Eutrophication Increases Phytoplankton Methylmercury Concentrations in a Coastal Sea : A Baltic Sea Case Study

Eutrophication is expanding worldwide, but its implication forproduction and bioaccumulation of neurotoxic monomethylmercury (MeHg) isunknown. We developed a mercury (Hg) biogeochemical model for the Baltic Seaand used it to investigate the impact of eutrophication on phytoplankton MeHg concentrations. For model evaluation, we measured total methylated Hg (MeHgT)in the Baltic Sea and found low concentrations (39 ± 16 fM) above the halocline and high concentrations in anoxic waters (1249 ± 369 fM). To close the Baltic Sea MeHgT budget, we inferred an average normoxic water column HgII methylationrate constant of 2 × 10−4 d−1. We used the model to compare Baltic Sea’s presentday(2005−2014) eutrophic state to an oligo/mesotrophic scenario. Eutrophication increases primary production and export of organic matter and associated Hgto the sediment effectively removing Hg from the active biogeochemical cycle; thisresults in a 27% lower present-day water column Hg reservoir. However, increasein organic matter production and remineralization stimulates microbial Hgmethylation resulting in a seasonal increase in both water and phytoplankton MeHg reservoirs above the halocline. Previous studies of systems dominated by external MeHg sources or benthic production found eutrophication to decrease MeHg levels in plankton. This Baltic Sea study shows that in systems with MeHg production in the normoxic water column eutrophication can increase phytoplankton MeHg content