Methylmercury dynamics at the upland-peatland interface: Topographic and hydrogeochemical controls

[1] Peatlands are important environments for the transformation of atmospherically deposited inorganic mercury into the bioaccumulative form, methylmercury (MeHg), which may accumulate in downstream aquatic biota, particularly in fish. In recent research, it was suggested that MeHg production and/or accumulation “hot spots” at the upland-peatland interface were the result of upland fluxes of sulfate and labile dissolved organic carbon (DOC) into the peatland margin. Along the upland-peatland interface, spatial heterogeneity of “hot spots” was thought to be a result of variations in upland hydrologic interaction with the peatland margin. This hypothesis was tested in this study. Pore water MeHg, sulfate, and dissolved organic carbon (DOC) concentrations were compared in peatland plots at the base of both topographically concave and linear upland subcatcments in Minnesota. Subcatchment contributing areas were 3–8 times larger in the peatland plots adjacent to areas of concave upland topography. Peat pore water MeHg concentrations were significantly higher in these plots. Fluxes of water, sulfate, and dissolved organic carbon (DOC) from the upland hillslope into the peatland margin were also generally much larger than those from below areas of concave upland topography. Taken together, these results suggest that watershed geomorphology plays an important role in controlling chemical fluxes into peatland margins and consequently MeHg production and accumulation. It may thus be possible to delineate areas of high MeHg production and/or accumulation in certain watersheds by using high-resolution topographic data. The resulting MeHg “hot spots” may be important for locally foraging biota and for downstream loading, especially in the spring and fall.

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