Biogeochemistry DOI 10.1007/s10533-012-9760-x
Depositional fluxes and sources of particulate carbon and nitrogen in natural lakes and a young boreal reservoir in Northern Que´bec Cristian R. Teodoru • Paul A. del Giorgio Yves T. Prairie • Annick St-Pierre
•
Received: 22 January 2012 / Accepted: 6 July 2012 Ó Springer Science+Business Media B.V. 2012
Abstract We investigated the depositional trends of total particles, carbon and nitrogen in a newly created, 600-km2 hydroelectric reservoir in Northern Que´bec, and compared the results with those observed in lakes of the surrounding region. We show that particulate fluxes exhibit a large degree of spatial heterogeneity in both the reservoir (68–548 mg POC m-2 d-1 and 5–33 mg PN m-2 d-1) and the natural lakes (30–150 mg POC m-2 d-1 and 3–12 mg PN m-2 d-1) and that on average, settling fluxes of the reservoir (211 ± 46 mg POC m-2 d-1 and 14 ± 3 mg PN m-2 d-1) exceeded lake deposition (79 ± 13 mg POC m-2 d-1 and 7 ± 1 mg PN m-2 d-1) by approximately two-fold. Our results also show that the nature of the organic matter reaching the sediments was significantly different between lakes and the reservoir, which can have consequences for benthic Electronic supplementary material The online version of this article (doi:10.1007/s10533-012-9760-x) contains supplementary material, which is available to authorized users. C. R. Teodoru (&) P. A. del Giorgio Y. T. Prairie A. St-Pierre De´partement des Sciences Biologiques, Universite´ du Que´bec a` Montre`al, CP 8888, Succ. Centre Ville, Montre`al, QC H3C 3P8, Canada e-mail:
[email protected] C. R. Teodoru Earth and Environmental Science Department, Division of Soil and Water Management, Katholieke Universiteit Leuven, Celestijnenlaan 200E, Box 2411, 3001 Leuven, Belgium
metabolism and the long-term storage. We found that sinking fluxes in the reservoir were mostly regulated by local morphological and hydrological conditions, with higher fluxes along or in the vicinity of the old riverbed (average 400 ± 73 mg POC m-2 d-1 and 24 ± 5 mg PN m-2 d-1) and lower fluxes in calmer zones such as side bays (average 106 ± 10 mg POC m-2 d-1 and 8 ± 1 mg PN m-2 d-1). In lakes, where settling fluxes were not linked to the trophy, or dissolved organic carbon, the actual nature of the sedimenting organic material was influenced by lake morphometry and the relative contribution of algal versus terrestrial sources. We conclude that re-suspension and erosion play a major role in shaping the reservoir sinking fluxes which explain both, the higher reservoir deposition and also some of the qualitative differences between the two systems. Despite all these differences, sinking particulate organic carbon fluxes were small and surprisingly similar relative to the surface carbon dioxide emissions in both the reservoir and lakes, representing approximately 16–17 % of the carbon efflux estimated for these same systems in 2008. Keywords Boreal ecosystem Carbon deposition Lake Particulate flux Reservoir Sediment trap
Introduction Freshwater ecosystems, primarily rivers, natural lakes, and artificial reservoirs, are transitional zones between
123
Biogeochemistry
the land and the ocean acting as funnels of terrestrially-derived materials. They are capable of processing, storing and delivering to the coastal seas large quantities of both organic and inorganic material originating from the surrounding terrestrial landscape, and therefore they are likely to play key roles in global carbon (C) cycling. Nevertheless, their contribution to C budgets remains unclear as inland waters have been rarely incorporated into terrestrial C models. Artificial reservoirs are of particular interest for regional and global carbon budgets as mounting evidences point to their disproportionately large capacity (relative to their areal extent) to both trap into the sediments and emit into the atmosphere significant amounts of C. Current figures based on literature data compilation (Cole et al. 2007; Downing et al. 2008; Tranvik et al. 2009) suggest that the annual amount of organic carbon (OC) trapped in artificial reservoirs alone (about 150–220 Tg C year-1, Dean and Gorham 1998; Mulholland and Elwood 1982) is 3–5 times higher than the global annual OC storage in natural lakes (about 30–70 Tg year-1, Einsele et al. 2001; Stallard 1998). This is similar to the amount of OC buried in the sediments of the global ocean (about 120–240 Tg year-1, Duarte et al. 2004; Sarmiento and Sundquist 1992; Sundquist 2003) and comparable in magnitude to the total riverine OC export to the seas (about 350–530 Tg C year-1, Meybeck 1993; Probst 2002; Schlu¨nz and Schneider 2000). Moreover, recent inventories of reservoir greenhouse gas (GHG) emissions indicate that worldwide impoundments may contribute with between 1 and 7 % to the global anthropogenic carbon dioxide (CO2) and methane (CH4) fluxes (Barros et al. 2011; St. Louis et al. 2000). Although large uncertainties may be associated with all these current estimates, these figures suggest that reservoirs play important ecological and biogeochemical roles in C cycles. They also demonstrate the need for future scientific investigation in order to understand the physical, hydrological and biogeochemical drivers behind such disproportionately large C sequestration and evasion from human-made impoundments. In general, the flooding of terrestrial landscapes associated with reservoirs construction has far-reaching consequences not only on the regional hydrology (Vo¨ro¨smarty et al. 1997, Vo¨ro¨smarty and Sahagian 2000), but also on the nutrient and C budget (Friedl and Wu¨est 2002; Rosenberg et al. 1997; Tranvik et al. 2009; Teodoru et al. 2012). Among the multiple
123
environmental and biogeochemical impacts associated with reservoir development, a series of ecological alterations result from the remobilization of nutrients from flooded soils due to large scale erosion (often leading to large spiked in in situ primary productivity) (Hall et al. 1999; Paterson et al. 1997), and from the remobilization of soil organic matter (OM) stored in the flooded terrestrial ecosystem (Bodaly et al. 2004; GalyLacaux et al. 1997; Rosenberg et al. 1997). Due to intense microbial degradation during the transit within the water column as well as at the sediment-water interface, only a portion of the settling OM becomes permanently buried in the bottom sediments (Meyers 1994; Sobek et al. 2009), while the remainder fueling often elevated levels of CO2 and CH4 production (Brothers et al. 2012; Paterson et al. 1997; Teodoru et al. 2010). Reservoirs also influence the transport of materials downstream, both by altering nutrient and contaminant mobilization, and by acting as effective traps of inorganic and organic particulates. Quantifying downward particle fluxes is thus important not only for understanding patterns of C sequestration and storage in reservoirs, but also for assessing the origin and the magnitude of C emissions from these man-made aquatic systems. A number of biological and environmental factors influence the quality and quality of particle deposition in freshwater aquatic systems (Hanson et al. 2004). The size and physical characteristics of the drainage basins influence the amount and nature of the particulate loading to freshwater systems, while physical aspects of the aquatic systems, such as morphometry and the associated hydrodynamic conditions, play a key role in determining the fate of the loaded materials (Hakanson and Jansson 1983). In addition to the physical environment, the source of sedimenting OM determines the chemical and physical properties and therefore the pathways that lead to C sequestration in reservoirs. Molar ratios of carbon to nitrogen (C:N) of the sedimenting OM have often been used in partitioning between terrestrial (soils, vascular plant debris) and aquatic (phytoplankton) sources (Hedges and Oades 1997; Hedges and Keil 1999). Molar C:N ratios of terrestrially derived OM are generally higher ([10) than those of freshwater phytoplankton (between 4 and 10, average *6.7, Meyers 1994; Sullivan et al. 2001). However, as bacteria are capable of altering the C:N ratio of soils OM by mineralizing nitrogen and respiring CO2 (He´lie and Hillaire-Marcel 2006), the identification
