MeHg Concentrations In Fish Research Articles

Hg bioaccumulation in the aquatic food web from tributaries of the Three Gorges Reservoir, China and potential consumption advisories

The Three Gorges Reservoir (TGR) holds the distinction of being China's largest reservoir, and the presence of pollutants in the fish from the reservoir have a direct impact on the health of local residents. Thus, 349 fish specimens of 21 species and 1 benthos (Bellamya aeruginosas) were collected from four typical tributaries of the TGR from 2019 to 2020. These specimens were analyzed for the concentrations of total mercury (THg) and methylmercury (MeHg), and some representative samples were tested for δ13C and δ15N values to reveal the characteristics of bioaccumulation and biomagnification. The maximum safe daily consumption was estimated based on the oral reference dose (0.1 μg kg−1 bw/day according to US-EPA, 2017). The results showed that the mean THg and MeHg concentrations in fish from the TGR tributaries were 73.18 ± 49.21 ng g−1 and 48.42 ± 40.66 ng g−1, respectively, with the trophic magnification factors (TMFs) of THg and MeHg being 0.066 and 0.060, respectively. Among all the fish species in the tributaries, the highest daily maximum safe consumption amount was 1253.89 g for S. asotus consumed by adults, while the lowest was 62.88 g for C. nasus consumed by children.

Experimental evidence for recovery of mercury-contaminated fish populations

Anthropogenic releases of mercury (Hg)1–3 are a human health issue4 because the potent toxicant methylmercury (MeHg), formed primarily by microbial methylation of inorganic Hg in aquatic ecosystems, bioaccumulates to high concentrations in fish consumed by humans5,6. Predicting the efficacy of Hg pollution controls on fish MeHg concentrations is complex because many factors influence the production and bioaccumulation of MeHg7–9. Here we conducted a 15-year whole-ecosystem, single-factor experiment to determine the magnitude and timing of reductions in fish MeHg concentrations following reductions in Hg additions to a boreal lake and its watershed. During the seven-year addition phase, we applied enriched Hg isotopes to increase local Hg wet deposition rates fivefold. The Hg isotopes became increasingly incorporated into the food web as MeHg, predominantly from additions to the lake because most of those in the watershed remained there. Thereafter, isotopic additions were stopped, resulting in an approximately 100% reduction in Hg loading to the lake. The concentration of labelled MeHg quickly decreased by up to 91% in lower trophic level organisms, initiating rapid decreases of 38–76% of MeHg concentration in large-bodied fish populations in eight years. Although Hg loading from watersheds may not decline in step with lowering deposition rates, this experiment clearly demonstrates that any reduction in Hg loadings to lakes, whether from direct deposition or runoff, will have immediate benefits to fish consumers.

Selenium in sediment and food webs of the Tapajós River basin (Brazilian Amazon) and its relation to mercury

BackgroundWe investigated Se levels along the Tapajós River basin - which is an important tributary of the Amazon River - and the possible antagonistic effect of Se in Hg availability. This is the first study to investigate Se transfer from abiotic to biotic compartments and along the food chain in aquatic ecosystems of the Amazon basin. MethodsSe concentrations were measured in superficial sediment (n = 29), plankton (n = 28) and fishes (n = 121) along two stretches of the Tapajós River basin (Tapup/mi and Taplow), comprising approximately 500 km with different hydrological characteristics. ResultsSe concentrations in sediment were significantly higher in the Taplow (345−664 μg kg−1) than in the Tapup/mi (60−424 μg kg−1). The seasonal flooding of the Amazon River probably helps to carry selenium-rich sediment to the Tapajós mouth (Taplow stretch). We suggest that Se in sediment could decrease the bioavailability of Hg resulting in lower MeHg concentrations in fish, as observed in the Taplow (45−934 μg kg−1). Sediment and plankton were positively correlated in relation to their Se concentrations (r = 0.62; p = 0.001) suggesting that sediment can possibly be the main source of Se to plankton. Our data indicate Se uptake by primary consumers, as noted in phytoplankton levels. The decrease of Se concentrations along the food chain was also noteworthy. ConclusionThis work elucidates some aspects of Se biogeochemistry in the Amazon basin and shows its importance regarding Hg cycles in aquatic ecosystems.

