2b: Sources and cycling of mercury in freshwater ecosystems

Following recent spills of coal combustion residue (fly ash) into rivers of the United States, questions have arisen as to the impact of this material on the ecosystem. Following the Dan River spill in 2014, we undertook a study to investigate how fly ash impacted mercury cycling in riverine sediments. First we used particle size separation methods and low level mercury analysis to test for the presence of fly ash. Second we assessed if the presence of fly ash affected total mercury (T-Hg) concentrations in sediment, as this system had already been impacted by industrial releases. We next tested how ash might impact Hg partitioning between sediment and water. Finally we examined whether the presence of ash impacted methylmercury concentration in sediment and sediment pore water.

We found that analyzing discrete particles size fractions for total mercury (T-Hg) is an effective means of determining the presence of fly ash in the sediment. Analysis of Hg concentrations in bulk phase sediment was not nearly as discerning. When ash was present in sediment, T-Hg concentrations (as defined by the sum of all mercury species) in sediment pore water were not significantly different than in pore water from sediment containing no fly ash. In the case of the methylated Hg species (MeHg); experimental enrichment of sediment with fly ash resulted in increased MeHg concentration with increasing ash concentration. Methylmercury concentrations also slightly increased in water overlying the sediment enriched in fly ash suggesting that the presence of ash increases the contribution of MeHg to overlying water.

Mercury methylation in the water column of lakes and the ocean may be an important source of monomethylmercury (MMHg), which bioaccumulates in food webs and poses a potential health threat to wildlife and humans. To better understand MMHg synthesis in the water column, the vertical distribution of mercury (Hg) species were examined in a spring-fed, eutrophic lake in western Ohio, USA. Crystal Lake is seasonally stratified, with a sulfidic profundal zone during the summer and fall. Ambient concentrations of monomethyl-, elemental, and total Hg were profiled almost monthly throughout the water column for one year. Water from five depths of the lake (surface water, chlorophyll maximum, above and in the MMHg maximum, and sulfidic bottom water) was amended with isotopically enriched inorganic Hg and MMHg, and incubated at the respective depths to quantify rates of Hg methylation and MMHg demethylation. Ambient concentrations of total Hg and MMHg were greatest at the oxic/anoxic boundary layer. Mercury was methylated in both oxic and anoxic portions of the water column, although rates were greatest in the boundary layer. These results suggest that redox transition zones in the water column are an important source of bioavailable MMHg and that methylation also occurs in oxic water, although the mechanism is unknown.

Wetland is an important sink of mercury, and its reducing substrate favors for the production of gaseous elemental mercury. In order to adapt to the anoxic condition, wetland plants usually have developed aerenchyma to transport oxygen from the atmosphere to the roots for cellular respiration. In this study, a typical wetland plant paddy rice is used to study whether its root and aerenchyma can also be a channel for the transport of rhizosphere gaseous mercury into the atmosphere. If it is, what are the mechanisms underlying. In this paper, the roots of rice were separated from the shoots by a well-air-tighten chamber. Roots were exposed to saturated mercury vapor in root chamber, then the gaseous mercury volatilized from the leave chamber was absorbed by the active carbon absorbent. The results showed that gaseous elemental mercury could be absorbed by rice roots and be transported to shoots. The mercury of roots was negatively correlated with root porosity (R=0.8309, P<0.01), while the mercury in the aboveground tissues showed a positive correlation with root surface area and root volume (R = 0.896, P <0.01; R = 0.871, P <0.01; respectively). The results also indicated that the mercury absorbed by the roots could be volatilized into the atmosphere through the leaves. The volatilization of the mercury from the leaves was increased positively with the leaf area (R = 0.897, P <0.01). There was a significantly positive correlation between the mercury volatilization per unit leaf area and transpiration intensity (R = 0.73, p <0.01). The results have proved that rice roots can not only absorb gaseous elemental mercury, but also transfer mercury to the aboveground, and then released into the air through the leaves of the stomata. The results provided a scientific basis for further investigations of revealing mercury behavior and regulation mechanisms in wetland ecosystems.

