Little seasonal variation of mercury concentrations and biomagnification in an Arctic pelagic food web

Despite numerous studies on mercury in Arctic biota, data from inaccessible, ice-covered regions − especially during the polar night and late winter − remain scarce. This scarcity results in poor understanding of the seasonal dynamics of mercury within the food web. From the Northern Barents Sea, we quantified total mercury and the dietary descriptors δ¹⁵N and δ¹³C as long-term dietary signals (weeks to months) in biota to a) investigate the seasonal pelagic food web structure, b) seasonality in total mercury concentration, c) and its biomagnification in the food web. Mercury and dietary descriptors were analyzed in copepods, macrozooplankton (krill, amphipods, arrow worms, and pteropods) and the fishes, Atlantic cod (Gadus morhua), polar cod (Boreogadus saida) and capelin (Mallotus villosus) during spring, late summer, early, and late winter. Seasonal changes were observed in δ¹⁵N values in some macrozooplankton and capelin, and some seasonal variation was observed across the food web with depleted δ¹³C values in spring and enriched δ¹³C values in late summer. Mercury concentrations were lower (range: 2.49 ng/g dw in the krill Thyssanossa sp. – 70.55 ng/g dw in the pelagic pteropod Clione limacina) than reported from other parts of the Arctic. We found a positive linear relationship between mercury and relative trophic position represented by δ¹⁵N, i.e., biomagnification, during all seasons, except in early winter. As Clione limacina likely had different turnover rates for mercury and stable isotopes resulting in low δ¹⁵N, but high mercury concentrations in early winter, compared to the other species in the food web, the pteropod was omitted from the regression. By omitting Clione limacina, biomagnification was similar across all seasons (R²_adj = 0.45). Thus, we saw clear mercury biomagnification with consistent and little seasonal variation in this high Arctic marine food web despite large seasonal fluctuations in abiotic and biotic conditions.