MS Thesis Defense
Identification of Methylmercury Export Hotspots in an Industrially-Influenced Great Lakes Coastal Wetland
The production and export of methylmercury (MeHg) is a critical first step to accumulation of mercury (Hg) in the lower food web and subsequent accumulation and magnification in game fish. The purpose of our study was to identify MeHg export hotspots using ecologically delineated areas in a freshwater estuary with a history of industrial influence. Sediment, porewater and surface water was collected over two seasons from eleven sites encompassing four high-carbon sheltered embayments, two intermediate-carbon clay-influenced bays, and five low-carbon industrially influenced bays in the St. Louis River Estuary (Duluth, Minnesota). Sediment total and methylmercury ranged 3 orders of magnitude with the highest average sediment MeHg and percent MeHg in the isolated, wetland-like sheltered embayments, which had total mercury concentrations lower than the industrially influenced bays and similar to non-point source contaminated lakes and wetlands in Northern Minnesota. Porewater MeHg was highest at sites with high porewater sulfide but low- to middle- range carbon. Sites isolated from the main river channel experienced surface water MeHg concentrations 3-4x higher than the river, suggesting these areas could be sources of MeHg to the larger St. Louis River Estuary. The ecologically-defined habitats contained characteristically different quantities of Hg and MeHg and we propose a conceptual model that summarizes the potential controls on MeHg production, portioning, and diffusive flux. The results provide a basis for understanding how MeHg can move through complex freshwater aquatic environments that could form the basis for effective resource managers decisions.