Biogeochemistry
of C sources must be paired with other tracers, such as carbon and nitrogen stable isotopes. Carbon and nitrogen isotope ratios (d13C and d15N) have been used extensively to distinguish between aquatic and terrestrial sources of sedimentary organic matter and to identify OM originating from different types of land plants. For instance, organic matter produced from atmospheric CO2 by most photosynthetic land plants (C3) has a d13C value ranging from -23 to -34 % (average -27 %) while d13C of OM produced by C4 plants ranges from -7 to -23 % (average -14 %, Bowen 1991; Diefendorf et al. 2010; Kohn 2010; Meyers 1994). However, with a similar range of between -24 and -42 % (average -30 %, Kendall et al. 2001) the isotopic composition of lake-derived OM is indistinguishable from that of the surrounding watershed. Nevertheless, more defined range in the nitrogen isotopic content of plankton (d15N of ?8 %) compared to land plants (d15N of ?1 %) makes the N stable isotope approach a complementary method for identifying OM sources (Meyers and Ishiwatari 1993). The boreal region of Canada has one of the highest densities of surface water (rivers, streams, and lakes) of any biome, and is also home to some of the largest hydroelectric reservoirs in the world. Considerable effort has been devoted in recent years to quantify CO2 and CH4 emissions from these systems (Demarty et al. 2009; Marchand et al. 2009; Roehm et al. 2009; Teodoru et al. 2009; Teodoru et al. 2010; Teodoru et al. 2012; Vachon et al. 2010), and to understand the pathways that lead to these C gas emissions (Brothers et al. 2012; Hesslein 2005). Much less is known about bulk particulate fluxes in reservoirs and lakes, and the associated rates of net C accumulation. The present study is a part of a large project aiming to assess the net impact of a newly created (2006) Eastmain-1 Reservoir (Northern Que´bec) on the regional C budget. In previous work we have shown that this young reservoir exhibits elevated rates of CO2 emissions to the atmosphere and that these CO2 fluxes, as well as water column and benthic respiration rates are extremely heterogeneous within the reservoir and linked to the spatial distribution of the pre-flood landscape (Brothers et al. 2012; Teodoru et al. 2010; Teodoru et al. 2012). Here we: (1) explore in detail spatial patterns and the magnitude of particle deposition within the reservoir and, for comparative purposes, within surrounding lakes of different physico-chemical characteristics; (2) investigate potential
sources of sedimenting C using a combination of stoichiometry and stable isotopes, and examine the major factors responsible for the observed patterns in C deposition; and (3) construct a particulate organic carbon (POC) mass balance for the reservoir over the study period comparing depositional fluxes with surface CO2 emissions of the same period. Finally, we place particle deposition in the context of the regional C budget and compare the observed processes in these northern ecosystems to other regions of the world, exploring how regional factors can explain the large variability in C deposition encountered worldwide.
Materials and methods Study area The investigated area lies in the boreal region of northwestern Que´bec, between 51 and 52°N, and 75 to 76°W (supplementary material, Fig. S1). The area is characterized by relatively flat topography (average altitude 250 m A.S.L), homogeneous surface geology (quaternary deposits), and has one of the highest freshwater densities in the world, which collectively occupy between 15 and 25 % of the landscape (Teodoru et al. 2009). The Eastmain River is one of half a dozen major rivers in the region. The river originates in northcentral Que´bec, flows west across roughly 800 km, drains a total area of about 46,400 km2, and discharges into the James Bay. Located about 200 km upstream from the river mouth, the Eastmain-1 Reservoir is a part of a large hydroelectric complex (La Grande complex) consisting of a network of 8 large reservoirs. However, there are no other reservoirs on the Eastmain River upstream Eastmain-1. Filling the reservoir at the end of 2005 resulted in the flooding of more than 600 km2 of terrestrial (68 %) and natural aquatic (32 %) ecosystems (Teodoru et al. 2010). With an average depth of about 11 m, the reservoir has a total volume (maximum storage capacity) of 6.94 km3. The hydraulic residence time (HRT) of the reservoir calculated for an annual flow rate of 550 m3 s-1 (4 years average) is about 145 days. With an installed capacity of 485 megawatts (MW), this hydroelectric station generates an annual output of 2.7 terawatt-hours (TWh). An increase in total energy output is however anticipated for the end of 2012 with the completion of the second powerhouse (Eastmain1A, 768 MW).
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Biogeochemistry Table 1 Physical characteristics (LA lake area, CA catchment area, Secchi) and biogeochemical parameters (chl-a chlorophyll a, TP total phosphorus, TN total nitrogen, DOC dissolved Lake
Sediment trap ID
LA (km2)
CA (km2)
Secchi (m)
organic carbon, DIC dissolved inorganic carbon) of lakes and average values for the Eastmain-1 reservoir chl-a (lg L-1)
TP (lg L-1)
TN (mg L-1)
DOC (ppm)
DIC (ppm)
Lake 8
L1
0.32
1.92
2.42
1.58
6.7
0.19
6.86
2.24
Brendan
L2
1.07
2.19
2.58
2.60
8.4
0.19
5.28
2.18
Labyrinthe
L3
2.57
6.35
2.17
2.24
6.5
0.21
9.41
2.23
Mistumis
L4
3.97
8.79
1.87
2.70
9.0
0.19
6.46
0.74
Lake 40
L5
0.16
1.86
2.67
2.38
8.1
0.18
5.36
1.55
Lake 34
L6
0.46
1.20
2.08
2.24
16.0
0.19
9.10
0.85
EM-320 Clarkie
L7 L8
0.48 21.40
1.48 32.03
1.83 2.67
2.51 1.99
9.5 10.0
0.23 0.17
8.00 6.44
1.02 0.74
Lake 66
L9
0.07
0.92
1.50
2.42
11.6
0.22
9.63
0.75
Natel
L10
3.87
6.75
2.83
2.03
6.5
0.18
6.83
1.95
Lake 60
L11
1.38
3.74
2.75
2.42
5.6
0.18
6.31
1.45
Average
Lakes
3.25
6.11
2.31
2.28
8.9
0.19
7.24
1.43
Reservoir
602.9
46400
2.09
2.78
14.3
0.20
6.22
0.93
The work reported in this study was carried out during the ice-free period of 2008 (3 years after flooding), and is based on particulate matter collected in open sediment traps. Trap moorings were deployed at the end of June 2008 for a period of varying between 74 and 85 days at 14 different locations within the Eastmain-1 reservoir and at the deepest site of 11 reference lakes located in the immediate vicinity of the reservoir (supplementary material, Fig. S1). At each location, water samples (in duplicate) were collected three times over the study period (at deployment, at recovering and in between) from three different depths (0.5 m below surface, middle, and 1–2 m above the sediment floor) and used for chemical analyses and total suspended matter (TSM) concentration. Physical and limnological characteristics of the lakes and reservoir are displayed in Table 1. Sediment traps The traps used in this study were designed and constructed in our lab at UQAM and consisted of a large PVC pipe (inner diameter of 10.2 cm) connected at the lower end to a removable collector of smaller diameter (Ø = 4 cm) (see supplementary material, Fig. S2a). A valve positioned above the collector allows for water release without re-suspending the collected material. The trap has a total length of 108 cm and an active (exposed) area of 81.7 cm2.
