MeHg In Sediments Research Articles

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.

Mercury (Hg) and methylmercury (MeHg) in sediment and biota: A case study in a lagoon in Central Italy

A quantification of total mercury (Hgtot) and methylmercury (MeHg) concentrations in sediment and mussels was carried out in the east basin of the Orbetello lagoon in order to assess their bioaccumulation potential. The sediment was sampled in four macroareas, mussels were transplanted in the same sites and collected after seven weeks. The results show that Hgtot concentrations in sediments exceeded (0.21–16.9 mg/kg dry weight (dw)) the environmental quality standard of the Italian legislation (0.3 mg/kg dw). The Hgtot concentration in mussels (0.050–0.324 mg/kg wet weight (ww)) does not exceed the limit values (0.5 mg/kg ww) of the European food legislation. The biota–sediment accumulation factors (BSAFs) derived for MeHg (80–306.7) and a biomagnification factor (BMF) greater than 1 for Hgtot demonstrate that in the lagoon, these compounds can be transferred in the upper levels of the trophic chain and pose a risk to human health.

Formation and availability of methylmercury in mercury-contaminated sediment: effects of activated carbon and biochar amendments

PurposeAs the formation of toxic and bioaccumulative methylmercury (MeHg) in Hg-contaminated sediments is of great concern worldwide, suitable remediation options are needed. Activated carbon (AC) amendment is a contested alternative due to uncertainties surrounding sorption efficiency and its potential role in aiding MeHg formation. The purpose of this study was therefore to demonstrate AC performance under favourable conditions for Hg-methylation and to further understand the role AC plays in the methylation process.Materials and methodsMercury-contaminated sediment (57.1 mg kg−1) was sampled from the Gunneklev fjord, a site known as the most heavily contaminated fjord in Norway. In a laboratory experiment, lignite AC (A-AC, 5%) or activated biochar (A-BC, 5%) along with dried algae biomass, serving as an excess source of easily degradable organic matter (OM) and sulphate, were added to sediment samples that were kept anoxic and dark over a period of 12 months.Results and discussionThe amount of MeHg in sediment and porewater of the amended samples were measured at 0, 1, 3, 6, and 12 months and compared to an unamended control. A net increase of MeHg in the sediment was observed in both control and amended samples, but contrary to expectations, sediment MeHg was 5 and 3 times higher in the A-AC and A-BC treatments, respectively, relative to the control after 12 months. As the stimulation of Hg-methylation could not be attributed to the sorbents supplying more available OM or sulphate for dissimilatory sulphate reduction, it is speculated that the sorbents rather aid this process through shuttling of electrons between the substrates involved. Meanwhile, the A-AC and A-BC amendments strongly reduced the available MeHg-concentration in porewater (by 87% for A-AC and by 93% for A-BC after 12 months), confirming that AC sorbents can be used to effectively limit the transport of MeHg from sediments.ConclusionWhen considering remediation of OM-rich Hg-contaminated sediments with AC, caution is thus warranted, as the overall effect of reducing MeHg-transport out of the sediment could partly be offset by an increased fraction of MeHg in the sediment. Thin-layer capping with AC might therefore be preferable to complete mixing of AC and sediment.

Dam cascade unveils sediment methylmercury dynamics in reservoirs

Methylmercury (MeHg) can be bioaccumulated through food chains and adversely affect human health. Reservoirs are reported to stimulate MeHg production, however, the characteristics of MeHg dynamics in cascade reservoirs and the associated relations to sedimentation as well as reservoir properties remained unclear. Here we investigated sediment MeHg dynamics in eight cascade reservoirs in the upper Mekong River. We found significant differences in sediment MeHg concentration between the reservoirs, showing an increase with fluctuations along the reservoirs cascade. However, a novel relationship was found between sediment%MeHg (MeHg/Hg) and the ratio of reservoir hydraulic residence time (HRT) to reservoir age. This relationship is formed by the joint effects of the original deposit of Hg and organic carbon (OC) before impoundment and the subsequent sedimentation of Hg and OC after impoundment. The original deposit is continuously transformed as the reservoir ages, whereas the latter is driven by the annual hydrological cycle and HRT, of which the HRT is dominant. This finding cannot be easily revealed in a single reservoir or by comparing multiple reservoirs in different rivers. The discovery is of great significance to understand Hg geochemical cycling in reservoirs, which is quickly increasing in rivers worldwide.

