Methyl mercury in pristine and impounded boreal peatlands, experimental Lakes Area, Ontario

Methyl mercury (MeHg) concentrations were measured in peat and peat porewat er of pristine wetlands and an impounded riparian wetland at the Experiment al Lakes Area in northwestern Ontario, Canada. In pristine wetlands, MeHg c oncentrations in peat ranged from 0.1 to 60 ng.g(-1) and in peat porewater from 0.02 (the detection limit) to 7.3 ng.L-1, with higher concentrations i n wetlands that received upland runoff. Impoundment increased the average M eHg concentration in the near-surface peat porewater from 0.2 to 1.0 ng.L-1 . As the increase was most dramatic near the peat - surface water interface , we suggest that the elevated MeHg concentrations resulted from an increas e in net MeHg production associated with the decomposition of inundated veg etation. Impoundment increases the area of potential Hg methylation by impo sing anoxia over the entire wetland surface and by facilitating the exchang e of nutrients and MeHg between the peat surface and the surface water. No clear chemical control on MeHg concentration was observed among the pristin e wetlands or in the impounded wetland. However, in laboratory incubations of peat, porewater MeHg concentration increased upon the addition of sulfat e. We propose that sulfate availability is an important variable in Hg meth ylation in pristine northern wetlands.