requirements and depth zona-

land use and management practices.

tion of marine macroalgae. Marine

Riemann, B. and Søndergaard M. (eds.) 1986. Carbon dynamics in eutrophic, temperate lakes. Elsevier, Amsterdam. Stanners, D. and Bourdeau, P. (eds.) 1995. Europe’s Environment – the

Geoderma 113, 357-380. Cole, J.J., Findlay S. and Pace M.C. 1988.

Markager, S. and Sand-Jensen, K. 1992.

Ecology Progress Series, 88, 83-92. Mattsson, T., Finer, L., Kortelainen, P.

Bacterial production in freshwater

and Sallantaus, T. 2003. Brook water

and saltwater ecosystems: a cross-

quality and background leaching


69

from unmanaged forested catch-

Stedmon, C. A., Markager, S., Borch, N.

ments in Finland. Water, Air, and Soil

The optics of chromophoric dissol-

H., Laubel, A., Søndergaard, M.,

Pollution, 147, 275-297.

ved organic matter (CDOM) in the

Vang, T. and Windelin, A. 2004. Dis-

Greenland Sea: An algorithm for dif-

solved organic matter (DOM) export

terns in dissolved organic carbon

ferentiation between marine and

to a temperate estuary: Seasonal var-

concentration, flux, and sources. In:

terrestrially derived organic matter.

iations and implications of land use.

Aquatic ecosystems: interactivity of

Limnol. Oceanogr. 46(8): 2087-2093.

Mulholland, P.J. 2003. Large-scale pat-

dissolved organic matter (Eds.

Stedmon, C. A. and Markager, S. 2003.

(Submitted) Søndergaard, M., Stedmon, C.A. and

Findlay, S.E.G. and Sinsabaugh, R.L.),

Behaviour of the optical properties

Borch, N.H. 2004. Fate of terrigenous

p. 139-159. Academic Press.

of Coloured Dissolved Organic Mat-

dissolved organic matter (DOM) in

ter (CDOM) under conservative mix-

estuaries: Aggregation and bioavaila-

ing. Estuarine Coastal and Shelf

bility. Ophelia, 57, 161-176.

Pomeroy, L.R. 1974. The ocean’s food web, a changing paradigm. Bioscience, 24, 499-504.

Science 57: 973-979.

Prepas, E.E., Planas, D.,Gibson, J.J.,Vitt,


Søndergaard, M., Williams, P.J leB., Cauwet, G., Riemann, B., Robinson, C.,

D.H., Prowse, T.D, Dinsmore, W.P.,

Resolving compositional changes in

Terzic, S., Woodward, E.M.S. and

Halsey, L.A., McEachern, P.M., Paquet,

dissolved organic matter (DOM) in a

Worm, J. 2000. Net accumulation and

S., Scrimgeour, G.J., Tonn, W.M.,

temperate estuary and its catchment,

flux of dissolved organic carbon and

Paszkowski, C.A., and Wolfstein, K.

using spectrofluorometry and PARA-

dissolved organic nitrogen in marine

2001. Landscape variables influenc-

FAC analysis. (Submitted).

plankton communities. Limnology

ing nutrients and phytoplankton


and Oceanography, 45, 1097-1111.

communities in Boreal Plain lakes of

Tracing the production and degrada-

northern Alberta: a comparison of

tion of autochthonous fractions of

models in the context of microbial

wetland- and upland-dominated

dissolved organic matter (DOM) us-

food webs. In S. Finley and R. L.

catchments. Canadian Journal of

ing optical analysis. (Submitted).

Sinsabaugh (eds.); Aquatic Ecosys-

Fisheries and Aquatic Science, 58,

Stedmon, C.A. Markager, S. and Bro, R.

1286-1299.

