Mercury cycling in stream ecosystems. 3. Trophic dynamics and methylmercury bioaccumulation

Habitat and dissolved organic carbon modulate variation in the biogeochemical drivers of mercury bioaccumulation in dragonfly larvae at the national scale

We paired mercury (Hg) concentrations in dragonfly larvae with water chemistry in 29 U.S. national parks to highlight how ecological and biogeochemical context (habitat, dissolved organic carbon [DOC]) influence drivers of Hg bioaccumulation. Although prior studies have defined influences of biogeochemical variables on Hg production and bioaccumulation, it has been challenging to determine their influence across diverse habitats, regions, or biogeochemical conditions within a single study. We compared global (i.e., all sites), habitat-specific, and DOC-class models to illuminate how these controls on biotic Hg vary. Although the suite of important biogeochemical factors across all sites (e.g., aqueous Hg, DOC, sulfate [SO42-], and pH) was consistent with general findings in the literature, contrasting the restricted models revealed more nuanced controls on biosentinel Hg. Comparing habitats, aqueous (filtered) total mercury (THg) and SO42- were important in lentic systems whereas aqueous (filtered) methylmercury (MeHg), DOC, pH, and SO42- were important in lotic and wetland systems. The ability to identify important variables varied among habitats, with less certainty in lentic (model weight (W) = 0.05) than lotic (W = 0.11) or wetland habitats (W = 0.23), suggesting that biogeochemical drivers of bioaccumulation are more variable, or obscured by other aspects of Hg cycling, in these habitats. Results revealed a contrast in the importance of aqueous MeHg versus aqueous THg between DOC-classes: in low-DOC sites (<8.5 mg/L), availability of upstream inputs of MeHg appeared more important for bioaccumulation; in high-DOC sites (>8.5 mg/L) THg was more important, suggesting a link to in-situ controls on bioavailability of Hg for MeHg production. Mercury bioaccumulation (indicated by bioaccumulation factor) was more efficient in low DOC-class sites, likely due to reduced partitioning of aqueous MeHg to DOC. Together, findings highlight substantial variation in the drivers of Hg bioaccumulation and suggest consideration of these factors in natural resource management and decision-making.

Country-Wide Ecological Health Assessment Methodology for Air Toxics: Bridging Gaps in Ecosystem Impact Understanding and Policy Foundations

Amid the growing concerns about air toxics from pollution sources, much emphasis has been placed on their impacts on human health. However, there has been limited research conducted to assess the cumulative country-wide impact of air toxics on both terrestrial and aquatic ecosystems, as well as the complex interactions within food webs. Traditional approaches, including those of the United States Environmental Protection Agency (US EPA), lack versatility in addressing diverse emission sources and their distinct ecological repercussions. This study addresses these gaps by introducing the Ecological Health Assessment Methodology (EHAM), a novel approach that transcends traditional methods by enabling both comprehensive country-wide and detailed regional ecological risk assessments across terrestrial and aquatic ecosystems. EHAM also advances the field by developing new food-chain multipliers (magnification factors) for localized ecosystem food web models. Employing traditional ecological multimedia risk assessment of toxics' fate and transport techniques as its foundation, this study extends US EPA methodologies to a broader range of emission sources. The quantification of risk estimation employs the quotient method, which yields an ecological screening quotient (ESQ). Utilizing Kuwait as a case study for the application of this methodology, this study's findings for data from 2017 indicate a substantial ecological risk in Kuwait's coastal zone, with cumulative ESQ values reaching as high as 3.12 × 103 for carnivorous shorebirds, contrasted by negligible risks in the inland and production zones, where ESQ values for all groups are consistently below 1.0. By analyzing the toxicity reference value (TRV) against the expected daily exposure of receptors to air toxics, the proposed methodology provides valuable insights into the potential ecological risks and their subsequent impacts on ecological populations. The present contribution aims to deepen the understanding of the ecological health implications of air toxics and lay the foundation for informed, ecology-driven policymaking, underscoring the need for measures to mitigate these impacts.

The influence of short-term temporal variability on the efficacy of dragonfly larvae as mercury biosentinels

Mercury (Hg) exposure to fish, wildlife, and humans is widespread and of global concern, thus stimulating efforts to reduce emissions. Because the relationships between rates of inorganic Hg loading, methylmercury (MeHg) production, and bioaccumulation are extremely complex and challenging to predict, there is a need for reliable biosentinels to understand the distribution of Hg in the environment and monitor the effectiveness of reduction efforts. However, it is important to assess how temporal and spatial variation at multiple scales influences the efficacy of specific biosentinels. Seasonal and interannual variation in total Hg (THg) concentrations of dragonfly larvae were examined in relation to spatial variability among 21 sites in two U.S. national parks with contrasting ecologies and Hg deposition patterns. Dragonfly THg differed among sampling events at 17 of the 21 sites, but by an average of only 20.4 % across events, compared to an average difference of 52.7 % among sites. Further, THg concentrations did not follow consistent seasonal patterns across sites or years, suggesting that the observed temporal variation was unlikely to bias monitoring efforts. Importantly, for a specific site, there was no difference in % MeHg in dragonflies among sampling events. Finally, there was significant temporal variability in the biogeochemical factors (aqueous inorganic Hg, aqueous MeHg, DOC, SO₄, and pH) influencing dragonfly THg, with the importance of individual factors varying by 2.4 to 4.3-fold across sampling events. Despite these results, it is noteworthy that the observed temporal variation in dragonfly THg concentrations was neither large nor consistent enough to bias spatial assessments. Thus, although this temporal variation may provide insights into the processes influencing biological Hg concentrations, it is unlikely to impair the use of dragonflies as biosentinels for monitoring spatial or temporal patterns at scales relevant to most mitigation efforts.

Tradeoffs and synergies in wetland multifunctionality: A scaling issue

Wetland area in agricultural landscapes has been heavily reduced to gain land for crop production, but in recent years there is increased societal recognition of the negative consequences from wetland loss on nutrient retention, biodiversity and a range of other benefits to humans. The current trend is therefore to re-establish wetlands, often with an aim to achieve the simultaneous delivery of multiple ecosystem services, i.e., multifunctionality. Here we review the literature on key objectives used to motivate wetland re-establishment in temperate agricultural landscapes (provision of flow regulation, nutrient retention, climate mitigation, biodiversity conservation and cultural ecosystem services), and their relationships to environmental properties, in order to identify potential for tradeoffs and synergies concerning the development of multifunctional wetlands. Through this process, we find that there is a need for a change in scale from a focus on single wetlands to wetlandscapes (multiple neighboring wetlands including their catchments and surrounding landscape features) if multiple societal and environmental goals are to be achieved. Finally, we discuss the key factors to be considered when planning for re-establishment of wetlands that can support achievement of a wide range of objectives at the landscape scale.

Declines of methylmercury along a salinity gradient in a low-lying coastal wetland ecosystem at South Carolina, USA

Wetlands are widely regarded as biogeochemical hotspots of highly toxic methylmercury (MeHg), mainly mediated by sulfate-reducing bacteria. In low-lying coastal wetlands, sea level rise, a phenomenon caused by global climate change, is slowly degrading numerous healthy freshwater forested wetlands into salt-degraded counterparts with a nickname "ghost forests", and eventually converting them to saltmarshes. However, little is known about the changes of mercury (Hg) methylation, bioaccumulation, and biomagnification along the forest-to-saltmarsh gradient. Here, we conducted extensive field sampling in three wetland states (healthy forested wetlands, salt-degraded forested wetlands, and saltmarsh) along a salinity gradient (from 0 to 9.4 ppt) in Winyah Bay, South Carolina, USA. We found that in our study wetland systems the saltmarshes had the lowest levels of both total Hg and MeHg in sediments and biota, as compared to healthy forested wetlands and saltwater-degraded ghost forests. Our results suggest that the slow conversion of forested wetland to saltmarsh could reduce net MeHg production in our study wetland systems, which we hypothesized that could be attributed to increased sulfate reduction and excessive buildup of sulfide in sediment that inhibits microbial Hg methylation, and/or reduced canopy density and increased photodegradation of MeHg. However, it should be noted that biogeochemical MeHg responses to salinity changes may be site-specific and we urge more similar studies in other wetland systems along a salinity gradient. Therefore, long-term salinization of coastal wetlands and the slow conversion of forests to marshes could decrease long-term exposure of toxic MeHg levels in coastal food webs that are similar to our system, and ultimately reduce human exposure to this neurotoxin.