Mercury and its form in a dammed reservoir ecosystem during the charging phase.

Throughout continents, reservoirs tend to have elevated methylmercury (MeHg) concentration transformed from mercury (Hg/total Hg). This impact may be pronounced in the reservoir with less velocity of water during the charging period resulted in the deposition of sediments. In sediments on favorable conditions, methylation may be enhanced by the decomposition of flood organic material, which can release Hg and enhance microbial activity. However, much less is known about the transfer ratio of Hg and its form MeHg from sediment to biota in the hydrological reservoir during the dam charging phase. The objective of our study was to understand the interrelationship between total Hg and MeHg in two key components sediment and fish in the reservoir ecosystem. This study was performed at the Three Gorges Reservoir (TGR) located on upstream of the Yangtze River in China. At the TGR charging phase, during winter time, the water level was high due to blockade of water by Three Gorges Dam (TGD). Sediment and fish samples were collected in winter season for total Hg, MeHg, and several ancillary parameters. The results showed that total Hg in sediment samples of the winter season were ranged from 6.2 ± 0.001 to 193.3 ± 0.001 × 10-3mg/kg, with an average value of 53.76 ± 51.80 × 10-3mg/kg, and for MeHg was ranged from 12.1 ± 0.04 to 348.7 ± 0.16 × 10-2ng/g, with an average value of 98.96 ± 93.07 × 10-2ng/g. Total Hg and MeHg in fish samples of the winter season were from 42.48 ± 6.71 to 166 ± 52.56ng/g, with an average value of 76.22 ± 31.23ng/g, and from 21.09 ± 2.31 to 61.60 ± 13.30ng/g, with an average value of 37.89 ± 11.96ng/g. The relationship of total Hg and MeHg concentrations in fish to those of sediments from corresponding sites showed a negative relationship. This might include a strong association of total Hg with an inorganic component of sediment (e.g., bound to sulfides or coprecipitated with other metal oxides such as manganese and iron). The average concentration of fish MeHg found in this study, at rates greater than 1.72g/day, was estimated hazardous to human health. This study concludes sediment was acting as sequestrate for total Hg and MeHg in TGR. The bioaccumulation of total Hg and MeHg in fish was not controlled by sediment further investigation about pathological routes and dietary habits of fish needed to be identified for total Hg and MeHg study in TGR.

Methylmercury sorption onto engineered materials.

Four commercially available sorbents (BioChar (BC), ThiolSAMMS® (TS), SediMite (SM), and Organoclay™ PM-199 (OC-199)) were tested for their ability to sorb methylmercury (MeHg) and MeHg complexed with dissolved organic matter (DOM). Testing sorption behavior with DOM is more representative of the environmental conditions and mercury speciation expected during in-situ remediation efforts. Isotherms were fit using a robust, iterative re-weighting scheme. This fitting approach improves upon the traditionally used indirect sorption method by removing the dependence between aqueous and solid phase concentrations in isotherm fitting. Developed isotherms show that without DOM, BC, TS, and SM adsorbed similar amounts of MeHg while OC-199 sorbed substantially less MeHg. Below an equilibrium concentration of 5.6 ng L-1 BC was the best performing sorbent, between 5.6 and 20.9 ng L-1 SM sorbed the most MeHg, and above an equilibrium concentration of 20.9 ng L-1 TS outperformed the other sorbents. BC and OC-199 showed indication of MeHg sorption saturation over the tested concentration range of 3.5-680 ng L-1. With DOM, SM outperformed the other sorbents at equilibrium concentrations less than 0.98 ng L-1 and TS was the superior MeHg:DOM sorbent at higher concentrations. MeHg:DOM sorption was controlled by DOM-sorbent interactions. DOM decreased MeHg sorption onto BC and SM whereas TS exhibited similar sorption with and without DOM. OC-199 had slightly higher MeHg uptake with DOM. East Fork Poplar Creek (EFPC), an industrially Hg contaminated site, was used as a case study example to build a relationship between aqueous and fish MeHg concentrations and subsequently compare the cost of sorbent materials required to meet regulatory objectives. For this case study, SM provided the most cost-effective sorbent option for in-situ remediation efforts to reduce aqueous MeHg concentrations.

