Methylated Mercury Species in Marine Waters of the Canadian High and Sub Arctic

Abstract

Distribution of total mercury (THg), gaseous elemental Hg(0) (GEM), monomethyl Hg (MMHg), and dimethyl Hg (DMHg) was examined in marine waters of the Canadian Arctic Archipelago (CAA), Hudson Strait, and Hudson Bay. Concentrations of THg were low throughout the water column in all regions sampled (mean ± standard deviation; 0.40 ± 0.47 ng L⁻¹). Concentrations of MMHg were also generally low at the surface (23.8 ± 9.9 pg L⁻¹); however at mid- and bottom depths, MMHg was present at concentrations sufficient to initiate bioaccumulation of MMHg through Arctic marine foodwebs (maximum 178 pg L⁻¹; 70.3 ± 37.3 pg L⁻¹). In addition, at mid- and bottom depths, the % of THg that was MMHg was high (maximum 66%; 28 ± 16%), suggesting that active methylation of inorganic Hg(II) occurs in deep Arctic marine waters. Interestingly, there was a constant, near 1:1, ratio between concentrations of MMHg and DMHg at all sites and depths, suggesting that methylated Hg species are in equilibrium with each other and/or are produced by similar processes throughout the water column. Our results also demonstrate that oceanographic processes, such as water regeneration and vertical mixing, affect Hg distribution in marine waters. Vertical mixing, for example, likely transported MMHg and DMHg upward from production zones at some sites, resulting in elevated concentrations of these species in surface waters (up to 68.0 pg L⁻¹) where primary production and thus uptake of MMHg by biota is potentially highest. Finally, calculated instantaneous ocean−atmosphere fluxes of gaseous Hg species demonstrated that Arctic marine waters are a substantial source of DMHg and GEM to the atmosphere (27.3 ± 47.8 and 130 ± 138 ng m⁻² day⁻¹, respectively) during the ice-free season.