Development of sensitive and portable immunosensors based on signal amplification probes for monitoring the mercury(II) ions

Mercury ion (Hg²⁺) as a major environmental pollutant threatens human health even at very low concentrations, so it is essential to monitor mercury residues in food. In this study, Hg²⁺ was conjugated with protein carrier using 1-(4-Isothiocyanobenzyl) ethylenediamine N, N, N', N'-tetraacetic acid (ITCBE) as a bifunctional chelator. 7A1 monoclonal antibody (mAb) against Hg²⁺-ITCBE with high affinity (7.3 × 10⁹ L/moL) and good specificity was obtained by cell fusion technology and performed to establish immunosensors. Immunochromatographic test strip using colloidal gold nanoparticles (AuNP with an average diameter of 18 nm) as signal reporter showed low sensitivity. Signal amplification probes including larger multi-branched gold nanoflowers (AuNF) and latex microspheres (LM) were employed to enhance the sensitivity of immunosensors. The visible limit of detection (vLOD) of the AuNF- and LM-based strip were determined to be 50 ng/mL and 25 ng/mL respectively, showing more sensitive than that of AuNP-based strip (200 ng/mL). Quantitative analysis showed that AuNF-based strip exhibited lower quantitative limit of detection (qLOD) (0.44 ng/mL) which was 20-fold lower than that of AuNP-based strip (8.92 ng/mL) for determination of Hg²⁺, and LM-based strip (0.49 ng/mL) was 18 times as sensitive as AuNP-based strip. In summary, the developed immunosensors using AuNF and LM as signal amplification probes exhibited excellent sensitivity and provided portable, on-site detection for Hg²⁺.

Stable carbon and nitrogen isotopes explain methylmercury concentrations in stream food webs of Lake George, New York (USA)

Mercury has been studied extensively in lakes due to health risks associated with the consumption of contaminated fish, while stream ecosystems have received less attention. To better understand mercury bioavailability in the lower food web of streams, we collected macroinvertebrates (predators and detritivore) along with autochthonous (epilithic algae) and allochthonous (leaf litter) basal resources in eight streams entering Lake George. Samples were analyzed for methylmercury (MeHg), total mercury, and carbon and nitrogen isotopes (δ¹³C & δ¹⁵N) to determine how mercury concentrations in basal resources, biomagnification rates, and environmental factors (watershed characteristics and water chemistry) effected MeHg concentrations in predatory macroinvertebrates. While biomagnification rates, calculated as trophic magnification slope, explained between 68% and 98% of MeHg variability within a stream food web, the range was small (0.310-0.387) resulting in the biotic components following a consistent pattern of increasing MeHg among streams. The stream order was negatively related to basin slope for all biotic components and explained 70% of MeHg variability in predatory macroinvertebrates. Methylmercury concentrations were significantly and negatively related to δ¹³C in predators, epilithic algae, and leaf litter. We believe the biofilms on leaf litter utilized bacterial-respired carbon dioxide decreasing δ¹³C (<-28‰) and increasing MeHg while epilithic algal δ¹³C increased due to enhanced primary production resulting in biodilution of MeHg. Methylmercury in basal resources responded to δ¹³C similarly but through different processes. Our findings show shallow slopes elevate MeHg in basal resources and explain most of the predator MeHg variation among streams with little influence of biomagnification rates.

Increased Mercury and Reduced Insect Diversity in Linked Stream-Riparian Food Webs Downstream of a Historical Mercury Mine

Historical mining left a legacy of abandoned mines and waste rock in remote headwaters of major river systems in the western United States. Understanding the influence of these legacy mines on culturally and ecologically important downstream ecosystems is not always straightforward because of elevated natural levels of mineralization in mining-impacted watersheds. To test the ecological effects of historic mining in the headwaters of the upper Salmon River watershed in Idaho (USA), we measured multiple community and chemical endpoints in downstream linked aquatic-terrestrial food webs. Mining inputs impacted downstream food webs through increased mercury accumulation and decreased insect biodiversity. Total mercury (THg) in seston, aquatic insect larvae, adult aquatic insects, riparian spiders, and fish at sites up to 7.6 km downstream of mining was found at much higher concentrations (1.3-11.3-fold) and was isotopically distinct compared with sites immediately upstream of mining inputs. Methylmercury concentrations in bull trout and riparian spiders were sufficiently high (732-918 and 347-1140 ng MeHg g^-1  dry wt, respectively) to affect humans, birds, and piscivorous fish. Furthermore, the alpha-diversity of benthic insects was locally depressed by 12%-20% within 4.3-5.7 km downstream from the mine. However, because total insect biomass was not affected by mine inputs, the mass of mercury in benthic insects at a site (i.e., ng Hg m^-2 ) was extremely elevated downstream (10-1778-fold) compared with directly upstream of mining inputs. Downstream adult aquatic insect-mediated fluxes of THg were also high (~16 ng THg m^-2  day^-1 ). Abandoned mines can have ecologically important effects on downstream communities, including reduced biodiversity and increased mercury flux to higher order consumers, including fish, birds, and humans. Environ Toxicol Chem 2022;41:1696-1710. Published 2022. This article is a U.S. Government work and is in the public domain in the USA.

Simple Acid Digestion Procedure for the Determination of Total Mercury in Plankton by Cold Vapor Atomic Fluorescence Spectroscopy

Plankton, at the bottom of the food web, play a central role in the entry of mercury into the aquatic biota. To investigate their role in mercury uptake, reliable analytical procedures for Hg analysis are highly sought. Wet digestion procedures for determining total mercury in different biological matrices have been established since years, however only few studies focused on planktonic samples. In the present work, a simple and cost-effective wet digestion method was developed for the determination of total mercury in samples of small plankton material using a cold vapor atomic fluorescence spectroscopy (CVAFS). The optimization of the digestion method was achieved by using glass vessels with Teflon caps, low amount of acids (3 mL w/w 65% HNO₃ or 3 mL 50% v/v HNO₃), a constant temperature of 85 °C, the presence and absence of pre-ultrasound treatment, and a continuous digestion period (12 h). Certified reference materials IAEA-450 (unicellular alga Scenedesmus obliquus) and BRC-414 (plankton matrix) were used to optimize and validate the digestion method. The recovery efficiency of the proposed method for IAEA-450 and BCR-414 (3.1 mg and 21.5 mg) ranged between 94.1 ± 7.6% and 97.2 ± 4.6%. The method displayed a good recovery efficiency and precision for plankton matrices of low size. Thus, allowing better digestion of planktonic samples for mercury analysis using CVAFS techniques.

Utility of Diffusive Gradient in Thin-Film Passive Samplers for Predicting Mercury Methylation Potential and Bioaccumulation in Freshwater Wetlands

Mercury is a risk in aquatic ecosystems when the metal is converted to methylmercury (MeHg) and subsequently bioaccumulates in aquatic food webs. This risk can be difficult to manage because of the complexity of biogeochemical processes for mercury and the need for accessible techniques to navigate this complexity. Here, we explored the use of diffusive gradient in thin-film (DGT) passive samplers as a tool to simultaneously quantify the methylation potential of inorganic Hg (IHg) and the bioaccumulation potential of MeHg in freshwater wetlands. Outdoor freshwater wetland mesocosms were amended with four isotopically labeled and geochemically relevant IHg forms that represent a range of methylation potentials (²⁰²Hg²⁺, ²⁰¹Hg-humic acid, ¹⁹⁹Hg-sorbed to FeS, and ²⁰⁰HgS nanoparticles). Six weeks after the spikes, we deployed DGT samplers in the mesocosm water and sediments, evaluated DGT-uptake rates of total Hg, MeHg, and IHg (calculated by difference) for the Hg isotope spikes, and examined correlations with total Hg, MeHg, and IHg concentrations in sediment, water, and micro and macrofauna in the ecosystem. In the sediments, we observed greater relative MeHg concentrations from the initially dissolved IHg isotope spikes and lower MeHg levels from the initially particulate IHg spikes. These trends were consistent with uptake flux of IHg into DGTs deployed in surface sediments. Moreover, we observed correlations between total Hg-DGT uptake flux and MeHg levels in periphyton biofilms, submergent plant stems, snails, and mosquitofish in the ecosystem. These correlations were better for DGTs deployed in the water column compared to DGTs in the sediments, suggesting the importance of vertical distribution of bioavailable MeHg in relation to food sources for macrofauna. Overall, these results demonstrate that DGT passive samplers are a relatively simple and efficient tool for predicting IHg methylation and MeHg bioaccumulation potentials without the need to explicitly delineate IHg and MeHg speciation and partitioning in complex ecosystems.

Bottom-Heavy Trophic Pyramids Impair Methylmercury Biomagnification in the Marine Plankton Ecosystems

Methylmercury (CH₃Hg⁺, MMHg) in the phytoplankton and zooplankton, which form the bottom of marine food webs, is a good predictor of MMHg in top predators, including humans. Therefore, evaluating the potential exposure of MMHg to higher trophic levels (TLs) requires a better understanding of relationships between MMHg biomagnification and plankton dynamics. In this study, a coupled ecological/physical model with 366 plankton types of different sizes, biogeochemical functions, and temperature tolerance is used to simulate the relationships between MMHg biomagnification and the ecosystem structure. The study shows that the MMHg biomagnification becomes more significant with increasing TLs. Trophic magnification factors (TMFs) in the lowest two TLs show the opposite spatial pattern to TMFs in higher TLs. The low TMFs are usually associated with a short food-chain length. The less bottom-heavy trophic pyramids in the oligotrophic oceans enhance the MMHg trophic transfer. The global average TMF is increased from 2.3 to 2.8 in the warmer future with a medium climate sensitivity of 2.5 °C. Our study suggests that if there are no mitigation measures for Hg emission, MMHg in the high-trophic-level plankton is increased more dramatically in the warming future, indicating greater MMHg exposure for top predators such as humans.

