Variations in Stable Isotope Fractionation of Hg in Food Webs of Arctic

Nov 10, 2009 - Wedeterminedisotoperatios(IRs)ofHginfoodwebs(zooplankton, chironomids, Arctic char) and sediments of 10 Arctic lakes from four regions ...
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Environ. Sci. Technol. 2009 43, 9148–9154

Variations in Stable Isotope Fractionation of Hg in Food Webs of Arctic Lakes N I K O L A U S G A N T N E R , * ,†,‡ HOLGER HINTELMANN,§ WANG ZHENG,§ A N D D E R E K C . M U I R †,‡ Department of Environmental Biology, University of Guelph, Gordon Street, Guelph, ON, N1G 2W1, Canada, Water Science and Technology Directorate, Environment Canada, 867 Lakeshore Drive, Burlington, ON, L7R 4A6, Canada, and Department of Chemistry, Trent University, 1600 West Bank Drive, Peterborough, ON, K9J 7B8, Canada

Received June 16, 2009. Revised manuscript received October 3, 2009. Accepted October 12, 2009.

Biotic and abiotic fractionation of mercury (Hg) isotopes has recently been shown to occur in aquatic environments. Wedeterminedisotoperatios(IRs)ofHginfoodwebs(zooplankton, chironomids, Arctic char) and sediments of 10 Arctic lakes from four regions and investigated the extent of Hg isotope fractionation. Hg IRs were analyzed by multicollector inductively coupled plasma mass spectrometry (MC-ICP/MS). Hg mass independent fractionation (MIF; ∆199Hg) and mass dependent fractionation (MDF; δ202Hg) were calculated and compared among samples. IRs of Hg in sediment were characterized mainly by MDF and low MIF (∆199Hg -0.37 to 0.74‰). However, all biota showed evidence of MIF, most pronounced in zooplankton (∆199Hg up to 3.40 ‰) and char (∆199Hg up to 4.87‰). Zooplankton takes up highly fractionated MeHg directly from the water column, while benthic organisms are exposed to sedimentaryHg,whichcontainslessfractionatedHg.Asevidenced by δ13C measurements, benthic chironomids make up a large proportion of char diet, explaining in part why MIFchar < MIFzooplankton in lakes, where both samples were measured. Hg IRs in char varied among regions, while char from lakes from each region showed similar degrees of MIF. A MIF-offset was derived representing the mean MIF difference between sediment and fish, and indicated that fish in two regions retain sediment signatures altered by a consistent offset. Due to its minimal laketo-catchment area and very high water retention time (∼330 years), the meteor impact crater lake (Pingualuk) reflects a “pure” atmospheric Hg signature, which is modified only by aqueous in-lake processes. All other lakes are also affected by terrestrial Hg inputs and sediment processes.

Introduction Mercury (Hg) is naturally present as seven stable isotopes (196Hg, 198Hg, 199Hg, 200Hg, 201Hg, 202Hg, and 204Hg), which differ in relative mass by up to 4%. Characterizing the Hg * Corresponding author current address: Water & Climate Impacts Research Centre Environment Canada, University of Victoria, Victoria, British Columbia, Canada V8W 3R4; phone: (250) 363 8947; fax: (250) 363 3586; e-mail: [email protected]. † University of Guelph. ‡ Environment Canada. § Trent University. 9148