Anoxia in the hypolimnion of lakes and reservoirs can promote the conversion of mercury (Hg) to the more toxic methylmercury (MeHg) form. In the 200-km Hells Canyon Reach of the Snake River along the Idaho-Oregon border, three deep (up to 90 m) reservoirs seasonally stratify for months at a time, creating anoxic conditions that promote MeHg production in the hypolimnion. As a result, both Idaho and Oregon have listed this reach of the Snake River as impaired for Hg, with fish-tissue samples regularly exceeding Idaho’s human health fish tissue criterion of 0.3 mg/kg wet weight.

In 2014, the U. S. Geological Survey and Idaho Power Company initiated a collaborative investigation of Hg cycling and fate in the Hells Canyon reach. Primary research questions for this project include understanding the mechanisms promoting MeHg production in the hypolimnion, and the fate of the MeHg accumulated in the hypolimnion subsequent to reservoir destratification and mixing. To help answer these questions, the mass loadings of Hg and MeHg into, within, and out of the Hells Canyon complex of reservoirs were estimated using discrete water-quality data and streamflow into and out of each reservoir.

Water samples were collected biweekly from four reservoir inflow/outflow locations between 2014 and 2016 and were analyzed for dissolved and particulate Hg and MeHg. Streamflow data collected continuously at the inflow and outflow of each of the three reservoirs were used to compute mean daily streamflow values during the sampling period. Using the R software package rloadest, regression models were developed to relate instantaneous Hg and MeHg loads to daily streamflow and seasonal variables. Loads were computed at various time steps over the sampling period to estimate the mass balance for each reservoir in the Hells Canyon Complex and for the complex as a whole. Results indicate that Hg inflow is ~2.7x greater than outflow, and MeHg inflow is ~2x greater than outflow, despite clear production and accumulation of dissolved MeHg in the hypolimnion within the reservoir complex. Thus, overall, the reservoirs act as Hg and MeHg sinks. Findings from this study are intended to help manage the Hells Canyon Complex to minimize downstream export of Hg and MeHg.

Vernal pools are ephemeral wetlands that provide essential habitats for amphibians. Our previous work demonstrated that inorganic mercury (HgI) in leaf litter was converted to methylmercury (MeHg) after spring flooding in a New England vernal pool. We further showed that this MeHg was bioaccumulated by resident invertebrates, including insect larvae. Based on these findings, we hypothesized that insect emergence might provide a vector for transferring MeHg into to the surrounding forest; therefore, we investigated the export of MeHg from a vernal pool via emergent insects. In the first year of the study we surveyed the types of emergent insects inhabiting the pool and measured their MeHg concentrations. In the second year we quantitatively collected insects in order to determine the seasonal flux of insect-associated MeHg. Duplicate emergence traps were deployed between May 5 and Aug 5, 2015, which spanned the period between full flooding in the spring and drying out in the summer. Insects were collected on a weekly basis. After insect samples were dried and massed, they were analyzed for MeHg and total mercury (HgT) concentrations using acid extraction and CVAFS. Our results showed similar MeHg concentrations for insects collected in the two traps, 34±6 and 29±9 mg/g, but different patterns of emergence led to differences in MeHg export rates over the study period. The average insect-associated MeHg export rates in the two traps were 40 and 140 pg MeHg/m^2 d. The total amounts of MeHg exported were 3 and 14 ng MeHg/m^2. This export represents about 0.1% of the HgI that was deposited onto the pool via leaf litter in the fall. Considering the abundance of vernal pools in some forested landscapes, our results suggest that these wetlands may be important sources of MeHg to forest food webs.

North American Laurentian Great Lakes are the largest surface freshwater system in the world presenting a valuable recreational and commercial fishery among many other benefits. Mercury levels in fish from Lake Erie, the shallowest of the five lakes, are weakly increasing over the last two decades. We conducted extensive analysis total mercury (THg) and methylmercury (MeHg) of water, sediment, seston and 3 fish species collected from 10 sites in the three basins of the lake (western, central and eastern) over a four month summer period to understand mercury dynamics in the system. Overall, our observations and reports in the literature suggest that THg have declined in the sediments of all three basins during the last two decades. THg and often MeHg water and sediment concentrations remain higher in the western basin than the central and eastern basins. This is especially true for the sediment, which may be acting as a continual source of Hg to both the water column and subsequently the primary biota of the basin. Despite the western basin having significantly higher Hg in the abiotic compartments, the %MeHg was often lower in the west and increased eastward, indicating that complex factors are contributing to higher MeHg production and comparable bioaccumulation in biota in the central and eastern basins. These data can be used for future reference as well as for input into Hg dynamics models, which require a certain minimum of accurate empirical data to be of use to aquatic ecology modellers.

