Distant drivers or local signals: where do mercury trends in western Arctic belugas originate?

Temporal trends of contaminants are monitored in Arctic higher trophic level species to inform us on the fate, transport and risk of contaminants as well as advise on global emissions. However, monitoring mercury (Hg) trends in species such as belugas challenge us, as their tissue concentrations reflect complex interactions among Hg deposition and methylation, whale physiology, dietary exposure and foraging patterns. The Beaufort Sea beluga population showed significant increases in Hg during the 1990 s; since that time an additional 10 years of data have been collected. During this time of data collection, changes in the Arctic have affected many processes that underlie the Hg cycle. Here, we examine Hg in beluga tissues and investigate factors that could contribute to the observed trends after removing the effect of age and size on Hg concentrations and dietary factors. Finally, we examine available indicators of climate variability (Arctic Oscillation (AO), the Pacific Decadal Oscillation (PDO) and sea-ice minimum (SIM) concentration) to evaluate their potential to explain beluga Hg trends. Results reveal a decline in Hg concentrations from 2002 to 2012 in the liver of older whales and the muscle of large whales. The temporal increases in Hg in the 1990 s followed by recent declines do not follow trends in Hg emission, and are not easily explained by diet markers highlighting the complexity of feeding, food web dynamics and Hg uptake. Among the regional-scale climate variables the PDO exhibited the most significant relationship with beluga Hg at an eight year lag time. This distant signal points us to consider beluga winter feeding areas. Given that changes in climate will impact ecosystems; it is plausible that these climate variables are important in explaining beluga Hg trends. Such relationships require further investigation of the multiple connections between climate variables and beluga Hg.

Transformation of mercury at the bottom of the Arctic food web: an overlooked puzzle in the mercury exposure narrative.

We show 2008 seasonal trends of total and monomethyl mercury (THg and MeHg, respectively) in herbivorous (Calanus hyperboreus) and predatory (Chaetognaths, Paraeuchaeta glacialis, and Themisto abyssorum) zooplankton species from the Canadian High Arctic (Amundsen Gulf and the Canadian Beaufort Sea) in relation to ambient seawater and diet. It has recently been postulated that the Arctic marine environment may be exceptionally vulnerable to toxic MeHg contamination through postdepositional processes leading to mercury transformation and methylation. Here, we show that C. hyperboreus plays a hitherto unrecognized central role in mercury transformation while, itself, not manifesting inordinately high levels of THg compared to its prey (pelagic particulate organic matter (POM)). Calanus hyperboreus shifts Hg from mainly inorganic forms in pelagic POM (>99.5%) or ambient seawater (>90%) to primarily organic forms (>50%) in their tissue. We calculate that annual dietary intake of MeHg could supply only ∼30% of the MeHg body burden in C. hyperboreus and, thus, transformation within the species, perhaps mediated by gut microbial communities, or bioconcentration from ambient seawater likely play overriding roles. Seasonal THg trends in C. hyperboreus are variable and directly controlled by species-specific physiology, e.g., egg laying and grazing. Zooplankton that prey on species such as C. hyperboreus provide a further biomagnification of MeHg and reflect seasonal trends observed in their prey.

Importance of Arctic zooplankton seasonal migrations for α-hexachlorocyclohexane bioaccumulation dynamics.

Like most zooplankton, Calanus hyperboreus undergoes seasonal migration spending late spring and summer grazing at the surface and the rest of the year in diapause at depth. As a result, in the Arctic Ocean this copepod resides for part of the year in the hexachlorocyclohexane (HCH) enriched surface water and for part of the year at depth where HCH undergoes significant microbial degradation resulting in far lower concentrations (~3 times for α-HCH). We collected C. hyperboreus from summer and winter from the Amundsen Gulf and measured their α-HCH concentrations, enantiomeric compositions, and bioaccumulation factors (BAFs) to investigate how this copepod responds to the change in exposure to α-HCH. C. hyperboreus collected in winter were also cultured for 5 weeks under surface water conditions without feeding to investigate bioconcentration dynamics following spring ascent. Concentration of α-HCH was 2-3 times higher in individuals from the summer than those from the winter. Log BAF from the summer (feeding period) does not exceed log BCF (bioconcentration factor) from the culturing experiment (no feeding) suggesting that α-HCH concentration in C. hyperboreus is maintained through equilibration rather than feeding. After the spring ascent from deep waters, C. hyperboreus approach equilibrium partitioning with the higher surface water concentrations of α-HCH within 3-4 weeks with about 60% of bioconcentration taking place in the first week. The C. hyperboreus α-HCH chiral signature also reflects ambient seawater and can therefore be used as a determinant of residence depth. Even though a single cycle of seasonal migration does not result in a significant redistribution of α-HCH in the water column, this process could have a significant cumulative effect over longer time scales with particular local importance where the zooplankton biomass is high and the ocean depth is great enough to provide substantial vertical concentration gradients.