Trophic transfer and dietary exposure risk of mercury in aquatic organisms from urbanized coastal ecosystems

In this study, 26 surface seawater samples, 26 surface sediment samples and 114 organisms were collected to study the trophic transfer and dietary exposure risk of mercury (Hg) in organisms from the Jiaozhou Bay, which is a typical semi-enclosed urbanized bay. The total mercury (THg) and methylmercury (MeHg) concentrations did not exceed the threshold limits and performed as: fish > crustaceans > mollusks. The trophic level values (TLs) were less than 3 in all the groups, indicating simple structure of food chain. With the increasing δ¹⁵N value, THg and MeHg were significantly biomagnified in the mollusks and fish but not in the crustaceans. In addition, the bioaccumulation and biomagnification of MeHg were higher than inorganic mercury (IHg) in the aquatic food chain. Target hazard quotient (THQ) and provisional tolerable weekly intake (PTWI) indicated that Hg exposure via consumption of seafood from the Jiaozhou Bay did not pose significant health risks for general population. Consuming fish will face the higher health risk than crustaceans and mollusks, especially in urban regions. Moreover, the risk of MeHg caused by intaking seafood deserved more attention. Trophic transfer function (TTF) explicated the transfer of Hg in the ecosystem and higher trophic transfer efficiency of MeHg than IHg. TTF interpreted the terrestrial input of Hg should be controlled to ensure the safety of consuming seafood from the Jiaozhou Bay.

Drivers of variability in mercury and methylmercury bioaccumulation and biomagnification in temperate freshwater lakes

The four largest freshwater lakes in southwestern France are of both ecological and economic importance. However, some of them are subjected to mercury (Hg) contamination, resulting in the ban of human consumption of piscivorous fish. Moreover, beyond predatory fish, little information exist regarding Hg levels in other species of these ecosystems. In this context, we used a food web analytical approach to investigate Hg bioaccumulation and biomagnification in relation to the trophic structure of these four lakes. More specifically, various organisms (macrophytes, epiphyton, invertebrates and fish) were collected at the four lakes and analysed for carbon and nitrogen stable isotopes as well as for total Hg (THg) and methylmercury (MeHg). A spatial variability of bioaccumulation in organisms was observed, particularly in carnivorous fish, with higher Hg levels being found in the two more northern lakes (median±SE: 3491 ± 474 and 1113 ± 209 ng THg.g^-1 dw in lakes HC and L, respectively) than in the southern pair (600 ± 117 and 911 ± 117 ng THg.g^-1 dw in lakes CS and PB, respectively). Methylmercury biomagnification was observed through the food webs of all four lakes, with different trophic magnification slopes (HC = 0.16; L = 0.33; CS = 0.27; PB = 0.27), even though the length of the food chains was similar between the lakes. Our results suggest that rather than the food web structure, anthropogenic inputs (sulfate in northern lakes and phosphorus inputs in southern ones) may have a strong impact, more or less directly, on Hg methylation in freshwater environments, and lead to concentrations exceeding environmental recommendations despite low Hg backgrounds in sediment and water.

Insights into Mercury Source Identification and Bioaccumulation Using Stable Isotope Approaches in the Hannibal Pool of the Ohio River, USA

Mercury contamination in river systems due to historic and current Hg releases is a persistent concern for both wildlife and human health. In larger rivers, like the Ohio River, USA, it is difficult to directly link Hg discharges to bioaccumulation due to the existence of multiple industrial Hg sources as well as the varied dietary and migratory habits of biota. To better understand how industrial effluent influences the cycling and bioaccumulation of Hg within the Ohio River, Hg stable isotope analysis was applied to various nonbiological and biological media. High Hg concentrations in suspended particulate matter suggest this vector was the largest contributor of Hg to the water column, and distinct Hg source signatures were observed in effluent particulates from different industrial processes, such as chlor-alkali activity (δ202 Hg = -0.52‰) and coal power plant discharge (δ202 Hg = -1.39‰). Despite this distinction, average sediments (δ202 Hg = -1.00 ± 0.23‰) showed intermediate isotopic signatures that suggest the accumulation of a mixed Hg source driven by multiple industrial discharges. Biota in the system were shown to have a conserved range of δ202 Hg and estimation approaches related these signatures back to particulate matter within Hannibal Pool. Mussels were found to conserve Hg isotopes signatures independently of food web drivers and served as ideal water column indicators of bioaccumulated Hg sources. This study highlights the complexity of Hg cycling within an industrialized river and shows that an isotope tracer approach can provide insight to water column sources of Hg. Integr Environ Assess Manag 2021;17:233-242. Published 2020. This article is a US Government work and is in the public domain in the USA.

Mercury bioaccumulation in stream fish from an agriculturally-dominated watershed

Bioaccumulation of mercury in freshwater fish is a complex process driven by environmental and biological factors. In this study, we assessed mercury in fish from four tributaries to the Red Deer River, Alberta, Canada, which are characterized by high surface water mercury concentrations. We used carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes to examine relationships between fish total mercury (THg) concentrations, food web dynamics and patterns in unfiltered THg and methylmercury (MeHg) concentrations. We found that THg concentrations exceeded the tissue residue quality guideline for the protection of wildlife consumers in 99.7% of fish sampled. However, while the surface water THg concentration was highest in Michichi Creek and the MeHg concentration was consistent across streams, patterns of fish THg concentrations varied depending on species. Furthermore, body size and trophic level were only correlated with THg concentrations in white sucker (Catostomus commersoni) and Prussian carp (Carrasius gibelio). The results of this study suggest that mercury poses a risk to the health of piscivorous wildlife in the Red Deer River watershed. Despite high THg concentrations in these streams, mercury bioaccumulation is not driven by environmental inorganic mercury concentrations. Additionally, commonly cited factors associated with mercury concentrations in fish, such as body size and trophic level, may not strongly influence bioaccumulation in these stream ecosystems.

The impact of lime additions on mercury dynamics in stream chemistry and macroinvertebrates: a comparison of watershed and direct stream addition management strategies

Acid deposition has declined across eastern North America and northern Europe due to reduced emissions of sulfur and nitrogen oxides. Ecosystem recovery has been slow with limited improvement in surface water chemistry. Delayed recovery has encouraged acid-neutralization strategies to accelerate recovery of impaired biological communities. Lime application has been shown to increase pH and dissolved organic carbon (DOC), which could also drive increased mobilization of mercury (Hg) to surface waters. A four-year study was conducted within Honnedaga Lake's watershed in the Adirondack region of New York to compare the effects of watershed and direct channel lime additions on Hg in stream water and macroinvertebrates. All treatments sharply increased stream pH and DOC concentrations, but large differences in the duration of impacts were apparent. The watershed treatment resulted in multi-year increases in concentrations and loads of total Hg (150%; 390%), DOC (190%; 350%) and nutrients, whereas total Hg and DOC increased for short periods (72-96 h) after channel treatments. No response of Hg in macroinvertebrates was evident following the watershed treatment, but a potential short-term and spatially constrained increase occurred after the channel treatment. Our observations indicate that both treatment approaches mobilize Hg, but that direct channel liming mobilizes considerably less than watershed liming over any period longer than a few days. During the final study year, increased methyl Hg concentrations were observed across reference and treated streams, which may reflect an extended dry period, highlighting that climate variation may also affect Hg dynamics.

Using a geographic information system to assess local scale methylmercury exposure from fish in nine communities of the Eeyou Istchee territory (James Bay, Quebec, Canada)

Exposure to methylmercury is a concern for those who rely on fish as a traditional food in the Eeyou Istchee territory of James Bay, Quebec, Canada, because industrial land uses overlap with community water bodies where fish are harvested. Consequently, this study assessed if traditional practices, particularly fishing, increased the risk of exposure to methylmercury from the consumption of locally harvested fish. We designed a geographic information system (GIS) that included land use and fish methylmercury tissue concentrations to assess clustering of potential hot spots. We also used generalized linear models to assess the association of fish consumption to blood organic-mercury concentrations, and logistic regression models to assess the probability of fish exceeding the safety threshold for methylmercury tissue concentrations in areas of high intensity land use. The GIS demonstrated significant clustered hot spots around regions of hydroelectric and mining land use. Our results also revealed that adult consumption of pike, lake trout and/or walleye, and child consumption of pike or walleye were significantly associated with blood organic-mercury concentrations. Further, large fish harvested in a community with high intensity land use yielded a 77% probability that the fish exceeded the safety threshold. From a human exposure perspective, our study highlights the need for further research on children who consume fish from this region.

