MERCURY CYCLING IN A PARTIALLY ANOXIC COASTAL ESTUARY: OYSTER POND, CAPE COD, MASSACHUSETTS

Oyster Pond is an ideal coastal environment in which to investigate numerous aspects of the biogeochemical behavior of mercury (Hg). 1. Sediments in the anoxic basins of the pond accumulate rapidly without mixing by benthic infauna, so sediment cores are useful for evaluating historical fluxes of contaminants to the pond. 2. The pond typically has a full range of redox conditions in both water and sediments that permits analysis of factors influencing methyl Hg production. 3. Oyster Pond hosts migratory fish (river herring, American eels, white perch), piscivorous birds (osprey), and other animals that participate in the cycling of Hg in this ecosystem.

²¹⁰Pb and ¹³⁷Cs profiles indicate a sedimentation rate over the last 50 years of about 4.4 mm/yr (28 mg/cm²/yr) in the north basin. Total Hg in sediment is lowest (<0.03 ppm) in horizons older than 1860 and reaches a maximum of 0.44 ppm in sediment from about 1948. Thereafter, concentrations decrease with time to about 0.15 ppm in surface sediments collected in 2007 and 2009, possibly reflecting diminishing regional and local industrial sources since the end of World War II. The concentrations of methyl Hg in sediments from the anoxic area are typically <0.4 ppb and represent <0.3 % of the total Hg, possibly due to reduced methyl Hg production in the presence of high sulfide or increased demethylation rates. By contrast, in oxic regions of the pond, methyl Hg concentrations in sediments are up to 6.5 ppb representing 3% of the total Hg. Experiments are planned to measure methyl Hg production rates in sediments and methyl Hg diffusion rates in pore water under different redox conditions.

Analyses of diadromous fish are underway in order to estimate transfer of Hg between fresh and salt water with fish migration. An opportunity exists to determine Hg in different life stages of river herring (Alosa pseudoharengus) that spawn in Oyster Pond and mature in coastal waters. We will contrast these results with analyses of American eels (Anguilla rostrata) that spawn in the Sargasso Sea and migrate to the eastern US to mature in fresh or brackish water locations like Oyster Pond. Combining Hg concentrations with estimates of biomass in transit will provide insight into the relative importance of migrating fish in the transport of Hg.