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The foundation of methylmercury (MeHg) bioaccumulation in aquatic systems is uptake and trophic transfer into phytoplankton and zooplankton. Phytoplankton and zooplankton samples were collected seasonally from Long Island Sound (LIS) and its adjacent shelf waters, located along the northeast coast of the United States. Phytoplankton samples were size fractioned into 0.2-5 μm, 5-20 μm, and seston of > 20 μm. Average phytoplankton MeHg concentrations were 0.01-1.5 pmol g−1 (wet weight), and MeHg bioconcentration factors (logBCF) ranged from 2.5 to 5.5. There was higher MeHg concentration and logBCF values with increasing phytoplankton size. Zooplankton samples were size fractioned into 0.2-0.5 mm, 0.5-1.0 mm, 1.0-2.0 mm, and > 2.0 mm classes and analyzed for Hg and MeHg as well as carbon (δ13C), nitrogen (δ15N), and sulfur (δ34S) isotopes. The %MeHg in organisms was highest in the > 2.0 mm size class for coastal LIS and shelf species, indicating higher MeHg bioaccumulation with increasing zooplankton sizes. Eastern LIS zooplankton populations yielded higher overall Hg accumulation than western LIS populations, in alignment with phytoplankton fractions. Seasonal differences were evident for plankton %MeHg, although only ELIS displayed an increase in summertime %MeHg throughout the fractions. The results suggest the importance of biodilution in determining plankton MeHg concentrations. The δ13C values infer that zooplankton were opportunistically feeding on food containing variable levels of Hg and MeHg. The δ15N isotopes indicated enrichment characteristic of anthropogenic input in populations at the western end of LIS. The δ34S isotopes signified that organisms were primarily feeding on pelagic plankton. [ABSTRACT FROM AUTHOR]

Estuarine water column methylmercury (MeHg) is an important driver of mercury (Hg) bioaccumulation in pelagic organisms and thus it is necessary to understand the sources and processes affecting environmental levels of MeHg. Increases in water column MeHg concentrations can ultimately be transferred to fish consumed by humans, but despite this, the sources of MeHg to the estuarine water column are still poorly understood. Here we evaluate MeHg sources across 4 estuaries and 10 sampling sites and examine the distributions and partitioning of sediment and water column MeHg across a geographic range (Maine to New Jersey). Our study sites present a gradient in the concentrations of sediment, pore water and water column Hg species. Suspended particle MeHg ranged from below detection to 187 pmol g − 1 , dissolved MeHg from 0.01 to 0.68 pM, and sediment MeHg from 0.01 to 109 pmol g − 1 . Across multiple estuaries, dissolved MeHg correlated with Hg species in the water column, and sediment MeHg correlated with sediment total Hg (HgT). Water column MeHg did not correlate well with sediment Hg across estuaries, indicating that sediment concentrations were not a good predictor of water MeHg concentrations. This is an unexpected finding since it has been shown that MeHg production from inorganic Hg 2 + within sediment is the primary source of MeHg to coastal waters. Additional sources of MeHg regulate water column MeHg levels in some of the shallow estuaries included in this study. [ABSTRACT FROM AUTHOR]

Mercury (Hg) speciation and the activity of Hg(II)-methylating bacteria are responsible for the rate of methylmercury production and thus bioaccumulation in marine foodwebs. Factors affecting porewater partitioning (Kd) and methylation of Hg(II) were examined at 11 sites in sediment of 4 biogeochemically diverse estuaries in the Northeast U.S. In Long Island Sound, 88% of total mercury (HgT) log Kd variability was described by porewater dissolved organic carbon concentration and sediment total sulfur (S) content. Whereas across all estuaries, regression analyses showed that S alone drives about 70% of Kd variability and 50% of changes in methylation rates; and the inclusion of DOC and sulfides did not improve the prediction. Thus, we demonstrated that S is a better predictor of HgT log Kd than the sediment organic matter across multiple estuaries, and while organic matter and S are interchangeable in small-scale studies, on a larger scale, sediment S content is the simplest and most effective variable to measure. [ABSTRACT FROM AUTHOR]

