The basis for ecotoxicological concern in aquatic ecosystems contaminated by historical mercury mining

Functional genes and microorganisms controlling in situ methylmercury production and degradation in marine sediments: A case study in the Eastern China Coastal Seas

Dominant microorganisms and functional genes, including hgcA, hgcB, merA, and merB, have been identified to be responsible for mercury (Hg) methylation or methylmercury (MeHg) demethylation. However, their in situ correlation with MeHg levels and the processes of Hg methylation and MeHg demethylation in coastal areas remains poorly understood. In this study, four functional genes related to Hg methylation and MeHg demethylation (hgcA, hgcB, merA, and merB) were all detected in the sediments of the Eastern China Coastal Seas (ECCSs) (representative coastal seas highly affected by human activities) using metagenomic approaches. HgcA was identified to be the key gene controlling the in situ net production of MeHg in the ECCSs. Based on metagenomic analysis and incubation experiments, sulfate-reducing bacteria were identified as the dominant microorganisms controlling Hg methylation in the ECCSs. In addition, hgcA gene was positively correlated with the MeHg content and Hg methylation rates, highlighting the potential roles of Hg methylation genes and microorganisms influenced by sediment physicochemical properties in MeHg cycling in the ECCSs. These findings highlighted the necessity of conducting similar studies in other natural systems for elucidating the molecular mechanisms underlying MeHg production in aquatic environments.

Review: Abandoned mines as a resource or liability for wildlife

Abandoned Mine Lands (AMLs) are areas where previous mineral extraction or processing has occurred. Hundreds of thousands of AMLs exist within the United States. Contaminated runoff from AMLs can negatively affect the physiology and ecology of surrounding terrestrial and aquatic habitats and species and can be detrimental to human health. As a response, several U.S. federal and state agencies have launched programs to assess health risks associated with AMLs. In some cases, however, AMLs may be beneficial to specific wildlife taxa. There is a relative paucity of studies investigating the physiological and ecological impacts of AMLs on wildlife. We conducted a systematic review examining published scientific articles that assessed the negative and positive impacts of AMLs across invertebrate and vertebrate taxa. We also offer suggestions on evaluating AMLs to develop effective mitigation strategies that reduce their negative tole on human and wildlife communities. Peer-reviewed publications were screened across WebofScience, PubMed and Google Scholar databases. Abandoned mine lands were generally detrimental to wildlife, with adverse effects ranging from bioaccumulation of heavy metals to decreased ecological fitness. Conversely, AMLs were an overall benefit to imperiled bat populations and could serve as tools for conservation. Studies were unevenly distributed across different wildlife taxa groups, echoing the necessity for additional taxonomically diverse research. We suggest that standardized wildlife survey methods be used to assess how different species utilize AMLs. Federal and state agencies can use these surveys to establish effective remediation plans for individual AML sites and minimize the risks to both wildlife and humans.

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.

Activated carbon thin-layer placement as an in situ mercury remediation tool in a Penobscot River salt marsh

The efficacy of thin layer in situ soil amendments was tested as a potential tool for methylmercury (MeHg) risk mitigation in Penobscot River, ME, salt marsh. Salt marshes are sites of high MeHg accumulation within the Penobscot, and key targets for remediation. The study was a fully-crossed small plot study, with four treatments (activated carbon (AC), biochar, FeCl₂, and lime) and unamended controls at two sites. Plots were monitored for two years. Porewater MeHg concentrations were the main endpoint, with impacts on sediment biogeochemistry as a secondary study goal. AC-based SediMite™ was effective in reducing MeHg, and to a less extent total Hg, in surficial pore waters. AC reduced MeHg concentrations by >90% at the one month time point, and continued to significantly reduce pore water MeHg through about a year. AC was less effective in reducing total Hg in pore water, yielding about 70% reduction at one month, and 50-65% reduction at 8months. Biochar provided lower, and more variable reduction in porewater MeHg, but was not effective in reducing total Hg. Biochar amendment also increased soil MeHg. Neither FeCl₂ nor lime amendments reduced pore water Hg or MeHg levels. About 50% of AC treatment applied as SediMite™ pellets was retained in marsh soils after one year. This study is one of the first field trials of in situ amendment for MeHg remediation. Our results show that thin-layer AC placement is a potential remediation tool for Hg risk to biota, especially in marshes where net MeHg accumulation is often strong.

