Dissolved and Particulate Methylated Mercury in a Highly Productive Area of the Southern Ocean

Abstract Methylated mercury (MeHg), including dimethylmercury and monomethylmercury (MMHg), is a pollutant of concern because it biomagnifies in marine biota. The formation of MeHg in the oceans, specifically at highly productive regions and at high oxygen levels, remains elusive. We investigated dissolved and particulate total (THg) and MeHg in the water column and sediments at six stations in a highly productive area of the Southern Atlantic Ocean. Total MeHg concentrations and proportions of THg in seawater were higher (50%–73%) at eutrophic stations. We found that the distribution of MeHg in the mixed layer is strongly controlled by the biological pump. Concentrations of THg and MMHg were highest in particles within the chlorophyll maximum, suggesting THg and MMHg scavenging or assimilation by phytoplankton and an oxic pathway of MMHg formation. Particle breakdown and respiration appeared to increase dissolved MeHg concentrations and MMHg concentrations in small particles (2–51 μm) at greater depths. MMHg concentrations in the sediments were consistently lower than in particles from the mixed layer, indicating MMHg release during particle sinking and that deep‐sea sediments are unlikely to be an important source of MeHg in the water phase. Our study identifies productive marine areas as hotspots of MeHg formation and suggests increasing Hg methylation with increasing ocean eutrophication and may amplify biomagnification in marine food webs.