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isotope fingerprints (i.e., ratios) in the environment has been proposed as a tool of tracing sources of Hg pollution (atmospheric or point sources) (1, 2). Stable isotope ratios (IRs) of a number of elements (Li, Mg, Ca, Fe, Cu, Pb, Zn, Se, and Mo) are being used more and more as isotopic tracers in the environment (3). Hg isotopes are probably fractionated by industrial processes prior to deposition in the environment, resulting in isotopic ratios that may be specific to a region or even to an industry (4). While Hg IRs could be highly useful in linking Hg found in the environment to a specific source, fractionation effects are likely to change Hg IRs, which may obscure the initial fingerprint. While earlier studies have focused on detecting mass-dependent fractionation (MDF), more recently the discovery of massindependent fractionation (MIF) of Hg in biota has received more attention. MIF is specifically associated with Hg isotopes of odd mass numbers (199Hg and 201Hg) and may be useful to track Hg transfer through the environment, as MIF is only induced by a few, selected transformation reactions and remains unchanged by simple transport or transfer processes. Environmental fractionation of Hg, aided by biotic and abiotic (e.g., photochemical) processes, and with varying degrees of both MDF and MIF has been experimentally demonstrated in water (1), Hg resistant bacteria (5), and confirmed in sediments (6-8), ore deposits (9, 10), and biota (1, 11-13). Jackson et al. (11) reported biotic MIF in food webs (including fish) of three lakes in Canada (Lakes Cli, Shipiskan, and Ontario) and MIF was documented in several fish species by Bergquist and Blum (1). MDF values range from ∼6‰ by bacteria (δ202Hg, experimental) and ∼5.5‰ in ores, to -1‰ to 4‰ in sediments (δ202Hg, natural) (8), and MIF values range from 2.5‰ (∆199Hg, photo reduction experiments) (1), up to ∼4‰ (∆201Hg) in various fish species (1, 11). The precision of isotope ratio measurements by MC-ICP/MS allows detection of fractionation of about 0.2‰ or better. The Arctic is an area of great concern because indigenous peoples, particularly the Inuit (of Alaska, Canada, and Greenland) largely depend on a subsistence harvest of apex predators (fish, birds, or marine mammals), which are known to contain high concentrations of Hg (14). The Arctic receives most of its anthropogenic Hg through the atmosphere (15, 16), but inputs into lakes can also occur from terrestrially derived (anthropogenic and natural) Hg sources (17). High Arctic regions (north of 70° N), are known to be influenced by Eurasian sources (18). Anthropogenic Hg deposition in Arctic lakes in Canada based on the analysis of dated sediment cores averaged 2.8 µg m-2 y-1 in good agreement with results from global mercury deposition modeling (19). The relative contribution from atmospheric and terrestrial Hg sources and their pathways into freshwater food webs including Arctic food webs are not fully understood, thus IRs, and in particular MIF, have been proposed as a possible tool to trace sources. However, information on Hg isotopic ratios in Arctic samples is limited to one sediment core from Romulus Lake, Ellesmere Island, Nunavut, Canada (6) and one study on Alaskan seabird eggs (20). Arctic lake food webs are low in biodiversity and the transfer of nutrients to fish is coupled to benthos (21, 22): from sediment (main site of Hg methylation) to benthic invertebrates (chironomids) to adult Arctic char (Salvelinus alpinus L.). Zooplankton (dominated by Copepoda) is part of the char diet, but may not contribute greatly to Hg uptake (22). Cannibalism is known to occur (23), and can effect Hg concentrations in adult char (24). We set out to determine the following: (1) if Hg IRs in sediments and char vary by lake and region; (2) if there is 10.1021/es901771r CCC: $40.75

 2009 American Chemical Society

Published on Web 11/10/2009

FIGURE 1. Map of northern Canada and part of Greenland showing the 10 sampled lakes (red dots indicate approximate locations of lakes) in four Arctic regions with three in Nunavut and one in Nunavik, northern Quebec. Detailed location and lake characteristics are given in SI Table 1.

fractionation along the food chain; and (3) whether the results can be used to distinguish sources from natural processes occurring in the lake. Hg isotope signatures in fish may not need to be identical to atmospheric signatures to identify sources, as long as all fractionation steps between Hg(II) deposited in sediments leading to MeHg in fish are consistent among lakes. Accordingly, we measured stable isotope ratios of Hg, N, and C in surface sediments (surface layer 0-2 cm), zooplankton (bulk), emerging insects (chironomids), and apex predator (char) from 10 lakes across a wide geographic range in the Canadian Arctic. The δ15N and δ13C isotope data were used to infer trophic relations within the biota of each lake and were compared with the Hg IRs to help clarify variations in the isotope composition of Hg along food webs.

