Benthic and Pelagic Pathways of Methylmercury Bioaccumulation in Estuarine Food Webs of the Northeast United States

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

Mercury cycling in sediments of Chesapeake Bay and the mid-Atlantic continental shelf and slope

Human activities have significantly altered the mercury (Hg) cycle, increasing elemental Hg emissions to the atmosphere and inorganic Hg loadings to aquatic systems. Increased production of methylmercury (MeHg), a neurotoxin formed in aquatic systems, and its bioaccumulation into aquatic foodwebs, is likely, but the potential magnitude of the increase is not known. With fish MeHg levels as the main driver of the Hg-based US EPA and FDA fish consumption advisories, and given the importance of ocean fisheries as a source of MeHg exposure to humans, a comprehensive understanding of the behavior and fate of MeHg in the marine environment is necessary. This dissertation was designed to enhance knowledge of the biogeochemical factors affecting MeHg production and distribution in coastal marine sediments and to determine the importance of these sediments as a source of MeHg to the coastal ocean. ^ Although the biogeochemical characteristics of the suboxic-anoxic sediments of the mid-Atlantic continental shelf and slope were significantly different than those of the productive Chesapeake Bay, the controls on Hg distribution and MeHg production were consistent. Hg concentration and partitioning in the bulk-phase were strongly controlled by the organic matter quantity and quality of the sediment. MeHg production was related to the concentration of aqueous neutrally charged Hg-S complexes in the porewater and, to a lesser extent, the activity of methylating bacteria. In offshore sediments, high MeHg production coincided with a decrease in the strength of Hg partitioning to the bulk-phase, a relative increase in Hg porewater concentrations, and an expansion of the oxic-anoxic transition zone down-core. ^ All shelf and slope sites were sources of MeHg to the water column over all seasons studied, with an average diffusive flux of 0.8 pmol m−2 day−1. By integrating the flux over the global area of the continental margin, the coastal diffusive flux of MeHg was estimated to be 0.01 Mmol per year. This flux, which is a minimum estimate and likely at least ten times higher due to bioturbation/biorirrigation, is on the same order as other quantified MeHg inputs to the ocean, and therefore is consistent with the hypothesis that the continental margin is a significant source of MeHg to the global ocean, and must be included in future ocean MeHg budgets.

Methylmercury Production in Estuarine Sediments: Role of Organic Matter

Methylmercury (MeHg) affects wildlife and human health mainly through marine fish consumption. In marine systems, MeHg is formed from inorganic mercury (Hg^II) species primarily in sediments, then accumulates and biomagnifies in the food web. Most of the fish consumed in the United States 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 U.S. (ME, NH, CT, NY, and NJ). Sedimentary total Hg concentrations ranged across 5 orders of magnitude, increasing in concentration from the pristine, sandy sediments of Wells (ME), to industrially contaminated areas such as 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

Benthic and Pelagic Pathways of Methylmercury Bioaccumulation in Estuarine Food Webs of the Northeast United States

<div><p>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.</p></div

Relationships between sediment and water column compartments and organic carbon at each site.

<p>Relationships between continuous variables assessed by linear regression after logarithmic transformations. (A) total and MeHg sediment concentrations vs. %LOI.</p

Multiple regression results for environmental predictors (MeHg in sediments, water column dissolved aqueous, and particulates) of biotic MeHg tissue concentrations.

<p>Full and reduced models for (A) killifish; (B) Atlantic silversides; and (C) worms.</p

Stable isotope signatures of individual taxonomic groups measured as delta ¹³C and delta ¹⁵N.

<p>Delta ¹⁵N values were adjusted for site differences. Because of significantly different patterns, worms were excluded from analysis. Crosshairs show +/− two standard errors.</p

Relationship between MeHg in worm tissues vs. sediment MeHg.

<p>Relationships between continuous variables assessed by linear regression after logarithmic transformations. Points show site means and error bars extend to the minimum and maximum site concentrations.</p

Relational diagram for total Hg and MeHg in sediment and water column and biotic compartments.

<p>The magnitude and sign of the coefficients represent the relative contribution of the independent variable in the prediction of the dependent variable located at the head of each arrow. These were calculated by multiplying the linear regression coefficients by the ratio of standard deviations of the independent and dependent variables, respectively.</p

Relationship between MeHg tissue concentrations in fish and MeHg in water column particulate.

<p>Relationships between continuous variables assessed by linear regression after logarithmic transformations. (A) killifish MeHg vs. particulate; (B) Atlantic silverside MeHg vs. particulate. Points show site means and error bars extend to the minimum and maximum site concentrations.</p