Evaluating blood and excrement as bioindicators for metal accumulation in birds

Birds are widely used to assess metal contamination in the environment and there are different approaches to determine the exposure level in individuals, some being destructive (collection of soft tissues) and some non-destructive (blood, feathers and excrement). The use of blood to detect internal concentrations of metals is an acknowledged method, but to what extent blood can predict the concentrations in soft tissues has been less well evaluated in wild terrestrial birds. The same is true for excrements. This study compares the non-destructive methods using blood and excrement with liver sampling, with respect to exposure and accumulation of the elements arsenic, cadmium, copper, lead and zinc in nestling pied flycatchers (Ficedula hypoleuca). Blood, liver and excrement reflected the environmental exposure of non-essential elements and were independent of nestling sex. There were asymptotic relationships between the concentration of arsenic, cadmium and lead in liver and blood, excrement and liver, and excrement and blood, but none for copper or zinc. Those relationships were generally stronger between liver and blood than between excrements and internal concentrations. Lead had the strongest associations for all matrixes. The conclusion is that blood is an appropriate tool to assess accumulation of arsenic, cadmium and especially lead, but that blood can underestimate the accumulation at highly contaminated sites. Excrement can also give an indication of metal accumulation, but may overestimate internal concentrations at high exposure, and individual variability makes direct comparisons between these matrices less appropriate.

Biomagnification of Mercury in Fish from Two Gold Mining-Impacted Tropical Marshes in Northern Colombia

Total mercury concentrations (T-Hg) and stable isotope ratios (δ¹⁵N and δ¹³C) were analyzed in the muscle of fish, collected at two marshes impacted by gold mining in Northern Colombia, to evaluate the seasonal and spatial behavior of these variables and mercury biomagnification based on their relationship with δ¹⁵N ratios in the fish. There was not significant (p > 0.05) seasonal differences (dry and rainy season) in δ¹⁵N and δ¹³C values in none marsh, suggesting that these fish species keep a similar feed pattern within each marsh along the year. However, there were significant differences (p < 0.05) between marshes in δ¹⁵N, δ¹³C, and T-Hg, as well as significant seasonal differences (p < 0.05) in T-Hg values for some species, suggesting the possible influence of factors, such as differences in length of food chains between marshes and different degree of anthropogenic impact in relation to human settlements, farmland, livestock (higher in Ayapel marsh), and gold mining activity (higher in La Raya marsh). Correlations analysis showed significant (p < 0.05) relationships between T-Hg concentrations and δ¹⁵N values, as well as trophic level, evidencing a T-Hg biomagnification process in the fish food chain from both marshes and consequently a potential health risk for human riverside inhabitants who eat these fish.

Comparisons of field and laboratory estimates of risk of DDTs from contaminated sediments to humans that consume fish in Palos Verdes, California, USA