Biotic interactions in temporal trends (1992-2010) of organochlorine contaminants in the aquatic food web of Lake Laberge, Yukon Territory.

Declines in 6 organochlorine (OC) contaminant groups; chlordane (CHL), DDT, HCH, toxaphene (CHB), PCB and chlorinated benzenes (CBz) were measured in biota of a sub-Arctic lake (Lake Laberge, YT) following the closure of a commercial fishery in 1991. This study examined morphological (length, weight, age), biochemical (lipid content, δ(13)C, δ(15)N), population and OC data for 9 fishes and zooplankton between 1993 and 2003 (2010 for lake trout) to investigate causes for the OC declines. Growth dilution was a major factor influencing the decrease of OCs in lake trout, round whitefish and possibly zooplankton most notably in the early 2000s. A decline in lipids of most fish species also contributed to OC declines, although no such change was evident for zooplankton. It is suspected that increases in fish populations or climate variations over the 1990s, may have contributed towards a shift in plankton community composition. From 1991 to 1999, CPUE increased for 7 of the fish species and declined for 2 others. Concurrently, the zooplankton community shifted from an abundance of C. scutifer in 1993 to dominance by D. pribilofensis in 2001. Nitrogen and carbon stable isotope data suggested that food web interactions for most fish species have not changed over time. Although concentrations of OCs have declined in many fishes, the "rate" of OC transfer (using slopes of log OC vs. nitrogen isotope ratios) through the food web was greater in 2001 than in 1993. Overall, the declines in OC concentrations in the fish from Lake Laberge occurred concurrently with changes in their growth, lipid, and abundance, suggesting that ecosystem responses to the closure of the fishery were in part responsible for the lower contaminants in these fishes. As a result of this study, the Yukon government rescinded the health advisory for limiting the consumption of fish from Lake Laberge.

Mechanisms and implications of α-HCH enrichment in melt pond water on Arctic sea ice.

During the summer of 2009, we sampled 14 partially refrozen melt ponds and the top 1 m of old ice in the pond vicinity for α-hexachlorocyclohexane (α-HCH) concentrations and enantiomer fractions (EFs) in the Beaufort Sea. α-HCH concentrations were 3 - 9 times higher in melt ponds than in the old ice. We identify two routes of α-HCH enrichment in the ice over the summer. First, atmospheric gas deposition results in an increase of α-HCH concentration from 0.07 ± 0.02 ng/L (old ice) to 0.34 ± 0.08 ng/L, or ~20% less than the atmosphere-water equilibrium partitioning concentration (0.43 ng/L). Second, late-season ice permeability and/or complete ice thawing at the bottom of ponds permit α-HCH rich seawater (~0.88 ng/L) to replenish pond water, bringing concentrations up to 0.75 ± 0.06 ng/L. α-HCH pond enrichment may lead to substantial concentration patchiness in old ice floes, and changed exposures to biota as the surface meltwater eventually reaches the ocean through various drainage mechanisms. Melt pond concentrations of α-HCH were relatively high prior to the late 1980-s, with a Melt pond Enrichment Factor >1 (MEF; a ratio of concentration in surface meltwater to surface seawater), providing for the potential of increased biological exposures.

Western Canadian Arctic ringed seal organic contaminant trends in relation to sea ice break-up.

