Methylmercury cycling in sediments on the continental shelf of southern New England

Abstract

Exposure of humans to monomethylmercury (MMHg) occurs primarily through consumption of marine fish, yet there is limited understanding concerning the bioaccumulation and biogeochemistry of MMHg in the biologically productive coastal ocean. We examined the cycling of MMHg in sediments at three locations on the continental shelf of southern New Moreover, the efflux of dissolved MMHg to overlying water (i.e., net production at steady state) is correlated with the gross potential rate of MMHg production in surface sediments. These results suggest that the production and efflux of MMHg from coastal marine sediments is limited by Hg(II), loadings of which presumably are principally from atmospheric deposition to this region of the continental shelf. The estimated diffusive flux of MMHg from the shelf sediments averages 9 pmol m −2 d −1 . This flux is comparable to that required to sustain the current rate of MMHg accumulation by marine fish, and may be enhanced by the efflux of MMHg from nearshore deposits contaminated more substantially with anthropogenic Hg. Hence, production and subsequent mobilization of MMHg from sediments in the coastal zone may be a major source of MMHg to the ocean and marine biota, including fishes consumed by humans.

Characterization of mercury contamination in the Androscoggin River, Coos County, New Hampshire

“…In the far-stream reaches, percent LOI alone explained 49 percent of the variability in sediment THg concentration. A positive relation between THg and percent LOI has been observed in a number of other studies (Hammerschmidt and Fitzgerald, 2006;Han and others, 2007;Marvin-DiPasquale and others, 2009a, b). In the higher gradient near-stream reaches, sediment BD became an important explanatory variable in addition to percent LOI, alluding to the nature of the particles that are likely to be deposited in the high-gradient environment.…”

A mass balance inventory of mercury in the Arctic Ocean

“…Assuming that MeHg comprises no more than 25% of total Hg in bacteria, phytoplankton and zooplankton, [90] and 100% in marine mammals and fish, [91] the MeHg mass in marine biota is therefore ∼4.5 t. We conservatively estimate that 47 t of abiotic MeHg (∼5% of the total Hg pool) is present in the upper Ocean, and ∼450 t in the whole Ocean, based on a dissolved MeHg concentration of 0.03 ng L −1 measured in North Atlantic continental shelf waters. [92] Thus, bioaccumulated MeHg represents <10% of the dissolved MeHg pool in the upper Ocean alone, and <1% of whole Ocean MeHg. If the extant pool of bioavailable MeHg is already >10-fold larger than that that has been bioaccumulated, then the effect of further additions to that pool must be limited proportionately.…”

“…[36] In temperate estuarine sediments, cycling of Hg by biotic methylation-demethylation was rapid with a turn-over time for MeHg of the order of days. [92] Photodemethylation was a potent process in an Alaskan lake, accounting for ∼80% of annual sedimentary MeHg production, even though it was limited to the 100-day ice-free season. [122] In essence, photodemethylation competed with freshwater biota for MeHg, thereby possibly inhibiting its uptake by the lake's food-web.…”

Regional variation in mercury and stable isotopes of red snapper (Lutjanus campechanus) in the northern gulf of Mexico, USA

“…Sulfate-reducing bacteria convert inorganic Hg in marine sediments to methylmercury (MeHg) [12], which can accumulate in food chains when it is absorbed by primary producers, resulting in fish being exposed to MeHg primarily through the diet [13]. Prey preferences, ontogenetic shifts in habitat, migration, and seasonal movements are factors that complicate the determination of Hg bioaccumulation in marine fish.…”