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Easily removable from the main body of the trap, the collector can store a total volume of 440 cm3. An ‘‘I’’-mooring string was used to deploy the traps in the reservoir and lakes. The mooring consists of an anchor weight ([40 kg), a pre-stretch 8 mm plastic rope and several buoys attached at the upper end of the string to maintain the trap in an upright position (supplementary material, Fig. S2b). Two traps were attached side by side to the mooring at about 1.5 m above the sediment floor. In order to avoid the loss of buoyancy by potentially large water level fluctuation in the reservoir, intermediate floats totaling up to 8 kg buoyancy were positioned at least 2 m below the lowest predicted water level. In addition, each pair of traps (weighing less than 1 kg in the water) had two additional floats (buoyancy of 1 kg each) attached to the top of the main body to ensure a constant upright position independent of the surface buoys. During deployment, the moorings were slowly lowered into the water to minimize disturbance of the bottom sediment. Analyses and sample preparation After retrieving the sediment traps from the water column, the collectors were removed from the main body of the unit, sealed and stored in the freezer until analysis at our laboratory in Montre`al. The collected material containing both water and sediment, was then
Biogeochemistry
transferred into plastic bottles and left in a dark, coolroom at 4–8 °C for at least 48 h to ensure the settling of the suspended particles. The supernatant water was then removed and filtered on pre-combusted and preweighed glass-fiber filters (Whatman GF/F, pore size of 0.7 lm). The dry filters were re-weighted to determine the dry-weight of the filtered material. The remaining homogenized suspension was centrifuged to remove residual water and then freeze-dried for up to 5 days to determine its total dry weight (dw). The dry weights of the filtered and freeze-dry fractions were summed to determine total dw. The dried material was manually grounded and homogenized for further analyses. Particulate carbon and particulate nitrogen The high-temperature catalytic combustion method was used to determine the total particulate carbon content of samples. Duplicate aliquots of each sample were weighted and wrapped in a tin capsule, which acts as a catalyst for the reaction, and combust at high temperature (1,000 °C). During the combustion, all forms of carbon were converted into CO2 gas, which was then measured with an elemental analyzer (CARLO ERBA NC2500). Simultaneously, particulate nitrogen (PN) was also oxidized. After combustion, the resulting gases were brought to a reducing environment and separated by gas chromatography. A thermal conductivity detector allows precise quantification of the amount of nitrogen (N2) and CO2 gases. Particulate organic and inorganic carbon For the particulate organic carbon (POC) content, a second aliquot was sub-sampled and acidified by fumigation to remove the carbonate fraction before analysis. For this, the aliquot was carefully weighted (4–6 mg) and placed in a silver capsule and left for 24 h in a closed glass vessel containing a beaker of concentrated HCl. After fumigation, the silver capsules were sealed and wrapped in tin capsules for the high-temperature combustion. Although the tin capsule acts as a better catalyst for combustion, its preferential reaction with acids prevents his direct use in the fumigation step. As in the case of total particulate carbon, the POC was converted into CO2 gas during combustion and measured with the CARLO ERBA NC2500 elemental analyzer. The particulate
inorganic carbon (PIC) content was obtained indirectly as the difference between the total and the organic carbon fraction. d13C and d15N of OM Organic carbon and nitrogen isotopic compositions were determined using a continuous flow isotope ratio mass spectrometer (Micromass, IsoPrime) coupled to an elemental analyzer (Vario MICRO cube Isoprime). The mass spectrometer has a universal triple collector, which allows it to measure masses of 44, 45 and 46 for CO2, and 30 and 32 for N2. For isotopic carbon composition of the organic matter, decarbonation was performed prior to analysis. Data were corrected using two internal laboratory reference materials (leucine: d13C = -28.50 %, d15N = -0.26 %; and DORM-2: d13C = -17.35 %, d14N = ? 14.36 %) interspersed in every analytical sequence, calibrated against international reference material [d13C vs. V-PDB (±0.1 % to 1r); d15N vs. AIR (±0.2 % to 1r)] and reported in parts per million (%). Chemical analyses In addition to measurements on sinking particles, a series of chemical analyses were performed on duplicate water samples collected at three different depths (0.5 m below surface, middle and 2 m above bottom) using a Van Dorn bottle: Total phosphorus (TP) was analyzed spectrophotometrically following potassium persulfate digestion. Total nitrogen (TN) concentration was analyzed as nitrates following alkaline persulfate digestion and measured on an Alpkem Flow solution IV autoanalyzer. Dissolved organic and inorganic carbon (DOC and DIC) concentrations were measured on 0.2-lm-filtered water samples in an OI-1010 Total C Analyzer using wet persulfate oxidation. Chlorophyll a (chl-a) was analyzed spectrophotometrically following filtration on Whatman (GF/F) filters and hot ethanol (90 %) extraction. For total suspended matter (TSM) water samples were filtered through pre-combusted and preweighed glass-fiber filters (Whatman GF/F, pore size of 0.7 lm) and the concentration (mg L-1) was determined by the weight difference divided by the volume of filtered sample. Statistical analysis of all variables was carried out using JMP 7 (SAS Institute, Cary, NC, USA).