The association between changing sea ice conditions and contaminant exposure to Arctic animals interests Inuvialuit harvesters, communities, and researchers. We examined organochlorine contaminant (OC) concentrations in the blubber of 90 male adult ringed seals (Phoca hispida) sampled from the subsistence harvest in Ulukhaktok (formerly Holman), NT, Canada, just prior to break-up of the sea ice (1993-2008). OC blubber concentrations were assessed with respect to year and sea ice break-up date. HCB and age- and blubber-adjusted concentrations of p,p'-DDT and ΣCHB (chlorobornane) significantly decreased over the study period. With respect to the timing of the spring break-up, highly lipophlic OCs, such as p,p'-DDE and PCB 153, were higher during years of early ice clearing (at least 12 days earlier than the mean annual break-up date), whereas no trends were observed for α, ß, and γ isomers of HCH, trans- and cis-chlordane, oxychlordane, or ΣCHB. The higher contaminant concentrations found in earlier break-up years is likely due to earlier and/or increased foraging opportunities. This situation also has potential for enhancing bioaccumulation and biomagnification of contaminants over the long-term if projected changes continue to result in lighter and earlier ice conditions.

Mercury toxicity in beluga whale lymphocytes: limited effects of selenium protection.

Increasing emissions of anthropogenic mercury represents a growing concern to the health of high trophic level marine mammals. In its organic form, this metal bioaccumulates, and can be toxic to several physiological endpoints, including the immune system. In this study, we (1) evaluated the effects of inorganic mercury (mercuric chloride, HgCl2) and organic mercury (methylmercuric chloride, MeHgCl) on the in vitro function of lymphocytes isolated from the peripheral blood of beluga whales (Delphinapterus leucas); (2) characterized the potential protective effects of sodium selenite (Na2SeO3) on cell proliferation of HgCl2 or MeHgCl-treated beluga whale lymphocytes; and (3) compared these dose-dependent effects to measurements of blood Hg in samples collected from traditionally harvested beluga whales in the western Canadian Arctic. Lymphocyte proliferative responses were reduced following exposure to 1 µM of HgCl2 and 0.33 µM of MeHgCl. Decreased intracellular thiol levels were observed at 10 µM of HgCl2 and 0.33 µM of MeHgCl. Metallothionein induction was noted at 0.33 µM of MeHgCl. Concurrent exposure of Se provided a degree of protection against the highest concentrations of inorganic Hg (3.33 and 10 µM) or organic Hg (10 µM) for T-lymphocytes. This in vitro protection of Se against Hg toxicity to lymphocytes may contribute to the in vivo protection in beluga whales exposed to high Hg concentrations. Current Hg levels in free-ranging beluga whales from the Arctic fall into the range of exposures which elicited effects on lymphocytes in our study, highlighting the potential for effects on host resistance to disease. The implications of a changing Arctic climate on Hg fate in beluga food webs and the consequences for the health of beluga whales remain pressing research needs.

α- and γ-Hexachlorocyclohexane measurements in the brine fraction of sea ice in the Canadian High Arctic using a sump-hole technique.

We used holes augered partially into first-year sea ice (sumps) to determine α- and γ-HCH concentrations in sea-ice brine. The overwintering of the CCGS Amundsen in the Canadian western Arctic, as part of the Circumpolar Flaw Lead (CFL) System Study, provided the circumstances to allow brine to accumulate in sumps sufficiently to test the methodology. We show, for the first time, that as much as 50% of total HCHs in seawater can become entrapped within the ice crystal matrix. On average, in the winter first-year sea ice HCH brine concentrations reached 4.013 ± 0.307 ng/L and 0.423 ± 0.013 ng/L for the α- and γ-isomer, respectively. In the spring, HCHs decreased gradually with time, with increasing brine volume fraction and decreasing brine salinity. These decreasing concentrations could be accounted for by both the dilution with the ice crystal matrix and under-ice seawater. We propose that the former process plays a more significant role considering brine volume fractions calculated in this study were below 20%. Levels of HCHs in the brine exceed under-ice water concentrations by approximately a factor of 3, a circumstance suggesting that the brine ecosystem has been, and continues to be, the most exposed to HCHs.