Calculating risk from seafood exposure to persistent organic pollutants continues to be problematic as estimates of exposure from diet require extensive monitoring of fish species and limited assessments of bioavailability from sediments where the contaminants tend to reside. Previous studies in our laboratory utilized a laboratory-based isotope dilution method (IDM) to estimate the bioavailability of DDT [1,1,1-trichloro-2, 2-bis(p-chloro-phenyl)ethane] and its metabolites from sediment to biota from a superfund site on the shelf of the Palos Verdes (PVS) Peninsula in California (USA). Using a biota-sediment accumulation factor (BSAF) derived from IDM and biomagnification factors (BMF) calculated from previous studies as well as seafood-consumption data specific to anglers in the PVS area, we estimated cancer and non-cancer risks for anglers and nursing infants representing sensitive groups. Predicted cancer risks from consumption of White croaker (Genyonemus lineatus) to the 50th and 95th percentile to all shore mode anglers were, respectively, 2×10-7 and 7×10-7, which were similar to field studies using fish concentrations of all DDT isomers and their environmental degradates (ΣDDT) from collected animals. The calculated non-cancer hazard quotient values for the 50th and 95th percentile shore mode anglers consuming White croaker from this study (0.008 and 0.023, respectively) were also of similar magnitude as those obtained from studies based on samples obtained solely from fish. For nursing infants, similar results were also observed. These results indicate that estimates of bioavailability using IDM from sediment could be used accurately to determine risk to ΣDDT in humans from fish consumption.

Characterisation of artisanal mine waste on Buru Island, Indonesia and toxicity to the brittle star Amphipholis squamata

Artisanal small-scale gold mining (ASGM) using mercury (Hg) amalgamation commenced on Buru Island, Indonesia, in 2012, but was halted in 2015 due to concerns of widespread Hg contamination. Much of the Hg used in the mining process is lost to trommel waste which is disposed of in settlement ponds that drain into adjacent waterways and into Kayeli Bay. Several thousand unmanaged trommel sites and associated tailing ponds exist on Buru Island. This study shows that waste from the Marloso trommel at the Gogrea site contained 203 mg/kg total Hg (THg), with a negligible proportion present as bioavailable methyl Hg (MeHg) and a low total organic carbon content. There are currently very few tools available for ecotoxicological risk assessment of mine tailings for tropical marine ecosystems, and we support the development of Tailings Toxicity Tests (TTTs) and describe laboratory toxicity test methods using the cosmopolitan benthic echinoderm Amphipholis squamata. Undiluted trommel waste caused 100% mortality of A. squamata within 48 h, and a 96-h LC50 of 6.7% w/w trommel waste (4 mg/kg THg) was estimated. Sub-lethal effects on the water vascular system of the brittle star were assessed by quantification of the Ability to Right Itself (ARI), and a 48-h EC50 of 7.3% w/w trommel waste (14.4 mg/kg THg) was estimated. The results show that trommel waste produced on Buru Island is highly contaminated with THg and is acutely toxic, raising serious concern for receiving ecosystems where Hg methylation to more toxic and bioavailable forms is likely.

A tiered assessment framework to evaluate human health risk of contaminated sediment

For sediment contaminated with bioaccumulative pollutants (e.g., PCBs and organochorine pesticides), human consumption of seafood that contain bioaccumulated sediment-derived contaminants is a well-established exposure pathway. Historically, regulation and management of this bioaccumulation pathway has focused on site-specific risk assessment. The state of California (United States) is supporting the development of a consistent and quantitative sediment assessment framework to aid in interpreting a narrative objective to protect human health. The conceptual basis of this framework focuses on 2 key questions: 1) do observed pollutant concentrations in seafood from a given site pose unacceptable health risks to human consumers? and 2) is sediment contamination at a site a significant contributor to seafood contamination? The first question is evaluated by interpreting seafood tissue concentrations at the site, based on health risk calculations. The second question is evaluated by interpreting site-specific sediment chemistry data using a food web bioaccumulation model. The assessment framework includes 3 tiers (screening assessment, site assessment, and refined site assessment), which enables the assessment to match variations in data availability, site complexity, and study objectives. The second and third tiers use a stochastic simulation approach, incorporating information on variability and uncertainty of key parameters, such as seafood contaminant concentration and consumption rate by humans. The framework incorporates site-specific values for sensitive parameters and statewide values for difficult to obtain or less sensitive parameters. The proposed approach advances risk assessment policy by incorporating local data into a consistent region-wide problem formulation, applying best available science in a streamlined fashion.

