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

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

Relationship of MeHg tissue concentrations of all taxonomic groups across all sites vs. delta ¹³C.

<p>Relationships between continuous variables assessed by linear regression after logarithmic transformations.</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

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

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

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

Map of field sites.

<p>Ten estuarine field sites in seven states (across a contamination gradient) sampled in summer 2008.</p

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