Importance of hydraulic residence time for methylmercury accumulation in sediment and fish from artificial reservoirs

Excessive methylmercury (MeHg) accumulation in dietary fish is a global concern due to its harmful effects on human health, however, environmental factors affecting MeHg accumulation in reservoir ecosystems are not clearly known. In this study, we aim to identify the main sources of MeHg in the water column and the critical factors related to MeHg concentration and methylation rate constant (km) in sediment and total Hg concentration in fish using five-year (2016–2020) monitoring data of the five artificial reservoirs. The preliminary mass budgets constructed using the measurement and online data showed that sediment transport dominated over runoff in the long residence time reservoirs (400–475 days), while runoff dominated over sediment transport in the short residence time reservoirs (10 days). Whereas the sediment km showed a comparable variation with the algal biomass, the sediment MeHg concentration and the length-normalized Hg concentration in the barbel steed and bluegill increased in the longer residence time reservoirs with lower algal biomass. As MeHg accumulation in sediment and fish tends to increase in the slowly overturning reservoirs, the hydraulic residence time should be carefully managed to meet the best protection of human health from chronic Hg exposure by fish consumption.

Spatial patterns and distributional controls of total and methylated mercury off the Lena River in the Laptev Sea sediments

A warmer climate is predicted to accelerate the export of mercury (Hg) from Siberian rivers to the Arctic Ocean, yet there is a dearth of process-oriented studies on the speciation and fate of Hg in the shelf sea system. Here, we present data on total Hg (HgT) and methylmercury (MeHg) in Laptev Sea surface sediments along a cross-shelf transect starting at the mouth of the Lena River. Concentrations of HgT along the 330 km cross-shelf transect ranged within a fairly narrow span from 480 to 150 pmol g−1 d.w., while concentrations of MeHg decreased one hundredfold from 13 pmol g−1 d.w. near the Lena river to 0.095 pmol g−1 d.w. in the more distall stations. The highest concentrations of HgT and MeHg were observed close to the river delta and were associated with a high supply of organic carbon (OC). Enrichment of the OC normalized HgT concentration (HgTOC) and depletion of the OC normalized MeHg concentration (MeHgOC) across the shelf suggests bulk OC content to not be the only driver of the HgT and MeHg spatial distributions. Based on correlations observed between HgTOC and MeHgOC and proxies for sediment physics and organic matter pools we suggest the spatial distribution of Hg and MeHg to also be influenced by hydrodynamic sorting of riverine-derived material. For MeHg, depletion of the MeHgOC across the shelf is likely driven by the trapping of terrestrial MeHg in sediments close to the river delta before it is degraded in the water column.

Decreased bioavailability of both inorganic mercury and methylmercury in anaerobic sediments by sorption on iron sulfide nanoparticles

