No abstract

We present a detailed study of the biogeochemical factors controlling mercury (Hg) distribution, methylmercury (MeHg) production, and MeHg efflux in sediments of the mid-Atlantic continental shelf and slope. The mildly reduced surface sediments of the shelf and slope provide ideal conditions for MeHg production. They are sufficiently reduced to support microbial sulfate reduction, but contain very low dissolved sulfide concentrations. The redox zonation of sediments determined the depth distribution of MeHg production, whereas the bioavailability of inorganic Hg for methylation appeared to be the dominant driver of spatial patterns across the shelf and slope. Sediment total Hg concentrations were well predicted by sediment organic matter (SOM) content, with the highest concentrations of Hg and MeHg in the fine-grained organic clays of the slope. However, SOM-normalized Hg concentrations decreased with distance from shore. The changing character of organic matter with distance from shore appeared to affect Hg partitioning and bioavailability for methylation. The percentage of Hg in sediments as MeHg was well predicted by measured methylation rates, but not by demethylation rates. On the basis of measured concentrations in bottom waters and surficial pore waters, the average diffusive efflux of Hg(II) and MeHg from sediments to coastal waters was estimated to be 26 and 0.8 pmol m 22 d 21 , respectively. Extrapolated globally, the diffusive input of MeHg from shelf and slope sediments is estimated to be 0.01 Mmol per year. As the actual fluxes can be substantially higher than diffusive fluxes, we suggest that shelf and upper slope sediments are a major source of MeHg to the coastal ocean.

Methylmercury (MeHg) is a contaminant of global concern that bioaccumulates and bioamagnifies in marine food webs. Lower trophic level fauna are important conduits of MeHg from sediment and water to estuarine and coastal fish harvested for human consumption. However, the sources and pathways of MeHg to these coastal fisheries are poorly known particularly the potential for transfer of MeHg from the sediment to biotic compartments. Across a broad gradient of human land impacts, we analyzed MeHg concentrations in food webs at ten estuarine sites in the Northeast US (from the Hackensack Meadowlands, NJ to the Gulf of Maine). MeHg concentrations in water column particulate material, but not in sediments, were predictive of MeHg concentrations in fish (killifish and Atlantic silversides). Moreover, MeHg concentrations were higher in pelagic fauna than in benthic-feeding fauna suggesting that MeHg delivery to the water column from methylation sites from within or outside of the estuary may be an important driver of MeHg bioaccumulation in estuarine pelagic food webs. In contrast, bulk sediment MeHg concentrations were only predictive of concentrations of MeHg in the infaunal worms. Our results across a broad gradient of sites demonstrate that the pathways of MeHg to lower trophic level estuarine organisms are distinctly different between benthic deposit feeders and forage fish. Thus, even in systems with contaminated sediments, transfer of MeHg into estuarine food webs maybe driven more by the efficiency of processes that determine MeHg input and bioavailability in the water column.

Methylmercury (MeHg) affects wildlife and human health mainly through marine fish consumption. In marine systems, MeHg is formed from inorganic mercury (HgII) species primarily in sediments then accumulates and biomagnifies in the food web. Most of the fish consumed in the US are from estuarine and marine systems highlighting the importance of understanding MeHg formation in these productive regions. Sediment organic matter has been shown to limit mercury methylation in estuarine ecosystems, as a result it is often described as the primary control over MeHg production. In this paper, we explore the role of organic matter by looking at the effects of its changing sediment concentrations on the methylation rates across multiple estuaries. We measured sedimentary MeHg production at eleven estuarine sites that were selected for their contrasting biogeochemical characteristics, mercury (Hg) content, and location in the Northeastern US (ME, NH, CT, NY, and NJ). Sedimentary total Hg concentrations ranged across five orders of magnitude, increasing in concentration from the pristine, sandy sediments of Wells (ME), to industrially contaminated areas like Portsmouth (NH) and Hackensack (NJ). We find that methylation rates are the highest at locations with high Hg content (relative to carbon), and that organic matter does not hinder mercury methylation in estuaries.

Estuaries in the northern Gulf of Mexico (GOM) provide habitat for many ecologically, commercially, and recreationally important fish and crustacean species (i.e., nekton), but patterns of nekton abundance and community assemblages across habitat types, salinity zones, and seasons have not been described region-wide. Recognizing the wealth of information collected from previous and ongoing field sampling efforts, we developed a meta-analytical approach to aggregate nekton density data from separate studies (using different gear types) that can be used to answer key research questions. We then applied this meta-analytical approach to separate nekton datasets from studies conducted in the Gulf of Mexico to summarize patterns in nekton density across and within several estuarine habitat types, including marsh, oyster reefs, submerged aquatic vegetation (SAV), and open-water non-vegetated bottom (NVB). The results of the meta-analysis highlighted several important patterns of nekton use associated with these habitat types. Nekton densities were higher in structured estuarine habitats (i.e., marsh, oyster reefs, SAV) than in open-water NVB habitat. Marsh and SAV community assemblages were relatively similar to each other, but different from those associated with open-water NVB and oyster habitats. Densities of commercially and recreationally important crustacean and fish species were highest in saline marshes, thus demonstrating the importance of this habitat in the northern GOM. The results of our meta-analysis are generally consistent with previous site-specific studies in the region (many of which were included in the meta-analysis) and provide further evidence for these patterns at a regional scale. This meta-analytical approach is easy to implement for diverse research and management purposes, and provides the opportunity to advance understanding of the value and role of coastal habitats to nekton communities.