Increasing contaminant burdens in an arctic fish, Burbot ( Lota lota ), in a warming climate.

The temporal patterns of mercury (Hg), polychlorinated biphenyls (PCBs), and other contaminants in Arctic aquatic biota are usually attributed to changing atmospheric sources. However, climate variability and change is another means of altering contaminant fate and bioavailability. We show here that the concentrations of Hg and PCBs in Mackenzie River burbot ( Lota lota ), a top predator fish and important staple food for northern Canadian communities, have increased significantly over the last 25 years despite falling or stable atmospheric concentrations, suggesting that environmental processes subsequent to atmospheric transport are responsible. Using a dated sediment core from a tributary lake near the Mackenzie River sampling site, we show that variations in Hg concentrations downcore are strongly associated with labile, algal-derived organic matter (OM). Strong temporal correlations between increasing primary productivity and biotic Hg and PCBs as reflected by burbot suggest that warming temperatures and reduced ice cover may lead to increased exposure to these contaminants in high trophic level Arctic freshwater biota.

Historical interrelated variations of mercury and aquatic organic matter in lake sediment cores from a subArctic lake in Yukon, Canada: further evidence toward the algal-mercury scavenging hypothesis.

This study investigates the role of climate-driven aquatic productivity increases in conrolling the sedimentary accumulation of mercury (Hg) in Kusawa Lake, Yukon, Canada. Organic geochemistry data reveals a significant link between the increasing trends of Hg and of labile algal-derived organic matter (OM) over the centuries and in particular over recent decades. Sedimentary Hg accumulation rates continue to increase even though atmospheric concentrations of Hg have remained the same or even declined. Together, these findings indicate that the significant rise in Hg content recorded in the sediments since the early 1950s may not be representative of increasing atmospheric Hg levels but are strongly influenced by elevated concentrations of algal-derived OM in the water-column. These likely increase the scavenging rate of "available" Hg from the water column. Therefore, recent increases in algal productivity may result in overestimation of the historical rates of atmospheric Hg deposition recorded in the sediments. This study confirms earlier published findings from two High Arctic lakes, and shows that the Hg-OM scavenging hypothesis may also apply to sub-Arctic lakes.

Mercury trends in ringed seals (Phoca hispida) from the western Canadian Arctic since 1973: associations with length of ice-free season.

We examined a unique time series of ringed seal (Phoca hispida) samples collected from a single location in the western Canadian Arctic between 1973 and 2007 to test for changes in total mercury (THg) in muscle tissue associated with (1) year and (2) length of ice-free season. We found no temporal trend with muscle THg whereas a curvilinear relationship existed with the length of ice-free season: seals attaimed higher THg in short (2 months) and long (5 months) ice-free seasons. delta 15N and delta13C in muscle tissue did not illustrate significant trends with ice-free days. We estimated that the turnover time of THg in muscle was about twice as long as stable isotope turnover in muscle, possibly explaining the lack of trend with stable isotopes in association with ice-free duration. Our discussion explains how summer environmental conditions may influence the composition of prey (mercury exposure) available to ringed seals. Results offer insight into how marine mammals may respond to directional changes in the Arctic ice-free season.

Size and biomagnification: How Habitat selection explains beluga mercury levels.

Mercury (Hg) levels in the Beaufort Sea beluga (Delphinapterus leucas) population increased during the 1990s; levels have since declined but remain higher than the 1980s. The diet of this beluga population is not well-known, thus it is difficult to assess dietary Hg sources. During the summer, the Beaufort Sea belugas segregate by length, sex, and reproductive status corresponding to habitat use that may result in feeding differences and ultimately Hg uptake. To test this hypothesis, we examine beluga dietary variation using fatty acid profiles and determine which biological variables best predict diet Relationships between biological variables and fatty acids were further evaluated with stable isotopes and Hg concentrations in liver and muscle. Hg concentrations in muscle were better related to liver delta15N than muscle delta15N. Stable isotopes and fatty acids are compared in their ability to describe dietary Hg processes in beluga. Fatty acids provided support for influences of whale behavior on dietary Hg uptake, whereas stable isotopes inferred tissue Hg metabolic rates. Here, we show beluga length drives diet variability leading to differences in Hg uptake and biomagnification processes dominate beluga Hg levels over Hg bioaccumulation over time.