Organic carbon content drives methylmercury levels in the water column and in estuarine food webs across latitudes in the Northeast United States

Estuaries are dynamic ecosystems which vary widely in loading of the contaminant methylmercury (MeHg), and in environmental factors which control MeHg exposure to the estuarine foodweb. Inputs of organic carbon and rates of primary production are important influences on MeHg loading and bioaccumulation, and are predicted to increase with changes in climate and land use pressures. To further understand these influences on MeHg levels in estuarine biota, we used a field study approach in sites across different temperature regions, and with varying organic carbon levels. In paired comparisons of sites with high vs. low organic carbon, fish had lower MeHg bioaccumulation factors (normalized to water concentrations) in high carbon sites, particularly subsites with large coastal wetlands and large variability in dissolved organic carbon levels in the water column. Across sites, MeHg level in the water column was strongly tied to dissolved organic carbon, and was the major driver of MeHg concentrations in fish and invertebrates. Higher primary productivity (chlorophyll-a) was associated with increased MeHg partitioning to suspended particulates, but not to the biota. These findings suggest that increased inputs of MeHg and loss of wetlands associated with climate change and anthropogenic land use pressure will increase MeHg concentrations in estuarine food webs.

Bioaccumulation characteristics of mercury in fish in the Three Gorges Reservoir, China

Newly constructed reservoirs were recognized as hotspot of mercury (Hg) methylation, and then methylmercury (MeHg) accumulation in food chains. The risk of elevated MeHg concentrations in fish is one of the most important concerns in newly constructed reservoirs. The Three Gorges Reservoir (TGR) is one of the largest reservoirs in the world. However, the distribution and bioaccumulation characteristics of Hg species within the food chains and its potential ecological risk in the TGR remain poorly understood. In this study, 264 fish individuals covering 18 species were collected from the TGR. Total mercury (THg) and MeHg concentrations in different organs (gill, heart, liver, muscle and swim bladder) of fish species were analyzed; the values of δ13C and δ15N in fish muscle were determined as well to reveal the biomagnification properties of Hg in food chains. Our results showed that concentrations of THg (0.5–272 ng g−1, w.w.) and MeHg (0.1–199 ng g−1, w.w.) in fish muscle from the TGR ubiquitously fall below the safe fish consumption limit on Hg recommended by WHO (500 ng g−1, w.w.) and the US-EPA Water Quality Criterion for MeHg (300 ng g−1, w.w.). The short food web jointly with the limited trophic magnification factor in the TGR explained the relatively low Hg concentrations in predators. Among the five fish organs, muscle represented the highest Hg concentrations, followed by heart, liver, swim bladder, and gill, suggesting that muscle has the highest ability to accumulate Hg compared to the other organs. More importantly, no discernible “reservoir effect” was observed in the TGR within the initial few years after impoundment due to its special eco-environment including: 1) neutral and slightly alkaline pH and low dissolved organic carbon of water, 2) less vegetation coverage in inundated areas, 3) simple food web.

Mercury bioaccumulation increases with latitude in a coastal marine fish (Atlantic silverside, Menidia menidia).

Human exposure to the neurotoxic methylmercury (MeHg) occurs primarily via the consumption of marine fish, but the processes underlying large-scale spatial variations in fish MeHg concentrations [MeHg], which influence human exposure, are not sufficiently understood. We used the Atlantic silverside (Menidia menidia), an extensively studied model species and important forage fish, to examine latitudinal patterns in total Hg [Hg] and [MeHg]. Both [Hg] and [MeHg] significantly increased with latitude (0.014 and 0.048 μg MeHg g-1 dw per degree of latitude in juveniles and adults, respectively). Four known latitudinal trends in silverside traits help explain these patterns: latitudinal increase in MeHg assimilation efficiency, latitudinal decrease in MeHg efflux, latitudinal increase in weight loss due to longer and more severe winters, and latitudinal increase in food consumption as an adaptation to decreasing length of the growing season. Given the absence of a latitudinal pattern in particulate MeHg, a diet proxy for zooplanktivorous fish, we conclude that large-scale spatial variation in growth is the primary control of Hg bioaccumulation in this and potentially other fish species.

