Total carbon and proximity to a hydropower dam modulate hydric soil mercury in upstream wetlands

Legacy mercury from historic atmospheric deposition poses a continued threat to aquatic ecosystems. A change in water level management plans for the Moses–Saunders hydropower dam on the Upper St. Lawrence River (USLR) resulted in water level fluctuations that more closely resemble natural fluctuations. Prior stable water levels (1958–2016) created conditions for overgrowth of cattail (Typha spp.) in riparian wetlands. This study of 81 wetlands during a flood year (2017) built upon a previous assessment (2016, non-flood year; 16 wetlands), quantified methylmercury content, and hgcA, a microbial gene responsible for mercury methylation. Here, total mercury, methylmercury, and hgcA copy number per gram of hydric soil did not differ significantly among geomorphological types of riparian cattail wetlands (p > 0.05). Total mercury, total carbon, and longitude (distance downstream) were the top three predictors of methylmercury content in hydric soils. After adjusting for organic matter, total and methylmercury concentrations increased with distance downstream (proximity to the dam), where younger wetlands were created by the impoundment. Historic flooding may have created sites of legacy mercury accumulation in wetlands close to the hydropower dam, while ongoing flooding generates a continued risk of mobilizing wetland mercury into adjacent food chains.