Evidence for control of mercury accumulation rates in Canadian High Arctic lake sediments by variations of aquatic primary productivity.

Climate warming in the 20th Century has had profound effects on the limnology of High Arctic lakes, including substantial increases in autochthonous primary productivity (APP). Here, we report organic carbon and Hg core profiles from two lakes which support the hypothesis that 20th Century increases in sedimentary Hg at these latitudes were largely driven by APP increases, via Hg scavenging by algae and/or suspended detrital algal matter. Hydrocarbons quantitatively released by thermal cracking of algal-derived organic matter ("S2" carbon) were used to reconstruct past APP. Variation of S2 flux accounted for 87-91% of the variance in total Hg flux in the study lakes since 1854. Mercury and S2 carbon were also associated during the pre-Industrial Period, co-varying by as much as 30% during past warm/cool periods. As a test of the hypothesis, predicted values for 20th Century [Hg] were derived from pre-1900 Hg-S2 relationships. Measured 20th Century [Hg] was on average only 6-11% higher than that predicted in one lake, and 33% higher in the other. S2-normalization of [Hg] in the latter lake suggested that 78% of the average increase in 20th Century [Hg] could be explained by scavenging. These findings suggest that the atmospheric contribution of long-range anthropogenic Hg to High Arctic lakes may have been overestimated by several-fold because of this climate-driven process, and was responsible for no more than 22% of the 20th Century [Hg] increase in the study lakes.

Temporal trends of organochlorine contaminants in burbot and lake trout from three selected Yukon lakes.

Historical studies have demonstrated that organochlorine (OC) concentrations in top predators can vary considerably from lake to lake within a small geographic region but temporal trends of these contaminants have rarely been monitored in a sub-Arctic area for a long period of time. This study examined OC concentrations, including chlordane (CHL), DDT, hexachlorocyclohexane (HCH), toxaphene (CHB), PCB and chlorinated benzenes (CBz) in lake trout and burbot, from three Yukon lakes (Laberge, Kusawa, Quiet), over a span of 11 years (1992-2003). Temporal and spatial differences continue to exist in the OC concentrations of burbot and lake trout between these lakes. There is strong evidence that these contaminants are declining at various rates in lake trout (Salveninus namaycush) in Laberge, Kusawa and Quiet Lakes. For example, SigmaDDT concentrations have decreased 39%, 85% and 84% in Kusawa, Quiet and Laberge Lakes, respectively. Conversely, no consistent trends were observed in OC concentrations for burbot (Lota lota). For example, there is no evidence of a decline in toxaphene concentrations of Kusawa burbot yet a 58% decrease was observed in Laberge samples. Increases were also observed in the SigmaHCH levels of Kusawa Lake burbot, as well as increases in all OC groups (except SigmaHCH) for the Quiet Lake burbot samples. Decreases in burbot were evident in SigmaHCH and SigmaCHB for Lake Laberge fish and in SigmaCHL for Kusawa Lake samples. Spatial variations in OC levels are quite evident as Lake Laberge trout and burbot continued to maintain the highest levels over the eleven-year period from 1992 to 2003 followed by Kusawa Lake and then Quiet Lake. These differences were related to a variety of factors especially the species morphological characteristics such as log age, log weights and fish lipid content. A decreasing trend in Quiet and Laberge Lake trout lipid content, coupled with fluctuating condition factors and increases in body masses, suggest biotic changes may be occurring within the food webs due to fish population variations related to the cessation of commercial fishing or potentially an increase in lake plankton productivity related to annual climate variation. It is suspected that biotic factors rather than atmospheric inputs are the primary factors affecting the contaminant concentrations in lake trout and burbot in the study lakes.

Spatial trends and factors affecting variation of organochlorine contaminants levels in Canadian Arctic beluga (Delphinapterus leucas).