2003. Tracing dissolved organic mat-

Sholkovitz, E.R. 1976. Flocculation of

Thingstad, T. F. 2002. Physiological

tems and Dissolved Organic Matter. Academic Press. p. 383 –395.

ter in aquatic environments using a

Williams, P.J leB. 1981. Incorporation of

dissolved organic and inorganic mat-

new approach to fluorescence spec-

microheterotrophic processes into

ter during the mixing of river water

troscopy. Marine Chemistry, 82,

the classical paradigm of the plank-

and seawater. Geochimica Cosmo-

239-254.

tonic food web. Kieler Meeresforc-

chimica Acta, 40, 831-845.

70


hung Sonderheft, 5, 1-28.


Glossary

Algae: Aquatic photosynthetic organ-

signed to show proposed relationships

trient poor and low productivity to high

isms that vary in size from less than 1µm

between groups of data such as sam-

nutrient concentrations and high pro-

to seaweeds over 50 m long. Algae are

pling sites within a catchment.

ductivity. This can be from the addition

plants having a quite different evolu-

Denitrification: The formation of re-

of nutrients, changing the residence

tional history, although many of the

duced nitrogen compounds (gaseous

time of a body of water, or other com-

photosynthetic and nutrient uptake

N2) from nitrate. Denitrification is a

plex interactions.

processes are similar to those of the

microbial respiration process that takes

Export: The amount of a substance

green plants. All lack roots, stems and

place in wet soil and sediment when

transported by a river or water body.

leaves, and no algae produce flowers or

oxygen is not available, but nitrate is.

Loosely, the term export has the same

seeds.

The common source of energy used by

meaning as the transport, the load, the

Allochthonous: Matter imported from

denitrification is organic matter, or in

loss, or the yield of a river. For instance

another system, e.g. terrestrial DOM in

some cases pyrite.

expressed as moles m-2 yr-1.

aquatic systems.

Diatom: Unicellular algae with a cell

Fluorescence: Emission of light (pho-

Attenuation: A decrease in the energy of

wall or frustule of silica. Adapted to a

tons) after a molecule has absorbed

light due to absorption and scattering in

wide range of pelagic and benthic hab-

light energy at a lower wavelength.

the water.

itats.

Half-life: The time it takes to degrade

Autochthonous: Matter produced within

DIN: Abbreviation used for dissolved in-

a substance or compound to half of the

a given system.

organic nitrogen; mainly nitrate, ammo-

starting amount or concentration.

Autotrophy: The capacity to produce or-

nia and nitrite.

Heterotrophy: Non autotrophic nutri-

ganic compounds from inorganic com-

DIP: Abbreviation used for dissolved in-

tion (see autotrophy).

pounds using light energy (photoauto-

organic phosphorus, i.e. phosphate.

Humic: Of, or relating to, or derived

trophs) or chemical energy (chemoauto-

Discharge: Quantity of water passing

from humus. The term humus is used by

trophs).

a certain cross section per unit of time.

some soil scientists synonymously with

Bacterioplankton: Bacteria cells in the

DOM: Abbreviation used for dissolved

soil organic matter (all organic material

plankton, e.g. comparable to phyto-

organic matter. DOC, DON and DOP are

in the soil) including humic substances.

plankton

the abbreviations used for dissolve or-

Humic substances: A series of relatively

Catchment: The area draining naturally

ganic carbon, dissolved organic nitro-

high-molecular-weight, brown to black

to a water course or to a given point.

gen and dissolved organic phosphorus,

colored substances formed by secondary

The total area from which a single river

respectively.

synthesis reactions. The term is used as

or estuary collects surface runoff. A

Ectoenzymes: Hydrolytic enzymes pro-

a generic name to describe colored mate-

catchment is composed of many sub-

duced and released by bacteria and al-

rial or its fractions obtained on the ba-

catchments that contribute to the cha-

gae that are active outside the cell

sis of solubility characteristics, including

racteristics of the whole.

membrane.

humic acids, fulvic acids and humins.