Methylmercury (MeHg), derived via inorganic mercury (Hg(II)) methylation by anaerobic microorganisms, is a neurotoxic contaminant causing concern worldwide. Establishing how to reduce Hg(II) methylation and MeHg bioavailability is essential for effective control of Hg pollution. Iron sulfide nanoparticles (FeSNP) is a promising passivator for Hg(II) methylation. However, its effect on the fate of MeHg in aquatic systems remains poorly understood. This study investigated the effect of FeSNP on Hg(II) bioavailability, MeHg production and bioavailability in aquatic environments. Results demonstrated that FeSNP rapidly sorbed Hg(II) and MeHg, with sorption affected by pH, chloride ion and dissolved organic matter. Hg-specific biosensor analysis showed that Hg(II) sorbed onto FeSNP significantly reduced its bioavailability to microorganisms. Double stable isotope (199Hg(II) and Me201Hg) addition revealed that FeSNP significantly inhibited MeHg production in anaerobic sediments. Furthermore, synthetic gut juice extraction suggested that FeSNP decrease concentrations of bioavailable MeHg and Hg(II), reducing their integration into food webs. However, the sorbed MeHg and Hg(II) in sediments can be released after FeSNP oxidation, potentially enhancing the risk of exposure to aquatic organisms. Overall, these findings increase our understanding of Hg transformation and exposure risks in aquatic systems, providing valuable information for the development of in situ Hg remediation systems.

Fulvic acid: A key factor governing mercury bioavailability in a polluted plateau wetland

Fulvic acids (FAs) are known to regulate the fate of mercury (Hg) in sediments, but the key effects of their properties are still unclear. In this study, field investigations and simulation experiments were conducted in a heavy metal-polluted wetland to identify FA characteristics and their association with the production and bioaccumulation of methylmercury (MeHg). Compared to permanently inundated areas (PIA), seasonally inundated areas (SIAs) had lower total Hg levels in sediments, whereas higher MeHg levels in sediments (0.20 ± 0.09 ng g−1 vs. 0.55 ± 0.31 ng g−1) and benthos (0.25 ± 0.22 ng g−1 vs. 1.62 ± 1.78 ng g−1). Meanwhile, the THg and MeHg concentrations in the same macrophyte species between PIA and SIA also followed a similar rule with benthos. FA-bound Hg in the sediment was significantly correlated with MeHg in the sediment (p < 0.01), as well as THg and MeHg in benthos (p < 0.05), indicating that FAs have the capacity to promote MeHg production and bioaccumulation. Moreover, the FAs in the sediments of the SIA had lower fractions and alkyl C/O-alkyl C ratios, but higher molecular weights and THg/MeHg concentrations than those in the PIA, indicating that FAs in SIA have increased bioavailability and enhanced competition for Hg, favoring significantly elevated FA-bound Hg levels. Biological exposure testing further demonstrated that FAs extracted from SIA had a greater ability to increase the production and bioaccumulation of MeHg than those extracted from PIA. Overall, these results highlight that the molecular composition and sources of FAs, excluding their concentrations, are one of important factors responsible for the obvious spatial heterogeneity of MeHg in sediments and aquatic organisms in the wetland.

Methylmercury in caddisflies and mayflies: Influences of water and sediment chemistry

Quantifying methylmercury (MeHg) concentrations and uptake at the base of the food web is useful for assessing mercury exposure risk to higher trophic level organisms. Higher MeHg concentrations near the base of the food web may result in more MeHg exposure and accumulation in higher trophic organisms. Here, we analyze MeHg in caddisflies, mayflies, lake water, and sediment collected from two temperate lakes and one brook in Kejimkujik National Park, Nova Scotia, Canada. Overall, caddisfly larvae MeHg (15.38–276.96 ng/g; n = 29) was not significantly correlated with water chemistry. Whereas mayfly naiads MeHg (14.28–166.82 ng/g; n = 31) was positively correlated with water MeHg (rs = 0.43), negatively correlated with pH (rs = −0.49), and positively correlated with dissolved organic carbon (DOC; rs = 0.48). Of the mercury in insect tissues, the %MeHg ranged from 56 to 75 % in caddisfly larvae and 38–47 % in mayfly naiads. MeHg bioaccumulation factors (BAF) varied greatly (water to tissue BAFs = 0.145 × 106–1.054 × 106; sediment to tissue BAFs = 0.017 × 106–0.541 × 106). This study highlights the importance of quantifying variations in MeHg bioaccumulation and BAFs of common aquatic insect bioindicators at the base of complex food webs.

