Organic matter and sulfide inhibit methylmercury production in sediments of New York/New Jersey Harbor

Sediments of New York/New Jersey (NY/NJ) Harbor are contaminated with Hg from modern discharges and the pollution legacy of the Industrial Revolution, and there is concern related to the bioavailability, transformation, and mobilization of monomethylmercury (MMHg) from these deposits. We investigated the production and distribution of MMHg in sediments of NY/NJ Harbor across wide and contrasting ranges of inorganic Hg (Hg(II)), sulfide, and organic matter in August 2002 and February and May 2003. Organic material largely controls the geographical distribution of Hg(II) and MMHg. Partitioning coefficients (KD) for MMHg and Hg(II) are related positively to the organic content of deposits with less than 10 µM dissolved sulfide. KD values for MMHg and Hg(II) in sediments of NY/NJ Harbor are about 10× greater than those in deposits of Long Island Sound (LIS) and the continental shelf, suggesting differences in the affinity of Hg species for allochthonous (Harbor) and planktonic organic matter (LIS, shelf). Dissolved sulfide enhances the solubility of MMHg and Hg(II) and inhibits MMHg production. Potential gross rates of Hg methylation, assayed by experimental addition of 200Hg to intact cores, are related positively to the level of Hg(II), presumably as HgS0, in 0.2-µm filtered pore fluids of sediments having less than 10 µM sulfide, and are reduced greatly in deposits with greater levels of dissolved sulfide. MMHg:Hg(II) concentration ratios, potential rates of 200Hg methylation, and diffusional sediment–water MMHg fluxes vary seasonally. The estimated whole-basin sediment–water flux of MMHg (i.e., net production at steady state) is about 1% relative to Hg(II) burial, a percentage much less than that in LIS and at three sites on the continental shelf (8%). Thus, it appears that allochthonous organic material (terrestrial and/or sewage) and dissolved sulfide reduce bioavailability of Hg and attenuate gross and net rates of MMHg production in the Harbor. Accordingly, changes in allochthonous organic inputs and microbial respiration of this material (i.e., sulfide production) could affect the production and mobilization of MMHg by altering the bioavailability of modern inputs and some portion of the large reservoir of “legacy Hg” buried in the sediment.