Dynamics of mercury and methylmercury in forest floor and runoff of a forested watershed in Central Europe

Forested watersheds are an important part of the terrestrialmercury and methylmercury cycle, and a link between theatmospheric and aquatic environment. This study was conducted todetermine the contribution of the forest floor to the pools andfluxes of total Hg (Hgtotal) and methylmercury (MeHg) in aforested catchment, and to identify factors influencing themobility of both compounds. Throughfall deposition, litterfall,runoff and fluxes with forest floor percolate of Hgtotal and MeHgwere sampled during one year in a coniferous catchment inGermany. Total deposition of Hgtotal was 552 mg ha−1 a−1 withlitterfall contributing one third. Nearly 60% of the total inputof Hgtotal reached the mineral soil with the forest floorpercolate, but less than half of this fraction was found in therunoff of the catchment. Total deposition of MeHg was 2.6 mg ha−1a−1, with litterfall as the dominating pathway. Only 19% of theMeHg deposition was discharged from the forest floor, but theflux of MeHg with runoff was nearly twice as high. Only fewcorrelations with other solution parameters were found. Fluxes ofboth compounds with forest floor percolates depended mainly onwater fluxes, which was not true for the runoff. The forest floorof the upland soil is an effective sink for MeHg, but not forHgtotal. Differences in the mobility of both compounds in theforest floor disappeared at the catchment scale, probably becauseother processes (i.e. Hgtotal immobilization and MeHg formation)dominated.