This paper is freely available via Open Access: http://pubs.acs.org/doi/abs/10.1021/es802694n
We studied total mercury (THg) and methylmercury (MeHg) in eight streams, located in Oregon, Wisconsin, and Florida, that span large ranges in climate, landscape characteristics, atmospheric Hg deposition, and water chemistry. While atmospheric deposition was the source of Hg at each site, basin characteristics appeared to mediate this source by providing controls on methylation and fluvial THg and MeHg transport. Instantaneous concentrations of filtered total mercury (FTHg) and filtered methylmercury (FMeHg) exhibited strong positive correlations with both dissolved organic carbon (DOC) concentrations and streamflow for most streams, whereas mean FTHg and FMeHg concentrations were correlated with wetland density of the basins. For all streams combined, whole water concentrations (sum of filtered and particulate forms) of THg and MeHg correlated strongly with DOC and suspended sediment concentrations in the water column.
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"The U.S. Geological Survey (USGS) has conducted extensive studies of national and regional fish-tissue contaminant data by combining results of several separate targeted studies conducted over time, analyzing the data and characterizing contaminant levels on the basis of region and watershed type, including propensity for methylation and fish characteristics (Schmitt, 2002; Scudder et al., 2009; Chalmers et al., 2011). These studies have led to identification of patterns and mechanisms of Hg accumulation in fish including the propensity for methylation to occur in watersheds with abundant wetlands (Brigham et al., 2009; Scudder et al., 2009). USGS continues to emphasize locally-focused studies which are national in their distribution (Brigham et al., 2014; Feaster et al., 2014; Chalmers et al., 2014) and explore causative mechanisms for the observed variations in Hg concentrations in a variety of media and settings. "
[Show abstract][Hide abstract] ABSTRACT: The U.S. EPA conducted a national statistical survey of fish fillet tissue with a sample size of 541 sites on boatable rivers =>5th order in 2008-2009. This is the first such study of mercury (Hg) in fish tissue from river sites focused on potential impacts to human health from fish consumption to also address wildlife impacts. Sample sites were identified as being urban or non-urban. All sample mercury concentrations were above the 3.33ugkg(-1) (ppb) quantitation limit, and an estimated 25.4% (±4.4%) of the 51663 river miles assessed exceeded the U.S. EPA 300ugkg(-1) fish-tissue based water quality criterion for mercury, representing 13144±181.8 river miles. Estimates of river miles exceeding comparable aquatic life thresholds (translated from fillet concentrations to whole fish equivalents) in avian species were similar to the number of river miles exceeding the human health threshold, whereas some mammalian species were more at risk than human from lower mercury concentrations. A comparison of means from the non-urban and urban data and among three ecoregions did not indicate a statistically significant difference in fish tissue Hg concentrations at p<0.05.
