MERCURY AND METHYLMERCURY DYNAMICS IN THE SEDIMENTS OF THE GREAT BAY (NH) AND PENOBSCOT RIVER (ME) ESTUARIES

Due to the significant sedimentation of river-borne particulate matter in estuarine systems, these zones act as repositories for particulate contaminants including Hg. The sediments of estuaries and salt marshes have a high degree of geochemical variability, especially with respect to sulfate and organic matter, and are subject to significant fluctuations in water level, oxygen and salinity that result in redox transitions in the zone close to the sediment-water interface (SWI). We performed pore water and sediment chemical and molecular analyses to study Hg cycling in the Penobscot River estuary (ME) and Great Bay (NH) sediments. In Penobscot we studied the mudflats, and in Great Bay we studied the mudflats and a salt panne. We observed a correspondence between the abundance of sulfate-reducing bacteria (SRB) and solid-phase and pore-water MeHg concentrations. These concentrations and the SRB density reached a maximum close to the SWI in the mudflats. In addition, intact sediment cores were collected from Penobscot River and the location of the redoxcline was manipulated by altering the ponding regimes of the overlying water on the cores. Induced shoaling of the redoxcline was observed in the absence of any active aeration, which corresponded to the shoaling of the net MeHg production zone. These results corroborated our field observations in the salt panne, where ponded water resulted in the shoaling of the redoxcline as well as the maximum sediment MeHg concentration. Based on this, environments such as salt pannes are likely to be Hg methylation and release hotspots in estuaries.