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Biogeochemistry
Results After a total exposed period varying between 74 and 85 days, the total dry-weight mass of bulk matter collected in the sediment traps of the reservoir ranged from 171 to 1,246 mg (dw) (supplementary material, Table S1). Over a similar period (68–81 days), the total dry-weight material accumulated in the sediment traps of 11 reference lakes ranged from 89 to 578 mg dw (supplementary material, Table S1). Despite the large variability in the total mass of collected material and the 2-fold difference between reservoir (average 533 mg dw) and lake (average 264 mg dw) samples, the relative contribution of total particulate carbon to the total dryweight (as dry-weight percentage) was fairly homogeneous, ranging form 16 to 37 % and from 14 to 27 %, in the reservoir and lake samples, respectively (supplementary material, Table S1). The total carbon content of sedimenting particles in the reservoir varied from 160 to 370 mg C g-1 sed., which was slightly higher than for lakes (140–270 mg C g-1 sed.). As these poorlybuffered systems in the boreal region of Que´bec contain a very small fraction of inorganic carbon, the relative contribution of POC to total carbon averaged 98.5 % in the reservoir and 98.8 % in lakes. As for particulate carbon, the contribution of PN to the total dry weight (as weight percentage) was uniform across all samples (1–2.7 % for both systems, Table S1) with average concentrations of 18.1 and 17.9 mg N g-1 sed., in the reservoir and lakes, respectively. Since inorganic nitrogen compounds in aquatic systems exist
(a) Total Particles
500
Total particle deposition rates were estimated to vary between 265 and 1,924 mg m-2 d-1 in the reservoir and between 152 and 957 mg m-2 d-1 in lakes (Fig. 1a). Despite deposition rates spanning one order of magnitude for both types of aquatic systems, the mean rate of the reservoir (810 ± 162 mg m-2 d-1) was almost twice as high as that of natural lakes (438 ± 90 mg m-2 d-1). Particulate organic carbon and particulate nitrogen fluxes followed the depositional patterns of total particles. POC fluxes were highly heterogeneous in both systems, ranging from 68 to 548 mg C m-2 d-1 in the reservoir and from 30 to 150 mg C m-2 d-1 in lakes (Fig. 1b). The average deposition rate of POC in the reservoir of 211 ± 46 mg C m-2 d-1 was more than twice as high as that of lakes (79 ± 13 mg C m-2 d-1, Fig. 1b). Similarly, PN deposition in the reservoir exceeded that of lakes by several-fold (range 5.1–32.5 mg N m-2 d-1, average 13.8 ± 2.7 mg N m-2 d-1; and 2.7–14.5 mg N m-2 d-1, average 7.0 ± 1.2 mg N m-2 d-1, for the reservoir and lakes, respectively, Fig. 1c). As the inorganic fraction represented only 1 % of total carbon, PIC fluxes in both the reservoir and the lakes were very small (mean
35
500
30
400
300 200 100
Eastmain-1 Reservoir
Lakes
25 20 15 10 5
0
0
(c) Particulate Nitrogen
600
PN flux [mg N m-2 d-1]
POC flux [mg C m-2 d-1]
Total particle flux [mg m-2 d-1]
1000
Total, C and N particle fluxes in lakes and in the reservoir
(b) Particulate Organic Carbon
2000
1500
overwhelmingly in the dissolved form, the particulate nitrogen described here represents almost entirely particulate organic nitrogen (PON).
0
Eastmain-1 Reservoir
Lakes
Eastmain-1 Reservoir
Lakes
Fig. 1 The range in: a Total particle deposition (dry-weight); b Particulate organic carbon (POC) flux; and c Particulate nitrogen (PN) in the Eastmain-1 reservoir and the 11 reference lakes. Box plot show range, percentile, median, mean, and outliers
123
Biogeochemistry
3.2 ± 1.6 mg C m-2 d-1 and 1.0 ± 0.3 mg C m-2 d-1, for reservoir and lakes respectively; data indirectly shown in Table S1). Spatial heterogeneity of particle fluxes in the Eastmain reservoir There was a large degree of spatial heterogeneity in total particle fluxes within the reservoir (Fig. 2). The highest deposition rates (600–1,900 mg m-2 d-1) were generally found in deeper areas of the reservoir corresponding to (or in the vicinity of) the old riverbed where water velocities are also expected to be the highest (dynamic hydrology) (Fig. 2, Table S1). In contrast, the lowest deposition rates (265–600 mg m-2 d-1) occurred generally in shallower and sheltered areas of the reservoir such as side bays with calm hydrology and generally away from the main stream (Fig. 2). In the complete absence of water velocity measurements, grouping of the reservoir sites into areas of dynamic and calm hydrological conditions was indirectly based on their distance relative to the old riverbed and water depth (Table S1). There was a weak exponential relationship between total particle flux and depth (Fig. 3a), which explained 37 % of the spatial heterogeneity within the reservoir. As deepest areas of the reservoir are generally associated with either old river channel or former lakes, preferential particle focusing and resuspention processes in such deep areas of the reservoir associated with more dynamic hydrology may explain their higher particle
fluxes and the general exponential fit between fluxes and depth. Total particle fluxes within the reservoir were also related to the total suspended matter, which varied 4-fold within the reservoir (Fig. 3b), but here again the influence of different hydrological conditions is apparent: For any given TSM concentration, the rates of particle deposition tended to be several-fold higher in areas of dynamic hydrology compared to protected areas of calm hydrological conditions (Fig. 3b). Patterns of particulate fluxes across lakes The total particle fluxes (and thus POC and PN) in lakes were not correlated to any chemical or biological variables and, contrary to the patterns observed in the reservoir, fluxes were not correlated to depth or to total suspended solids (which varied less than 2-fold across lakes). Total particle, POC and PN fluxes were weakly positively correlated to lake area (data not shown), but these relationships were entirely driven by a single point from a large lake, whereas the remaining lakes showed no relationship with lake or catchment size. Comparison of the nature and sources of particulate flux between lakes and the reservoir The molar C:N ratio ranged from 13.7 to 25.5 in the reservoir and from 10.7 to 16.0 in lakes (Fig. 4). The highest C:N ratio of over 25 was measured at reservoir station R4 (Fig. 4), and corresponded to samples
Fig. 2 Spatial variability of total particle depositional flux (mg m-2 d-1) in the Eastmain-1 Reservoir based on sediment traps data collected from June to September 2008
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Biogeochemistry
Total particle flux [mg m-2 d-1]
(a)
(b)
2000
R11
R5
R13
1500
0.066x
y=174.73e
2000
1500
, r 2 = 0.37, p>0.001
1000
R6
R1 R14
R7
R10
500
2
y = 259.9x + 153.1,r = 0.88, p>0.0001
0 10
15
20
25
30
35
0.4
0.6
0.8
R1 R2