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"Interactions between Hg and organic matter influence the transport and bioavailability of Hg in riverine and estuarine environments (Laurier et al., 2003; Choe et al., 2003; Conaway et al., 2003; Han et al., 2007; Lee et al., 2011). Dissolved Hg distribution across watersheds is explained by dissolved organic carbon (DOC) distribution in a number of river systems (Peckenham et al., 2003; Schuster et al., 2008; Brigham et al., 2009). In estuarine systems, complexation of Hg with dissolved organic matter , coupled with colloidal coagulation, is reported to influence estuarine mixing behavior (Stordal et al., 1996; Choe et al., 2003; Lee et al., 2011) and bioavailability of Hg (Pan and Wang, 2004; Zhong and Wang, 2009). "
[Show abstract][Hide abstract] ABSTRACT: The lower Mekong and Saigon River Basins are dominated by distinctive monsoon seasons, dry and rainy seasons. Most of the Mekong River is a freshwater region during the rainy season, whereas during the dry season, salt water intrudes approximately 70 km inland. To understand the role of salinity intrusion controlling Hg behavior in the Mekong and Saigon River Basins, Hg and monomethylmercury (MMHg) in surface water and sediment of the Mekong River and in sediment of the Saigon River were investigated in the dry season. Sediment Hg distribution, ranging from 0.12 to 0.76 nmol g À1 , was mainly controlled by organic carbon distribution in the Mekong River; however, the location of point sources was more important in the Saigon River (0.21–0.65 nmol g À1). The MMHg concentrations in Mekong (0.16–6.1 pmol g À1) and Saigon (0.70–8.7 pmol g À1) sediment typically showed significant increases in the estuarine head, with sharp increases of acid volatile sulfide. Unfiltered Hg (4.6–222 pM) and filtered Hg (1.2–14 pM) in the Mekong River increased in the estuarine zone due to enhanced particle loads. Conversely, unfiltered MMHg (0.056–0.39 pM) and filtered MMHg (0.020–0.17 pM) was similar between freshwater and estuarine zones, which was associated with mixing dilution of particulate MMHg by organic-and MMHg-depleted resuspended sediment. Partitioning of Hg between water and suspended particle showed tight correlation with the partitioning of organic carbon across study sites, while that of MMHg implied influences of chloride: enhanced chlo-ride in addition to organic matter depletion decreased particulate MMHg in the estuarine zone. Primary production was an important determinant of inter-annual variation of particulate Hg and sediment MMHg. The bloom year showed relatively low particulate Hg with low C/N ratio, indicating biodilution of Hg. In contrast, the percentage of MMHg in sediment increased significantly in the bloom year, likely due to greater availability of metabolizable fresh organic matter. The overall results emphasize that Hg behavior in the lower Mekong River Basin is strongly connected to the local monsoon climate, via alterations in particle loads, biological productivity, and availability of sulfate, chloride and organic matter.
"Each sample was collected from the approximate centroid of flow. Ultra-trace-level clean techniques, as described in Brigham et al. (2009), were used to collect and process samples for FMeHg analysis. The reporting limit for FMeHg was 0.04 ng/L. "
[Show abstract][Hide abstract] ABSTRACT: We studied lower food webs in streams of two mercury-sensitive regions to determine whether variations in consumer foraging strategy and resultant dietary carbon signatures accounted for observed within-site and among-site variations in consumer mercury concentration. We collected macroinvertebrates (primary consumers and predators) and selected forage fishes from three sites in the Adirondack Mountains of New York, and three sites in the Coastal Plain of South Carolina, for analysis of mercury (Hg) and stable isotopes of carbon (δ13C) and nitrogen (δ15N). Among primary consumers, scrapers and filterers had higher MeHg and more depleted δ13C than shredders from the same site. Variation in δ13C accounted for up to 34 % of within-site variation in MeHg among primary consumers, beyond that explained by δ15N, an indicator of trophic position. Consumer δ13C accounted for 10 % of the variation in Hg among predatory macroinvertebrates and forage fishes across these six sites, after accounting for environmental aqueous methylmercury (MeHg, 5 % of variation) and base-N adjusted consumer trophic position (Δδ15N, 22 % of variation). The δ13C spatial pattern within consumer taxa groups corresponded to differences in benthic habitat shading among sites. Consumers from relatively more-shaded sites had more enriched δ13C that was more similar to typical detrital δ13C, while those from the relatively more-open sites had more depleted δ13C. Although we could not clearly attribute these differences strictly to differences in assimilation of carbon from terrestrial or in-channel sources, greater potential for benthic primary production at more open sites might play a role. We found significant variation among consumers within and among sites in carbon source; this may be related to within-site differences in diet and foraging habitat, and to among-site differences in environmental conditions that influence primary production. These observations suggest that different foraging strategies and habitats influence MeHg bioaccumulation in streams, even at relatively small spatial scales. Such influence must be considered when selecting lower trophic level consumers as sentinels of MeHg bioaccumulation for comparison within and among sites.
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The online version of this article (doi:10.1007/s10646-012-1003-3) contains supplementary material, which is available to authorized users.