Methylmercury Concentration in Fish and Risk-Benefit Assessment of Fish Intake among Pregnant versus Infertile Women in Taiwan.

This study examined methylmercury (MeHg) concentrations in fish, the daily MeHg exposure dose, and the risk–benefit of MeHg, ω-3 polyunsaturated fatty acid (ω-3 PUFA), docosahexaenoic acid (DHA), and eicosapentaenoic acid (EPA) related to fish intake among pregnant and infertile women in Taiwan. The measured MeHg concentrations in fish did not exceed the Codex guideline level of 1 mg/kg. Swordfish (0.28 ± 0.23 mg/kg) and tuna (0.14 ± 0.13 mg/kg) had the highest MeHg concentrations. The MeHg concentration in the hair of infertile women (1.82 ± 0.14 mg/kg) was significantly greater than that of pregnant women (1.24 ± 0.18 mg/kg). In addition, 80% of infertile women and 68% of pregnant women had MeHg concentrations in hair that exceeded the USEPA reference dose (1 mg/kg). The MeHg concentrations in hair were significantly and positively correlated with the estimated daily MeHg exposure dose. Based on the risk–benefit evaluation results, this paper recommends consumption of fish species with a low MeHg concentration and high concentrations of DHA + EPA and ω-3 PUFA (e.g., salmon, mackerel, and greater amberjack).

The risk of mercury exposure to the people consuming fish from Lake Phewa, Nepal.

The risk of mercury exposure through consumption of fish from Lake Phewa, Nepal was investigated. A total of 170 people were surveyed to know their fish consumption levels. The weekly mercury (Hg) intake in the form of methylmercury (MeHg) through fish was calculated by using the data on average MeHg concentrations in fish, the average consumption of fish per week, and an average body weight of the people. Hotel owners were consuming significantly high amounts of fish, followed by fishermen, in comparison to the government staff, army/police, locals and others (visitors). Some individuals exceeded the Provisional Tolerable Weekly Intake (PTWI) of 1.6 µg per kg body weight of MeHg (FAO/WHO). The minimum intake of MeHg (0.05 µg/kg/week) was found in the visitors (others) category, whereas the hotel owners had the maximum intake (3.71 µg/kg/week). In general, it was found that a person of 60 kg can consume at least 2 kg of fish per week without exceeding PTWI such that it does not pose any health risk associated with Hg poisoning at the present contamination level. Hg based PTWI values for Nepal has not been proposed yet in fishery resources so as to reduce health risk of the people.

Effects of urbanization on mercury deposition and accumulation in New England

We compare total mercury (HgT) loading and methylmercury (MeHg) accumulation in streams and lakes from an urbanized area (Boston, Massachusetts) to rural regions of southern New Hampshire and Maine. The maximum HgT loading, as indicated by HgT atmospheric deposition, HgT emissions, and sediment HgT concentrations, did not coincide with maximum MeHg concentrations in fish. Urbanized ecosystems were areas of high HgT loading but had low MeHg concentrations in fish. Controls on MeHg production and accumulation appeared to be related primarily to HgT loading in undeveloped areas, while ecosystem sensitivity to MeHg formation appeared to be more important in regulating accumulation of MeHg in the urban area.

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 Methylation Rates for Geochemically Relevant HgII Species in Sediments

