Seasonal biogeochemical cycling of mercury on the Arctic Ocean shelf

The toxic pollutant mercury accumulates in marine food webs on a global scale, with consumption of seafood as the primary exposure pathway for humans. Since 2017, the Minamata Convention on Mercury has aimed to reduce human exposure to mercury by limiting anthropogenic mercury emissions to the environment. However, climate change threatens to alter the fundamental biogeochemical cycling of mercury. In the Arctic, local populations have the highest exposure to mercury globally. Furthermore, the Arctic region is experiencing rapid climate change compared to the rest of the global ocean. Despite intensive study in the last decades on mercury in the Arctic, seasonal studies in the Arctic Ocean are limited to the sunlit months. After the dark Arctic winter, the return of sunlight and climbing temperatures facilitate the delivery of significant amounts of mercury to Arctic surface waters. These mercury inputs during spring and summer coincide with the Arctic’s highest biological productivity. In situ production of methylmercury in the Arctic Ocean is known to contribute to mercury bioaccumulation in the Arctic marine food web. However, the biogeochemical cycling of mercury during the polar night remains largely unexplored. In this study, we investigated the full seasonal cycle of mercury in an Arctic shelf sea in both seawater and sediment. We observed that mercury distributions in the northern Barents Sea during summer were similar to those previously reported in literature. During polar night, we measured lower total mercury concentrations than the summer, and highlighted that seasonal particle scavenging limits the amount of mercury in the northern Barents Sea during winter. We linked seasonal scavenging mechanisms in the water column to mercury concentrations in surface sediment, with mercury exhibiting correlations to other scavenged metals like manganese and lead. After the polar night, we observed potential demethylation of methylmercury during the spring. Declines of methylmercury coincided with both seasonal inputs of inorganic mercury and the Arctic spring bloom, suggesting an important biological demethylation mechanism that may mitigate mercury uptake into the food web. Overall, our seasonal data suggest that vertical distributions of mercury in the northern Barents Sea show distinct seasonal variations that are coupled to major physical, chemical, and biological changes in the Arctic Ocean.