Tidal exchange of total mercury and methylmercury between a salt marsh and a Chesapeake Bay sub-estuary

Abstract

We examined the net exchange of total mercury (THg) and methylmercury (MeHg) between a tidal marsh and its adjacent estuary over a 1-year period from August 2007 to July 2008. Our objectives were to estimate the importance of tidal salt marshes as sources and sinks of mercury within the Chesapeake Bay system, and to examine the hydrologic and biogeochemical controls on mercury fate and transport in tidal marshes. Tidal flows and water chemistry were measured at an established tidal flume at the mouth of the principal tidal creek of a 3-ha marsh section at the Smithsonian Environmental Research Center. Fluxes were estimated by combining continuous tidal flow measurement for the entire study year, with discrete, hourly, flow-weighted measurements of filterable and particulate THg and MeHg, dissolved organic carbon (DOC), and suspended particulate matter (SPM) made over 20 tidal cycles during the year. We found that the marsh was a relatively small net tidal source of MeHg, mainly during the warmer growing season. We also confirmed that the marsh was a substantial source of DOC to the adjacent estuary. DOC was a significant predictor of both filterable THg and MeHg fluxes. However, although the marsh was a source of filterable THg, it was overall a net sink for THg because of particulate trapping. The net per-area annual flux of MeHg from tidal marshes is greater than other MeHg pathways within Chesapeake Bay. The annual load of MeHg from tidal marshes into Chesapeake Bay, however, is likely small relative to fluvial fluxes and efflux from bottom sediment. This study suggests that MeHg production within the tidal marsh has greater consequences for biota inhabiting the marsh than for the efflux of MeHg from the marsh.