Pools and fluxes of mercury and methylmercury in two forested catchments in Germany

Mercury (Hg) and methylmercury (CH(3)Hg(+)) are global pollutants, but little information is available on rates of atmospheric input, distribution and mobility in soils and catchments of central Europe. The objectives of this study were to investigate input and output fluxes of these compounds in a deciduous and a coniferous catchment in NE Bavaria (Germany), and to estimate pools and mobility of total Hg (Hg(tot)) and CH(3)Hg(+) at the catchment scale. Bulk precipitation, throughfall, litterfall and runoff were collected biweekly from April 1998 to April 1999. Several soil profiles were sampled to estimate pools of Hg compounds in the catchments. In both catchments highest contents of Hg(tot) were found in the Oa layer of the forest floor (up to 500 ng g(-1)) acid the soil storage of Hg(tot) calculated for a soil depth of 60 cm was approximately 890 g ha(-1) in the coniferous and 190 g ha(-1) in the deciduous catchment. Highest contents of CH(3)Hg(+) in upland soils were observed in the Oi layer of the forest floor, and soil storage of CH(3)Hg(+) was 4.35 g ha(-1) in the coniferous and 0.59 g ha(-1) in the deciduous catchment. The annual total deposition of Hg(tot) (total deposition not measured directly but calculated from throughfall and litterfall) was 541 mg ha(-1) year In the coniferous and 618 mg ha(-1) year(-1) in the deciduous catchment. Total deposition rates of CH(3)Hg(+) were 3.5 and 2.6 mg ha(-1) year(-1). The contribution of litterfall to the total deposition of Hg(tot) and CH(3)Hg(+) was 55% in the deciduous catchment. In the coniferous catchment, the contribution of litterfall to total deposition was only 29% for Hg(tot), but 55% for CH(3)Hg(+). By far the largest proportion of the deposited CH(3)Hg(+) and Hg(tot) remained in the catchments (85% in the coniferous, 95% in the deciduous). As compared to remote Swedish catchments, deposition and output via runoff of Hg(tot) were higher, but deposition and output of CH(3)Hg(+) were lower in our catchments. In contrast to other studies, the annual budget revealed no differences in the mobility between the two species at the catchment scale. However, temporal patterns of the runoff fluxes and converse gradients of CH(3)Hg(+) and Hg(tot) contents in the forest floor indicated differences in mobility on shorter time scales. (C) 2000 Elsevier Science B.V. All rights reserved.