Monomethylmercury (MeHg) in fish from freshwater, estuarine, and marine environments is a major global environmental issue. Mercury levels in biota are mainly controlled by the methylation of inorganic mercuric mercury (Hg(II)) to MeHg in water, sediments, and soils. There is, however, a knowledge gap concerning the mechanisms and rates of methylation of specific geochemical Hg(II) species. Such information is crucial for a better understanding of variations in MeHg concentrations among ecosystems and, in particular, for predicting the outcome of currently proposed measures to mitigate mercury emissions and reduce MeHg concentrations in fish. To fill this knowledge gap we propose an experimental approach using Hg(II) isotope tracers, with defined and geochemically important adsorbed and solid Hg(II) forms in sediments, to study MeHg formation. We report Hg(II) methylation rate constants, k(m), in estuarine sediments which span over 2 orders of magnitude depending on chemical form of added tracer: metacinnabar (β-(201)HgS(s)) < cinnabar (α-(199)HgS(s)) < Hg(II) reacted with mackinawite (≡FeS-(202)Hg(II)) < Hg(II) bonded to natural organic matter (NOM-(196)Hg(II)) < a typical aqueous tracer ((198)Hg(NO(3))(2)(aq)). We conclude that a combination of thermodynamic and kinetic effects of Hg(II) solid-phase dissolution and surface desorption control the Hg(II) methylation rate in sediments and cause the large observed differences in k(m)-values. The selection of relevant solid-phase and surface-adsorbed Hg(II) tracers will therefore be crucial to achieving biogeochemically accurate estimates of ambient Hg(II) methylation rates.

Mercury stable isotopes in sediments and largemouth bass from Florida lakes, USA

Humans and wildlife can be exposed to mercury (Hg) through the consumption of fish with elevated concentrations of methylmercury (MeHg). Studies have shown that increased atmospheric deposition of Hg often leads to increased MeHg concentrations in aquatic organisms. However, depending on the ecosystem characteristics, reductions in Hg emissions may not always lead to immediate decreases in fish MeHg concentrations. Measurements of natural abundance Hg stable isotope ratios may enable a better understanding of these complex relationships. To gain insight into the sources of Hg to sport fish in central Florida, we measured the Hg isotopic compositions of surface sediments and largemouth bass from freshwater lakes. We found that fish collected from lakes located near the large Crystal River coal-fired power plant did not display evidence of anomalous negative δ202Hg values that were observed in nearby precipitation. This suggests that Hg recently deposited from the atmosphere is not preferentially methylated and bioaccumulated in these lakes relative to previously deposited Hg accumulated in the lake sediments. We also observed significant positive Δ199Hg values in the fish that were correlated with light penetration depth in the lakes from which they were collected. This indicates that a significant amount of photochemical degradation of MeHg (up to ~40%) occurred prior to uptake of the remaining MeHg into the food webs. These results suggest that depending on physical lake characteristics and biogeochemical factors, decreased atmospheric Hg deposition may not lead to immediate short-term reductions in fish MeHg concentrations. Instead, recovery of some freshwater fish populations to baseline MeHg concentrations may take decades to centuries.

Mercury and methylmercury concentrations in Mediterranean seafood and surface sediments, intake evaluation and risk for consumers

Total mercury and methylmercury concentrations were measured in sediments and marine organisms from the Taranto Gulf to understand their distribution and partitioning. Sediment concentrations ranged from 0.036 to 7.730 mg/kg (mean: 2.777 mg/kg d.w.) and from 1 to 40 μg/kg (mean: 11 μg/kg d.w.) for total mercury (THg) and methylmercury (Me-Hg), respectively. In mollusks THg ranged from n.d. to 1870 μg/kg d.w. while in fish from 324 to 1740 μg/kg d.w. Me-Hg concentrations in fish ranged from 190 to 1040 μg/kg d.w. and from n.d. to 1321 μg/kg d.w. in mollusks. THg exceeded the maximum level fixed by the European Commission (0.5 mg/kg w.w.) only in gastropod Hexaplex t. The calculated weekly intake was in many cases over the Provisional Tolerable Weekly Intake established by EFSA for all edible species. These results seem to indicate that dietary consumption of this seafood implicates an appreciable risk for human health.

Biomarkers of low-level mercury exposure through fish consumption in pregnant and lactating Slovenian women