Abstract: Mercury (Hg) contamination, which includes its toxic byproduct monomethylmercury (MMHg), is ubiquitous and often severe in estuarine and near-shore systems of industrialized countries. However, Hg cycling is not well studied in anthropogenically impacted systems such as New York/New Jersey Harbor Estuary (NY/NJ Harbor), and more generally, knowledge concerning the biogeochemical cycling of toxic metals is limited for estuaries and coastal regions. Here, we are reporting results from a 1-year seasonally focused field investigation on the behavior and fate of Hg and MMHg in NY/NJ Harbor. Filtered (<0.2-μm) and unfiltered reactive (labile) and total Hg, as well as filtered and particulate MMHg, were measured in surface and bottom waters (5–9 stations) in August 2002, February 2003, and May 2003. Scavenging by particulate matter is a major control on the partitioning of total Hg in the vertically well-mixed waters of the Harbor; 89–99% of total Hg (log K D =5.3–6.5) and 56–89% of MMHg (log K D =4.5–5.6) were in the particle phase. Mean levels of unfiltered total Hg (30–550 pM) and total MMHg (0.2–1.8 pM; filtered+particulate) were greater and more variable seasonally at the sites in Newark Bay and the Upper Harbor, as compared to the Lower Harbor and Jamaica Bay, consistent with fluvial inputs (Hudson River yields 90% of freshwater input) as a principal source of total Hg to the Harbor. Water-column particle-specific concentrations (mean±SE) of total Hg (4.8±0.5 nmol g−1) and MMHg (20±2 pmol g−1; 0.4% of total Hg) are comparable to levels in surface sediment, suggesting significant sediment suspension and deposition. The major sources of total Hg (∼2440 mol y−1) to NY/NJ Harbor are rivers (∼1640 mol y−1; 67% of total inputs), the East River (∼630 mol y−1), water pollution control facilities (WPCFs; ∼140 mol y−1), and direct atmospheric deposition (∼30 mol y−1). Although direct atmospheric deposition to NY/NJ Harbor is small, leaching (ca. 25–30% watershed delivery) of atmospheric Hg deposition to the NY/NJ Harbor watershed is a significant source (22–69%), but additional unknown sources also appear to contribute to fluvial Hg loadings. The MMHg flux to the Harbor from internal and external sources is estimated as 33 mol y−1 with 18 mol y−1 from rivers (55%), and 8 mol y−1 (25%) from in situ sedimentary production and mobilization. While fluvial MMHg inputs are dominant, the relative source strength of watershed versus sedimentary riverine production is not yet clear. [Copyright &y& Elsevier]

Methylmercury (MeHg) is the only species of mercury (Hg) to biomagnify in aquatic food-webs to levels that are a widespread concern for human and ecological health. Here we investigate the association between dissolved organic matter (DOM) in seawater and Hg speciation and uptake using experimental data and field measurements from Long Island Sound (LIS) and the Northwestern Atlantic continental margin. We measured differences in DOM composition across sampling stations using excitation emission matrix fluorescence spectroscopy and further separated DOM into terrestrial and marine components using Parallel Factor Analysis (PARAFAC). Highest MeHg concentrations were found in the estuarine stations (LIS) with highest DOM concentrations due to enhanced external inputs from the watershed and rivers. For stations on the shelf and slope, MeHg in plankton increased linearly with a decreasing fraction of fluorescence attributable to DOM components with a terrestrial rather than marine origin. These results are corroborated by experimental data showing higher MeHg uptake by cells in the presence of predominantly marine DOM compared to terrestrial DOM. Highest fractions of dissolved gaseous mercury were alsofound at stations with the highest marine DOM content, suggesting a greater reducible fraction of divalent inorganic Hg. These data suggest DOM composition is a critical driver of Hg reactivity and bioavailability in offshore marine waters. [ABSTRACT FROM AUTHOR]

Methylmercury (MeHg) affects wildlife and human health mainly through marine fish consumption. In marine systems, MeHg is formed from inorganic mercury (Hg11) 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. [ABSTRACT FROM AUTHOR]

