EFFECTS OF TIDAL INFLUX ON MERCURY METHYLATION IN HAMBURG COVE, CT

Monomethylmercury is the only form of mercury that bioaccumulates in the environment, but the pathways and controls on monomethylmercury cycling are still fairly unknown. Biological methylation is the primary factor driving methylmercury concentrations in bodies of water, and in particular, sulfate-reducing bacteria play a key role in this process. We seek to evaluate the effect of tidal influx of saline water in a Connecticut River embayment on microbial mercury methylation rates. We hypothesize that periodic salt-water encroachment introduces metabolites and substrates that drive higher levels of anaerobic respiration and thus higher levels of mercury methylation in the areas of the cove where tidal water collects. Four sediment cores (40-60 cm) were collected from two different sites in Hamburg Cove, CT; two from the area of the cove where saline water collects, and two from the shallow area of the cove where freshwater dominates. Porewaters were extracted from 1-2 cm horizons and analyzed for salinity, alkalinity, and sulfate, and sediment from each horizon was analyzed for total organic carbon content (TOC) and total mercury concentration. Using previously well-defined total mercury chronologies, the sedimentation rate of the saline site was found to be seven times greater than that of the freshwater site. Anaerobic respiration rates were also much greater at the saline site, likely due to the introduction of sulfate ions and organic substrates by tidal influx. In the saline site alkalinity increased with depth to a maximum concentration of 12 meq/L, whereas in the freshwater site alkalinity did not exceed 3 meq/L. However, both sites had similar total mercury concentrations. Overall, our data suggest that though total mercury is the same throughout Hamburg Cove, tidal influx affects the porewater composition and microbial activity in different areas of the cove, driving higher anaerobic respiration rates in saline areas and potentially higher mercury methylation rates and total methylmercury concentrations. Predicted sea level rise will increase the areas affected by tidal influx and could thus increase microbial activity and have serious implications for methylmercury levels in coastal environments.