| We studied the interactions between organic matter (OM) and mercury (Hg) in the sediment and water column of four biogeochemically contrasting estuaries, with varying levels of Hg contamination (Maine, New Hampshire, Long Island Sound and New Jersey). Total Hg (HgT) and methylmercury (MeHg) sediment concentrations were highest in the highly contaminated sediments of Hackensack, NJ. However, the MeHg fraction of HgT in the sediment was significantly higher in the pristine sandy sediments of Maine. The commonly reported positive linear correlation between the sedimentporewater HgT partition coefficient and OM was only found at pristine to mildly impacted sites, suggesting that the binding capacity of the sediment was not the major determinant of HgT concentration at contaminated sites.;With the exception of Long Island Sound, where the lowest methylation rates were measured in organic rich sediments, MeHg production increased with increased sediment OM content in surficial sediments across all the sites. We found that net methylation rates were higher in organic matter rich sediments and that the methylation potential (defined as the capacity of a particular environment to convert HgII to MeHg; calculated as in situ porewater HgII concentration multiplied by the methylation rate) was comparable in pristine and highly contaminated sites. We show that sediment sulfur content inhibits Hg methylation and is a more important factor across these sites than sediment organic content. The lower methylation rates measured at locations displaying low carbon to sulfur ratios (molar ratio 6.6) could be explained by the presence of iron sulfides, which is known to limit the availability of inorganic Hg to sulfate reducing bacteria.;Finally, we show that not all fractions of dissolved organic matter (DOM) participate in Hg complexation. There is evidence, in short excitation wavelengths (characteristic of reduced compounds), and a conditional stability constant comparable to those calculated for RSH-DOM complexes (∼39), that fractions of DOM in a reduced redox state are the most important with regard to Hg complexation. |
