An examination of the factors influencing the bioaccumulation of methylmercury at the base of the estuarine food web.

Estuarine systems have received ongoing mercury (Hg) inputs from both point sources and regional contamination and have high legacy Hg in sediments. This is an environmental concern given that coastal seafood is an important vector for human exposure to methylmercury (MeHg). The base of the food chain represents the most important trophic steps for MeHg bioaccumulation. The magnitude of the uptake by phytoplankton, and their consumers, is influenced by many factors, in addition to sediment and water MeHg concentrations, that impact MeHg assimilation into phytoplankton and the trophic transfer to higher trophic levels, both benthic and pelagic. For forage fish, such as mummichogs (Fundulus heteroclitus), abiotic and biotic (bioenergetic) factors can influence their MeHg content, and diet is also important as they feed both on benthic and pelagic prey. Given that the importance of sediment MeHg versus pelagic MeHg sources has been debated, we updated a phytoplankton bioaccumulation model, and coupled this with a bioaccumulation model for MeHg concentration in mummichog tissue to examine the controlling factors for sites, from Maine to Maryland, USA, ranging widely in their Hg concentrations and other variables. The study highlighted the importance of DOC in modulating uptake into the pelagic food web, but also demonstrated the importance of diet in controlling mummichog MeHg. Finally, the relative importance of MeHg source - sediment or water column - was correlated with the level of Hg contamination. Sediment-derived MeHg was a more important source for highly Hg contaminated systems. As water column and sediment MeHg are not strongly correlated for the studied ecosystems, their importance as a source of MeHg to mummichogs varies with location. The study highlights the differences across ecosystems in MeHg bioaccumulation pathways, and that uptake into phytoplankton is an important variable controlling forage fish concentration.

Historic contamination alters mercury sources and cycling in temperate estuaries relative to uncontaminated sites.

Mercury (Hg) is a global and persistent pollutant which can be methylated to more toxic forms (methylmercury; MeHg) in natural systems. Both forms pose a health risk to humans and wildlife, and exposure often begins in aquatic environments. Therefore, quantifying aquatic concentrations and identifying source pathways is important for understanding biotic exposure. In this study, data from estuaries in the Northeast United States were combined to evaluate how point source contamination impacts the concentration and source dynamics of water column total and MeHg with an emphasis on sediment versus non-sediment sources. Partial least squares regression models were implemented to identify a set of variables most related to water column MeHg and total Hg (HgT) across the estuaries. The main findings suggest that contaminated sites have strong internal recycling of HgT that dominates over external inputs, and this leads to elevated concentrations of HgT and MeHg in the local water columns. However, HgT sources in uncontaminated estuarine systems have a strong connection to the local watershed with dissolved HgT linked to dissolved organic carbon, and particulate HgT linked to watershed land use and estuarine mixing. There was little correlative evidence that water column MeHg concentrations were linked to sediment in such systems, but unlike HgT, the concentrations were also not clearly linked to the watershed. Instead, in situ methylation of dissolved water column HgT appeared to dominate the MeHg source pathway. The results suggest that Hg point-source contaminated sites should be considered independently from non-contaminated sites in terms of management, and that land use plays an important indirect role in coastal MeHg dynamics.

The interaction of mercury and methylmercury with chalcogenide nanoparticles.

Mercury (Hg) and methylmercury (CH3Hg) bind strongly to micro and nano (NP) particles and this partitioning impacts their fate and bioaccumulation into food webs, and, as a result, potential human exposure. This partitioning has been shown to influence the bioavailability of inorganic Hg to methylating bacteria, with NP-bound Hg being more bioavailable than particulate HgS, or organic particulate-bound Hg. In this study we set out to investigate whether the potential interactions between dissolved ionic Hg (HgII) and CH3Hg and NPs was due to incorporation of Hg into the core of the cadmium selenide and sulfide (CdSe; CdS) nanoparticles (metal exchange or surface precipitation), or due purely to surface interactions. The interaction was assessed based on the quenching of the fluorescence intensity and lifetime observed during HgII or CH3Hg titration experiments of these NP solutions. Additional analysis using inductively coupled plasma mass spectrometry of CdSe NPs and the separated solution, obtained after HgII additions, showed that there was no metal exchange, and X-ray photoelectron spectroscopy confirmed this and further indicated that the Hg was bound to cysteine, the NP capping agent. Our study suggests that Hg and CH3Hg adsorbed to the surfaces of NPs would have different bioavailability for release into water or to (de)methylating organisms or for bioaccumulation, and provides insights into the behavior of Hg in the environment in the presence of natural or manufactured NPs.

Role of Sediment Resuspension on Estuarine Suspended Particulate Mercury Dynamics.

Coastal sediments are an important site for transient and long-term mercury (Hg) storage, and they foster a geochemical environment optimal for Hg methylation. Therefore, efforts have been taken to constrain the role of sediments as a source of methylmercury (MeHg) to the estuarine water column. This study employed the Gust Microcosm Erosion Core system capable of quantifying particle removal from undisturbed cores under measurable shear stress conditions to assess particulate Hg and MeHg exchange between sediments and the water column. Samples were collected from organic-rich and organic-poor sediment types from the mid- and lower Delaware Bay. It was found that bulk sediment samples from organic-rich systems overpredict total Hg and MeHg release to the water column, whereas organic-poor sediments underpredict the exchange. In general, organic-rich sediments in shallow environments have the most impact on surface particle dynamics. There is little evidence to suggest that MeHg formed in the sediments is released to the water column via particulate exchange, and therefore, nonsedimentary sources likely control MeHg levels in this estuarine water column.

Contemporary Mobilization of Legacy Pb Stores by DOM in a Boreal Peatland.

We examined how different landscape areas in a catchment containing a northern ombrotrophic peatland and upland mineral soils responded to dramatic decreases in atmospheric deposition of lead (Pb). Pb concentrations in the outflow stream from the peatland measured from 2009-2015 indicated continued mobilization and export of Pb derived from historic inputs to the bog. In contrast, Pb concentrations in surface peat and runoff from upland mineral soils have declined in response to reductions in atmospheric deposition. Relative to the early 1980s, Pb concentrations in the streamflow decreased only ∼50%, while Pb in surface peat and upland subsurface runoff decreased by more than 90%. Water level fluctuations in the slow-accumulating peat have allowed dissolved organic matter (DOM) to continue mobilizing Pb deposited in the peatland decades earlier. Strong correlations between dissolved organic carbon (DOC) and Pb concentrations in outflow from the peatland and in bog porewaters demonstrate Pb mobility related to DOM production. Peat stores of Pb in 2016 were less than or equal to those reported in the early 1980s despite the dry mass inventory increasing by 60-80%. Much of the loss in Pb stored in peat can be accounted for by stream runoff from the peatland.