Assessment of mercury mobilization potential in Upper St. Lawrence River riparian wetlands under new water level regulation management

Mercury is a contaminant of concern in freshwater ecosystems. Riparian wetlands in the Upper St. Lawrence River (USLR) are dominated by monotypic (Typha spp.) marshes due to water level management since 1958. A new management plan will make water levels more closely resemble natural fluctuations and restore riparian wetland biodiversity by eroding some existing marshes. If Typha abundance is reduced there is potential for Hg, from legacy atmospheric deposition, contained in hydric soils to become mobilized into the food web. To evaluate this potential, the capacity of sixteen USLR wetlands to support Hg-methylating microbes was determined by quantifying total Hg, C, N, S, P and organic matter content in conjunction with genomic analysis in hydric soils from four riparian wetland types: barrier beaches, drowned river mouths, protected embayments, and open embayments. Drowned river mouth hydric soils contained significantly (p < 0.05) higher average Hg concentrations, 110 ± 79 ng/g, compared to protected and open embayments that contained 43 ± 26 ng/g and 40 ± 28 ng/g, respectively. Hg concentrations within hydric soil at the open water interface, the upland soil interface, soil horizons, and across longitudes were also compared, but only a statistically insignificant trend of higher Hg concentrations in hydric soil near shore was observed. Bacteria and Archaea capable of mercury methylation were identified, suggesting a fraction of the 86 ± 50 kg of Hg estimated to be mobilized from riparian wetland soils due to the new water level management plan could be transformed to bioavailable methylmercury in-situ.