Urban sediment contamination in a former Hg mining district, Idrija, Slovenia

Road sediments from gully pots of the drainage system and stream sediments from local streams were investigated for the first time in the urban area of Idrija town, the central part of the second largest and strongly contaminated Hg mining district in the world. Hg concentrations in road sediments were lower than in stream sediments. They ranged from 16 to 110 mg/kg (Md = 29 mg/kg) for <0.125 mm particles and from 7 to 125 mg/kg (Md = 35 mg/kg) for <0.04 mm particles, while Hg concentrations in stream sediments ranged from 10 to 610 mg/kg (Md = 95 mg/kg) for <0.125 particles and from 10 to 440 mg/kg (Md = 105 mg/kg) for <0.04 mm particles. High Hg loadings in stream sediments were successfully linked with identified mercury sources (rocks containing mercury ore, areas of former ore roasting sites, ore residue dumps), because they are located in the drainage areas of streams, from which the sediments were collected. Links between Hg loadings in road sediments and identified mercury sources were not recognized. Solid phases of Hg were determined by thermo-desorption technique and are similar for both types of sediments. Results show the occurrence of three different forms: elemental mercury, mercury bound to matrix components and cinnabar. Approximately 50 % of Hg in samples consist of non-cinnabar fractions. This is important, since they are potentially bioavailable. An interesting new discovery according to previous research of environmental media from Idrija area by solid-phase Hg thermo-desorption technique is that elemental mercury was determined in almost all investigated sediments in minor amounts (Md = 3 %).

Pre-anthropocene mercury residues in North American freshwater fish

Mercury (Hg) has been entering the environment from both natural and anthropogenic sources for millennia, and humans have been influencing its environmental transport and fate from well before the Industrial Revolution. Exposure to Hg (as neurotoxic monomethylmercury [MeHg]) occurs primarily through consumption of finfish, shellfish, and marine mammals, and regulatory limits for MeHg concentrations in fish tissue have steadily decreased as information on its health impacts has become available. These facts prompted us to consider 2 questions: 1) What might the MeHg levels in fish tissue have been in the pre-Anthropocene, before significant human impacts on the environment? and 2) How would these pre-Anthropocene levels have compared with current regulatory criteria for MeHg residues in fish tissue? We addressed the first question by estimating pre-Anthropocene concentrations of MeHg in the tissues of prey and predatory fish with an integrated Hg speciation, transport, fate, and food web model (SERAFM), using estimated Hg concentrations in soil, sediment, and atmospheric deposition before the onset of significant human activity (i.e., ≤2000 BCE). Model results show MeHg residues in fish varying depending on the characteristics of the modeled water body, which suggests that Hg in fish tissue is best considered at the scale of individual watersheds or water bodies. We addressed the second question by comparing these model estimates with current regulatory criteria and found that MeHg residues in predatory (but not prey) fish could have approached or exceeded these criteria in some water bodies during the pre-Anthropocene. This suggests that the possibility of naturally occurring levels of Hg in fish below which it is not possible to descend, regardless of where those levels stand with respect to current regulatory limits. Risk management decisions made under these circumstances have the potential to be ineffectual, frustrating, and costly for decision makers and stakeholders alike, suggesting the need for regulatory flexibility when addressing the issue of Hg in fish.

