Monomethylmercury is a neurotoxin that accumulates in marine organisms, with serious implications for human health1. The toxin is of particular concern to northern Inuit peoples, for example, whose traditional diets are composed primarily of marine mammals and fish2. The ultimate source of monomethylmercury to marine organisms has remained uncertain, although various potential sources have been proposed, including export from coastal3 and deep-sea4 sediments and major river systems, atmospheric deposition and water-column production8,9. Here, we report results from incubation experiments in which we added isotopically labelled inorganic mercury and monomethylmercury to seawater samples collected from a range of sites in the Canadian Arctic Archipelago. Monomethylmercury formed from the methylation of inorganic mercury in all samples. Demethylation of monomethylmercury was also observed in water from all sites. We determined steady-state concentrations of monomethylmercury in marine waters by incorporating the rate constants for monomethylmercury formation and degradation derived from these experiments into a numerical model. We estimate that the conversion of inorganic mercury to monomethylmercury in the water column accounts for around 47% (±62%, standard deviation) of the monomethylmercury present in polar marine waters, with site-to-site differences in inorganic mercury and monomethylmercury levels accounting for most of the variability. We suggest that water-column methylation of inorganic mercury is a significant source of monomethylmercury in pelagic marine food webs in the Arctic, and possibly in the world's oceans in general.
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