Seasonal patterns of methylmercury production, release, and degradation in profundal sediment of a hypereutrophic reservoir

Fuhrmann B, Beutel M, Ganguli P, Zhao L, Brower S, Funk A, Pasek J. 2021. Seasonal patterns of methylmercury production, release, and degradation in profundal sediment of a hypereutrophic reservoir. Lake Reserv Manage. 37:360–377. Profundal lake sediment is an important site of toxic methylmercury (MeHg) production by anaerobic bacteria. This study tracked sediment MeHg cycling in hypereutrophic Hodges Reservoir, in the United States. Sediment-associated MeHg was most elevated in the early spring during moderately reduced conditions. In the late spring, MeHg and iron were released into hypolimnetic water during the progression from iron-reducing to sulfate-reducing conditions at the sediment–water interface. A decrease in porewater sulfate in the early summer indicated enhanced sulfate-reducing bacteria (SRB) activity and was associated with sediment buildup of MeHg, likely due to enhanced sorption to iron sulfides. Depletion of sulfate in the sediment during the fall corresponded with a substantial decrease in both sediment and water-column MeHg, suggesting that methanogenic conditions led to enhanced MeHg degradation. Shortly afterward, MeHg increased in the hypolimnion, indicating an upward shift in the zone of SRB methylation. Our study suggests 2 “hot moments” of MeHg entry into the water column, where it is susceptible to uptake into the pelagic food web: a spring window of mildly reduced conditions that promote MeHg release from sediment, and a fall window where MeHg is produced in the upper hypolimnion. These hot moments may potentially be managed to lower mercury bioaccumulation via redox enhancement of the profundal zone with approaches such as bottom water oxygenation, which began operation in Hodges Reservoir in 2020. However, the complexity of biogeochemical responses to such management perturbations in regard to MeHg cycling makes it difficult to predict the ultimate effect of oxygenation on mercury bioaccumulation. Supplemental data for this article is available online at https://doi.org/10.1080/10402381.2021.1940397 .

Mercury methylation in cyanide influenced river sediments: A comparative study in Southwestern Ghana.

Studies on the influence of CN on Hg methylation rates in aquatic systems draining gold mining (artisanal and small-scale) communities in Africa are rare. The study assessed the influence of CN on Hg methylation in aquatic sediments of two major river systems draining artisanal and small-scale gold mining (ASGM) communities of the Prestea-Huni Valley district, Southwestern Ghana. The miners extract gold (Au) through exclusive amalgam [Hg-Au] formation or cyanidation of Au-rich Hg-contaminated tailings, or a combination of both techniques. Hg water solubility and probable mercuric compounds in sediments of Hg-contaminated CN-loaded (River Aprepre) and Hg-contaminated non-CN (River Ankobra) aquatic systems within the district were investigated. THg was determined by CV-AAS after HF/HNO3/HCl digestion. MeHg in sediments were extracted with H2SO4/KBr/CuSO4-CH2Cl2; followed by aqueous-phase propylation, preconcentration-on-Tenax, and GC-CV-AFS. River Aprepre showed 4.58-14.83 ngMeHg/g as Hg (1.4-3.7% THg as MeHg), with 241-415 ngTHg/g, and 0.05-0.21 mgCN/kg. For River Ankobra, MeHg ranged 0.24-1.21 ngMeHg/g (0.08-0.35% THg as MeHg) with 162-490 ngTHg/g dw and CN <0.001mg/kg. There was positive correlation (r2=0.5974; p<0.01) between MeHg and CN in River Aprepre. The water-soluble fraction of Hg in sediment from both rivers was <1% of THg. Hg in sediments from River Aprepre were generally more soluble than that from River Ankobra, indicating that Hg in sediments from River Aprepre were potentially more bioavailable for methylation. Accordingly, the presence of CN in Hg-dominated river sediments potentially influences and enhances the solubility and mobility of Hg, resulting in increased Hg methylation rates.