Materials and Methods Sampling Sites. Three lakes in three primary sampling locations in the Canadian Arctic were selected over a range of latitudes and longitudes (Figure 1, SI Table 1). The tenth lake is the meteor-impact crater lake, Lake Pingualuk, which has a minimal catchment and thus influence from the terrestrial environment is minimal. Further details on all lakes are provided in Supporting Information and in Gantner et al. (25). Sample Collection. Arctic char were collected from 10 lakes in spring and summer of 2006 and 2007. Five to six char per lake were selected for Hg IR analysis; the selection was based on similar δ15N signatures and total Hg concentrations ([THg]) in muscle tissue to reduce skewing of results through trophic variability or differences in [THg]. To test for influences of δ15N, two morphotypes (with distinguishable δ15N signature) of char from Lake Hazen (26) were selected (i.e., three cannibal and three piscivorous individuals). For Amituk Lake, char were selected from a wide range of [THg] (0.9-3.0 µg/g ww), allowing us to investigate any effects [THg] may have on fractionation of Hg isotopes. All analysis for

char (Hg IRs, [THg], δ15N, and δ13C) was performed on subsamples of dorsal muscle tissue. To investigate Hg IRs, samples of the principal prey items of char (i.e., chironomid midges and zooplankton) of known MeHg content were obtained from four lakes. Zooplankton from Gavia Faeces and Notgordie Lakes, and Lake Hazen were collected in 2006 and 2007, respectively. Unfortunately, the amount of zooplankton obtained from Lake Pingualuk was not sufficient to allow the measurement of IRs. For Resolute Lake, zooplankton samples from 2005 to 2007 were analyzed to evaluate annual variability of IRs. Adult chironomid midges were collected from lakes Resolute and Hazen. High Arctic chironomid species stop feeding several months prior to emergence (27), thus Hg concentrations and IRs measured reflect body burden and not gut contents. Sediment samples were obtained using a Ponar sampler, and the upper 2 cm was subsampled for analysis. Analysis of THg and IRs. The THg content of fish and sediment samples was determined by direct combustion using a Milestone DMA 80 (Milestone Inc., Shelton, CT; EPA method 7473). MeHg concentrations in zooplankton and chironomids were determined by GC CV-AFS, following methods described in Gantner et al. (25). Stable isotope analysis of carbon and nitrogen was conducted using Delta continuous-flow isotope ratio mass spectrometry (Delta, Thermo-Scientific) connected to an elemental analyzer (Carlo Erba) at the Environmental Isotope Laboratory, University of Waterloo (Canada). Lake area (LA) and catchment area (CA) were available for all sites (SI Table 1), allowing us to investigate the potential effect of inputs of Hg from the terrestrial surroundings (17, 28, 29) of a lake by comparing CA/LA ratio and MIF. For Hg isotope analysis, all samples were digested in open, precleaned glass vials on a hot plate at ∼120 °C using ∼5 mL of aqua-regia (HNO3/HCl, 1:3 v/v). Following digestion, the sample extracts were made up to 40 mL with Milli-Q water, VOL. 43, NO. 24, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. (A) δ199Hg versus δ202Hg in sediments, Arctic char (by region), zooplankton, and chironomid samples (means ( 1 SD). The continuous line represents the theoretical mass-dependent fractionation (MDF). Deviation from this line (red arrow) represents mass-independent fractionation (MIF). (B) MIF ∆199Hg versus δ202Hg in Arctic char, sediments, and standard reference materials (SRMs). aiming for final concentrations of 0.5, 1.0, or 2.0 ng/mL. Hg isotope ratios were measured by MC-ICP-MS (Neptune, ThermoScientific), employing the method of Foucher and Hintelmann (8). Here, we report the total isotope fractionation using the delta notation (δ) as calculated from δXHg ) 1000 ×

(XHgsample/ 198Hg - XHgstandard/ 198Hg) X

Hgstandard / 198Hg

using NIST 3133 as standard, and are reporting MDF in terms of δ202Hg and specifically MIF as ∆199Hg ) δ199Hg - δ202Hg × 0.252 (2) (unless otherwise noted), assuming a kinetic fractionation law (30). The confidence level (type I error rate) for statistical significance was set to R ) 0.05 for all analyses. Linear regression was used to infer relationships among IRs and analysis of variance (ANOVA) was used to determine differences of mean IRs among sites.