Organochlorine pesticides and PCBs were analysed in blubber from beluga (Delphinapterus leucas), or white whales, collected at 15 sites in the Canadian Arctic between 1993 and 2001. The objective of the study was to define and interpret the spatial trends of major organic contaminants in northern beluga in terms of sources and transport pathways, and the biological factors influencing accumulation. When compared on a lipid weight basis, the concentrations of beta-HCH, cis-CHL and SigmaCHL, cis-nonachlor, heptachlor epoxide and p,p'-DDT were significantly higher in males than females at all five sites in the eastern Arctic where the two sexes were harvested. The differences were attributed to losses from the females during fetal development and lactation as reported in previous studies. Major compounds increased with age in males at most sites, however the lack of a significant increase with age at some sites was in part due to high organochlorine concentrations in young year classes (2-5 years), particularly at eastern sites such as Iqaluit and Pangnirtung. Lower concentrations of SigmaHCH and SigmaDDT compounds in young males in 2001 relative to 1995 at Hendrickson Island could be due to declining levels in the environment, changes in the diet, or differences in organochlorine loads transferred from the female after birth. Age-corrected least square mean concentrations in males showed significantly higher levels of many compounds, such as p,p'-DDE and SigmaCHB, at south Baffin Island sites than those in the west. Two notable exceptions were HCBz and beta-HCH which were higher in the west. Methoxyclor was detected in males at Sanikiluaq (58 ng g-1) and in both sexes at Kimmirut, but at no other sites. Principal component analysis grouped the 16 sites into five major groupings based on the similarity of normalised organochlorine pesticide and PCB levels. Sites from the western Arctic were grouped by higher proportions of HCBz, beta-HCH and gamma-HCH and higher chlorinated PCBs. Endosulfan and alpha-HCH comprised a larger proportion of total organochlorine residues in the northern Hudson Bay sites, while methoxychlor, chlordane compounds and octachlorobiphenyls were enriched at Sanikiluaq in eastern Hudson Bay. The analysis showed that the relative amounts of several key compounds are similar in the beluga stocks over large spatial areas (i.e. eastern versus western sites), however, some stocks have distinct fingerprints which can be used to differentiate them from adjacent stocks. Ratios of major HCH isomers largely corresponded with air and surface water measurements conducted during the 1990s, but low alpha-/beta- and alpha-/gamma-HCH ratios in all three western Arctic collections indicate rapid losses of the alpha-isomer from the food web, proportionately higher beta- and gamma-isomers in the Beaufort Sea, or a combination of the two processes. Chlordane residue patterns generally correspond to those from previous studies, however, interpretation of spatial trends are difficult due to the aging of the probable sources in the south, possible atmospheric input from new sources and complex transport pathways.

A history of total mercury in edible muscle of fish from lakes in northern Canada.

Subsistence fishing has been an important source of food for Native People in northern Canada since prehistoric time. Measurements of the levels of mercury in edible muscle of northern fish have been undertaken for over three decades in efforts to evaluate the risks of consuming northern fish. This report summarizes the data obtained from 7974 fish of 25 species from sites distributed from the Yukon to Labrador. The most abundant species were lake trout, lake whitefish, arctic char, walleye, northern pike and burbot. The question being asked was essentially "Are the fish safe to eat?" The results were used to support decisions on fishing and consumption of fish. They were sorted in several ways, into concentration ranges corresponding to human consumption guidelines, into political jurisdictions and into types of bedrock geology. Overall walleye, northern pike and lake trout, usually exceeded the subsistence consumption guideline of 0.2 microg g-1 total mercury and often exceeded the higher guideline of 0.5 microg g-1 total mercury for commercial sales of fish. Mercury in burbot, another facultative predator, was often lower but several still exceeding a guideline. Arctic char collections were mostly from anadromous populations and these had very low levels of mercury, presumably reflecting marine food sources. Lake whitefish were among the cleanest fish examined with 69 of 81 collections falling in the lowest range. Most collections were from sites in sedimentary rock. However a few sites were in metamorphic, intrusive or volcanic rocks and these, taken together, tended to have a higher proportion of sites in the higher ranges of mercury. These results indicate a widespread problem with mercury in subsistence fisheries for predator species of fish with the problem being most problematic for Nunavut.