Mercury in fish from streams and rivers in New York State: Spatial patterns, temporal changes, and environmental drivers

Mercury (Hg) concentrations in freshwater fish across the state of New York frequently exceed guidelines considered harmful to humans and wildlife, but statewide distribution and temporal changes are not well known for the state's streams and rivers. We analyzed existing data to describe recent spatial patterns, identify key environmental drivers, and assess temporal changes. Size classes within sportfishes and prey fishes formed 'functional taxa' (FT), and standardized scores were generated from 2007-2016 data for 218 sites. Muscle Hg in ≥1 sportfish FT exceeded human-health guidelines of 50 ng/g (sensitive populations) and 300 ng/g (general population, GP) at 93 and 56% of sites, respectively, but exceeded 1000 ng/g (a state threshold) at only 10% of sites. Whole-body Hg in ≥1 prey fish FT exceeded wildlife thresholds of 40 ng/g and 100 ng/g at 91 and 51% of sites, respectively. Environmental drivers of recent spatial patterns include extent of forest cover and storage, the latter an indicator of wetlands. Standardized Hg scores increased with increasing atmospheric Hg deposition and storage across rural 'upland' regions of New York. However, scores were not related to atmospheric deposition in more-developed 'lowland' regions due to the limited methylation potential of urban landscapes. Comparisons of 2010-2015 sportfish Hg concentrations with those of 1998 and 2000-2005 showed inconsistent temporal changes both among and within eight sites examined. Some recent stream and river fish Hg spatial patterns differed from those of lake-based studies, highlighting the importance of New York's flowing waters to future Hg monitoring and risk assessment.

Spatial variation in aquatic invertebrate and riparian songbird mercury exposure across a river-reservoir system with a legacy of mercury contamination

Mercury (Hg) loading and methylation in aquatic systems causes a variety of deleterious effects for fish and wildlife populations. Relatively little research has focused on Hg movement into riparian food webs and how this is modulated by habitat characteristics. This study characterized differences in Hg exposure in aquatic invertebrates and riparian songbirds across a large portion of the Willamette River system in western Oregon, starting at a Hg-contaminated Superfund site in the headwaters (Black Butte Hg Mine) and including a reservoir known to methylate Hg (Cottage Grove Reservoir), all downstream reaches (Coast Fork and Willamette River) and off-channel wetland complexes (Willamette Valley National Wildlife Refuge Complex). After accounting for year, date, and site differences in a mixed effects model, MeHg concentrations in aquatic invertebrates varied spatially among habitat categories and invertebrate orders. Similarly, THg in songbird blood varied by among habitat categories and bird species. The highest Hg concentrations occurred near the Hg mine, but Hg did not decline linearly with distance from the source of contamination. Birds were consistently elevated in Hg in habitats commonly associated with enhanced MeHg production, such as backwater or wetlands. We found a positive but weak correlation between aquatic invertebrate MeHg concentrations and songbird THg concentrations on a site-specific basis. Our findings suggest that Hg risk to riparian songbirds can extend beyond point-source contaminated areas, highlighting the importance of assessing exposure in surrounding habitats where methylmercury production may be elevated, such as reservoirs and wetlands.

A National-Scale Assessment of Mercury Bioaccumulation in United States National Parks Using Dragonfly Larvae As Biosentinels through a Citizen-Science Framework

We conducted a national-scale assessment of mercury (Hg) bioaccumulation in aquatic ecosystems, using dragonfly larvae as biosentinels, by developing a citizen-science network to facilitate biological sampling. Implementing a carefully designed sampling methodology for citizen scientists, we developed an effective framework for a landscape-level inquiry that might otherwise be resource limited. We assessed the variation in dragonfly Hg concentrations across >450 sites spanning 100 United States National Park Service units and examined intrinsic and extrinsic factors associated with the variation in Hg concentrations. Mercury concentrations ranged between 10.4 and 1411 ng/g dry weight across sites and varied among habitat types. Dragonfly total Hg (THg) concentrations were up to 1.8-fold higher in lotic habitats than in lentic habitats and 37% higher in waterbodies with abundant wetlands along their margins than those without wetlands. Mercury concentrations in dragonflies differed among families but were correlated (r² > 0.80) with each other, enabling adjustment to a consistent family to facilitate spatial comparisons among sampling units. Dragonfly THg concentrations were positively correlated with THg concentrations in both fish and amphibians from the same locations, indicating that dragonfly larvae are effective indicators of Hg bioavailability in aquatic food webs. We used these relationships to develop an integrated impairment index of Hg risk to aquatic ecosytems and found that 12% of site-years exceeded high or severe benchmarks of fish, wildlife, or human health risk. Collectively, this continental-scale study demonstrates the utility of dragonfly larvae for estimating the potential mercury risk to fish and wildlife in aquatic ecosystems and provides a framework for engaging citizen science as a component of landscape Hg monitoring programs.

Nano-selenium functionalized zinc oxide nanorods: A superadsorbent for mercury (II) removal from waters

In this study, nano selenium functionalized zinc oxide nanorods, NanoSe@ZnO-NR, was prepared, characterized and investigated for Hg(II) removal from waters of different types. The study results revealed that the material showed a superior adsorption capacity (q_m, 1110 mg g^-1) and excellent distribution coefficient (K_d, 9.11 × 10⁸ mL g^-1) which is two or more orders above most of the adsorbents reported in the literature. It should be also known that, 30 mg of the adsorbent can quickly reduce 10 mg L^-1 Hg(II) to undetectable level from 10 mL of sample solution. The adsorption data were well explained with the pseudo-second order kinetic model and Langmuir adsorption isotherm model. Besides, the capturing capability of the material is independent on the pH change (2-12), selective against interfering cations, and exhibited fast kinetics (equilibrium time, ∼1 min). The NanoSe@ZnO-NR performance was also tested on real samples from different origin, surface waters (tap, lake and river) and wastewaters (effluent and influent), and complete removal and ≥99.2% removal efficiency was observed at 0.01 and 10 mg L^-1 spiking levels, respectively. Therefore, NanoSe@ZnO-NR can be considered as a potential adsorbent in advancing the wastewater treatment technology.

Review of stable mercury isotopes in ecology and biogeochemistry

Due to the advent of cold vapor-multicollector-inductively coupled plasma mass spectrometry (CV-MC-ICP-MS) in the past two decades, many research groups studying mercury (Hg) biogeochemistry have integrated stable Hg isotopes into their research. Currently, >200 studies using this technique have been published and this has greatly enhanced our understanding of the Hg biogeochemical cycle beyond what Hg concentration and speciation analyses alone can provide. These studies are largely divided into two groups: (i) controlled experiments investigating fractionation of Hg isotopes and refining tools of isotopic analyses, and (ii) studies of natural variations of Hg isotopes. It is now known that Hg isotopes undergo both mass dependent fractionation (MDF; reported as the ratio of mass ²⁰²Hg to ¹⁹⁸Hg) and mass independent fractionation (MIF), with MIF occurring at odd masses (¹⁹⁹Hg, ²⁰¹Hg) to a larger magnitude and at even masses (²⁰⁰Hg, ²⁰⁴Hg) to a much smaller magnitude. The two types of MIF are controlled by different photochemical processes. The range of isotopic variations of MDF, odd-MIF, and even-MIF are now well documented in a diverse set of environmental samples, and researchers are continuing to explore how the field of Hg isotope biogeochemistry can be further developed and taken to the next level of understanding. One application that has received considerable attention is the use of Hg isotopes to examine the environmental controls on the production and degradation of methylmercury (MeHg), the most toxic and bioaccumulative form of Hg. Since MeHg is efficiently assimilated and biomagnified along food chains, MeHg has the potential to be a robust ecological tracer. In this review, we give an updated overview of the field of Hg isotopes and focus on how Hg isotopes of MeHg can be used to address fundamental ecological questions, including energy transfer across ecosystem interfaces and as a tracer for animal movements.

Five decades of declining methylmercury concentrations in boreal foodwebs suggest pivotal role for sulphate deposition

Widespread declines in mercury (Hg) in fish in pristine lakes in Fennoscandia since the 1970s are unexplained. Interactions between climate, atmospheric deposition, and elemental cycling of carbon (C), sulphur (S) and Hg are complex and affect Hg bioaccumulation. A parallel significant decline in methyl-Hg (MeHg) concentrations in aquatic macroinvertebrates (Chironomidae) was found between 1976-78 and 2004-15 in an intensely studied, pristine boreal lake (Langtjern, boreal Fennoscandia). Monitoring at Langtjern demonstrated a four-fold decrease in aqueous sulphate concentrations (SO₄, 50-year record), significant lake browning (30-year records), increasing sediment Hg concentrations (50-year record), warming (45-year record) and increased runoff (40-year record). Contrasting Hg trends in biota (downward) and sediment (upward) indicated a disconnect between lake Hg loading and foodweb Hg bioaccumulation. We suggest that reduced SO₄-deposition has 1) constrained substrate availability for SO₄-reducing methylating bacteria (causing reduced foodweb MeHg exposure despite increased Hg loading to the lake), and 2), increased the binding affinity between aqueous organic matter and Hg species (leading to reduced MeHg bioavailability). The downward MeHg trend at the base of the foodweb at Langtjern is mirrored at higher trophic levels by strong declines in perch (Perca fluviatilis) and pike (Esox lucius) Hg concentrations in boreal Fennoscandia. A plausible explanation is that declining SO₄-deposition, rather than climate change or reduced atmospheric Hg, is currently driving reduced MeHg contamination in northern freshwater foodwebs.