Results and Discussion Surface Sediments. The 13 sediments revealed mostly MDF and little MIF for Hg isotopes (Figure 2). Ratios of δ202Hg ranged from -2.03 to 0.25‰ (average -0.59 ( 0.66‰) overall. Regional comparison of sediment IR signatures from Ellesmere (n ) 6), Cornwallis (n ) 3), and Kent Peninsula (n ) 3) revealed small differences within regions, with δ202Hg averaging -0.94 ( 0.11‰ (average ( SE), -1.23 ( 0.41‰, and -1.02 ( 0.32‰, respectively. Our results are in agreement with literature values for two subarctic lakes of -2.3 and -1.6‰ δ202Hg in Cli and Shipiskan (0-0.5 cm), respectively (converted to 202/198Hg notation used here from Jackson et al. (31)). However, sediments in subarctic lakes appear slightly more depleted in heavier isotopes. The Hg signal detected in lake sediments is probably a combination of Hg derived from the atmosphere directly and from the catchment (atmospheric + terrestrial/natural) of the lakes, and the isotope signatures of these sources may have been different. The upper ∼2 cm of sediment layer analyzed in our study likely reflects an integrated Hg signal of 15-30 years of deposition and sedimentation (19), which could explain the variation among lake sediments. Variation of up to 1.5‰ in δ-values in the upper 15 cm was reported for one Arctic sediment core (Romulus Lake, Ellesmere Island) (6), however, variation was small ( 0.05). Variability of Hg isotopic composition within lakes was low in terms of ∆201Hg (-1.12 ( 0.01‰, mean ( 1 SD) and ∆199Hg (-0.2 ( 0.1‰), as indicated by three samples obtained from Lake Hazen in 2007 (mean of three samples ( SD). The mean ∆199Hg/∆201Hg ratio of sediments for the lakes as a whole was 1.35 ( 0.77. The highest regional ∆199Hg/ ∆201Hg ratio was calculated for the most southern sediments on Kent Peninsula a ratio of 2.04 ( 0.71, while Cornwallis lake sediments had a ratio of 1.32 ( 0.70, and the most northern lake sediments on Ellesmere Island (1.00 ( 0.64). Pingualuk sediment had the lowest ratio (0.49) of all individual lakes. These ratios are in very good agreement within the range (0.54-2.00) reported by Ghosh et al. (12) for peat. The variation of this ratio is likely caused by different mechanisms of MIF, and in turn may be utilized to distinguish different MIF processes (12). Hg IRs in Arctic Char. The Hg signatures of 50 char muscle samples varied considerably overall (δ202Hg: -1.68 to 1.27‰) (SI Table 2). However, char from lakes within each region revealed strikingly similar degrees of fractionation (Figure 2). Char from Ellesmere lakes (Lake D, Lake G, and Hazen) showed the overall lowest fractionation (δ202Hg: -1.68 to -0.62‰), while char from Ungava Peninsula (Pingualuk) had the highest δ-values (δ202Hg: 1.13 to 1.27‰). Char from lakes on Cornwallis and Kent Peninsula had similar isotopic composition, ranging from δ202Hg: -0.86 to 0.18‰, and δ202Hg: 0.03 to 0.41‰, respectively. Interestingly, δ202Hg in fish decreased with increasing latitude (linear regression, r2 ) 0.84, p < 0.05) (SI Figure 2). This could be due to different degree of fractionation of Hg during atmospheric transport to the Arctic, reflect Hg derived from different air masses, or because of differences in terrestrial Hg sources, or a combination of all three.