R6
R7
R9
R14
R8
R3
R12
R2
R12
5
R13
y=1211.2x-206.9, r 2 = 0.97, p>0.001
1000
R9
R8
R3
R5
R4
R10
500
R11
dynamic hydrology "calm" hydrology
1.0
1.2
1.4
1.6
1.8
TSM [mg L-1]
Depth [m]
Fig. 3 The relationship between total particle flux and a depth of the water column; and b total suspended matter (TSM) concentration (average over the entire water column)
(a)
(b)
26 24
C:Nratio
22
R4
R11
20 18
Dominant autochthonous R9 R7
16 14
R12
R2 R6 R8
12
R5 R10
R13 L3
R14 L9
L7 L1
L6
24
R1 R3
L4 L5
R4
Dominant allochthonous
y = 2.07x+ 76.96,r 2 = 0.85, p>0.0001
22
y = 3.64x + 10.51, r 2 = 0.31, p>0.03 R11
20
R10
18
R13
16 14
L11
Reservoir Lakes
26
C:Nratio
Terrestrial end-meber
Reservoir Lakes
L2
L8 L10
y = 0.37x + 23.75, r 2 = 0.05, p>0.5
10 -31.0 -30.5 -30.0 -29.5 -29.0 -28.5 -28.0 -27.5 -27.0 -26.5
δ13C [‰]
L10 L11 L4 L1
12
L6
10 -1.0
-0.5
0.0
R5
R1
R3 R7 L3 R9 R14 L9 R6 R2 L7 R8 R12 L5 L2 L8 y = 0.77x + 12.92, r 2 = 0.13, p>0.2
0.5
1.0
1.5
2.0
2.5
3.0
δ15N [‰]
Fig. 4 Molar C:N ratio of reservoir and lakes samples in relation to the isotopic composition of: a d13C-POC [%]; and d15N-PON [%]. The empirical relationships for the reservoir samples does not include sampling station R4 values are separated into two main areas with different hydrology and hydrodynamic conditions: (i) dynamic hydrology (square dots)—correspond to deepest areas of the reservoir generally
distributed along the old riverbed where water velocity is expected to be the highest; and (ii) calm hydrology (diamond dots) correspond to side bays and all the other areas of the reservoir away from the main channel where water flow is expected to be minimal. In the absence of water velocity measurements, the selection of the two subsample sets was done on the base of depth and their proximity to the old riverbed
consisting almost entirely of black spruce needles, as this trap was unintentionally deployed in an area of flooded dense conifer forest. Most other high C:N ratio of reservoir samples ranged from 16.9 and 19.7, and generally correspond to stations characterized by high overall particle fluxes (R5, R10, R11, R13). However, even excluding station R4 from the overall calculation, the mean C:N ratio of the reservoir of 16.3 was significantly higher (p \ 0.0008) than that of lakes (13.3). The d13C of POC in the reservoir varied from -30.7 % to -27.9 %, and a similar range was observed in lakes (-30.0 % to -26.8 %) (Fig. 4a). With an average value of -29.2 %, the reservoir
samples were significantly different (p \ 0.023) from mean lake samples (-28.3 %). While d13C values overlapped between the lake and reservoir samples, the relationships between the isotopic signature and the C:N ratio were strikingly different between the two environments. In the reservoir the two were strongly positively related:
123
C : N ¼ 2:07 d13 C þ 76:96; r 2 ¼ 0:85; p\0:0001
ð1Þ
Reservoir sites ranged from high d13C-POC (-28 %) and high C:N ratios (around 20, R5, R10, R11, Fig. 4a) to more negative values (-30 %) and much lower C:N ratios (around 14.5, R2, R6, R7, R8,
Biogeochemistry
R9, Fig. 4a), suggesting a shift from predominantly terrestrial to autochthonous origin of the sedimenting POM. In lakes, on the other hand, this relationship was not significant (p [ 0.495), and the C:N of POM remained relatively constant across a fairly wide range of d13C values (Fig. 4a). With both high C:N ratio and high d13C, station R4 (dominated by black spruce needles) can be seen as an end-member of terrestrial OM origin, whereas the side bay station R12, with lowest C:N ratio and the lowest d13C concentration, can be viewed as an end-member of autochthonous origin (Fig. 4a). However, while there was a strong relationship between d13C and C:N ratio of the reservoir particulate fluxes (Eq. 1), there was no such relationship for lakes, where, despite a similar range in d13C values as in the reservoir, C:N ratios remained around a mean value of 13 (Fig. 4a). This would suggest that chemical composition of POC is fundamentally different between lakes and the reservoir, a conclusion further supported by the patterns in nitrogen isotopes. The d15N of reservoir samples were all positive and characterized by a relatively narrow range, from 1.0 to 2.2 % (Fig. 4b). In contrast, lake samples spanned a larger range, from -0.6 % up to 2.6 % (Fig. 4b). With an average value of 1.5 %, the d15N of reservoir samples are significantly different (p \ 0.001) and generally 1 % heavier than the mean value of the surrounding lakes (0.5 %, Fig. 4b). Differences in the origin of settling organic matter in the two systems are further supported by the contrasting relationships between organic C content of settling particles (in mg C g-1 sed.) and the d13C of the settling POC (Fig. 5a). In lakes, the lowest organic C content of sinking particles was associated with a lighter
Discussion Total particle sinking fluxes were on average twice as high in the reservoir than in lakes, as were the particulate C and N sinking fluxes. In addition, there were marked differences between the reservoir and the lakes in term of the nature of sinking material and the OM sources. However, particle deposition rates exhibit a large spatial heterogeneity, both across natural lakes, and within this young boreal reservoir. Patterns in particulate organic C and N sinking flux across lakes Our results show that nearly all ([99 %) of the particulate C in sinking flux was in the form of POC, indicating that variability in total particulate C deposition was largely driven by variability in POC deposition, rather than by changes in the OC content of the particulate carbon. The five-fold range in POC deposition observed across lakes (from 30 to 152 mg C m-2 d-1, average of 79 mg C m-2 d-1) is well within the reported range for other boreal lakes (Jonsson and Jansson 1997; von Wachenfeldt and Tranvik 2008), but is at the lower end of organic C
(a)
(b)
-27.0
L10 L8
-27.5 -28.0
L3
L2 L11
-28.5 L7
-29.0
R13
L5
L4
R3 R2
R8
-30.0
R6
R7
L6
R9 R12
-30.5 -31.0 150
200
R4 R11
R1 R14 L1
-29.5
250
Reservoir Lakes
30 R5
L9
35
Resevoir Lakes
y = 25.11 - 0.016x r2 = 0.51, p0.08
PN flux[mg N m-2 d-1]
-26.5
δ13C-POC [‰]
Fig. 5 The relationships between a the organic carbon (OC) content of lake and reservoir samples and the isotopic signature of POC (d13C-POC); and b POC versus PN flux
terrestrial POC signature, and settling particles became C-enriched as the POC isotopic signature become more d13C-depleted, suggesting a greater proportion of algal-derived C. In the reservoir, on the other hand, the most organic-rich particles were associated to a terrestrial d13C signature, and particles contained less organic C as the d13C signature of the POC become more algal (Fig. 5a).
25 20 15 10 5
(R):y = 0.056x+ 1.98, r 2 = 0.95, p>0.0001 (L):y = 0.088x+ 0.048, r 2 = 0.95, p>0.0001
0 300
350
OC concentration [mg C g-1 sed.]