Cluster analysis: A statistical analysis

Eutrophic: Water that contains high

Lysis: Dissolution or destruction of cells.

which allows the description of like-

concentrations of inorganic nutrients

Macrophytes: Large visable aquatic

nesses in large data sets comprised of

and typically supports high primary

plants, mostly used for angiosperms.

many parameters.

production and plankton biomass. Spe-

Microbial loop: The regeneration of

Concentration: Number of molecules of

cies diversity in eutrophic waters tends

nutrients and their return to the food

a substance in a given volume. Generally

to be low.

chain that is mediated by bacteria and

Eutrophication: The process by which

protozoans.

natural waters are converted from nu-

Mineralisation: See remineralisation.

expressed as moles per litre = mol

l-1.

Dendrogram: A tree-like diagram de-


71

72

Multivariate Statistics: A technique en-

net amount of organic carbon that re-

cause it is a weight divided by another

abling the researcher to examine the

sults from photosynthesis. It is usually

weight it is in fact a unit-less unit, but

patterns of relationships within data si-

expressed per unit volume or area and

often presented as ppt = parts per thou-

multaneously. These methods are often

per unit of time.

sand.

used to summarise data and to reduce

Protozoa: Unicellular animals.

Trophic level: The nutritional position

the number of variables necessary to

Redfield ratio: The ratio between orga-

occupied by an organism in a food chain

describe it.

nic carbon, nitrogen and phosphorus in

or food web.

Nutrient: Elements, inorganic or orga-

plankton. It is reported as 106:16:1

Turn over time: The time it takes to con-

nic compounds or ions used primarily in

(carbon:nitrogen:phosphorus), and is a

sume the amount of a given substance

the nutrition of primary producers, e.g

generally accepted ration for these ele-

present, e.g. the concentration of glu-

carbon, nitrogen and phosphorus.

ments in planktonic organisms.

cose is 10 nM and the rate of bacterial

Oligotrophic: Water that is low in nutri-

Remineralisation: Sometimes called mi-

uptake is 2 nM hour-1, then the turn over

ents and subsequently low in primary

neralisation. Transformation of elements

time is 5 hours.

production and plankton biomass. Typi-

from organic to inorganic form, e.g. the

Water Framework Directive: EU direc-

cally high in plankton species diversity.

conversion of organic carbon to inorga-

tive for the protection of water quality

PARAFAC: A statistical method to search

nic carbon.

and with guidelines for how it should

and identify patterns.

Respiration: A metabolic process carried

be achieved

Periphyton: Alga, bacteria and associated

out by all organisms in which organic

Wetland: An area dominated by water

microorganisms growing attached to any

substances are broken down to yield

but often shallower than a lake. Its for-

submerged surface.

energy. It is the opposite reaction to

mation has been dominated by water,

Phytoplankton: Floating or swimming

photosynthesis and results in the release

and its processes and characteristics are

mostly microscopic algae.

of carbon dioxide.

largely controlled by water. A wetland

Plankton: Organisms that are suspended

Runoff: The proportion of precipitation

is a place that has been wet enough for

or swim in the water column. These or-

that flows towards the stream on the

a long time to develop specially adapted

ganisms are not capable of swimming

ground surface or within the soil (per

vegetation and other organisms. The

against a water current, but rather drift

unit of area in a specified time).

soil is always saturated with water.

with water masses.

Salinity: The number of grams of salts

They include areas of fen or peatland

Primary production/ productivity: The

dissolved in 1000 grams of water. Be-

with permanent or temporary water.


(Blank page)

Dissolved Organic Matter (DOM) in Aquatic Ecosystems: A Study of European Catchments and Coastal Waters Edited by Morten Søndergaard & David N. Thomas

Finland Partner 2 Bergen Partner 5

Bangor Partner 6

Norway

Finland

Horsens Denmark Partners 1-3-7

Wales

Prepared by Pascal Conan

France Banyuls-sur-Mer Partner 4

Substantial amounts of nutrients are leaving terrestrial environments as dissolved organic matter (DOM). Neglecting the effects of DOM in coastal waters could lead to environmentally damaging management strategies. Here we summarise the results concerning DOM export from four selected European catchments and advocate, why we find it important to study DOM. www. domaine.ku.dk ISBN 87-89143-25-6