FIGURE 3. Regression equations of ∆201Hg versus ∆199Hg relationships for all lakes, and per region. The legend in the insert applies to both figures. MIF was evident in all char samples, ∆199Hg ranging from 0.18 to 4.87‰ (SI Table 2). These results are in agreement with results for lake trout (Salvelinus namaycush) from subarctic lakes (∆199Hg, 2.2-5.5‰) (11) and burbot (Lota lota) from three Michigan lakes (∆199Hg, 3.0-5.0‰) (5). The lowest MIF occurred in the most northerly lakes on Ellesmere (range of ∆199Hg: 1.28-2.20‰), while the greatest degree of MIF was measured in Lake Pingualuk char (∆199Hg: 4.53-4.87‰). Again, Cornwallis and Kent Peninsula char had similar degrees of MIF (∆199Hg: 0.18-2.28‰; and 1.02-2.33‰, respectively). MIF in Pingualuk was higher than all three regions (ANOVA p < 0.05), while there were no statistical differences among the other three regions (p > 0.05). Among the Cornwallis char, Amituk Lake char (n ) 5) deviated from the other two sites within this region, as they showed low MIF (∆199Hg: 0.18-0.56‰), which curiously resembles the range of MIF in sediments presented here. This may be due to the large catchment area of Amituk, or different light conditions in this lake, leading to varying degrees of MIF inducing photodemethylation (5). The clear delineation of ∆199Hg in char from the 4 regions is contrasted against δ202Hg in Figure 2B. The relationship between ∆199Hg and ∆201Hg in all char combined was strong (r2 ) 0.99, p < 0.05) (Figure 3), and did not differ greatly among the four regions (Ellesmere r2 ) 0.97; Cornwallis r2 ) 0.99; Kent Peninsula r2 ) 0.98, Pingualuk r2 ) 0.96, all p < 0.05). Slopes of the linear regressions ranged from 1.09 (Ellesmere) to 1.33 (Kent Peninsula) among regions, while the slope was 1.21 for all char combined. Regression parameters for each region and for all fish combined are presented in SI Table 4. The mean ∆199Hg/∆201Hg ratio for all char was 1.32 ( 0.06 overall (excluding Amituk Lake char). Amituk Lake had a ratio of 19.3 ( 39.1, and influenced the overall ratio greatly, prompting us to look at this site separately and to exclude it. Mean ∆199Hg/∆201Hg ratios of char were lower (pTukey’s < 0.05) on Ellesmere (1.27 ( 0.04) and at Pingualuk (1.27 ( 0.01) than on Cornwallis (1.39 ( 0.04, Amituk char removed) and Kent Peninsula (1.34 ( 0.06). These ratios are lower than previously reported for fish-eating arctic birds (1.19) (20). In comparison, photoreduction experiments yielded a ∆199Hg/ ∆201Hg ratio of ∼1 to 1.3 (1, 32), while abiotic, nonphotoinduced reduction gave values from ∼1.5 to 1.6 (32). Our ∆199Hg/∆201Hg ratios in char are consistent with the pho-