Concentrations of mercury in tissues of beluga whales (Delphinapterus leucas) from several communities in the Canadian Arctic from 1981 to 2002.

Beluga whales have been hunted for food by Native People in the Canadian Arctic since prehistoric time. Here we report the results of analyses of total mercury in samples of liver, kidney, muscle and muktuk from collections over the period 1981-2002. We compare these results with human consumption guidelines and examine temporal and geographic variation. Liver has been analyzed more frequently than other organs and it has been used as the indicator organ. Mercury accumulates in the liver of the whales over time so that the whale ages are usually linked statistically to their levels of mercury in liver. Virtually all the samples of 566 animals analyzed contained mercury in liver at concentrations higher than the Canadian consumption guideline of 0.5 microg g-1 (wet weight) for fish. (There is no regulatory guideline for concentrations in marine mammals in Canada.) Samples from locations in the Mackenzie Delta in the western Canadian Arctic and from Pangnirtung in the eastern Canadian Arctic were obtained more often than from other location and these offered the best chances to determine whether levels have changed over time. Statistical outlier points were removed and the regressions of (ln) mercury in liver on age were used to calculate the level of mercury in whales of age 13.1 years in order to compare age-adjusted levels at different locations. These age-adjusted levels and also the slopes of regressions suggested that levels have increased in the Mackenzie Delta over the sampling period although not in a simple linear fashion. Other locations had fewer collections, generally spread over fewer years. Some of them indicated differences between sampling times but we could not establish whether these differences were simply temporal variation or whether they were segments of a consistent trend. For example, the levels in whales from Arviat were considerably higher in 1999 than in 1984 but we have only two samples. Similarly, samples from Iqaluit in 1994 exceeded considerably those in 1993 and the interval seems too short to reflect any regional temporal trend and more likely represent an extreme case of year-to-year variation. Previous analyses of data from geographically distinct groups had suggested that whales in the western Canadian Arctic had higher levels of mercury than those from the eastern Canadian Arctic. The present analysis suggests that such regional differences have diminished and are no longer statistically significant. No site has indicated significant decreases in more recent samples. The levels of total mercury in the most analyzed organs fell in the order of liver (highest levels), kidney, muscle and muktuk (lowest level). While muktuk had the lowest level of the organs most frequently analyzed, it is the preferred food item from these whales and it still exceeded the consumption guideline in most instances.

Biogeographic provinces of total and methyl mercury in zooplankton and fish from the Beaufort and Chukchi seas: results from the SHEBA drift.

Samples of copepods (Calanus hyperboreus) and arctic cod (Boreogadus saida) were collected along the SHEBA (Surface HEat Budget of the Arctic) drift track, which commenced in the Canada Basin (October 1997) and finished in the Mendeleev Basin (October 1998). Here, we report total mercury (HgT) and CH3Hg concentrations in these biological samples and examine concentration variability along the drift track in the context of trophic variation, inputs from land, spring mercury depletion events (MDEs), and oceanographic provinces. We find background concentrations of HgT in C. hyperboreus as low as 0.02 microg/g (dw), with the Canada Basin samples exhibiting approximately 2-fold higher mercury concentrations than those from the Chukchi Plateau and Mendeleev Abyssal Plain. This east-to-west trend in mercury concentration is punctuated by two and possibly three intervals of elevated mercury (HgT, 0.10-0.12 microg/g (dw); CH3Hg, 0.023-0.028 microg/g (dw)) along the drift track. One interval of elevated HgT and CH3-Hg levels occurred during and shortly after melt. %CH3Hg reached a maximum of 60% during this time period, three times higher than any other time during the drift. This transient rise in C. hyperboreus CH3Hg concentration seems to strongly point to mercury accumulated in snow during MDEs. However, the alignment of elevated mercury samples with oceanographic fronts and the observed regional differences between basins suggest that variation of mercury concentration is primarily a consequence of ocean structure. Given that large animals such as whales selectively forage in regions of higher food concentration such as fronts, recent change in the ice climate of the western Arctic Ocean, perhaps mediated by changes in heat storage, may provide the means to change their exposure to mercury thus explaining observed increases in mercury concentrations in western beluga whales during the 1990s.