Methylmercury determination in freshwater biota and sediments: Static headspace GC-MS compared to direct mercury analyzer

We developed and compared two analytical methods for determination of MeHg in freshwater biota and sediments, by: I) simplified static headspace GC-MS using internal standard (IS) isotope dilution quantification, after microwave acid digestion and aqueous phase NaBEt4 ethylation; II) Automated Mercury Analyzer, after double toluene extraction followed by back-extraction with L-cystein. The performance was evaluated by analysis of certified reference materials. For biota, mean recovery was 100 ± 2% and relative standard deviation (RSD) ≤ 6.8% for method I, and mean recovery was 98 ± 7% and RSD ≤13% for method II. For sediments, recovery of 94.5% and RSD of 8.8% were obtained with method I, and recovery of 90.3% and RSD of 9.4% with method II. Limits of detection (LOD) were 0.7 µg kg−1 and 6 µg kg−1, respectively. Both techniques were tested for MeHg analysis in freshwater invertebrates, fish and sediments, covering a large range of MeHg values (1.9–670 µg kg−1 d.w.).• Both protocols proved to be suitable for MeHg analysis in complex environmental matrices, even if, for method II, interferences in the extraction phase and limited sensitivity may hinder sediment analysis.• Passing-Bablock regression revealed a slight disproportion between methods, with line slope = 1.058 (95% CI ranging from 1.001 to 1.090).

Patterns in forage fish mercury concentrations across Northeast US estuaries

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

Production of methylmercury by methanogens in mercury contaminated estuarine sediments.

Anaerobic bacteria are known to produce neurotoxic methylmercury [MeHg] when elemental mercury [Hg(0)] is provided as the sole mercury source. In this study, we examined the formation of MeHg in anaerobic incubations of sediment collected from the San Jacinto River estuary (Texas, USA) amended with aqueous Hg(0) to investigate the microbial communities involved in the conversion of Hg(0) to MeHg. The results show that the addition of the methanogen inhibitor 2-bromoethanesulfonate (BES) significantly decreased MeHg production. The mercury methylation gene, hgcA, was detected in these sediments using archaeal specific primers, and 16S rRNA sequencing showed that a member of the Methanosarcinaceae family of methanogens was active. These results suggest that methanogenic archaea play an underappreciated role in the production of MeHg in estuarine sediments contaminated with Hg(0).

Examination of mercury contamination from a recent coal ash spill into the Dan River, North Carolina, United States

Coal ash spills occasionally occur due to the accidental failure of surface impoundments, and toxic metal-laden ash can pose a serious health threat to adjacent aquatic ecosystems. Here, we performed an investigation into longitudinal variations of mercury (Hg) contamination in the Dan River (North Carolina, United States) about 17 and 29 months after a February 2014 coal ash spill incident, in which the reported Hg concentrations in the spilled coal ash (210 ng/g) were 1–2 orders of magnitude higher than the river sediments (2–61 ng/g). We examined total Hg (THg) and methyl Hg (MeHg) in sediments from 0 to 65 km downstream of the spill, and found that most of the variations of THg and MeHg in surface sediments (0–16 cm) could be well accounted by the organic matter content and appeared to be not contaminated by Hg derived from coal ash. In examining MeHg bioaccumulation in invertebrates (aquatic and riparian) and fish in the Dan River and fish in a reservoir downstream of Dan River, we found no evidence of elevated MeHg bioaccumulation due to the 2014 coal ash spill. Thus, we concluded that Hg contamination from the coal ash spill is largely absent in the Dan River for both surface sediments and biota within the first three years of spill (until 2017), even though the majority of coal ash may be buried deeper in the sediment in the river channel and/or the downstream reservoir. Alternatively, the Hg associated with the coal ash is largely not bioavailable for extensive microbial Hg methylation. The findings provide useful insights into remediation strategies for this incident and other coal ash spills.

