Methyl and Total Mercury Budget of a Mid-Atlantic Estuarine Salt Marsh

Coastal and estuarine salt marshes are both efficient accumulators of particulate-bound inorganic mercury (Hg) and transformers of inorganic Hg to methylmercury (MeHg). As part of continuing studies on the biogeochemical controls, sources, and fate of Hg and MeHg in the Chesapeake Bay region, we have recently expanded our research to examine Hg and MeHg cycling in Chesapeake tidal marshes. Our main study site is the Kirkpatrick Marsh, a salt marsh at the Smithsonian Environmental Research Center (SERC) on the shores of a Chesapeake Bay sub-estuary, the Rhode River. Kirkpatrick Marsh is dominated by Spartina patens, Scirpus olneyi, Phragmites australis, and several other species and is influenced by a mean tidal range of approximately 30 cm. The marsh is currently and has previously been the subject of various biogeochemical studies, thus basic biogeochemistry, carbon, and nutrient cycling for this system is well understood. Research goals for our study include an estimation of the contribution of salt marshes to MeHg budgets in the Chesapeake specifically and in coastal zones more generally; a first look at the sources of Hg for methylmercury production in tidal marshes; and an improved understanding of the biogeochemical controls on net MeHg production and flux in these wetlands. The research study has two major components. One is a spatially-distributed investigation of the geochemical and microbial controls on MeHg production in this high sulfate/high sulfide wetland system. Detailed biogeochemical measurements were made across three marsh zones distinguished by vegetation/elevation characteristics. Microbial activity in the three zones peaks at depths approximately equal to the mean water table depth, but always in the upper 5-10 cm of soil. Pore water sulfide concentrations increase substantially with depth in marsh soil cores, with highest sulfide concentrations (up to 1.5 M) found deeper in the least frequently flooded site. Initial data show that MeHg concentrations are maximal in the top 5-10 cm of soil, right above the transition into high sulfide zones. In contrast to some other marine systems, our initial data reveals high MeHg concentrations (up to 2.5 ng/L) in marsh pore water across a wider range of sulfide concentrations between 5 and 400 uM. The other component of this research is the construction of comprehensive and temporally-intensive water, total mercury, and methylmercury budgets for the salt marsh. This includes local Hg deposition, continuous flow- weighted Hg/MeHg flux measurements through the main tidal channel, monthly Hg/MeHg measurements along a salinity gradient in the adjacent Rhode River, and various other hydrologic and climatologic measurements. Initial results indicate, as expected, that the marsh is a major sink for particulate bound Hg. Linking process scale measurements with larger-scale hydrology is a key step in attributing the source/sink characteristics of the marsh to spatial and temporal variability of processes within it.