Abstract: Sediments of New York/New Jersey (NY/NJ) Harbor are contaminated with Hg from modern discharges and the pollution legacy of the Industrial Revolution, and there is concern related to the bioavailability, transformation, and mobilization of monomethylmercury (MMHg) from these deposits. We investigated the production and distribution of MMHg in sediments of NY/NJ Harbor across wide and contrasting ranges of inorganic Hg (Hg(II)), sulfide, and organic matter in August 2002 and February and May 2003. Organic material largely controls the geographical distribution of Hg(II) and MMHg. Partitioning coefficients (K D) for MMHg and Hg(II) are related positively to the organic content of deposits with less than 10 µM dissolved sulfide. K D values for MMHg and Hg(II) in sediments of NY/NJ Harbor are about 10× greater than those in deposits of Long Island Sound (LIS) and the continental shelf, suggesting differences in the affinity of Hg species for allochthonous (Harbor) and planktonic organic matter (LIS, shelf). Dissolved sulfide enhances the solubility of MMHg and Hg(II) and inhibits MMHg production. Potential gross rates of Hg methylation, assayed by experimental addition of 200Hg to intact cores, are related positively to the level of Hg(II), presumably as HgS0, in 0.2-µm filtered pore fluids of sediments having less than 10 µM sulfide, and are reduced greatly in deposits with greater levels of dissolved sulfide. MMHg:Hg(II) concentration ratios, potential rates of 200Hg methylation, and diffusional sediment–water MMHg fluxes vary seasonally. The estimated whole-basin sediment–water flux of MMHg (i.e., net production at steady state) is about 1% relative to Hg(II) burial, a percentage much less than that in LIS and at three sites on the continental shelf (8%). Thus, it appears that allochthonous organic material (terrestrial and/or sewage) and dissolved sulfide reduce bioavailability of Hg and attenuate gross and net rates of MMHg production in the Harbor. Accordingly, changes in allochthonous organic inputs and microbial respiration of this material (i.e., sulfide production) could affect the production and mobilization of MMHg by altering the bioavailability of modern inputs and some portion of the large reservoir of “legacy Hg” buried in the sediment. [Copyright &y& Elsevier]

• Sedimentary sources of mercury dominated in mercury contaminated estuaries • Watershed land-use most strongly influenced estuarine particulate mercury concentrations • Water column methylmercury tightly coupled with total Hg suggesting in situ formation • Hg contaminated and uncontaminated sites are unique and should be evaluated separately 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. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Developing Canadian hydroelectric resources is a key component of North American plans for meeting future energy demands. Microbial production of the bioaccumulative neurotoxin methylmercury (MeHg) is stimulated in newly flooded soils by degradation of labile organic carbon and associated changes in geochemical conditions. We find all 22 Canadian hydroelectric facilities being considered for near-term development are located within 100 km of indigenous communities. For a facility in Labrador, Canada (Muskrat Falls) with planned completion in 2017, we probabilistically modeled peak MeHg enrichment relative to measured baseline conditions in the river to be impounded, downstream estuary, locally harvested fish, birds and seals, and three Inuit communities. Results show a projected 10-fold increase in riverine MeHg levels and a 2.6-fold increase in estuarine surface waters. MeHg concentrations in locally caught species increase 1.3 to 10-fold depending on time spent foraging in different environments. Mean Inuit MeHg exposure is forecasted to double following flooding and over half of the women of childbearing age and young children in the most northern community are projected to exceed the U.S. EPA's reference dose. Equal or greater aqueous MeHg concentrations relative to Muskrat Falls are forecasted for 11 sites across Canada, suggesting the need for mitigation measures prior to flooding. [ABSTRACT FROM AUTHOR]

