MERCURY AND METHYLMERCURY IN DECOMPOSING VEGETATION OF A PRISTINE ANDIMPOUNDED WETLAND

Mass loss, methylmercury (MeHg), and total mercury (T-Hg) concentratio ns were measured in sedge, spruce needles, and Sphagnum moss decomposi ng in a headwater bog and an impounded riparian wetland. No significan t differences in the rates of decomposition were apparent for like tis sues among sites. Although concentrations of T-Hg increased in some ti ssues, T-Hg mass was lost from most tissues during decomposition. In t he reservoir, approximately 75, 50, and 45% of the initial T-Hg mass w as released from the sedge, needles, and moss, respectively, compared to 45, 0, and 40% in the headwater wetland. Therefore, we propose impo undment promotes the release of T-Hg during decomposition, Concentrati ons, masses, and percentages of MeHg decreased in all tissue types at the headwater-dry site and MeHg concentrations increased in all tissue types at the headwater-wet and impounded sites. At the headwater-wet site, the mass of MeHg only increased in the moss (500%) and T-Hg occu rring as MeHg averaged 6% in sedge, 1% in needles, and 7% in moss comp ared to initial values of 4, 0.5, and 1%, At the impounded sites, the amount of MeHg in the needles and moss increased by approximately 700 and 500%, respectively. Also, T-Hg occurring as MeHg averaged 12% in s edge, 6% in spruce needles, and 17% in moss. We conclude that decompos ition of fresh plant tissue under anoxic conditions increases their Me Hg mass and impoundment promotes this process, This increase in tissue MeHg mass is must likely the result of in situ methylation of previou sly accumulated inorganic Hg.