Methylmercury exposure in wildlife: A review of the ecological and physiological processes affecting contaminant concentrations and their interpretation

Exposure to methylmercury (MeHg) can result in detrimental health effects in wildlife. With advances in ecological indicators and analytical techniques for measurement of MeHg in a variety of tissues, numerous processes have been identified that can influence MeHg concentrations in wildlife. This review presents a synthesis of theoretical principals and applied information for measuring MeHg exposure and interpreting MeHg concentrations in wildlife. Mercury concentrations in wildlife are the net result of ecological processes influencing dietary exposure combined with physiological processes that regulate assimilation, transformation, and elimination. Therefore, consideration of both physiological and ecological processes should be integrated when formulating biomonitoring strategies. Ecological indicators, particularly stable isotopes of carbon, nitrogen, and sulfur, compound-specific stable isotopes, and fatty acids, can be effective tools to evaluate dietary MeHg exposure. Animal species differ in their physiological capacity for MeHg elimination, and animal tissues can be inert or physiologically active, act as sites of storage, transformation, or excretion of MeHg, and vary in the timing of MeHg exposure they represent. Biological influences such as age, sex, maternal transfer, and growth or fasting are also relevant for interpretation of tissue MeHg concentrations. Wildlife tissues that represent current or near-term bioaccumulation and in which MeHg is the predominant mercury species (such as blood and eggs) are most effective for biomonitoring ecosystems and understanding landscape drivers of MeHg exposure. Further research is suggested to critically evaluate the use of keratinized external tissues to measure MeHg bioaccumulation, particularly for less-well studied wildlife such as reptiles and terrestrial mammals. Suggested methods are provided to effectively use wildlife for quantifying patterns and drivers of MeHg bioaccumulation over time and space, as well as for assessing the potential risk and toxicological effects of MeHg on wildlife.

Relationships between mercury concentrations in fur and stomach contents of river otter (Lontra canadensis) and mink (Neovison vison) in Northern Alberta Canada and their applications as proxies for environmental factors determining mercury bioavailability

The fur of piscivorous animals such as river otter (Lontra canadensis) and mink (Neovison vison) has been proposed to be used as a biomarker medium to assess mercury (Hg) exposure, but the relationship with dietary and environmental Hg exposure has not been fully characterized. The objective of this study was to investigate the relationship between fur total mercury (THg) and stomach content THg in river otter and mink, and their relationships with environmental factors. THg concentrations were measured in fur and stomach contents of river otter (n = 35) and mink (n = 30) collected from northern Alberta, Canada between 2014 and 2017. The fur THg concentration (mean ± standard deviation) was 6.36 ± 4.12 μg/g fur weight and 5.25 ± 3.50 μg/g fur weight and the average stomach content THg was 0.95 ± 0.56 μg/g dry weight and 0.71 ± 0.54 μg/g dry weight in river otter and mink respectively. There was a positive relationship between the log fur THg and log stomach contents THg for both species (p < 0.05). There was a positive relationship between the log THg of stomach contents and the percent of deciduous forest and a negative relationship with soil pH. There was a positive relationship between the log THg of fur and the total area burned by forest fire and a negative relationship with the percentage of wetlands. These results provide field evidence that fur can be used to reflect dietary Hg exposure and to identify sources and environmental factors that affect the bioavailable Hg in the habitats of these wildlife species.

Disentangling the effects of habitat biogeochemistry, food web structure, and diet composition on mercury bioaccumulation in a wetland bird

Methylmercury (MeHg) is a globally pervasive contaminant with known toxicity to humans and wildlife. Several sources of variation can lead to spatial differences in MeHg bioaccumulation within a species including: biogeochemical processes that influence MeHg production and availability within an organism's home range; trophic positions of consumers and MeHg biomagnification efficiency in food webs; and individual prey preferences that influence diet composition. To better understand spatial variation in MeHg bioaccumulation within a species, we evaluated the effects of habitat biogeochemistry, food web structure, and diet composition in the wetland-obligate California black rail (Laterallus jamaicensis coturniculus) at three wetlands along the Petaluma River in northern San Francisco Bay, California, USA. The concentration of MeHg in sediments differed significantly among wetlands. We identified three sediment and porewater measurements that contributed significantly to a discriminant function explaining differences in habitat biogeochemistry among wetlands: the porewater concentration of ferrous iron, the percent organic matter, and the sediment MeHg concentration. Food web structure and biomagnification efficiency were similar among wetlands, with trophic magnification factors for MeHg ranging from 1.84 to 2.59. In addition, regurgitation samples indicated that black rails were dietary generalists with similar diets among wetlands (percent similarity indices > 70%). Given the similarities in diet composition, food web structure, and MeHg biomagnification efficiency among wetlands, we concluded that variation in habitat biogeochemistry and associated sediment MeHg production was the primary driver of differences in MeHg concentrations among black rails from different wetlands.

Isolation of methylmercury using distillation and anion-exchange chromatography for isotopic analyses in natural matrices

The development of mercury (Hg) stable isotope measurements has enhanced the study of Hg sources and transformations in the environment. As a result of the mixing of inorganic Hg (iHg) and methylmercury (MeHg) species within organisms of the aquatic food web, understanding species-specific Hg stable isotopic compositions is of significant importance. The lack of MeHg isotope measurements is due to the analytical difficulty in the separation of the MeHg from the total Hg pool, with only a few methods having been tested over the past decade with varying degrees of success, and only a handful of environmentally relevant measurements. Here, we present a novel anion-exchange resin separation method using AG 1-X4 that further isolates MeHg from the sample matrix, following a distillation pretreatment, in order to obtain ambient MeHg stable isotopic compositions. This method avoids the use of organic reagents, does not require complex instrumentation, and is applicable across matrices. Separation tests across sediment, water, and biotic matrices showed acceptable recoveries (98 ± 5%, n = 54) and reproducible δ²⁰²Hg isotope results (2 SDs ≤ 0.15‰) down to 5 ng of MeHg. The measured MeHg pools in natural matrices, such as plankton and sediments, showed large deviations from the non-speciated total Hg measurement, indicating that there is an important isotopic shift during methylation that is not recorded by typical measurements, but is vital in order to assess sources of Hg during bioaccumulation. Graphical abstract.

Contaminant Concentrations in Sediments, Aquatic Invertebrates, and Fish in Proximity to Rail Tracks Used for Coal Transport in the Pacific Northwest (USA): A Baseline Assessment

Railway transport of coal poses an environmental risk, because coal dust contains polycyclic aromatic hydrocarbons (PAHs), mercury, and other trace metals. In the Pacific Northwest of the United States, proposed infrastructure projects could result in an increase in coal transport by train through the Columbia River corridor. Baseline information is needed on current distributions, levels, and spatial patterns of coal dust-derived contaminants in habitats and organisms adjacent to existing coal transport lines. To that end, we collected aquatic surface sediments, aquatic insects, and juvenile fish in 2014 and 2015 from Horsethief Lake State Park and Steigerwald National Wildlife Refuge, both located in Washington state close to the rail line and within the Columbia River Gorge National Scenic Area. Two subsites in each area were selected: one close to the rail line and one far from the rail line. Detected PAH concentrations were relatively low compared with those measured at more urbanized areas. Some contaminants were measured at higher concentrations at the subsites close to the rail line, but it was not possible to link the contaminants to a definitive source. Trace metal concentrations were only slightly higher than background concentrations, but a few of the more sensitive benchmarks were exceeded, including those for arsenic, lead, and selenium in fish tissue and fluoranthene, cadmium, copper, manganese, nickel, zinc, iron, and arsenic in sediments. At Horsethief Lake, Chinook salmon and yellow perch showed lower total mercury body burdens than other species, but PAH body burdens did not differ significantly among species. Differences in the species caught among subsites and the low number of invertebrate samples rendered food web comparisons difficult, but these data show that the PAHs and trace metals, including mercury, are accumulating in these wetland sites and in some resident organisms.

Tissue content of thiol-containing amino acids predicts methylmercury in aquatic invertebrates

Aquatic invertebrates vary in methylmercury (MeHg) levels among systems which has been attributed, in part, to environmental conditions, but may also be linked to differences in their biochemical composition. As MeHg is known to bind to thiol-containing amino acids such as cysteine in proteins of fish, our objective was to determine if these amino acids explain MeHg variability among aquatic invertebrate taxa. Benthic macroinvertebrates from diverse functional feeding groups and bulk zooplankton were collected from six acidic lakes in Kejimkujik National Park, Nova Scotia, Canada, and analyzed for MeHg, cysteine (as cysteic acid), methionine (as methionine sulfone), and nitrogen (relative trophic level, δ¹⁵N) and carbon (carbon source, δ¹³C) isotopes. MeHg was significantly and positively related to cysteine or methionine in zooplankton, caddisfly and stonefly tissues (R² from 0.24 to 0.57). In addition, methionine or cysteine in combination with δ¹⁵N and/or δ¹³C were better predictors of MeHg levels in stoneflies, mayflies, caddisflies and zooplankton among these lakes (R²_adj = 0.25-0.91). Overall, these novel findings suggest that the variability in MeHg of aquatic invertebrates can be explained, in part, by their tissue levels of thiol-containing amino acids.

