Bioconcentration as a key driver of Hg bioaccumulation in high-trophic-level fish

Abstract. The ability of monomethylmercury (MMHg+) to bioaccumulate in seafood is of concern due to its neurotoxic properties. The challenge in understanding MMHg+ bioaccumulation lies in the fact that its levels in higher trophic levels result from both bioconcentration and biomagnification. Furthermore, Hg can occur in several chemical species, including Hg2+ and MMHg+, which both bioaccumulate. Although the dominant pathway for MMHg+ bioaccumulation into seafood is the bioconcentration of MMHg+ in primary producers and the subsequent biomagnification to higher trophic levels, other pathways can contribute to MMHg+ bioaccumulation. In this study, we use a fully coupled 1D water column Hg bioaccumulation model to quantify how total bioaccumulation of Hg2+ and uptake of MMHg+ from the water (bioconcentration) in consumers affects the bioaccumulation of MMHg+ in high-trophic-level fish. The study is performed in three setups representing hydrodynamic conditions representative of the North and Baltic Seas. We find that Hg2+ bioaccumulation does not influence the bioaccumulation of MMHg+ but the bioconcentration of MMHg+ plays an important role. Although direct bioconcentration accounts for < 15 % of MMHg+ bioaccumulation in cod, the cumulative effect of bioconcentration on all trophic levels increases the MMHg+ content of cod by 28 %–49 %. These results demonstrate that bioconcentration in consumers is essential to accurately model the bioaccumulation of MMHg+ at higher trophic levels.