Previous studies have revealed that beaver impoundments increase methylmercury (MeHg) concentrations and alter dissolved organic matter (DOM) composition in rivers, especially in the first years after flooding. Although DOM controls mercury (Hg) availability for methylating bacteria, the impact of DOM quantity and composition in Hg methylation processes in aquatic systems remains still unclear. Here, we studied Hg methylation rates and DOM composition in 9 beaver ponds differentially aged across Sweden. We found a decrease in MeHg production with increasing age of the ponds, ponds older than 18 years featuring on average 65% lower methylation compared to those younger than 7 years. Our results show that increased MeHg concentrations in surface waters is a consequence of enhanced Hg methylation in the pond sediments. Elevated chlorophyll a concentrations found in young ponds imply internally produced organic matter as an important driver of Hg methylation. Qualitative analysis of the DOM with excitation-emission-matrix fluorescence spectroscopy, further suggests that unprocessed humic DOM also enhances Hg methylation in young pounds. Our study provide novel insights into the drivers of Hg methylation processes in boreal aquatic ecosystems.

Seasonal flooding changes chemical and morphometric characteristics of the tropical floodplain lakes, affecting stratification which can influence mercury dynamics. We investigate the influence of flooding on the mercury dynamics in an Amazon floodplain lake. Three points on the lake, the Solimes River and the connecting channel to the river were sampled along the annual flood-pulse. During high-water, the lake was deep (12.7 m, on average) and the area farthest from the river was stratified with low oxygen and high methylmercury concentrations ([MeHg]) at depths below 7 m (0.37 ngL-1, on average). The two sites closest to the river were destratified and the [MeHg] were lower (0.077 ng L-1, on average along all depths). At low-water, the lake was shallower (3 m, on average) and destratified, with higher oxygen concentrations, and no difference in [MeHg] along the water column at all lake sites. Independent of season, the [MeHg] in the connecting channel were higher than those in the river. The [MeHg] in phytoplankton varied in response to changes in biomass, showing a biodilution effect. Variations in lake morphometry controlled the stratification and [MeHg] in the lake during the high-water. Floodplain lakes export hypolimnetic MeHg to their associated rivers during high-water.Seasonal flooding changes chemical and morphometric characteristics of the tropical floodplain lakes, affecting stratification which can influence mercury dynamics. We investigate the influence of flooding on the mercury dynamics in an Amazon floodplain lake. Three points on the lake, the Solimes River and the connecting channel to the river were sampled along the annual flood-pulse. During high-water, the lake was deep (12.7 m, on average) and the area farthest from the river was stratified with low oxygen and high methylmercury concentrations ([MeHg]) at depths below 7 m (0.37 ngL-1, on average). The two sites closest to the river were destratified and the [MeHg] were lower (0.077 ng L-1, on average along all depths). At low-water, the lake was shallower (3 m, on average) and destratified, with higher oxygen concentrations, and no difference in [MeHg] along the water column at all lake sites. Independent of season, the [MeHg] in the connecting channel were higher than those in the river. The [MeHg] in phytoplankton varied in response to changes in biomass, showing a biodilution effect. Variations in lake morphometry controlled the stratification and [MeHg] in the lake during the high-water. Floodplain lakes export hypolimnetic MeHg to their associated rivers during high-water.