Mercury distribution in different environmental matrices in aquatic systems of abandoned gold mines, Western Colombia: Focus on human health

Total mercury (THg), methylmercury (MeHg) in water, sediments, macrophytes, fish and human health risks were analyzed and assessed from abandoned gold mining ponds (AGMPs)/ mining areas in Western Colombia to know its present environmental condition. Concentrations of THg in water (avg. 13.0 ± 13.73 ng L-1) was above the EPA threshold level (12 ng L-1), suggesting possible chronic effects. Sediment sample revealed that the ponds are methylated (%MeHg: 3.3–11). Macrophyte Eleocharis elegans presented higher THg content in the underground biomass (0.16 ± 0.13 µg g-1 dw) than in the aerial biomass (0.05 ± 0.04 µg g-1 dw) indicating accumulation of THg. MeHg was the most abundant chemical species in fish (MeHg/THg: 83.2–95.0%), signifying higher bioavailability and its risk towards human health. Fish samples (15%) indicate that THg were above WHO limit (0.5 µg g), particularly in Ctenolucius beani, Hoplias malabaricus and lowest in Sternopygus aequilabiatus and Geophagus pellegrini. Bioaccumulation and biomagnification of MeHg were higher in the carnivores representing a source of exposure and potential threat to human health. Fulton's condition factor (K) for bioaccumulation indicate a decrease with increasing trophic level of fishes. Overall results suggest, mercury species found in different AGMPs compartments should be monitored in this region.

The effect of beaver dams on organic carbon, nutrients and methyl mercury distribution in impounded waterbodies

The European beaver Castor fiber is well‐known as an ecosystem engineer that greatly affects landscape structure, biodiversity as well as physical and chemical properties of surface water bodies. Beaver ponds alter surface water bodies by raising water elevation, decreasing flow velocity and altering the morphology of streams or drainage ditches, which can reduce the concentrations of organic carbon (OC) and nutrients (N, P). Recent studies indicated that mercury transforms into hazardous and neurotoxic methylmercury (MeHg) in beaver impoundments by biological processes in anaerobic conditions.However, the knowledge about nutrients and MeHg levels in impounded forest waterbodies is scarce in Lithuania. We aimed to ascertain the alteration in concentrations and stocks of OC, nutrients and MeHg in water and sediments from upstream and downstream, as well as within beaver dams and ponds during the growing seasons of 2016–2018. Results showed higher concentrations of dissolved organic carbon (DOC) and nutrients (P and N) in upstream water samples compared to those of downstream from beaver dams. Meanwhile, in sediments mean stocks of OC, P and N were the highest in the middle part of the ponds and in beaver dams. Moreover, the concentrations and stocks of MeHg in sediments were higher in beaver dams than in any other parts of beaver impoundments (upstream, mid‐pond, pond periphery and downstream). We conclude that dam bottom sediments were rich in OC, N and P, and at the same time, contained toxic MeHg. Therefore, beaver dams could act as a trickle filter by improving water quality, in our case, DOC, N and P leaching, from riparian forests and soils, but may also act as hotspots of mercury methylation.

Cycling of methylmercury and other redox-sensitive compounds in the profundal zone of a hypereutrophic water supply reservoir

