Environmental controls on the speciation and distribution of mercury in coastal sediments

Methylmercury production by sulfate reducing bacteria in coastal sediments leads to bioaccumulation of mercury in fish, shellfish, and ultimately humans. Sulfur, organic carbon, and sediment structure and composition can all affect methylmercury production by changing the amount of bioavailable inorganic mercury and by stimulating the activity of methylating microbes. This study investigates total and methylmercury in solids and porewaters relative to total sulfide concentration, redox potential, sediment grain size, and total organic carbon in a range of sediment types from the Bay of Fundy region of Canada. Using these data, we construct a conceptual model of the biogeochemical environment surrounding methylating microbes in high sulfide, organically enriched sediments. Whereas other studies of methylmercury dynamics measured porewater sulfide concentrations in relatively low-sulfide systems (∼20–300 μM), we measured total sulfide levels using a method developed to indicate organic enrichment across a much wider range of sulfidic sediments (10–4000 μM). We observed that higher sulfide concentrations correspond to an elevated fraction of mercury in methylated form suggesting higher net methylation rates in these sediments. This relationship is strongest in sediments that are moderately impacted by organic enrichment, but weak in less impacted, aerobic sediments. Higher sulfide concentrations in porewaters containing dissolved organic matter appear to yield a geochemical environment that is conducive to uptake of Hg(II) by methylating bacteria. Data collected in this study imply that moderate levels of organic enrichment through fish farming may enhance methylmercury production in the Bay of Fundy.