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 terrestrial mercury and methylmercury cycle, and a link between the atmospheric and aquatic environment. This study was conducted to determine the contribution of the forest floor to the pools and fluxes of total Hg (Hg-total) and methylmercury (MeHg) in a forested catchment, and to identify factors influencing the mobility of both compounds. Throughfall deposition, litterfall, runoff and fluxes with forest floor percolate of Hg(t)otal and MeHg were sampled during one year in a coniferous catchment in Germany. Total deposition of Hg(t)otal was 552 mg ha(-1) a(-1) with litterfall contributing one third. Nearly 60% of the total input of Hg(t)otal reached the mineral soil with the forest floor percolate, but less than half of this fraction was found in the runoff of the catchment. Total deposition of MeHg was 2.6 mg ha(-1) a(-1), with litterfall as the dominating pathway. Only 19% of the MeHg deposition was discharged from the forest floor, but the flux of MeHg with runoff was nearly twice as high. Only few correlations with other solution parameters were found. Fluxes of both compounds with forest floor percolates depended mainly on water fluxes, which was not true for the runoff. The forest floor of the upland soil is an effective sink for MeHg, but not for Hg-total. Differences in the mobility of both compounds in the forest floor disappeared at the catchment scale, probably because other processes (i.e. Hg-total immobilization and MeHg formation) dominated.