Spatial characteristics of net methylmercury production hot spots in peatlands.

Many wetlands are sources of methylmercury (MeHg) to surface waters, yet little information exists about the distribution of MeHg within wetlands. Total mercury (THg) and MeHg in peat pore waters were studied in four peatlands in spring, summer, and fall 2005. Marked spatial variability in the distribution of MeHg, and %MeHg as a proxy for net MeHg production, was observed, with highest values occurring in discrete zones. We denote these zones "MeHg hot spots", defined as an area where the pore water %MeHg exceeded the 90th percentile of the data set (n=463) or >22% of THg as MeHg. MeHg hot spots occurred near the interface between peatland and the upland watershed with few exceptions. The %MeHg in pore water was significantly less in peatland interiors compared to upland-peatland interface zones, with the significance of these differences related to the delineation of the boundary between the two areas. Although further research is necessary, our data suggest that the occurrence of MeHg hot spots is related to the transport of solutes in upland runoff to the peatland perimeter and not to the accumulation of MeHg in this zone as a result of transport from either the peatland interior or the surrounding upland watershed. These findings augment the understanding of peatland MeHg production in upland-peatland watersheds, provide guidance for more accurate quantification of MeHg pool sizes in the landscape, and a spatial framework forthe further study of mercury methylation processes in peatlands.