MERCURY AND LEAD FLUXES FROM A PEATLAND EXPERIENCING INCREASED DISSOLVED ORGANIC CARBON EXPORT

Many wetland ecosystems around the world are experiencing increases in dissolved organic carbon (DOC) concentration and changes in hydrology due to shifting rainfall patterns. DOC concentration in the outflow of the S2 ombrotrophic peatland at the Marcell Experimental Forest (MEF) has more than doubled since the mid 1980s while DOC concentration in upland runoff has not changed. Increased DOC concentration and export has impacted Pb export from the peatland more dramatically than Hg. Pb and DOC concentrations in the S2 outflow are strongly correlated, while Hg concentration is weakly correlated with DOC. Most of the Pb and DOC flowing from the S2 peatland is derived from the raised bog region of the peatland. Hg, on the other hand, has significant contributions from upland runoff. During high flow events, Hg concentrations increase while Pb and DOC are diluted due to the increased hydrologic contribution of upland runoff. Supporting these results, upland runoff was found to be depleted in Pb and DOC, but contained elevated Hg levels relative to the porewater concentrations in the raised bog. Porewater transects exhibited increasing Pb concentrations, but decreasing Hg concentrations moving from the upland to the raised portion of the bog. Geochemical differences in binding to soil minerals and organic matter could explain the differences observed between Hg and Pb. In addition, we could be observing different ecosystem response times to decreased Pb atmospheric deposition with the uplands being depleted of Pb available for transport via runoff while the larger stores of Pb in the bog continues to be mobilized, potentially in relation to the enhanced generation of DOC in S2. These results are also useful in constraining inherently challenging hydrologic models in peatlands.