0
100
200
300
400
500
600
POC flux [mg C m-2 d-1]
123
Biogeochemistry
Spatial heterogeneity of sinking particle fluxes in the reservoir
and close to the river mouth (Eldardir 1994; Kunz et al. 2011; Morris and Fan 1998). Our most upstream reservoir station (R3) had one of the lowest total particle sinking fluxes recorded in the reservoir (360 mg m-2 d-1, Fig. 2). Spatial differences in the hydrodynamic regime have been previously invoked to explain the strong heterogeneity of sediment dynamics in reservoirs with large storage capacities and dendritic morphology (Thornton 1990; Osidele and Beck 2004). The role of local hydrology on the dynamics of particle deposition in the Eastmain-1 is well depicted in the longitudinal transect from the upper station R3 to the downstream station R8 (Fig. 7) where there was a sharp increase in total particle fluxes between side bay station R3 and the mainstream station R13 (from 360 mg m-2 d-1 to 1,920 mg m-2 d-1), followed by a progressively decrease in depositional rates with increasing distance downstream (to 435 mg m-2 d-1 at R8) (Fig. 6). These increases
(a) 0
1.4 1.0
Depth [m]
deposition previously reported for temperate lakes at the same time of year (\10–300 mg C m-2 C d-1, Baines and Pace 1994). There was a tight coupling between particulate C and N fluxes, such that particulate N deposition in our lakes (2.5–14.5 mg N m-2 d-1) was also at the lower end of the range reported for temperate lakes (Baines and Pace 1994). However, the molar C:N ratio of sinking particles averages 13.3 in our lakes, considerably higher than the average of 8 reported by Baines and Pace (1994) for temperate lakes which span a wider trophic range. Contrary to previous studies of temperate lakes (Baines and Pace 1994), we did not find any relationship between POC sinking flux and lake productivity, TP or chlorophyll concentrations, most likely due to the relatively narrow trophic range in lakes of our studied region. Likewise, we did not find a relationship between POC sinking flux and DOC concentration (figure not shown), as was reported by von Wachenfeldt and Tranvik (2008) for boreal lakes in Sweden. Again, this is likely due to the fact that, although there was considerable overlap in the range of DOC observed in both studies, this range was much narrower (6–10 mg DOC L-1) in our study as compared to the Swedish study (4–22 mg DOC L-1).
1.4
1.8
2.6 2.2
3.0
1.0
-5
0.6
-10 R8
R10
R11
R3
R13
-15 35
123
25
20
15
10
50
Distance [km]
Total particle flux [mg Cm-2 d-1]
There was a 7-fold range in sinking fluxes within the reservoir, exceeding the variability found across all sampled lakes. Considering the great extent of the Eastmain-1 reservoir ([600 km2) such large heterogeneity is not entirely unexpected (Thornton 1990). Similarly, previous studies on an older (80 year) boreal reservoir (Cabonga Reservoir) in northern Que´bec reported a wider range in particle sedimentation from 350 to 2,465 mg m-2 d-1 (Houel et al. 2006) based on radiometric measurements of 210Pb activity in sediments. The highest particle fluxes in the Eastmain-1 reservoir were generally distributed along or in the vicinity of the old riverbed, whereas lower fluxes occurred mainly in areas away from the main watercourse (such as in side bays), suggesting that local hydrology plays a critical role in defining the depositional pattern. This is in contrast to tropical reservoirs or systems dominated by large riverine particle loads, where the highest sedimentation commonly occurs generally in the upstream areas of the reservoir
30
(b) 2000
30%
21% % POC
1500 1000 500
16% 17%
33%
0
R8
R10
R11
R13
R3
Fig. 6 (a) Depth-profile of total suspended matter (TSM) concentration (mg L-1) during September 14, 2008 along a hydrological gradient in the direction of the main water flow of the reservoir from the side bay station (R3) away from the main flow where water velocity must be minimal to station R13 located in the narrowest stretch of the reservoir where water velocities are expected to be highest and further to open water stations R11, R10 and R8 where water flow is expected to have reduced considerably. (b) Total particle flux and % contribution of POC to the total fluxes along the same hydrological gradient
Biogeochemistry
in particle fluxes of more than 5-fold from R3 to R13, as well as the gradual decrease downstream of station R13 can be attributed to profound hydrological changes that occur along this transect. The highest flux along the transect (as well as of the entire reservoir) corresponds to station R13 (Fig. 6), probably due to its position in the main channel and in the narrowest area of the reservoir (Fig. 2). Increased water velocity there due to a substantial reduction in the reservoir section is expected to cause enhanced lateral erosion. This hypothesis is supported by the elevated TSM concentrations found at this site, with values reaching up to 3.1 mg L-1 compared to only 0.9–1.1 mg L-1 measured at the upstream R3 and nearby downstream stations (R11, R10, R8, Fig. 6). Moreover, the expected slowdown in the water flow rates downstream of station R13 due to the widening of the reservoir section (Fig. 2) would encourage settling. This is supported by the observed reduction in surface water TSM concentrations and the gradual decrease in particle deposition (Fig. 6). Therefore, by controlling the balance between erosion, resuspension and particles transport, spatial variation in hydrological conditions within the reservoir can result in hot spots for sedimentation. Composition and origin of the sinking particles The chemical and isotopic composition of the sedimenting particulate flux varied considerably within the reservoir and among lakes, suggesting that fundamental differences exist in the biogeochemistry of the two aquatic systems. The contribution of autochthonous and allochthonous origin of settling material can be roughly estimated if assuming a terrestrial end member with a d13-POC isotopic signal of -27 % (Jonsson et al. 2001; Karlsson et al. 2003; von Wachenfeldt and Tranvik 2008) and an autochthonous end member with a d13-POC of -31 % (average algal d13C signal measured in 13 natural lakes and 6 hydropower reservoirs in this same region of northern Que´bec, Marty and Planas 2008). The linear regression between the isotopic signature (d13C) and % allochthonous (where -27 % and -31 % represent 100 % and 0 %, respectively) was then used to calculate the fraction of algal and terrestrial-derived C for all reservoir and lake samples. This first order calculation suggests a large heterogeneity within the reservoir in terms of the dominant origin of
sedimenting material: The sinking POC in areas of calm hydrology (and lower POC fluxes) was largely (75 %) of algal origin, in contrast to the sinking POC in areas of dynamic hydrology (and higher POC fluxes) which appeared to be dominantly ([65 %) of terrestrial origin. Also, there was a relatively large variability in the relative contribution of allochthonous C origin among lakes (20–100 %), but overall, the settling POC in these lakes appeared to be dominated by terrestrial OC (average 65 % allochthonous), in agreement with the results of von Wachenfeldt and Tranvik (2008) for Swedish boreal lakes. The C:N ratio of the sedimenting particles (10.7 to 25.5, average 15.4) were all above the typical range of freshwater phytoplankton (4–10, average 6.7, Sullivan et al. 2001), and within the reported range of pure soil OM ([10) and vascular plant debris ([20) (Meyers 1994, Sullivan et al. 2001). There were, however, striking differences between lake and reservoir samples in terms of patterns in C:N ratio. In the reservoir, the C:N of sinking particles was positively related to their d13C and d15N signatures and thus declined as the material became more dominated by algal fraction (Fig. 4). In contrast, the C:N of sinking particles in lakes was relatively constant around an average of 13.3, in spite of a relatively wide range in both d13C and d15N values (Fig. 4), suggesting that sinking material in lakes has roughly the same degree of alteration regardless of whether this material is mostly from algal or terrestrial origin. This seems to be in contrast with the results of Sobek et al. (2009) which suggest that terrestrial POC material has quite different diagenetic behavior compared to the organic matter of aquatic origin. Moreover, as many chemical compounds display generally a greater variability in smaller systems rather than in large environments, this may also apply to stable isotopes and explain the wider range of d13C and d15N in lakes as compared to the reservoir, probably related to greater variability in primary producers between lakes and over time in the same lake. POC fluxes of the reservoir were positively correlated to both the C:N ratio and to the d13C of the sinking material (Fig. S3a), suggesting that high reservoir sinking fluxes are driven mostly by terrestrial material. In contrast, in the lakes, neither POC nor PN sinking fluxes were significantly correlated to the C:N ratio or isotopic d13C signature (Fig. S3b), and therefore higher sinking C fluxes were not necessarily
123
Biogeochemistry
associated to a particular OC source in these lakes. Further evidence that the pathways that generate the sinking particle fluxes differ markedly between lakes and the reservoir comes from the fact that in the reservoir, the highest organic C content of the sedimenting matter was associated with material of a predominantly terrestrial origin, while in lakes the pattern was the opposite, with the OC content of sedimentaing material declining with increasing terrestrial contribution. Although the exact implication of particle resuspension in the present estimates of reservoir fluxes relative to lakes as well as in the overall budget calculation is not well understood, both isotopic compositions and molar C:N ratio together with the OC content of reservoir samples may suggest its limited significance mainly to those areas of more dynamic hydrology. Higher terrestrial signature there associated with soil particle resuspension may explain their overall two-fold higher settling fluxes compare to side bay stations as well as some of the quantitative differences between depositional fluxes of much smaller and more wind-protected lakes, and the larger and open reservoir.