Abstract: The Black Sea is a unique natural laboratory for studying the biogeochemical cycling of mercury (Hg), a toxic metal present at trace levels in natural waters. The strong vertical density gradient in the Black Sea results in little ventilation of deep waters and therefore anoxic and sulfidic conditions at depth. In this way, the water column of the Black Sea resembles the geochemical transition that is observed in coastal sediments. This is of relevance to Hg studies, as coastal sediments are known to be locations of vigorous Hg methylation. Furthermore, the Black Sea supports a rich commercial fishery that could be at particular risk for accumulation of Hg as a result of its unique hydrography and anthropogenic inputs. The concentration of monomethylHg (CH3Hg+) was highest at the top of the suboxic zone in the Black Sea, where dissolved oxygen and sulfide were very low. This is circumstantially consistent with the hypothesis that it is the chemical form of Hg which determines bioavailability and therefore methylation rate and not the activity of sulfate reducing bacteria. Hg concentrations below the suboxic zone increased in tandem with increasing sulfide, and appear to be approximately consistent with cinnabar solubility control. These concentrations (ca. 12 pM) are among the highest reported for an open ocean environment. A preliminary mass balance for Hg in the Black Sea, including source terms such as precipitation, river and Mediterranean water inflow as well as the evasional sink and Bosphorus outflow indicates that a large additional input term is required that is greater than 68 kmol year− 1. Potential candidates include tectonic-related sources and river-derived pollution. Scaling the total inputs to the Black Sea by area of the basin indicates that this system is comparably impacted by Hg as an urbanized embayment such as Long Island Sound or Chesapeake Bay. [Copyright &y& Elsevier]

The fate of atmospherically deposited and environmentally active Hg is uncertain in the Arctic, and of greatest toxicological concern is the transformation to monomethylmercury (MMHg). Lake/watershed mass balances were developed to examine MMHg cycling in four northern Alaska lakes near the ecological research station at Toolik Lake (68° 38′ N, 149° 36′ W). Primary features of the cycle are watershed runoff, sedimentary production and mobilization, burial, and photodecomposition in the water column. The principal source of MMHg is in situ benthic production with 80–91% of total inputs provided by diffusion from sediments. The production and contribution of MMHg from tundra watersheds is modest. Photodecomposition, though confined to a short ice-free season, provides the primary control for MMHg (66–88% of total inputs) and greatly attenuates bioaccumulation. Solid-phase MMHg and gross potential rates of Hg methylation, assayed with an isotopic tracer, vary positively with the level of inorganic Hg in filtered pore water, indicating that MMHg production is Hg-limited in these lakes. Moreover, sediment-water fluxes of MMHg (i.e., net production at steady state) are related to sediment Hg loadings from the atmosphere. These results suggest that loadings of Hg derived from atmospheric deposition are a major factor affecting MMHg cycling in arctic ecosystems. However, environmental changes associated with warming of the Arctic (e.g., increased weathering, temperature, productivity, and organic loadings) may enhance MMHg bioaccumulation by stimulating Hg methylation and inhibiting photodecomposition. [ABSTRACT FROM AUTHOR]

Cycling of toxic monomethylmercury (MMHg) in biologically productive coastal marine systems is affected by geochemical and macro/microbiological factors that control its production and mobilization from sediment. We examined the biogeochemistry of MMHg in sediment at three sites in Long Island Sound (LIS) in March, June, and August. Bioturbation homogenizes profiles of inorganic Hg (Hg(II)=total Hg-MMHg) in the upper 10 cm of sediment, whereas MMHg is enriched in surface material, indicating rapid and dynamic cycling of the toxic species. Sediment–water partitioning of MMHg and Hg(II) is controlled largely by sedimentary organic matter. Distribution coefficients (KD; l kg-1) of MMHg and Hg(II) are correlated with those for organic carbon, which suggests that adsorption/desorption of Hg species with sediment particles is mediated by partitioning of associated organic ligands. A fraction of MMHg and Hg(II) on sediment particles is associated with Fe complexes, presumably oxyhydroxides. Bioturbation enhances Hg methylation; subsurface peaks in 200Hg methylation potentials coincide with anomalies in a bioturbation index (d[organic matter]/dz) that indicate non-local disturbance of sediment. The estimated diffusive sediment–water flux of MMHg in LIS (55±20 mol year-1) exceeds inputs from external sources. This indicates that in situ production is the major source of MMHg to LIS, and by extension, other comparable near-shore systems. Furthermore, sedimentary methylation can account for most of the MMHg in primary producers of LIS. Hence, through dietary bioaccumulation, much of the MMHg in higher trophic levels within the Sound may be attributed to MMHg production in the sediments. [Copyright &y& Elsevier]