FIGURE 4. MIF-offset Φ ) (MIFchar - MIFsediments) for ∆199Hg (black bars) and ∆201Hg (gray bars) in all 10 lakes. Capital letters indicate significant differences of regional mean MIF-offset (ANOVA p < 0.05), small letters indicate significance of ANOVA Tukey’s HSD within regions (statistic results shown refer to ∆199Hg). toreduction ratio, which is thought to be the result of magnetic isotope effects (MIE). Since the nonphotoinduced reduction ratio is more likely caused by nuclear volume effects (NVE) (32), our data suggest that MIE is the dominating reason for the MIF observed in char. The only unknown in this process is the MeHg bioaccumulaton process, which may or may not cause MIF with unknown contributions of NVE and MIE. Previous studies showed that MIF varied among fish species (1, 11), which complicated regional comparisons. Our study is the first to highlight regional differences in Hg isotope fractionation utilizing only one fish species over a large geographic range. We demonstrate here that the degree of Hg fractionation can vary considerably between geographic regions even within one species, which may be reflective of true regional differences of Hg signal. Among lakes of two of the three regions in which multiple lakes were sampled, however, fractionation was similar, indicating that regional Hg signals are consistent in neighboring lakes and that region may be more important than trophic position of fish (Figure 4, SI Figure 1A). Assuming that biomagnification is similar in neighboring lakes, which is plausible considering the typically simple food chains in this region, the differences among regions are mainly caused by differences in Hg sources, of either atmospheric or geogenic origin. Watershed characteristics may also need to be taken in account, as demonstrated by the differences between Amituk and the other two lakes on Cornwallis Island. MIF in Fish and Sediments. Since >95% of all Hg in fish muscle is typically in the form of MeHg (33), fractionation of Hg isotopes during food web transfer should be associated with MeHg (11). We compared the results of fractionation in char with sediment Hg IRs, where changes should reflect the integrated effects of processes occurring between Hg deposition (to sediments through sedimentation) and food web transfer to top predator. We calculated the difference in MIF (∆199Hg and ∆201Hg) between mean sediment signature and mean char signature, and refer to this measure as MIF-offset Φ: Φchar-sediment ) (MIFchar - MIFsediments), and compared the results among lakes and regions (Figure 4). All of our lakes showed detectable MIF-offsets, which is presumably the result of at least one of Hg methylation, MeHg bioaccumulation, or MeHg photoreduction, with the latter the most likely cause and would lead to an enrichment of odd isotopes in MeHg remaining in lake water. Additionally, significant differences in MIFVOL. 43, NO. 24, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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offset (∆199Hg and ∆201Hg) were not only detected among all 10 lakes (ANOVA p < 0.05), but also between different regions (ANOVA p < 0.05). Ellesmere Island had higher MIF-offset than Cornwallis and Kent Peninsula sites (pTukey’s 0.05). However, the MIF-offset of lakes on Ellesmere Island did not differ among each other (pTukey’s > 0.05), and the same was determined for Kent Peninsula lakes (pTukey’s > 0.05) (Figure 4). Cornwallis lakes appeared more variable, showing significant differences (pTukey’s 260 m), and a very steep shoreline. Stomach contents and δ15N values indicate that adult Pingualuk char are cannibals, moreover their δ13C signature implies a pelagic habitat, and their connection with the benthic environment is minimal (22). Thus, the high degree of Hg isotope fractionation in general, and MIF (∼4.5‰) in particular, in Pingualuk char that were analyzed suggest that the isotopic signature of this population is controlled by atmospheric inputs and aqueous transformation processes (e.g., photoreduction). Furthermore, Pingualuk was the only lake whose sediment yielded a positive δ202Hg signature (0.25‰). This could be also due to its depth and its extraordinary clear water column (Secchidepth of 33 m and a profundal depth of 87 m () 1% of surface light) (34). Possibly, extensive photoreduction removed a significant fraction of light isotopes from the water column, leading to an enrichment of heavy isotopes in sediment. Moreover, the sedimentation rate is extremely low (1.2 mm/ 100 years) as determined most recently from sediment cores (R. Pienitz, University of Laval, Quebec, Canada, Pers. comm.), indicating that Hg may remain in the water column (and thus exposed to light) much longer in this lake compared to other sites. This would allow photochemical reactions such 9152