Timber harvest alters mercury bioaccumulation and food web structure in headwater streams

Timber harvest has many effects on aquatic ecosystems, including changes in hydrological, biogeochemical, and ecological processes that can influence mercury (Hg) cycling. Although timber harvest's influence on aqueous Hg transformation and transport are well studied, the effects on Hg bioaccumulation are not. We evaluated Hg bioaccumulation, biomagnification, and food web structure in 10 paired catchments that were either clear-cut in their entirety, clear-cut except for an 8-m wide riparian buffer, or left unharvested. Average mercury concentrations in aquatic biota from clear-cut catchments were 50% higher than in reference catchments and 165% higher than in catchments with a riparian buffer. Mercury concentrations in aquatic invertebrates and salamanders were not correlated with aqueous THg or MeHg concentrations, but rather treatment effects appeared to correspond with differences in the utilization of terrestrial and aquatic basal resources in the stream food webs. Carbon and nitrogen isotope data suggest that a diminished shredder niche in the clear-cut catchments contributed to lower basal resource diversity compared with the reference of buffered treatments, and that elevated Hg concentrations in the clear-cut catchments reflect an increased reliance on aquatic resources in clear-cut catchments. In contrast, catchments with riparian buffers had higher basal resource diversity than the reference catchments, indicative of more balanced utilization of terrestrial and aquatic resources. Further, following timber harvest THg concentrations in riparian songbirds were elevated, suggesting an influence of timber harvest on Hg export to riparian food webs. These data, coupled with comparisons of individual feeding guilds, indicate that changes in organic matter sources and associated effects on stream food web structure are important mechanisms by which timber harvest modifies Hg bioaccumulation in headwater streams and riparian consumers.

Mercury in aquatic fauna contamination: A systematic review on its dynamics and potential health risks

Mercury is an important pollutant, released into aquatic ecosystems both naturally and by anthropogenic action. This element is transferred to aquatic organisms in different ways, causing potential health risks. In addition, mercury can be accumulated by humans, especially through the consumption of contaminated food. This systematic review aims to present mercury pathways, the major routes through which this element reaches the aquatic environment and its transformations until becoming available to living animals, leading to bioaccumulation and biomagnification phenomena. The key biotic and abiotic factors affecting such processes, the impact of mercury on animal and human health and the issue of seafood consumption as a source of chronic mercury contamination are also addressed. A total of 101 articles were retrieved from a standardized search on three databases (PubMed, Emabse, and Web of Science), in addition to 28 other studies not found on these databases but considered fundamental to this review (totaling 129 articles). Both biotic and abiotic factors display fundamental importance in mediating mercurial dynamics, i.e., muscle tropism, and salinity, respectively. Consequently, mercurial contamination in aquatic environments affects animal health, especially the risk of extinction species and also on human health, with methylmercury the main mercury species responsible for acute and chronic symptomatology.

Factors affecting MeHg bioaccumulation in stream biota: the role of dissolved organic carbon and diet

The bioaccumulation of the neurotoxin methylmercury (MeHg) in freshwater ecosystems is thought to be mediated by both water chemistry (e.g., dissolved organic carbon [DOC] and dissolved mercury [Hg]) and diet (e.g., trophic position and diet composition). Hg in small streams is of particular interest given their role as a link between terrestrial and aquatic processes. Terrestrial processes determine the quantity and quality of streamwater DOC, which in turn influence the quantity and bioavailability of dissolved MeHg. To better understand the effects of water chemistry and diet on Hg bioaccumulation in stream biota, we measured DOC and dissolved Hg in stream water and mercury concentration in three benthic invertebrate taxa and three fish species across up to 12 tributary streams in a forested watershed in New Hampshire, USA. As expected, dissolved total mercury (THg) and MeHg concentrations increased linearly with DOC. However, mercury concentrations in fish and invertebrates varied non-linearly, with maximum bioaccumulation at intermediate DOC concentrations, which suggests that MeHg bioavailability may be reduced at high levels of DOC. Further, MeHg and THg concentrations in invertebrates and fish, respectively, increased with δ¹⁵N (suggesting trophic position) but were not associated with δ¹³C. These results show that even though MeHg in water is strongly determined by DOC concentrations, mercury bioaccumulation in stream food webs is the result of both MeHg availability in stream water and trophic position.

Wetland Management Strategy to Reduce Mercury in Water and Bioaccumulation in Fish

Wetland environments provide numerous ecosystem services but also facilitate methylmercury (MeHg) production and bioaccumulation. We developed a wetland-management technique to reduce MeHg concentrations in wetland fish and water. We physically modified seasonal wetlands by constructing open- and deep-water treatment cells at the downstream end of seasonal wetlands to promote naturally occurring MeHg-removal processes. We assessed the effectiveness of reducing mercury (Hg) concentrations in surface water and western mosquitofish that were caged at specific locations within 4 control and 4 treatment wetlands. Methylmercury concentrations in wetland water were successfully decreased within treatment cells during only the third year of study; however, treatment cells were not effective for reducing total Hg concentrations. Furthermore, treatment cells were not effective for reducing total Hg concentrations in wetland fish. Mercury concentrations in fish were not correlated with total Hg concentrations in filtered, particulate, or whole water; and the slope of the correlation with water MeHg concentrations differed between months. Fish total Hg concentrations were weakly correlated with water MeHg concentrations in April when fish were introduced into cages but were not correlated in May when fish were retrieved from cages. Fish total Hg concentrations were greater in treatment wetlands than in control wetlands the year after the treatment wetlands' construction but declined by the second year. During the third year, fish total Hg concentrations increased in both control and treatment wetlands after an unexpected regional flooding event. Overall, we found limited support for the use of open- and deep-water treatment cells at the downstream end of wetlands to reduce MeHg concentrations in water but not fish. We suggest that additional evaluation over a longer period of time is necessary. Environ Toxicol Chem 2019;38:2178-2196. Published 2019 Wiley Periodicals, Inc. on behalf of SETAC. This article is a US government work, and as such, is in the public domain in the United States of America..

Effects of water impoundment and water-level manipulation on the bioaccumulation pattern, trophic transfer and health risk of heavy metals in the food web of Three Gorges Reservoir (China)

The Three Gorges Reservoir (TGR) of China, the largest hydropower project over the world, has attracted much attention to the water impoundment and water-level manipulation. In this study, we evaluated potential effects of water impoundment and seasonal water-level manipulation on the bioaccumulation, trophic transfer and health risk of HMs (Cu, Fe, Zn, Hg, Cd and Pb) in food web components (seston, aquatic invertebrate and fish) in TGR. Our results show that, after the impoundment for eight years (2003-2010), all of the six metal concentrations in aquatic biota fell within the criteria of safety quality guidelines. The concentrations of Cu, Fe, Zn and Hg in fish and aquatic invertebrates were higher than those before impoundment, whereas Cd and Pb were lower than those before impoundment. Nonetheless, Hg, Cd and Pb in aquatic consumers underwent an increasing trend during the entire impoundment, implying potential reservoir effect in the future. Only the concentrations of Hg, Cd and Pb in aquatic consumers exhibited a declining trend towards the dam, showing consistent with the background level at the three reaches. Seasonal variations in HM concentrations of fish and aquatic invertebrates were ascribed to the water-level manipulation associated with reservoir management. Our findings show that Hg or Cd biomagnified through aquatic food web during different hydrological periods, whereas Pb, Cu, Fe and Zn exhibited weak biomagnification power. Overall, Hg, Cd and Pb showed a higher risk than that of Cu, Fe and Zn.

Vanadium and thallium exhibit biodilution in a northern river food web

Trophic transfer of contaminants dictates concentrations and potential toxic effects in top predators, yet biomagnification behaviour of many trace elements is poorly understood. We examined concentrations of vanadium and thallium, two globally-distributed and anthropogenically-enriched elements, in a food web of the Slave River, Northwest Territories, Canada. We found that tissue concentrations of both elements declined with increasing trophic position as measured by δ¹⁵N. Slopes of log [element] versus δ¹⁵N regressions were both negative, with a steeper slope for V (-0.369) compared with Tl (-0.099). These slopes correspond to declines of 94% with each step in the food chain for V and 54% with each step in the food chain for Tl. This biodilution behaviour for both elements meant that concentrations in fish were well below values considered to be of concern for the health of fish-eating consumers. Further study of these elements in food webs is needed to allow a fuller understanding of biomagnification patterns across a range of species and systems.

Chemical and Physical Controls on Mercury Source Signatures in Stream Fish from the Northeastern United States

Streams in the northeastern U.S. receive mercury (Hg) in varying proportions from atmospheric deposition and legacy point sources, making it difficult to attribute shifts in fish concentrations directly back to changes in Hg source management. Mercury stable isotope tracers were utilized to relate sources of Hg to co-located fish and bed sediments from 23 streams across a forested to urban-industrial land-use gradient within this region. Mass-dependent isotopes (δ²⁰²Hg) in prey and game fish at forested sites were depleted (medians -0.95 and -0.83 ‰, respectively) in comparison to fish from urban-industrial settings (medians -0.26 and -0.38 ‰, respectively); the forested site group also had higher prey fish Hg concentrations. The separation of Hg isotope signatures in fish was strongly related to in-stream and watershed land-use indicator variables. Fish isotopes were strongly correlated with bed sediment isotopes, but the isotopic offset between the two matrices was variable due to differing ecosystem-specific drivers controlling the extent of MeHg formation. The multivariable approach of analyzing watershed characteristics and stream chemistry reveals that the Hg isotope composition in fish is linked to current and historic Hg sources in the northeastern U.S. and can be used to trace bioaccumulated Hg.