Activated carbon mitigates mercury and methylmercury bioavailability in contaminated sediments

There are few available in situ remediation options for Hg contaminated sediments, short of capping. Here we present the first tests of activated carbon and other sorbents as potential in situ amendments for remediation of mercury and methylmercury (MeHg), using a study design that combined 2 L sediment/water microcosms with 14 day bioaccumulation assays. Our key end points were pore water concentrations, and bioaccumulation of total Hg and MeHg by a deposit-feeding oligochaete Lumbriculus variegatus. Four amendments were tested: an activated carbon (AC); CETCO Organoclay MRM (MRM); Thiol-SAMMS (TS), a thiol-functionalized mesoporous silica; and AMBERSEP GT74, an ion-exchange resin. Amendments were tested in four separate microcosm assays using Hg-contaminated sediments from two freshwater and two estuarine sites. AC and TS amendments, added at 2-7% of the dry weight of sediments significantly reduced both MeHg concentrations in pore waters, relative to unamended controls (by 45-95%) and bioaccumulation of MeHg by Lumbriculus (by between 30 and 90%). Both amendments had only small impacts on microcosm surface water, sediment and pore water chemistry, with the exception of significant reductions in pore water dissolved organic matter. The effectiveness of amendments in reducing bioaccumulation was well-correlated with their effectiveness in increasing sediment:water partitioning, especially of MeHg. Sediments with low native sediment:water MeHg partition coefficients were most effectively treated. Thus, in situ sediment sorbent amendments may be able to reduce the risk of biotic Hg and MeHg uptake in contaminated sediments, and subsequent contamination of food webs.

Mercury methylation by novel microorganisms from new environments

Microbial mercury (Hg) methylation transforms a toxic trace metal into the highly bioaccumulated neurotoxin methylmercury (MeHg). The lack of a genetic marker for microbial MeHg production has prevented a clear understanding of Hg-methylating organism distribution in nature. Recently, a specific gene cluster (hgcAB) was linked to Hg methylation in two bacteria.1 Here we test if the presence of hgcAB orthologues is a reliable predictor of Hg methylation capability in microorganisms, a necessary confirmation for the development of molecular probes for Hg-methylation in nature. Although hgcAB orthologues are rare among all available microbial genomes, organisms are much more phylogenetically and environmentally diverse than previously thought. By directly measuring MeHg production in several bacterial and archaeal strains encoding hgcAB, we confirmed that possessing hgcAB predicts Hg methylation capability. For the first time, we demonstrated Hg methylation in a number of species other than sulfate- (SRB) and iron- (FeRB) reducing bacteria, including methanogens, and syntrophic, acetogenic, and fermentative Firmicutes. Several of these species occupy novel environmental niches for Hg methylation, including methanogenic habitats such as rice paddies, the animal gut, and extremes of pH and salinity. Identification of these organisms as Hg methylators now links methylation to discrete gene markers in microbial communities.

Environmental exposure to lead and mercury in Mexican children: a real health problem

Exposure to lead (Pb) and mercury (Hg) remains a world public health problem, particularly for young children in developing countries. In Mexico, the main sources of exposure to Pb and Hg are wastes from human activities that increase the natural sources of these metals. Pb and Hg are highly toxic during development and maturation periods of the central nervous system (CNS); these effects are associated with the risk for neurodegenerative diseases. Mexico has numerous exposure sources to Pb and Hg; nevertheless, information on exposure in children is limited, particularly for Hg. Therefore, we conducted a review of the studies performed in children exposed to Pb and Hg. Data presented support that an important proportion of Mexican children have Pb levels above values associated with dangerous effects. On the other hand, studies on Hg-exposure are scarce, so we need more studies to estimate the magnitude of the problem and to determine exposure levels in Mexican children. Available data support the urgent need for coordinated actions among researchers, and health and environmental government authorities to implement education and nutritional campaigns, as well as to decrease exposure and effects of Pb and Hg. In addition, there must be a priority for the implementation of educational campaigns directed to the general population, but with emphasis in parents, education staff and health care providers to decrease both the risk of exposure of children to Pb and Hg and the effects of the exposure to these metals.