Sedimentation rates and annual sediment yield To assess the impacts of particle deposition on the potential loss of storage capacity, areal sinking particle fluxes were converted into sedimentation rates (SR in mm year-1) using Eq. 2: SR ¼
AR 10 q ð1 uÞ
ð2Þ
where SR is the sedimentation rate in mm year-1; AR is the areal accumulation rate in g cm-2 year-1; q is the dry bulk density of the sediment in g cm-3, and u is the porosity (between 0 and 100 %). An estimate of the bulk dry density of the reservoir sediments of 0.9 ± 0.001 g cm-3 was calculated from the organic carbon content of our sample (158 and 350 mg OC g-1, mean 255 mg C g-1) described in Avnimelech et al. (2000). Assuming a porosity of the fresh accumulated sediment of about 90 %, and extrapolating daily summer deposition rates over the entire year, the annual sedimentation rate in the reservoir would range between 1.0 and 7.5 mm year-1. For the reservoir area of 603 km2, the average annual
123
sedimentation rate of 3.3 mm year-1 would translate into an annual sediment yield of 2.0 9 106 m3 year-1, and this value likely represents an upper limit. Sediment management therefore does not appear to be an issue for boreal reservoirs as, at this estimated rate, the reservoir would lose its total storage capacity of 6.94 km3 in more than 3,500 years. In the lakes, the dry bulk density was slightly higher (0.95 ± 0.01 g cm-3) due to a consistently lower organic carbon content of sedimenting particles (mean 200 mg OC g-1). This combined with sinking fluxes that were half those of the reservoir, yields upper limit sedimentation rates of between 0.5 and 3.5 mm year-1 (average 1.7 mm year-1), which are well within the literature estimates for lakes in northern latitudes (Brothers et al. 2008). POC mass balance for the Eastmain-1 reservoir To assess the role of the reservoir in C dynamics, we assembled a POC mass balance for the study period (June 20 to September 15, 2008) using a simple box model where, at steady state, the amount of C imported from the watershed plus the autochthonous produced C should be balanced by the settling C at the reservoir floor and by the amount that is exported to the river downstream via the dam. Although this simplified mass balance approach described by Eq. 3, assumes that no POC is consumed over the time frame of the study, in reality, changes in POC concentration occur naturally over relatively short period of time as POC is respired during both the transition within the water column as well as in the sediment trap. However, such changes have been already accounted in our data as the sediment traps used in this study were not poisoned or treated with any preservatives before deployment to inhibit the biological activity. FIN þ FPP FDEP FOUT ¼ 0
ð3Þ
where FIN represents the inflowing river POC load; FPP is the OC produced in situ from primary production (PP) which was derived from Chlorophyll a concentration using the published relationship between chl-a (mg m-3) and volumetric rates of PP (mg C m-3 d-1) described in del Giorgio and Peters (1993) for lakes worldwide (PP = 10.3 9 Chl1.19). Volumetric production rates were converted into areal fluxes (mg C m-2 d-1) by multiplying the volumetric PP with the depth of the photic zone; FDEP is gross
Biogeochemistry CO2 flux: 622±30 t C d-1 CH4 flux: 5.5±3 t C d-1
FIN= 17±4 t C d-1
FOUT= 21±2 t C d-1 1±0.5 t C d-1 (5% of FIN )
1±0.5 t C d-1 (5% of FIN )
Outflow 20±1.5 t C d-1 (20% of FPP )
Inflow
FPP= 95±7 t C d-1 75±5 t C d-1 (80% of FPP )
16±4 t C d-1 (95% of FIN )
FREM= 34±10 t C d-1
FDEP= 125±19 t C d-1 FSEQ = 54±8 t C d-1 (43% of FDEP) 3
QIN= QOUT= 750 m s
-1
Fig. 7 Schematic representation of POC mass balance for the Eastmain-1 Reservoir during the summer period of 2008 indicating the inflow and outflow loads (FIN and FOUT), POC produced during primary production (FPP), total deposition (FDEP) and estimated permanent C sequestration (FSEQ). For comparison reasons, diffusive CO2 and CH4 surface fluxes measured during the same period of 2008 are also added. The balance between all C sources (FIN ? FPP) and sinks (FDEP ? FOUT) imply the existence of an additional C source
of 34.7 ± 10.0 t C d-1 which could originate from remobilization of terrestrial C within the reservoir (FREM). Assuming that 95 % of inflowing POC load is trapped behind the dam (see text) together with all remobilized material, and that 20 % of FPP is exported below the dam, then FDEP consist of 60 % is in situ produced, 13 % is terrestrial C imported from the watershed and 27 % is terrestrial C originated from remobilization of flooded soils and/or DOC flocculation
POC deposition; and FOUT represents the OC exported downstream through the dam. The inflowing load (FIN) was estimated as 16.9 ± 4.3 t C d-1 (Fig. 7) based on the average POC concentration of 0.26 ± 0.06 mg L-1 measured at the river inflow and the mean discharge over the study period of 750 m3 s-1 (484–1,093 m3 s-1). Similarly, the outflowing POC load (FOUT) exported through the dam, estimated at 21.1 ± 2.0 t C d-1 (Fig. 7) was obtained from the average POC concentration of 0.33 ± 0.03 mg L-1 measured at reservoir stations close to the outlet (R2, R4, R5, R6, R9) and the same mean discharge of 750 m3 s-1. A similar POC concentration of 0.36 mg L-1 was measured between July and August 2008 in the Eastmain River downstream of the dam. To incorporate the effect of distinct hydrology and hydrodynamic conditions on the overall reservoir productivity and POC deposition, we divided the reservoir area into: 1) an area of dynamic hydrology (35 % of reservoir area) calculated as twice the area of the old riverbed and characterized by an average primary production of 132.9 ± 3.6 mg C m-2 d-1 and average POC deposition of 399.7 ± 72.7 mg C m-2 d-1; and 2) an area of calm hydrodynamic conditions (65 % of the reservoir surface) characterized by higher C productivity (average 169.4 ± 15.6 mg C m-2 d-1) but lower POC deposition (average
106.3 ± 10.3 mg C m-2 d-1). This approach allowed us to calculate an area-weighted average primary production (FPP) for the entire reservoir of 156.8 ± 11.5 mg C m-2 d-1 (94.5 ± 6.9 t C d-1), and an area-weighted POC total deposition of 207.5 ± 31.8 mg C m-2 d-1 (FDEP = 125.1 ± 19.2 t C d-1, Fig. 7). Nevertheless, this separation of the reservoir surface into calm and dynamic hydrology areas may have minor influence on the overall budget as a normal average primary production and POC deposition of 153.8 ± 10.4 and 211.1 ± 46.3 mg C m-2 d-1, respectively, extrapolated over the entire reservoir area would result in comparable rates of total reservoir primary production (FPP) of 92.7 ± 6.3 t C d-1 and total POC deposition (FDEP) of 127.3 ± 27.9 t C d-1. However, these values suggest that, over the study period, the sum of C sources (FIN ? FPP = 111.4 ± 11.2 t C d-1) was lower than the sum of the outflowing load and sedimentary C sink (FDEP ? FOUT = 146.2 ± 21 t C d-1), implying the existence of an additional C source of 34.7 ± 10.0 t C d-1 (or 38.7 ± 19.3 t C d-1 if using normal averages for the entire reservoir). Considering that the reminder of approximately 35 t C d-1 does not reflect the degree of uncertainty of the budget and further assuming that the input of airborne C through the reservoir surface is negligible, this additional C source (FREM. Fig. 7) may originate from remobilization of C from flooded
