Total mercury and methylmercury dynamics in upland–peatland watersheds during snowmelt

Wetlands, and peatlands in particular, are important sources of methylmercury (MeHg) to susceptible downstream ecosystems and organisms, but very little work has addressed MeHg production and export from peatland-dominated watersheds during the spring snowmelt. Through intensive sampling, hydrograph separation, and mass balance, this study investigated the total mercury (THg) and MeHg fluxes from two upland–peatland watersheds in Minnesota, USA during the 2005 spring snowmelt and proportionally attributed these fluxes to either peatland runoff or upland runoff. Between 26% and 39% of the annual THg flux and 22–23% of the annual MeHg flux occurred during the 12-days snowmelt study period, demonstrating the importance of large hydrological inputs to the annual mercury flux from these watersheds. Upland and peatland runoff were both important sources of THg in watershed export. In contrast to other research, our data show that peatland pore waters were the principal source of MeHg to watershed export during snowmelt. Thus, despite cold and mostly frozen surface conditions during the snowmelt period, peatland pore waters continued to be an important source of MeHg to downstream ecosystems.