Fate and developmental effects of dietary uptake of methylmercury in Silurana tropicalis tadpoles

Adverse effects of methylmercury (MeHg) exposure during amphibian metamorphosis remain to be fully characterized. Most previous investigations determined effects of short-term exposure to elevated dose rates, without information on mercury (Hg) depuration and degradation pathways. Since metamorphosis is primarily controlled by thyroid hormones (TH), alterations in this process suggest a disruption of the TH endocrine axis. The aim of this research was to (1) characterize patterns of MeHg accumulation and depuration in tadpoles and (2) examine effects of MeHg accumulation on metamorphosis and the TH axis. Silurana tropicalis tadpoles were exposed to environmental levels of dietary MeHg until metamorphic climax. Whole-body MeHg and total Hg (THg) levels were measured, as well as the number of metamorphs, rate of metamorphosis, body size, and whole-body triiodothyronine (T3) levels at metamorphosis. Tadpoles exposed to a higher level of MeHg exhibited increased mortality and size, and reduced metamorphosis. At lower levels of MeHg, body burdens increased rapidly and eventually reached a plateau, whereas no plateau was reached at a higher level of MeHg exposure. T3 levels were not affected. Data indicate that at low and medium levels of exposure, depuration of MeHg may prevent toxicity in tadpoles. However, depuration mechanisms may be insufficient at high doses, producing disruption of metamorphosis and death. Although there were no marked effects of MeHg on whole-body T3 levels, further investigation of other components of the TH axis is warranted.

Spatial and seasonal mercury distribution in the Ayapel Marsh, Mojana region, Colombia

Geographical and temporal changes of total mercury (T-Hg) concentrations in sediments, macrophytes and fish were evaluated in the Ayapel Marsh, Mojana, Colombia. Sampling was conducted during 2006-2007, including both rainy and dry seasons, and T-Hg was measured using atomic absorption spectroscopy. Distribution of T-Hg in sediments and the macrophyte Eichhornia crassipes showed that higher concentrations were found along the flooding pathway of the Cauca River. Hg has also polluted the food chain. Highest T-Hg concentrations in fish were detected for the carnivorous Pseudoplatystoma fasciatun (0.432 ± 0.107 μg/g, fresh weight [fw]), and lowest in the non-carnivorous Prochilodus magdalenae (0.143 ± 0.053 μg/g, fw), with differences between trophic levels. T-Hg for fish samples (0.298 ± 0.148 μg/g, fw) did not exceed the limit consumption level (0.50 μg/g, fw). It is concluded that mercury-polluted sediments from the Cauca River, contribute to Hg deposition into the Ayapel Marsh. Accordingly, interventions must be conducted to decrease Hg accumulation in fish.

Mercury bioaccumulation in a stream network

Mercury (Hg) contamination is common in stream and river ecosystems, but factors mediating Hg cycling in the flowing waters are much less understood than inthe lakes and wetlands. In this study, we examined the spatial patterns of methylmercury (MeHg) concentrations in the dominant groups of aquatic insect larvae across a network of streams (drainage area ranging from 0.5 to 150 km2) in northern California during summer baseflow conditions. We found that, with the exception of water striders, all invertebrate groups showed significant (p < 0.05) increases in MeHg concentrations with drainage area. The largest stream in our study watershed, the South Fork Eel River, had the highest aqueous MeHg concentration (unfiltered: 0.13-0.17 ng L(-1)) while most of the upstream tributaries had aqueous MeHg concentrations close to or below the established detection limits (0.02 ng L(-1)). A filamentous alga abundant in South Fork Eel River (Cladophora glomerata) had an exceptionally high fraction of total-Hg as MeHg (i.e., %MeHg from 50-100%). Since other potential hotspots of in-stream Hg methylation (e.g., surface sediment and deep pools) had %MeHg lower than or similar to surface water (approximately 14%), we hypothesize that Cladophora and possibly other autotrophs may serve as hotspots of in-stream MeHg production in this bedrock-dominated stream. Recent studies in other regions concluded that wetland abundance in the watershed is the predominant factor in governing Hg concentrations of stream biota. However, our results show that in the absence of wetlands, substantial spatial variation of Hg bioaccumulation can arise in stream networks due to the influence of in-stream processes.