In order to assess the mercury exposure of pregnant and lactating women in Slovenia, levels of total mercury (THg) and methylmercury (MeHg) were determined in hair, cord blood and breast milk. In addition, the frequency of fish consumption was estimated, because fish is generally the main pathway for human exposure to MeHg. Hair samples were collected from 574 women participating in this study, while cord blood and breast milk samples were collected from 446 and 284 women, respectively. As expected, the levels of THg in hair (median (Med)=297ng/g, 10th percentile (P10)=73ng/g, 90th percentile (P90)=781ng/g), cord blood (Med=1.5ng/g, P10=0.5ng/g, P90=4.2ng/g) and breast milk (Med=0.2ng/g, P10=0.06ng/g, P90=0.6ng/g) were low, due to low consumption of fish (X=25g/day). A significant linear correlation was found between levels of lnTHg in hair and lnTHg in cord blood (r=0.87, 95% confidence interval (CI): 0.84–0.89), between levels of lnTHg in hair and lnMeHg in cord blood (r=0.94, 95% CI: 0.90–0.96) and between lnTHg levels in cord blood and lnTHg levels in breast milk (r=0.36, 95% CI: 0.25–0.47). Spearman's rank correlations between the frequency of fish consumption and THg in hair (rs=0.35, 95% CI: 0.28–0.42), and between the frequency of fish consumption and THg in cord blood (rs=0.43, 95% CI: 0.36–0.51) or MeHg in cord blood (rs=0.31, 95% CI: 0.06–0.52) were weak. This could be due to the approximate information on fish consumption obtained from the questionnaires, the high variability of MeHg concentrations in fish and a relatively high proportion of inorganic mercury in the biomarkers which originates from sources other than fish. In conclusion, THg levels in cord blood, THg levels in hair and MeHg levels in cord blood are suitable biomarkers of low-level Hg exposure through fish consumption. Compared to cord blood, hair samples are easy to collect, store and analyse.

Mercury concentrations at a historically mercury-contaminated site in KwaZulu-Natal (South Africa)

A mercury (Hg) processing plant previously operating in KwaZulu-Natal Province (South Africa) discharged Hg waste into a nearby river system causing widespread contamination since the 1980s. Although the processing plant ceased operation in the 1990s, Hg contamination (due to residual Hg) remains significant. Previous studies in the area since the plant's closure have found elevated Hg concentrations in fish, and that these concentrations were as a direct consequence of widespread contamination of the Hg processing plant operations conducted between the 1980s and 1990s. This study aimed at investigating the impacts of residual Hg almost 20years after the plant's closure. Water, sediment and biota (invertebrates and fish) were collected in water resources in the vicinity of the processing plant to determine the Hg concentrations in these compartments, as a proxy for assessing the extent to which residual Hg that is reintroduced to the water column becomes bioavailable to biota. For water and sediment samples, higher total mercury (TotHg) and methylmercury (MeHg) concentrations were measured at sampling sites immediately downstream of the Hg processing plant when compared to the upstream sites, while concentrations decreased with distance from the plant. Fish MeHg concentrations measured just below the US EPA guideline for Hg in fish muscle tissue. The results show that the historically Hg-contaminated river system is a potential Hg pollution source due to the residual Hg present in sediment. Any dredging of sediment as a form of remediation in the Mngceweni River is not recommended; however, a Hg monitoring programme is recommended for assessing the bioavailability of resuspended Hg from sediment.

Bioaccumulation syndrome: identifying factors that make some stream food webs prone to elevated mercury bioaccumulation

Mercury is a ubiquitous contaminant in aquatic ecosystems, posing a significant health risk to humans and wildlife that eat fish. Mercury accumulates in aquatic food webs as methylmercury (MeHg), a particularly toxic and persistent organic mercury compound. While mercury in the environment originates largely from anthropogenic activities, MeHg accumulation in freshwater aquatic food webs is not a simple function of local or regional mercury pollution inputs. Studies show that even sites with similar mercury inputs can produce fish with mercury concentrations ranging over an order of magnitude. While much of the foundational work to identify the drivers of variation in mercury accumulation has focused on freshwater lakes, mercury contamination in stream ecosystems is emerging as an important research area. Here, we review recent research on mercury accumulation in stream-dwelling organisms. Taking a hierarchical approach, we identify a suite of characteristics of individual consumers, food webs, streams, watersheds, and regions that are consistently associated with elevated MeHg concentrations in stream fish. We delineate a conceptual, mechanistic basis for explaining the ecological processes that underlie this vulnerability to MeHg. Key factors, including suppressed individual growth of consumers, low rates of primary and secondary production, hydrologic connection to methylation sites (e.g., wetlands), heavily forested catchments, and acidification are frequently associated with increased MeHg concentrations in fish across both streams and lakes. Hence, we propose that these interacting factors define a syndrome of characteristics that drive high MeHg production and bioaccumulation rates across these freshwater aquatic ecosystems. Finally, based on an understanding of the ecological drivers of MeHg accumulation, we identify situations when anthropogenic effects and management practices could significantly exacerbate or ameliorate MeHg accumulation in stream fish.

