The importance of hydrogeomorphic setting for total mercury and methylmercury export from fen wetlands in western Canada

The export of neurotoxic mercury (Hg) and its bioavailable form, methylmercury (MeHg), from wetland-dominated catchments is common throughout boreal regions. Wetlands vary significantly, however, in their degree of minerotrophy, which affects wetland Hg cycling and is related to their hydrogeomorphic setting. In this field study, we highlight how hydrogeomorphic setting, expressed as degree of minerotrophy, impacts streamwater total-Hg (THg) and MeHg export from Canadian Western Boreal Plain, fen-dominated catchments along a series of increasing minerotrophy (poor fen < moderate fen < channel fen < rich fen). Streamwater from the catchments dominated by less minerotrophic poor and moderate fens had the highest study period MeHg yields (13 and 19 mg km⁻², respectively) and percent MeHg in exported streamwater, while THg yield was greatest from the rich fen headwaters (260 mg km⁻²). MeHg and THg yields decreased within the rich fen. Decreases in the rich fen MeHg concentrations and yields coincides with greater total manganese, suggesting manganese redox chemistry may be important in regulating MeHg cycling and/or mobility in more minerotrophic wetlands. We extend previous studies showing some swamps to be net MeHg importers to include rich fens as another wetland type that removes MeHg from streamwaters and provides another possible mechanism, manganese reduction, that influences MeHg cycling.