Biogeochemical conditions and landscape can have strong influences on mercury bioaccumulation in fish, but these effects across regional scales and between sites with and without point sources of contamination are not well understood. Normal means clustering, a type of unsupervised machine learning, was used to analyze relationships between forage fish (Fundulus heteroclitus and Menidia menidia) mercury (Hg) concentrations and sediment and water column Hg and methylmercury (MeHg) concentrations, ancillary variables, and land classifications within the sub-watershed. The analysis utilized data from 38 sites in 8 estuarine systems in the Northeast US, collected over five years. A large range of mercury concentrations and land use proportions were observed across sites. The cluster correlations indicated that for Fundulus , benthic and pelagic Hg and MeHg concentrations were most related to tissue concentrations, while Menidia Hg was most related to water column MeHg, reflecting differing feeding modes between the species. For both species, dissolved MeHg was most related to tissue concentrations, with sediment Hg concentrations influential at contaminated sites. The models considering only uncontaminated sites showed reduced influence of bulk sediment MeHg for both species, but Fundulus retained sediment drivers at some sites, with dissolved MeHg still highly correlated for both. Dissolved organic carbon (DOC), chlorophyll, land use, and other ancillary variables were of lesser importance in driving bioaccumulation, though DOC was strongly related within some clusters, likely in relation to dissolved Hg. Land use, though not of primary importance, showed relationships opposite to those observed in freshwater, with development positively correlated and forests and agriculture negatively correlated with tissue concentrations across clusters and species. Clusters were composed of sites from geographically distinct systems, indicating the greater importance of small scale drivers of MeHg formation and uptake into the food web over system or region-wide influences. Image 1 • Estuarine forage fish Hg concentrations are strongly correlated with abiotic Hg. • Fish are most strongly influenced by sediment Hg at contaminated sites. • Fundulus reflect benthic and pelagic Hg sources, while Menidia reflect pelagic. • Local, small-scale processes dominate over regional-scale in driving bioaccumulation. [ABSTRACT FROM AUTHOR]

High exposures of mammalian species to inorganic mercury (HgII) and methylmercury (MeHg) have been associated with adverse effects on behavior and reproduction. Different mammalian species exhibit varying responses to similar external exposure levels, reflecting potential differences in Hg toxicokinetics. Here, we use Hg stable isotopes, total Hg, MeHg and selenium (Se) concentrations measured in multiple tissues of North Atlantic pilot whales (Globicephala melas) to investigate processes affecting the distribution and accumulation of HgII and MeHg. We find that simple mixing of two distinct isotopic end-members: MeHg (1.4‰) and HgII (−1.6‰) can explain the observed variability of δ 202Hg in brain tissue. A similar isotopic composition for the MeHg end-member in the brain, muscle, heart, and kidney suggests efficient exchange of MeHg in blood throughout the body. By contrast, the Hg isotopic composition of the liver of adult whales is different from younger whales and other tissues that follow the two-end member mixing model. Measured Se:Hg ratios are lowest in adult whales with the highest levels of MeHg exposure. In these individuals, Se availability is likely reduced by complexation with demethylated HgII. We speculate that this results in a higher fraction of labile HgII eliminated from the liver of adult whales compared to young whales and subsequent redistribution to other tissues, potentially affecting toxicity. Unlabelled Image • The mechanism of Hg metabolism in marine mammals is not fully understood. • Stable Hg isotopes and Se concentrations in multiple whale tissues were measured. • The brain δ202Hg values are determined by the fraction of Hg present as HgII. • We observe variability in δ202Hg values in liver tissues across whale life stages. • Reduced Se availability may drive Hg redistribution among pilot whale tissues. [ABSTRACT FROM AUTHOR]