PRODUCTION AND LOSS OF METHYLMERCURY AND LOSS OF TOTAL MERCURY FROM BOREAL FOREST CATCHMENTS CONTAINING DIFFERENT TYPES OF WETLANDS

Four terrestrial boreal forest catchments containing different types o f wetlands were studied to determine their strength as sources or sink s of methylmercury (MeHg) and total mercury (THg) to downstream ecosys tems and to determine if patterns seen in one year were consistent ove r several years. All catchments were sinks for THg. The wetland type, percentage wetland area (0-25%), or annual water yield did not appear to have a consistent effect on the magnitude of this retention., Wetla nd areas of the catchments were always net sources of MeHg. Unlike for THg, there were large and consistent differences in the source streng th among wetland types for MeHg, These differences appeared to be rela ted to differences in the internal hydrology of the wetlands. All type s of wetlands were greater sources of MeHg during years of high water yield, but even during years of low flow all wetland types were source s of MeHg. Thus, we conclude that wetlands are important sites of MeHg production in boreal ecosystems on the long term. Upland areas of cat chments were consistently sinks for MeHg, and so whole catchment sink/ source values were strongly affected by the percentage of wetland area s within a catchment. Mass balance estimates of MeHg input from wetlan d areas to a lake indicate that the annual input of MeHg from wetlands is larger than the annual uptake of tig by fish and is similar to the amount of MeHg produced in the lake. Because of the predictable patte rns between terrestrial catchments in their strength as sources or sin ks of MeHg, it is possible to model inputs of MeHg from lake catchment s with knowledge of the percentage wetland area in a catchment, the ty pe of wetland contained in a catchment, and the annual water yield of a catchment.