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as photoreduction and -demethylation to induce MIF in Hg species (1), which may subsequently transfer to sediments via sedimentation processes. On the other end of the MIF spectrum in this study is Amituk Lake. Char here are known to have high [THg] (35). The THg profile in sediments cores suggest an anthropogenic signal (19), which may be derived via its large catchment (17). Another study suggests that deposition of THg in sediments is a result of autochtonous carbon production and scavenging of Hg by algae (36). However, the Amituk watershed is large (∼26 km2), compared to the lakes surface area (0.38 km2), leading us to consider the effect of catchment inputs to the lake to be significant. The difference in fractionation of Hg (i.e., low MIF) in Amituk Lake char may thus be a result of a more terrestrially influenced Hg signal. Trophic Relationships and MIF in Food Webs. Our δ13C results confirm that zooplankton occupied a pelagic habitat and were subject to a pelagic dietary regime. Furthermore, zooplankton and chironomid midges had distinct δ13C signatures, indicating utilization of different carbon sources (SI Figure 4A). Zooplankton in the study lakes inhabit pelagic zones (δ13C range: -35.7‰ to -29.7‰), while chironomid midges occupy benthic habitat (δ13C range: -20.1‰ and -23.7‰). The δ13C values of char (-19.9‰ to -25.3‰) indicate that chironomids make up a large proportion of char diet (SI Figure 4A) (22, 25). High MIF in zooplankton was found in all four lakes and even exceeded the degree of MIF in char in lakes Hazen and Resolute (SI Figure 4B). This is surprising, as previous studies in subarctic and midlatitude lakes have shown zooplankton to have low MIF compared to fish (11). Greater MIF in zooplankton may be due to a different source of Hg utilized by pelagic zooplankton. Zooplankton are likely receiving Hg directly from the water column (37), in contrast to benthic invertebrates. Microcosm experiments using enriched Hg undertaken during the METAALICUS experiment, also confirm rapid uptake of Hg by pelagic zooplankton (38). MIF in zooplankton could thus reflect a (pelagic) water Hg signal, which may be more representative of recent atmospheric inputs compared to sediments, which integrate over longer time periods. Possibly, MeHg present in the water column may also be photodemethylated, resulting in a different MIF signature relative to sediments, as suggested by Lake Pingualuk sediment and fish results. Annual variation of δ202Hg in three zooplankton samples from Resolute Lake (2005-2007) was considerable from -0.23‰ (2005), to -0.13‰ (2006), and -0.14‰ (2007) (all δ202Hg), indicating some variability of the signal over time. It is possible that light conditions may have varied due to the extent of ice cover at time of sampling (July). Resolute Lake was mostly ice-covered in 2005, partly ice-covered in 2006, and ice-free in 2007. The degree of photodemethylation (leading to stronger MIF) in the water column may have varied. During this period, mean (monthly) air temperatures in July of 3.8 °C (2005), to 4.3 °C (2006), and 7.4 °C (2007) were recorded at the nearby Environment Canada weather station. While these climatic indicators do not confirm our above hypothesis, they also do not eliminate the possibility that precipitation and thus optical properties of the ice-cover may have influenced light conditions in Resolute Lake prior to July. The degree of MIF in zooplankton varied between 2.2-3.4 ‰ (∆199Hg) and 1.5-2.7 ‰ (∆201Hg) over the same period (SI Figure 4B). This could indicate a response to variable proportions of terrestrial versus atmospheric (anthropogenic) inputs under the prevailing climatic conditions (17, 19). Chironomid midges from Lake Hazen and Resolute Lake showed MIF as well (∆199Hg: 0.32‰ and 1.31‰), although to a much lesser degree than zooplankton. At both sites, adult chironomid IRs were intermediate between those of

sediment and char (SI Figure 4B). No chironomids were obtained at Lake Pingualuk and Kent Peninsula lakes. In contrast to pelagic zooplankton, chironomids are benthic organisms, mostly or partly buried in the sediment throughout their larval stage. It is thus intuitive that these organisms would carry an Hg IR signal similar to the sediments, which is confirmed by the low MIF in chironomids. Jackson et al. (11) report similar results for invertebrates from Lake Ontario (Ontario, Canada). Catchment-to-Lake Area and MIF. Although the MIFoffset (Φ) was generally not related to the ratio of catchment area (CA)/lake area (LA) (linear regression p > 0.05) (SI Figure 3), results of Lake Pingualuk (which had the smallest CA/LA ratio and greatest MIF) and Amituk lake (which had the greatest CA/LA ratio and smallest MIF) indicate that this factor may nonetheless affect Hg isotope fractionation to some extent. The size of the catchment and precipitation determine the amount of terrestrial and atmospheric input a lake receives, but the design of this study did not allow detailed conclusions to be drawn from our observation. However, conventional Hg models assign catchment area a big influence (29, 39). On the other hand the influence may be greater in temperate lakes since Muir et al. (19) did not see a relationship of Hg fluxes in dated sediment cores with CA/LA or catchment area in high arctic lakes but did see a relationship in midlatitude lakes.

Acknowledgments We thank Network of Centres of Excellence ArcticNet, Environment Canada, the Canadian International Polar Year Project - Climate Variability and Change on Chars in the Arctic, the Canadian Foundation for Climate and Atmospheric Sciences (Grant GR-623), and Indian and Northern Affairs Canada (NCP) for financial support of this work. We are grateful to Greg Lawson for his valuable help during digestion of samples in the laboratory (NWRI, Burlington). Wendy Michaud (University of Waterloo) aided during analysis and interpretation of Pingualuk char trophic isotope analysis. We thank Reinhard Pienitz for providing the Pingualuk sediment sample for analysis. Togwell Jackson provided valuable comments on an early version of the manuscript.

Supporting Information Available Additional explanatory text, tables, and figures. This material is available free of charge via the Internet at http:// pubs.acs.org.

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