Mercury and methylmercury in decomposing vegetation of a pristine and impounded wetland

Mass loss, methylmercury (MeHg), and total mercury (T-Hg) concentrations were measured in sedge, spruce needles, and Sphagnum moss decomposing in a headwater bog and an impounded riparian wetland. No significant differences in the rates of decomposition were apparent for like tissues among sites. Although concentrations of T-Hg increased in some tissues, T-Hg mass was lost from most tissues during decomposition. In the reservoir, approximately 75, 50, and 45% of the initial T-Hg mass was released from the sedge, needles, and moss, respectively, compared to 45, 0, and 40% in the headwater wetland. Therefore, we propose impoundment promotes the release of T-Hg during decomposition, Concentrations, 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 occurring as MeHg averaged 6% in sedge, 1% in needles, and 7% in moss compared 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 sedge, 6% in spruce needles, and 17% in moss. We conclude that decomposition of fresh plant tissue under anoxic conditions increases their MeHg mass and impoundment promotes this process, This increase in tissue MeHg mass is must likely the result of in situ methylation of previously accumulated inorganic Hg.