Bioaccumulation of newly deposited mercury by fish and invertebrates: an enclosure study using stable mercury isotopes

Enriched stable mercury (Hg) isotopes were added to four 10 m diameter enclosures in Lake 239 at the Experimental Lakes Area to increase inorganic Hg loading. Our main objectives were to (i) follow low-level additions (spikes) of isotope-enriched Hg through the biogeochemical cycle and into the food web and (ii) determine the relative contribution of newly deposited Hg to methyl Hg (MeHg) accumulation by fish and other biota. The experiment ran for two summers (2000, 2001), with different enriched Hg isotopes being added each year. Within 1 month of beginning additions in 2000, spike Hg was detected in water, zooplankton, and benthic invertebrates as MeHg, and in fish as total Hg (THg; the sum of inorganic and organic Hg). In 2001, concentrations in water of inorganic spike Hg added in 2000 were near detection limits, but concentrations of 2000 spike MeHg in water and biota remained unchanged or greater. Despite comparatively large increases in inorganic Hg loading, accumulation of ambient, non-spike MeHg predominated in all organisms, and spike MeHg never comprised more than 15%, even after 1 year. Our results suggest that changes in Hg loading will affect MeHg concentrations in fish and other biota, but that steady state may not be achieved for at least 10–30 years under conditions similar to our enclosures.

Assessing Mercury Exposure and Effects to American Dippers in Headwater Streams near Mining Sites

To evaluate mercury (Hg) exposure and possible adverse effects of Hg on American dipper (Cinclus mexicanus) reproduction, we collected eggs and nestling feathers and the larval/nymph form of three Orders of aquatic macroinvertebrates (Ephemeroptera, Plecoptera and Trichoptera = EPT) important in their diet from three major headwater tributaries of the upper Willamette River, Oregon in 2002. The Coast Fork Willamette River is contaminated with Hg due to historical cinnabar (HgS) mining at the Black Butte Mine; the Row River is affected by past gold-mining operations located within the Bohemia Mining District, where Hg was used in the amalgamation process to recover gold; and the Middle Fork Willamette River is the reference area with no known mining. Methyl mercury (MeHg) concentrations (geometric mean) in composite EPT larvae (111.9 ng/g dry weight [dw] or 19.8 ng/g wet weight [ww]), dipper eggs (38.5 ng/g ww) and nestling feathers (1158 ng/g ww) collected from the Coast Fork Willamette were significantly higher than MeHg concentrations in EPT and dipper samples from other streams. Total mercury (THg) concentrations in surface sediments along the same Hg-impacted streams were investigated by others in 1999 (Row River tributaries) and 2002 (Coast Fork). The reported sediment THg concentrations paralleled our biological findings. Dipper breeding territories at higher elevations had fewer second clutches; however, dipper reproductive success along all streams (including the lower elevation and most Hg-contaminated Coast Fork), was judged excellent compared to other studies reviewed. Furthermore, MeHg concentrations in EPT samples from this study were well below dietary concentrations in other aquatic bird species, such as loons and ducks, reported to cause Hg-related reproductive problems. Our data suggest that either dipper feathers or EPT composites used to project MeHg concentrations in dipper feathers (with biomagnification factor of 10-20x) may be used, but with caution, to screen headwater streams for potential Hg-related effects on dippers. When actual feather concentrations or projected feather concentrations are equal to or lower than concentrations reported for the Coast Fork, dippers are expected to reproduce well (assuming adequate prey and suitable nest sites). When Hg concentrations are substantially higher, more detailed investigations may be required. Birds feeding almost exclusively on fish (e.g., osprey [Pandion haliaetus]) and usually found further downstream from the headwaters would not be adequately represented by dippers given the higher MeHg concentrations in fish resulting from biomagnification, compared to lower trophic level invertebrates.