SINKS AND SOURCES OF METHYLMERCURY IN A BOREAL CATCHMENT

A simple, catchment-scale, cascade model was used to assess the import ance of sinks and sources of methylmercury (MeHg) in a boreal catchmen t that contains a forested upland, a lowland peatland and a small lake . The three compartment model was run using realistic flow rates and a tmospheric loading of MeHg, and the model was constrained by observed concentrations of MeHg in each compartment. Assuming no internal sinks and sources of MeHg, modelled catchment yields showed reasonable agre ement with field observation, but the predicted internal MeHg concentr ations in each compartment were implausible. Only when sources and sin ks of MeHg are added to the three compartments do MeHg-pool concentrat ions fall into the range of those measured in the field. To maintain b oth catchment-scale and compartment-scale continuity, the upland and p eatland were net sources of MeHg (0.0007 and 0.1065 mg ha(-1) d(-1) re spectively), and the lake a net sink (-0.2215 mg ha(-1) d(-1)). These source/sink rates are 1.73, 259 and -539 times the input of MeHg via w et precipitation input for the modelled ice-free season. Sensitivity a nalysis revealed that the volume of runoff delivered to the peatland b y the upland area, peatland size and porewater MeHg concentration in t he peatland an important controls on catchment MeHg yield, and that co ntemporary atmospheric deposition of MeHg is insignificant compared to the sources of MeHg within the catchment.