Hg-Stable Isotope Variations in Marine Top Predators of the Western Arctic Ocean

Recent studies on mercury (Hg)-stable isotopes in Alaskan seabird eggs and ringed seal livers illustrated the control of sea ice cover on marine methyl-Hg photochemistry. Here, complementary marine mammal tissues have been analyzed to document variations in Hg-, carbon (C)-, and nitrogen (N)-stable isotope compositions of Arctic marine food webs. Hg-stable isotope ratios were measured in liver samples of 55 beluga whales (Delphinapterus leucas) and 15 polar bears (Ursus maritimus) collected since 1990. Large variations in delta Hg-202 (approximate to 2.1 parts per thousand) and Delta Hg-199 (approximate to 1.7 parts per thousand) are observed between species and within species stocks covering the Gulf of Alaska-Bering Sea-Arctic Ocean regions. Polar bears, mainly feeding on ringed seal (delta N-15 shift of 4.2 parts per thousand), show identical liver Delta Hg-199 of 0.5 parts per thousand, confirming the absence of metabolic mass-independent fractionation, and 0.33 +/- 0.11 parts per thousand enrichment in heavy Hg isotopes. Beluga whale liver total Hg concentrations increase with age, reflecting lifetime bioaccumulation, while Hg speciation shifts to inorganic Hg with age as a result of hepatic methyl-Hg breakdown. Delta Hg-200 variations in biota show a small, 0.1 parts per thousand decrease from North Pacific Ocean to Arctic Ocean habitats, suggesting atmospheric Hg deposition to be important in the Pacific and terrestrial Hg inputs to dominate in the Arctic Ocean. Similar to seabird eggs, a consistent south to north gradient in Delta Hg-199 baseline is seen in mammal liver tissues, confirming sea ice cover as a control factor on marine Hg photoreduction and Delta Hg-199. Arctic Ocean beluga whales have near zero Delta Hg-199, indicating that terrestrial Hg and in-situ-produced methyl-Hg are not measurably photoreduced in the Arctic Ocean before entering the marine food web.