123
Biogeochemistry
terrestrial soils within the reservoir and/or DOC flocculation, as recently reported for boreal lakes in Sweden (von Wachenfeldt and Tranvik 2008). We can further speculate on the relative contribution of different C sources to the total POC sinking flux by examining the isotopic composition of sedimenting POC, which suggests that roughly 40 % (50 t C d-1) is of allochthonous and 60 % (75 t C d-1) is of autochthonous origin. Knowing that most of the C imported from the watershed is terrestrial (d13C of -27.3 %) and assuming that the majority of it (up to 95 %) will be trapped in the reservoir (based on the relationship between % retention of the inflowing particle load and hydraulic residence time, see Teodoru et al. 2006; Kunz et al. 2011), then 80 % of all in situ produced C must settle to the reservoir floor (75 t C d-1) while 20 % (20 t C d-1) must be exported downstream of the dam (Fig. 7). In other words, out of the entire 125 t C d-1 (equivalent of 207 mg C m-2 d-1) that settles to the reservoir floor, approximately 60 % is in situ produced, 13 % is terrestrial C imported from the watershed and 27 % is terrestrial C originated from remobilization of flooded soils and/ or DOC flocculation (Fig. 7). Although highly speculative, these numbers suggest that the reservoir is an important sink of autochthonous produced C, and that most of the C exported to the river downstream may be of aquatic origin. Due to the young age of the reservoir (3 years) and therefore the impossibility of successful sediment coring, we estimate the fraction of settled C that may become permanently incorporated into bottom sediments (long-term C sequestration) based on the relationship between chl-a concentration and the percentage of the sinking C that will be eventually respired ( %Cresp = 71.4 Chl-0.22) described in Pace and Prairie (2005). For a weighted average chla concentrations of 2.8 lg L-1, this inferred relationship suggests that up to 57 % of fresh POC that settles to the reservoir floor will be eventually respired (71 t C d-1 equivalent of 120 mg C m-2 d-1) leading to a long-term C sequestration (FSEQ) of roughly 54 t C d-1 (Fig. 7). Although, accurate estimates of the two coupled processes (long-term C sequestration and sediment respiration) and their evolution in time should be followed once a sufficient sediment layer has formed at the reservoir floor, this predicted rate of permanent C sequestration in the reservoir of roughly 90 mg C m-2 d-1 is well within the range reported for
123
boreal and temperate lakes of other regions (40 to 250 mg C m-2 d-1; Mulholland and Elwood 1982; Molot and Dillon 1996; Campbell et al. 2000; Squires et al. 2006) but higher than the calculated range of between 3.3 and 21.6 mg C m-2 d-1 for our studied lakes based on dated sediment cores (Ferland et al. 2012). Moreover, these rates of both, short and longterm C accumulation in the reservoir are similar to the reported rates of CO2 uptake by the terrestrial ecosystems in the region (-117 mg C m-2 d-1 for forests, and -102 mg C m-2 d-1 for wetlands, Teodoru et al. 2012) suggesting the ecological significance of reservoir C deposition at the landscape level. Moreover, our estimated rate of sediment respiration of about 120 mg C m-2 d-1 is one order of magnitude lower than the surface CO2 flux to the atmosphere reported for the Eastmain-1 reservoir during the icefree period 2008 (1,330 mg C m-2 d-1, Teodoru et al. 2010). These comparisons suggest not only that reservoir C deposition plays an important role at the regional scale but also demonstrates that high CO2 emissions immediately following reservoir construction in boreal areas are largely the result of preferential degradation of flooded terrestrial biomass and/or C soils rather than the respiration of new settling OM at the reservoir floor.
Conclusions POC fluxes in these boreal lakes do not appear to be linked to trophic state, DOC or morphometry of the system, whereas the actual nature of the sedimenting material is clearly influenced by lake morphometry and the relative contribution of algal versus terrestrial sources. Sinking fluxes in the reservoir are overwhelmingly regulated by morphometry and local hydrology, and our results suggest a major role of sediment focusing and resuspension/erosion in shaping reservoir sinking fluxes, which may explain both, the higher average fluxes and some of the qualitative differences in sinking material observed between the reservoir and the surrounding lakes. Interestingly, sinking POC fluxes were small relative to CO2 emissions to the atmosphere representing on average 16 % of the reported average CO2 efflux of lakes (471 mg C m-2 d-1, Brothers et al. 2012), and 17 % of the reservoir CO2 flux (1,330 mg C m-2 d-1, Teodoru et al. 2010; Brothers et al. 2012; Teodoru et al., 2012).
Biogeochemistry
This similarity between lakes and the reservoir suggests that POC sinking flux and CO2 efflux in boreal aquatic ecosystems may be well coupled for example, via DOC inputs, which influence both processes in similar manner in both lakes and reservoir. However, the nature of the organic matter reaching the sediments is quite different in lakes and in reservoir, which may have consequences for both the long-term storage of the sedimenting material, and for the benthic metabolism of boreal ecosystems. Our calculations suggest that the reservoir is a large sink for both terrestrial and aquatic material but the export below the dam of mostly algal material produced in the reservoir can have large implications on the functioning of the systems downstream. Acknowledgments This study was supported both, financially and logistically by Hydro-Que´bec, through the Hydro-Que´bec/UQAM Eastmain-1 Research Project. We would like to thank to the co-ordinator of the project, A. Tremblay (Hydro-Que´bec) for his continuing support as well as S. Barette, S. Brothers and D. Marchand for field assistance.
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