The reduction of mercury in lake surface water plays an important role in the mercury biogeochemical cycle by decreasing Hg(II) availability to aquatic organisms and increasing the release of Hg(0) to the atmosphere. The formation of dissolved gaseous mercury (DGM) is a photochemical process mediated by dissolved organic matter (DOM). The aims of this study are: 1) to measure DGM concentrations and reduction (kre) and oxidation (kox) rate constants in oligotrophic, mesotrophic, and eutrophic lakes, and 2) to correlate DGM and redox rate constants with environmental variables to understand the mechanisms of DGM production in surface lake water. The composition of fluorescent DOM in surface water samples was also analyzed using excitation-emission matrix (EEM) fluorescence combined with a parallel factor (PARAFAC) model. Three target monitoring sites were chosen based on self-organizing map (SOM) results: Jangsung Lake (JSL) is an oligotrophic lake surrounded by forest (85% of catchment area), Kumho Lake (KHL) is a mesotrophic lake located by the coastal sea, and Yeongsan Lake (YSL) is an eutrophic lake located downstream of Gwangju city. The EEM-PARAFAC identified three components of DOM in surface lake waters: humic-like (C1), fulvic-like (C2), and protein-like (C3). Seasonal monitoring results showed that surface water DGM was highest in the oligotrophic JSL and lowest in the eutrophic YSL. The site trend of DGM agreed well with the site trend of the net reduction rate: kre/kox was highest in the JSL and lowest in the YSL. The Pearsons correlation analysis results demonstrated a significantly positive correlation between kre/kox and %C3 and a significantly negative correlation between kre/kox and %C1, indicating that the net reduction of Hg(II) is promoted by protein-like DOM or inhibited by humic-like DOM. Currently, we are carrying out field measurements of individual kre and kox to determine which reaction (i.e., reduction or oxidation) rate varies as a function of DOM composition.

Mercury pollution of the aquatic ecosystems, especially agricultural wetlands is of major concern owing to the highly toxic and persistent nature of mercury and subsequent accumulation in the aquatic biota. The present study was conducted in a unique tropical agricultural ecosystem, situated in the South west coast of India, where rice cultivation is practiced below sea level. Most of these areas are water logged throughout the year and subjected to flood during monsoon period. The geochemical analysis revealed the acidic as well as anaerobic nature of the system. The THg content in the soil and various tissues of rice plants were studied during the entire rice growing season (i.e., from 30th day to 120th day).The analysis was done using Direct Mercury Analyzer (DMA 80, Milestone, USA). The mercury concentration in the soils were within the range of uncontaminated sites prescribed by USEPA. The mean THg concentration obtained for soils is 0.088mg/kg. The average concentration of THg in the plant tissues followed the pattern root>leaf>grain>husk>stem. The correlation analysis as well as translocation factor supported the possible translocation of mercury from the root to the other parts of the rice plant. The comparative study showed that, the THg concentration obtained in the present study (0.0219 mg/kg) is less than the values reported in the literature. However it slightly exceeds the maximum limit (0.02 mg/kg) recommended by the Chinese National Standard Agency for food stuff. Since studies on mercury cycling in the aquatic ecosystems of tropical countries is very less, it is highly crucial to develop a thorough understanding of the behavior of mercury in this complex system. Varying geochemistry and enormous land area of the agricultural wetland ecosystem greatly influences the speciation of mercury and thus its toxic impacts. Hence the present study focused on the detection, bioavailability and accumulation of mercury.

In aquatic ecosystems the mercury (Hg) participates of numerous chemical reactions, being able to go out of the chemical form inorganic to organic form and quickly incorporate in the biota. The Madeira River has been exploited by gold mining for decades, being an anthropic route of introduction of mercury in the aquatic environment. The objective of the study is to evaluate the concentration of total mercury (THg) and methylmercury (MeHg) in the water sample (total and filtered fraction) of the Madeira River, upstream and downstream of a hydroelectric plant. Quarterly data were collected in six points (3 upstream and 3 downstream), between the years of 2013 and 2016 on the Madeira River. The water samples were collected in amber glass bottles, stored on ice and transported to the laboratory. THg and MeHg were quantified by gas chromatography coupled to cold vapor atomic fluorescence spectrometry following the methodology EPA 1631 and EPA 1630, respectively. Upstream of the hydroelectric plant, the waters of the total fraction presented the following medians for THg and MeHg, respectively: 7.21 and 0.09 ng/L (2013); 6.94 and 0.05 ng/L (2014); 5.16 and 0.11 ng/L (2015); 6.13 and 0.15 ng/L (2016). In the dissolved fraction the medians were 1.84 and 0.03 ng/L (2013); 0.70 and 0.03 ng/L (2014); 0.73 and 0.04 ng/L (2015); 0.04 and 0.04 ng/L (2016). At the downstream of the hydroelectric plant, in the total fraction, the following median values were found for THg and MeHg, respectively: 7.51 and 0.10 ng/L (2013); 5.86 and 0.07 ng/L (2014); 5.55 and 0.09 ng/L (2015); 7.17 and 0.014 ng/L (2016). In the dissolved fraction the medians were 1.76 and 0.04 ng/L (2013); 0.54 and 0.03 ng/L (2014); 0.62 and 0.04 ng/L (2015); 0.19 and 0.04 ng/L (2016). The highest THg (2103) and MeHg (2015) concentration in the water (total fraction) was observed upstream of the hydroelectric plant. The highest percentage ratio of MeHg: HgT occurred in the total fraction (9.16%) in downstream of hydroelectric (2014). There was a significant increase in the concentration of MeHg in the water (total fraction) upstream of the hydroelectric plant over the years, as well as the increase in the relation between the concentration of THg and MeHg. The highest concentration in the total water fraction was attributed to a large load of suspended particulate matter (SPM) transported by the Madeira River (annual average of SPM= 376.67 mg/L).