The legacy of mercury cycling from mining sources in an aquatic ecosystem: from ore to organism

Clear Lake is the site of an abandoned mercury (Hg) mine (active intermittently from 1873 to 1957), now a U.S. Environmental Protection Agency Superfund Site. Mining activities, including bulldozing waste rock and tailings into the lake, resulted in approximately 100 Mg of Hg entering the lake's ecosystem. This series of papers represents the culmination of approximately 15 years of Hg-related studies on this ecosystem, following Hg from the ore body to the highest trophic levels. A series of physical, chemical, biological, and limnological studies elucidate how ongoing Hg loading to the lake is influenced by acid mine drainage and how wind-driven currents and baroclinic circulation patterns redistribute Hg throughout the lake. Methylmercury (MeHg) production in this system is controlled by both sulfate-reducing bacteria as well as newly identified iron-reducing bacteria. Sediment cores (dated with dichlorodiphenyldichlorethane [DDD], 210pb, and 14C) to approximately 250 cm depth (representing up to approximately 3000 years before present) elucidate a record of total Hg (TotHg) loading to the lake from natural sources and mining and demonstrate how MeHg remains stable at depth within the sediment column for decades to millenia. Core data also identify other stresses that have influenced the Clear Lake Basin especially over the past 150 years. Although Clear Lake is one of the most Hg-contaminated lakes in the world, biota do not exhibit MeHg concentrations as high as would be predicted based on the gross level of Hg loading. We compare Clear Lake's TotHg and MeHg concentrations with other sites worldwide and suggest several hypotheses to explain why this discrepancy exists. Based on our data, together with state and federal water and sediment quality criteria, we predict potential resulting environmental and human health effects and provide data that can assist remediation efforts.

Changes in fish diets and food web mercury bioaccumulation induced by an invasive planktivorous fish

The invasion, boom, collapse, and reestablishment of a population of the planktivorous threadfin shad in Clear Lake, California, USA, were documented over a 20-year period, as were the effects of changing shad populations on diet and mercury (Hg) bioaccumulation in nearshore fishes. Threadfin shad competitively displaced other planktivorous fish in the lake, such as inland silversides, young-of-year (YOY) largemouth bass, and YOY bluegill, by reducing zooplankton abundance. As a result, all three species shifted from a diet that was dominated by zooplankton to one that was almost entirely zoobenthos. Stable carbon isotopes corroborated this pattern with each species becoming enriched in delta13C, which is elevated in benthic vs. pelagic organisms. Concomitant with these changes, Hg concentrations increased by approximately 50% in all three species. In contrast, obligate benthivores such as prickly sculpin showed no relationship between diet or delta13C and the presence of threadfin shad, suggesting that effects of the shad were not strongly linked to the benthic fish community. There were also no changes in Hg concentrations of prickly sculpin. The temporary extirpation of threadfin shad from the lake resulted in zooplankton densities, foraging patterns, isotope ratios, and Hg concentrations in pelagic fishes returning to pre-shad values. These results indicate that even transient perturbations of the structure of freshwater food webs can result in significant alterations in the bioaccumulation of Hg and that food webs in lakes can be highly resilient.

Spatiotemporal trends in fish mercury from a mine-dominated ecosystem: Clear Lake, California