The byproducts of anaerobic biogeochemical processes, many mediated at the sediment–water interface, can degrade water quality in the bottom of reservoirs. Using both experimental sediment–water chambers and field monitoring, this study assessed nutrient and metals cycling in the profundal zone of hypereutrophic Hodges Reservoir, San Diego (maximum depth = 29.1 m; maximum surface area = 6.0 km2). A focus of the study was methylmercury (MeHg), a toxic form of mercury that bioaccumulates in aquatic food webs. Results confirmed that oxic conditions repressed release of redox-sensitive compounds (e.g., MeHg, ammonia, phosphate, manganese) from profundal sediment, though high sediment oxygen demand complicated experimental efforts to maintain a well-oxygenated sediment–water interface. In both experimental chambers and in the reservoir, patterns of MeHg cycling correlated with manganese cycling, suggesting that moderately low redox conditions that stimulate reductive dissolution of manganese also enhance net Hg methylation. Sediment MeHg flux was tightly coupled with sulfate uptake under moderately reduced conditions, reinforcing the link between sulfate-reducing bacteria and Hg methylation. Results highlight the fact that hypolimnetic oxygenation in Hodges Reservoir using pure oxygen gas, which is planned for spring 2020, must maintain high oxygen concentrations at the profundal sediment–water interface. In addition, results indicate that the presence of manganese in surface water can be used as an indicator of oxygenation’s effectiveness in lowering rates of internal nutrient and MeHg loading.

Occurrence and risk assessment of total mercury and methylmercury in surface seawater and sediments from the Jiaozhou Bay, Yellow Sea

The Jiaozhou Bay is a semi-enclosed bay located in the middle of the Yellow Sea. Effluents from wastewater treatment plants have been carried into the bay, which has significantly increased the deposition of mercury. The spatial distributions of total mercury (THg) and methylmercury (MeHg) in dissolved state, in suspended matters of seawater and surface sediments at 26 locations inside the Jiaozhou Bay and five surrounding rivers in April 2018 were examined. The contents of THg and MeHg found along the eastern coast were higher than those found along the western coast, which indicated the impact of human activities (river input) on the Jiaozhou Bay. The partition coefficient (LogKd) was used to express the distribution relationships of THg and MeHg in suspended matters and dissolved state, and it was concluded that suspended matter was the main reservoir of mercury in Jiaozhou Bay seawater. The correlations between contents and physicochemical properties of seawater showed that THg and MeHg concentrations in seawater decreased with increasing salinity and pH. The effects of the mean grain diameter (MGD) and sediment organic matter (SOM) on the THg and MeHg in surface sediments were also discussed. Principal component analysis (PCA) was used to obtain the factors determining the methylation proportion in the surface sediments, indicating that the combination of human activities and natural processes affected the degree of methylation in the sediments. The spatial distribution of THg, MeHg and MeHg% was suggested to be disturbed by the interaction of natural processes and human activities (river input) by the correlation analysis of the corresponding pollutant concentrations among seawater and. Although the concentrations of THg and MeHg in seawater and sediments of the Jiaozhou Bay did not exceed the Chinese regulatory standards, the pollution levels of THg and MeHg were comparable to those in other bays in the world.

Dynamics of (total and methyl) mercury in sediment, fish, and crocodiles in an Amazonian Lake and risk assessment of fish consumption to the local population

Increasing concerns have been raised about the toxicity of mercury (Hg) to humans, especially for those that consume a great amount of fish. High Hg concentrations have previously been measured in Amazonian waterbodies, both resulting from natural and anthropogenic sources. However, few studies have been conducted so far in Amazonian lakes that are fished by local populations. In addition, few of those studies included methylmercury (MeHg), the most toxic and bioaccumulative Hg form, and evaluated the influence of physico-chemical conditions and season on Hg dynamics. In the present study, total Hg (THg) and MeHg concentrations were measured in bottom sediment as well as in two fish and two crocodile species of the Amazonian Cuniã Lake. Bottom sediment MeHg concentrations were higher in the dry season than in the wet season, which is related to differences in physico-chemical (pH and electrical conductivity) conditions. Diet appeared to be related with animal tissue MeHg concentrations, with the herbivorous fish having lower MeHg levels than the predatory fish and crocodiles. Based on the measured tissue concentrations and published data on local person weight and fish consumption, MeHg risk to Cuniã Lake populations was estimated. Although the MeHg fish tissue concentrations did not exceed national and international standards, a significant risk to the local population is anticipated due to their high fish consumption rates. Graphical abstract.