Bioaccumulation as a driver of high MeHg in the North and Baltic Seas

Abstract. Mercury (Hg) is a toxic pollutant that poses significant risks to marine ecosystems and human health as a result of bioaccumulation. Despite its known hazards, the processes that govern Hg bioaccumulation within the marine food web are poorly understood. This study examines the role of the marine ecosystem in Hg cycling in highly productive coastal seas. We integrate Hg biotic uptake, release and transformation into the ECOSMO E2E marine ecosystem model, coupled with the MERCY v2.0 marine Hg cycling model. Our results show that bioaccumulation can increase total methylmercury (tMeHg) in coastal pelagic waters from 0.059 to 0.092 pM, a 44 % increase. Bioaccumulation and binding of Hg to organic matter contribute to elevated Hg levels in surface waters. Furthermore, cyanobacteria-driven reduction of Hg²⁺ to Hg⁰ reduces average marine Hg concentrations by up to 9 % above the mixed layer depth in the Gotland Deep and 20 % in shallow Baltic Sea regions, and increases Hg⁰ evaporation in the Baltic Sea, reducing Hg inflow into the North Sea. We quantify a 1 % increase in tMeHg per 4.5 mgC m^-3 biota biomass. Finally, we show that bioaccumulation decreases the burial of Hg by 13 kg y^-1 increasing Hg export to the Atlantic Ocean and the English Channel. These findings highlight the importance of ecosystem feedback on marine Hg cycling and demonstrate the need to integrate biological processes into Hg cycling models.