In this study, we examined MeHg biogeochemistry in the S2 peatland, an ombrotrophic bog in the Marcell Experimental Forest, MN, USA. Biogeochemical “hot spots” have been identified as the interface between terrestrial and aquatic systems, and they are characterized by temporally higher ‘reaction rates’. Methylmercury (MeHg) hot spots are areas with a high percentage of total mercury existing as MeHg. Other studies have found MeHg hot spots in the lagg area of peatlands. These higher %MeHg biogeochemical hot spots may be explained by local conditions where sulfate reduction occurs and bioavailable Hg and carbon are present. A sampling scheme was devised to characterize MeHg hot spots in the S2 peatland and describe their occurrence temporally and spatially over three years (2010-2012). Intensive sampling of peat porewaters was conducted on six transects extending from the upland, to the lagg, to the center bog area. Three transects were downstream of a convex upland area and three were adjacent to a concave upland area. MeHg, total Hg, trace elements, and dissolved organic carbon were also measured weekly following spring thaw at the exit weir, one lagg location, and in flow from runoff collectors. Additionally, occasional samples were collected at lagg points around the perimeter of the peatland.
The presence of MeHg hot spots in S2 was highly transient, but several general trends were observed. THg concentrations were highest in the subsurface runoff and then generally decreased moving from lagg to bog porewaters, particularly in the concave transect areas. When water levels were high in the peatland, MeHg concentrations were typically highest in the lagg wells and decreased moving out towards the bog, again most consistently in the concave areas. During low water level conditions in the peatland, MeHg concentration trends were not as clear, and MeHg in bog waters often exceeded lagg porewater concentrations. As expected, %MeHg was low in subsurface upland runoff and in the upland porewaters. Due to the differences between high and low water conditions and concave versus convex transect areas, the data suggests zones of methylation within the lagg are influenced by local topography and hydrologic conditions.