Factors influencing methylmercury contamination of black bass from California reservoirs

Understanding how mercury (Hg) accumulates in the aquatic food web requires information on the factors driving methylmercury (MeHg) contamination. This paper employs data on MeHg in muscle tissue of three black bass species (Largemouth Bass, Spotted Bass, and Smallmouth Bass) sampled from 21 reservoirs in California. During a two-year period, reservoirs were sampled for total Hg in sediment, total Hg and MeHg in water, chlorophyll a, organic carbon, sulfate, dissolved oxygen, pH, conductivity, and temperature. These data, combined with land-use statistics and reservoir morphometry, were used to investigate relationships to size-normalized black bass MeHg concentrations. Significant correlations to black bass MeHg were observed for total Hg in sediment, total Hg and MeHg in surface water, and forested area. A multivariate statistical model predicted Largemouth Bass MeHg as a function of total Hg in sediment, MeHg in surface water, specific conductivity, total Hg in soils, and forested area. Comparison to historical reservoir sediment data suggested there has been no significant decline in sediment total Hg at five northern California reservoirs during the past 20 years. Overall, total Hg in sediment was indicated as the most influential factor associated with black bass MeHg contamination. The results of this study improve understanding of how MeHg varies in California reservoirs and the factors that correlate with fish MeHg contamination.

Examining toxic trace element exposure in American alligators

Toxic trace element exposure occurs through release of the ubiquitous and naturally occurring elements arsenic (As), cadmium (Cd), lead (Pb), and mercury (Hg). The unique environmental conditions of the wetland ecosystems along the southeastern Atlantic coast of the United States lead to the accumulation of Hg which is greater than in most other ecosystems in the country. There are also point sources of As, Cd, and Pb in this region. To effectively monitor trace element concentrations, and consequently the potential human exposure, accessible local sentinel species are needed. In this study, concentrations of As, Cd, Pb, Hg and six other trace elements (Al, Ni, Cu, Zn, Se, Mo) were examined in American alligators (Alligator mississippiensis) from seven wetland sites in South Carolina and Florida and assessed for their utility as a sentinel species for human trace element exposure. Alligators were chosen as a potential sentinel as they share a common exposure with the local human population through their aquatic diet, and they are directly consumed commercially and through recreation hunting in this region. Sex was significantly related to the concentration of Zn, Mo, and Al, but not As, Pb, Hg, Cd, Se, or Cu. Site specific differences in element concentrations were observed for As, Pb, Hg, Cd, Se, Zn, and Mo. Size/age was significantly related to the element Hg and Pb concentrations observed. The observed concentration ranges for the four toxic elements, As (6-156 ng/g), Cd (0.3-1.3 ng/g), Pb (3-4872 ng/g), and Hg (39-2765 ng/g), were comparable to those previously reported in diverse human populations. In this region alligators are hunted recreationally and consumed by the local community, making them a vehicle of direct human toxic element exposure. We propose that the similarity in As, Cd, Pb, and Hg concentrations between alligators observed in this study and humans underscores how alligators can serve as a useful sentinel species for toxic element exposure.

Impact of biochar on mobilization, methylation, and ethylation of mercury under dynamic redox conditions in a contaminated floodplain soil

Mercury (Hg) is a highly toxic element, which is frequently enriched in flooded soils due to its anthropogenic release. The mobilization of Hg and its species is of ultimate importance since it controls the transfer into the groundwater and plants and finally ends in the food chain, which has large implications on human health. Therefore, the remediation of those contaminated sites is an urgent need to protect humans and the environment. Often, the stabilization of Hg using amendments is a reliable option and biochar is considered a candidate to fulfill this purpose. We tested two different pine cone biochars pyrolyzed at 200 °C or 500 °C, respectively, with a view to decrease the mobilization of total Hg (Hg_t), methylmercury (MeHg), and ethylmercury (EtHg) and/or the formation of MeHg and EtHg in a contaminated floodplain soil (Hg_t: 41 mg/kg). We used a highly sophisticated automated biogeochemical microcosm setup to systematically alter the redox conditions from ~-150 to 300 mV. We continuously monitored the redox potential (E_H) along with pH and determined dissolved organic carbon (DOC), SUVA₂₅₄, chloride (Cl^-), sulfate (SO₄^2-), iron (Fe), and manganese (Mn) to be able to explain the mobilization of Hg and its species. However, the impact of biochar addition on Hg mobilization was limited. We did not observe a significant decrease of Hg_t, MeHg, and EtHg concentrations after treating the soil with the different biochars, presumably because potential binding sites for Hg were occupied by other ions and/or blocked by biofilm. Solubilization of Hg bound to DOC upon flooding of the soils might have occurred which could be an indirect impact of E_H on Hg mobilization. Nevertheless, Hg_t, MeHg, and EtHg in the slurry fluctuated between 0.9 and 52.0 μg/l, 11.1 to 406.0 ng/l, and 2.3 to 20.8 ng/l, respectively, under dynamic redox conditions. Total Hg concentrations were inversely related to the E_H; however, ethylation of Hg was favored at an E_H around 0 mV while methylation was enhanced between -50 and 100 mV. Phospholipid fatty acid profiles suggest that sulfate-reducing bacteria may have been the principal methylators in our experiment. In future, various biochars should be tested to evaluate their potential in decreasing the mobilization of Hg and to impede the formation of MeHg and EtHg under dynamic redox conditions in frequently flooded soils.

Mercury bioaccumulation in stream food webs of the Finger Lakes in central New York State, USA

Studies of mercury (Hg) bioaccumulation in streams draining both forested and agricultural watersheds are not common. Sixteen streams were sampled in the Finger Lakes region in central New York State with a mean agricultural land cover of 48%. Stream fish (Blacknose Dace, Rhinichthys atratulus, an invertivore; and Creek Chub, Semotilus atromaculatus, an omnivore) were collected and analyzed for total Hg (THg), and macroinvertebrates and periphyton were collected and analyzed for methylmercury (MeHg) determination. The effect of water chemistry, land cover, and macroinvertebrate MeHg was assessed as predictors of fish Hg concentrations. Blacknose Dace had significantly higher THg concentrations compared to Creek Chub (229 ng/g vs. 195 ng/g dry weight, respectively), and predatory and omnivorous macroinvertebrates (i.e., Perlidae and Cambaridae) were found to have significantly higher MeHg concentrations compared to other functional feeding groups. Mixed effects models identified macroinvertebrate MeHg concentrations as predictors of stream fish THg concentrations. Partition modeling found fish total length and total suspended solids predicted Blacknose Dace with 'High' vs 'Low' Hg (≥ or < 90 ng/g wet weight, respectively). Overall, stream fish THg concentrations observed were not of concern, unlike other regions in New York State such as the Adirondack Mountains, but a significant proportion of Blacknose Dace (22 - 73%) and Creek Chub (5 - 69%) would be considered a risk to a range of sensitive consumers.

Element Concentrations in Tree Swallows (Tachycineta bicolor) from the U.S. and Binational Great Lakes Areas of Concern

Selected elements were targeted in state Remedial Action Plans as one group of chemicals affecting the Beneficial Use Impairments of Great Lakes Areas of Concern (AOCs). Livers of nestling tree swallows, Tachycineta bicolor, were harvested from 76 sites in the Great Lakes, which included multiple sites at 27 AOCs and 12 reference sites from 2010 to 2015, and were analyzed for 21 elements. Mercury concentrations were at background levels at all sites. Elevated cadmium (Cd) concentrations were associated with industry. The highest Cd values were from the Black River, OH AOC and were associated with historic coke production but were not at toxic levels. Lead (Pb) concentrations were highest on the Rouge River, MI AOC-the oldest and most heavily populated and industrialized area in southeast Michigan. Individual Pb concentrations were elevated to a level associated with delta-aminolaevulinic acid dehydratase inhibition but not to a level considered toxic. In contrast, livers harvested from sites on the southwest shore of Lake Michigan had selenium (Se) concentrations elevated to levels associated with reduced avian reproduction. One likely source of the high Se concentrations was pollution from a local coal-fired power plant. Concentrations of the remaining elements were at background levels.

Controls of Methylmercury Bioaccumulation in Forest Floor Food Webs

Compared to the extensive research on aquatic ecosystems, very little is known about the sources and trophic transfer of methylmercury (MeHg) in terrestrial ecosystems. In this study, we examine energy flow and trophic structure using stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios, respectively, and MeHg levels in basal resources and terrestrial invertebrates from four temperate forest ecosystems. We show that MeHg levels in biota increased significantly ( p < 0.01) with δ¹³C and δ¹⁵N at all sites, implying the importance of both microbially processed diets (with increased δ¹³C) and trophic level (with increased δ¹⁵N) at which organisms feed, on MeHg levels in forest floor biota. The trophic magnification slopes of MeHg (defined as the slope of log₁₀MeHg vs δ¹⁵N) for these forest floor food webs (0.20-0.28) were not significantly different ( p > 0.05) from those observed for diverse temperate freshwater systems (0.24 ± 0.07; n = 78), demonstrating for the first time the nearly equivalent efficiencies with which MeHg moves up the food chain in these contrasting ecosystem types. Our results suggest that in situ production of MeHg within the forest floor and efficient biomagnification both elevate MeHg levels in carnivorous invertebrates in temperate forests, which can contribute to significant bioaccumulation of this neurotoxin in terrestrial apex predators.