Clear Lake, California, USA, receives acid mine drainage and mercury (Hg) from the Sulphur Bank Mercury Mine, a U.S. Environmental Protection Agency (U.S. EPA) Superfund Site that was active intermittently from 1873 to 1957 and partially remediated in 1992. Mercury concentrations were analyzed primarily in four species of Clear Lake fishes: inland silversides (Menidia beryllina, planktivore), common carp (Cyprinus carpio, benthic scavenger/omnivore), channel catfish (Ictalurus punctatus, benthic omnivorous predator), and largemouth bass (Micropterus salmoides, piscivorous top predator). These data represent one of the largest fish Hg data sets for a single site, especially in California. Spatially, total Hg (TotHg) in silversides and bass declined with distance from the mine, indicating that the mine site represents a point source for Hg loading to Clear Lake. Temporally, fish Hg has not declined significantly over 12 years since mine site remediation. Mercury concentrations were variable throughout the study period, with no monotonic trends of increase or decrease, except those correlated with boom and bust cycles of an introduced fish, threadfin shad (Dorosoma petenense). However, stochastic events such as storms also influence juvenile largemouth bass Hg as evidenced during an acid mine drainage overflow event in 1995. Compared to other sites regionally and nationally, most fish in Clear Lake exhibit Hg concentrations similar to other Hg-contaminated sites, up to approximately 2.0 mg/kg wet mass (WM) TotHg in largemouth bass. However, even these elevated concentrations are less than would be anticipated from such high inorganic Hg loading to the lake. Mercury in some Clear Lake largemouth bass exceeded all human health fish consumption guidelines established over the past 25 years by the U.S. Food and Drug Administration (1.0 mg/kg WM), the National Academy of Sciences (0.5 mg/kg WM), and the U.S. EPA (0.3 mg/kg WM). Mercury in higher trophic level fishes exceeds ecotoxicological risk assessment estimates for concentrations that would be safe for wildlife, specifically the nonlisted Common Merganser and the recently delisted Bald Eagle. Fish populations of 11 out of 18 species surveyed exhibited a significant decrease in abundance with increasing proximity to the mine; this decrease is correlated with increasing water and sediment Hg. These trends may be related to Hg or other lake-wide gradients such as distribution of submerged aquatic vegetation.

Mercury in abiotic matrices of Clear Lake, California: human health and ecotoxicological implications

Mercury (Hg) from Hg mining at Clear Lake, California, USA, has contaminated water and sediments for over 130 years and has the potential to affect human and environmental health. With total mercury (TotHg) concentrations up to 438 mg/kg (dry mass) in surficial sediments and up to 399 ng/L in lake water, Clear Lake is one of the most Hg-contaminated lakes worldwide. Particulate Hg in surface water near the mine ranges from 10,000 to 64,000 ng/g; TotHg declines exponentially with distance from the Sulphur Bank Mercury Mine. From 1992 to 1998, no significant long-term trends for TotHg or methylmercury (MeHg) in sediments or water were observed, but peaks of both TotHg and MeHg occurred following a 1995 flooding event. Sediments and water exhibit summer/fall maxima and winter/spring minima for MeHg, but not TotHg. Sediment TotHg has not declined significantly a decade after remediation in 1992. At the mine site, aqueous TotHg reached 374,000 ng/L in unfiltered groundwater. Pore water sulfate in sediments varies seasonally from 112 mg/L in summer/fall (when Hg methylation is highest) to 3300 mg/L in winter. While TotHg is exceptionally high in both sediments and water, MeHg is substantially lower than would be expected based on the bulk Hg loading to the lake and in comparison with other sites worldwide. Total mercury in Clear Lake water does not exceed the Safe Drinking Water Act criteria, but it sometimes greatly exceeds human health criteria established by the Great Lakes Initiative, U.S. Environmental Protection Agency water quality guidelines, and the California Toxics Rule criterion. Methylmercury concentrations exceed the Great Lakes Initiative criterion for MeHg in water at some sites only during summer/fall. Relative to ecological health, Clear Lake sediments greatly exceed the National Oceanic and Atmospheric Administration's benthic fauna Sediment Quality Guidelines for toxic effects, as well as the more concensus-based Threshold Effects Concentration criteria. Based on these criteria, Hg-contaminated sediments and water from Clear Lake are predicted to have some lethal and sublethal effects on specific resident aquatic species. However, based on unique physical and chemical characteristics of the Clear Lake environment, MeHg toxicity may be significantly less than anticipated from the large inorganic Hg loading.