Evaluating the role of local mercury sources on spatial and temporal pollution trends in marine predators living in a changing arctic

The consequences of a rapidly warming Arctic on environmental mercury (Hg) exposure in marine biota remains largely unknown. Stable carbon (C), nitrogen (N), sulphur (S) and Hg isotope ratios are increasingly used to harmonice and trace Hg sources, pathways and concentrations in marine top predators. Our objective was to identify Hg sources in marine Arctic predators at spatial and temporal scale and evaluate the consequences on Hg exposure over time. We measured THg concentrations, C, N, S, and Hg isotope ratios in muscle tissue of hooded Cystophora cristata and ringed seal Pusa hispida from East Greenland, collected between 1985 and 2019. We created multi-isotopic niches (standard ellipse areas, SEAs), after which we applied linear mixed models to relate temporal variation in Hg exposure and sources to environmental change (e.g. sea-ice, NAO). Hooded seals presented a larger SEA (mode, 95% CI: 0.93, 0.60 – 1.41) than ringed seals (mode, 95% CI: 0.22, 0.14 – 0.37). Ringed seals showed higher THg concentrations (Mean ± SD: 1.10 ± 0.48 µg g-1 dw) than hooded seals (0.88 ± 0.92 µg g-1 dw) on average. THg levels in hooded seal muscle decreased by 1.5% y-1, while no significant change was found for ringed seals. At spatial scale, habitat use (oceanic vs. coastal) was the most important driver of THg variability, with landfast ice-associated ringed seals being potentially influenced by enhanced coastal MeHg emission and accumulate higher THg levels. Attemporal scale, the shift in local marine biogeochemistry (represented by δ15N values) and prey biomass were the most important drivers of THg in ringed and hooded seals, respectively. The different Hg trends shown by the two species suggested how Hg sources are important drivers of bioaccumulation in Arctic seals, underling the necessity to reevalute their importance for the forecasting of future Hg trends in the Arctic Ocean.