Mercury cycling and bioaccumulation in a changing coastal system: From water to aquatic organisms

This study evaluated -for the first time- the occurrence and distribution of total Hg in abiotic (dissolved and particulate water fractions) and biotic (gills and muscle tissues of six commercial fish species) compartments within a South American coastal environment with anthropogenic pressure. This study dealt with environmental issues on developing countries which, as they are in continuous growth, face the highest rate of coastal transformation. Total Hg content was determined by acid digestion and measured using a cold vapor atomic absorption spectrophotometer. Results revealed that dissolved Hg exceeded the recommended levels for superficial saline waters in 67% of the cases. Hg concentrations varied among fish species and its tissues. The results suggested that metal burden in fishes achieved a mean maximum accumulation towards the muscle, with concentrations below the international maximum permitted values. The estimated daily intakes calculated suggest that people would not experience significant health risks through fish consumption.

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.

Identifying contaminants of potential concern in remote headwater streams of Tennessee's Appalachian Mountains

The susceptibility of Tennessee's Appalachian Mountains to anthropogenic stressors has remained largely uninvestigated likely due to a lack of known point source contamination. However, a growing body of scientific evidence suggests that depositional inputs can lead to concerning levels of contamination, even in remote areas. To investigate potential concerns, water quality parameters, contaminants in water (nitrogen, TSS, and metals), and contaminants in eastern brook trout (mercury, polychlorinated biphenyls [PCBs], organochlorine [OC] pesticides, dioxins, furans, and phthalates) were measured in four Appalachian Mountain streams from 2015 to 2017. Concentrations were compared to literature and/or model-derived (e.g., biotic ligand model) threshold values to determine whether levels exceeded those acceptable for stream health. Dioxins and furans were detectable in fish tissue at all sites with an average 2,3,7,8-tetrachlorodinbenzodioxin toxicity equivalence (TEQ) of 0.0015 ng/kg. Concentrations of PCBs, phthalates, and organochlorine pesticides were never above analytical quantitation limits, although several OC pesticides (e.g., alpha-chlordane) were detectable in fish. Aluminum concentrations in water were found at levels shown previously to cause mortality in brook trout during acidic rain events. The average whole-body methylmercury concentrations in fish among sites were 0.037 ± 0.003 μg/kg and were on average 75 ± 2% of total mercury.

Improved Environmental Status: 50 Years of Declining Fish Mercury Levels in Boreal and Subarctic Fennoscandia

Temporally (1965-2015) and spatially (55°-70°N) extensive records of total mercury (Hg) in freshwater fish showed consistent declines in boreal and subarctic Fennoscandia. The database contains 54 560 fish entries ( n: pike > perch ≫ brown trout > roach ≈ Arctic charr) from 3132 lakes across Sweden, Finland, Norway, and Russian Murmansk area. 74% of the lakes did not meet the 0.5 ppm limit to protect human health. However, after 2000 only 25% of the lakes exceeded this level, indicating improved environmental status. In lakes where local pollution sources were identified, pike and perch Hg concentrations were significantly higher between 1965 and 1990 compared to values after 1995, likely an effect of implemented reduction measures. In lakes where Hg originated from long-range transboundary air pollution (LRTAP), consistent Hg declines (3-7‰ per year) were found for perch and pike in both boreal and subarctic Fennoscandia, suggesting common environmental controls. Hg in perch and pike in LRTAP lakes showed minimal declines with latitude, suggesting that drivers affected by temperature, such as growth dilution, counteracted Hg loading and food web exposure. We recommend that future fish Hg monitoring sampling design should include repeated sampling and collection of pollution history, water chemistry, fish age, and stable isotopes to enable evaluation of emission reduction policies.

Do biofilms affect the measurement of mercury by the DGT technique? Microcosm and field tests to prevent biofilm growth

The diffusive gradients in thin films (DGT) technique has been used routinely for monitoring the dissolved, bioavailable fraction of trace metals in freshwater during field campaigns. Nevertheless, for long deployment times, the biofilm formed on the filter of the DGT devices restricts trace metal uptake and hence interferes with the DGT measurements. In this work, we design different experiments to evaluate the potential of silver nanoparticles (AgNPs) in preventing the formation of biofilms on in-house manufactured mercury-specific DGTs. Laboratory tests were carried out by a microcosm system in independent glass containers, where biofilms obtained from field inocula were grown for weeks. Afterward, several experiments were performed with Hg-spiked river water, biofilms and DGTs treated and untreated with AgNPs to better understand biofilm colonization, inhibition and Hg uptake. The results showed that the treatment is very useful, since the mass of the biofilm accumulated at the surface of the treated DGT is significantly (p < 0.05) lower than in control (untreated) devices. Tests in colonized environments and Hg-spiked river water showed that the Hg uptake by the treated DGT matched the theoretical values and prevented biofilm formation up to 24 days post-deployment. Conversely, in deployments longer than two weeks using the untreated DGT, measurements could be underestimated by 35%. The results in the field reveal that in sampling stations with high levels of suspended matter, the filter becomes clogged despite there being no biofilm, thereby explaining its low efficiency for the uptake of Hg. In summary, the use of AgNPs inhibits biofilm formation and their use is especially recommended in eutrophic freshwaters with low amounts of suspended particulate matter.

Mercury Biomagnification Through a Coral Reef Ecosystem

Total mercury (Hg) and stable isotopes of nitrogen and carbon were determined in the muscle tissue of 50 species of fishes and invertebrates collected at two sites along the Florida reef tract from April 2012 to December 2013. The objective was to test the hypothesis that high biodiversity in coral reefs leading to complex food webs with increased lateral links reduces biomagnification. However, Hg levels ranged as high 6.84 mg/kg. Interestingly, it was not highest in great barracuda (Sphyraena barracuda), considered the top predatory fish, but instead in small porkfish (Anisotremus virginicus), possibly due to their role as a cleaner fish. Trophic magnification slopes (TMS; from regression of log Hg on δ¹⁵N) as a measure of biomagnification did not differ between sites, ranging from 0.155 ± 0.04 (± 95% CI) to 0.201 ± 0.07. These TMS also were within the ranges of slopes reported for food webs in other ecosystems; thus, biomagnification of Hg in muscle tissue was not reduced in the system.

Drought promotes increases in total mercury and methylmercury concentrations in fish from the lower Paraíba do Sul river, southeastern Brazil

Bioaccumulation of mercury and methylmercury in fish represents a serious risk to human beings. Extreme climate events like droughts may increase the trophic transfer of contaminants and net methylation of mercury. The present study assessed the influence of the 2014 drought on total mercury and methylmercury levels in fish from the lower Paraiba do Sul river basin. Contaminant levels were compared for Pimelodus fur, Pachyurus adspersus, Pimelodella lateristriga, Oligosarcus hepsetus, and Crenicichla lacustris captured in five sites in 2013 (N = 212) and 2014 (N = 231). The results indicate that levels of contaminants were higher during the drought in most species. Rainfall was weakly and negatively correlated with total mercury levels in most of the species. The weak relationship between these two variables was due to the indirect influence of rainfall on mercury bioaccumulation. In summary, drought increased the levels of two contaminants in fish.

Mercury in fish from Norwegian lakes: The complex influence of aqueous organic carbon

Mercury (Hg) concentrations in water and biota are often positively correlated to organic matter (OM), typically measured as total or dissolved organic carbon (TOC/DOC). However, recent evidence suggests that higher OM concentrations inhibit bioaccumulation of Hg. Here, we test how TOC impacts the Hg accumulation in fish in a synoptic study of Methyl-Hg (MeHg) in water and total Hg (THg) in perch (Perca fluviatilis) in 34 boreal lakes in southern Norway. We found that aqueous MeHg (r² = 0.49, p < 0.0001) and THg (r² = 0.69, p < 0.0001), and fish THg (r² = 0.26, p < 0.01) were all positively related with TOC. However, we found declining MeHg bioaccumulation factors (BAF_MeHg) for fish with increasing TOC concentrations. The significant correlation between fish THg concentrations and aqueous TOC suggests that elevated fish Hg levels in boreal regions are associated with humic lakes. The declining BAF_MeHg with increasing TOC suggest that increased OM promotes increased aqueous Hg concentrations, but lowers relative MeHg bioaccumulation. A mechanistic understanding of the response from OM on BAF_MeHg might be found in the metal-complexation properties of OM, where OM complexation of metals reduces their bioavailability. Hence, suggesting that MeHg bioaccumulation becomes less effective at higher TOC, which is particularly relevant when assessing potential responses of fish Hg to predicted future changes in OM inputs to boreal ecosystems. Increased browning of waters may affect fish Hg in opposite directions: an increase of food web exposure to aqueous Hg, and reduced bioavailability of Hg species. However, the negative relationship between BAF_MeHg and TOC is challenging to interpret, and carries a great deal of uncertainty, since this relationship may be driven by the underlying correlation between TOC and MeHg (i.e. spurious correlations). Our results suggest that the trade-off between Hg exposure and accumulation will have important implications for the effects of lake browning on Hg transport, bioavailability, and trophodynamics.