Thaw lakes (TLs) form a unique and widely distributed landscape in permafrost regions. The Hg biogeochemistry in climate-sensitive TLs has not been extensively investigated, but there is evidence that climate warming is altering the structure and function of TLs. In this study, we combined field measurements, in situ incubation experiments, remote sensing, and a previous water isotopic tracer study to document the characteristics of Hg and MMHg cycling in three TLs and to obtain insight into the Hg biogeochemistry of TLs under rapid climatic warming. We found that Hg(0) in water displayed higher concentrations during daytime and warm seasons and lower concentrations during nighttime and cold seasons (including under ice). But MMHg concentrations in water during cold seasons and under ice were much higher than those in warm seasons and in open water. These results suggest an important role of solar radiation on the Hg and MMHg cycle. In situ incubation experiments further confirmed that solar radiation greatly regulates Hg redox reactions and MMHg degradation. Field measurements showed that TLs were a Hg(0) emission source, and ebullition (bubbling) further increased the Hg(0) emission flux during ice-free periods. The simple Hg mass balance, incubation experiments, and previous water isotopic tracer study suggest that the groundwater and lake margin are important Hg sources to the lake water, and that the lake expansion will induce an increase in Hg(0) emission from the TL system. This study suggests that climate change may potentially affect the cycling of Hg in TL ecosystems.

A recent review of mercury concentration data from the Red Deer River (Alberta, Canada) revealed a number of exceedences to Canadian water quality guidelines over the past decade. Total Hg (THg) concentrations reaching as high as 860 ng/L have been measured in Michichi Creek (a tributary to the Red Deer River) and >250 ng/L within the Red Deer River itself. Concentrations >100 ng/L have been routinely observed within the Red Deer River during the open water season. These THg concentrations are generally at the upper range of values reported elsewhere, including streams and rivers impacted by legacy mining or intensive urban and industrial activities. Here we will present the results of an investigation into the potential cause(s) of these high THg concentrations.

The Red Deer River watershed covers 49,650 km2, which includes the Alberta badlands. Median (open-water) THg and total suspended solids (TSS) concentrations at sites draining the badlands were frequently more than double those of stations upstream. THg was significantly related to TSS (r2=0.66; p<0.05), indicating that THg concentrations in the Red Deer River and its tributaries are driven primarily by sediment transport. However, the concentration of THg associated with suspended sediment (Csed) did not increase downstream of the badlands, suggesting that the high THg concentrations observed are driven primarily by increases in sediment mass. In addition, THg to Al ratios of suspended sediment in the Red Deer River watershed indicate negligible (< 2) to moderate (2-5) anthropogenic enrichment.

Contributions to THg loading from upstream of the badlands were low (2-8%) despite contributing most of the flow to the river (≈70%). In contrast, four tributaries draining the badlands contributed 22% of the THg loading, despite providing only ≈9% of the flow to river. Sediment loading was highly episodic, reflecting the rapids erosional response of the Alberta badlands to precipitation events. Our results suggest that erosional processes in the Alberta badlands have played a key role in driving Hg dynamics within this river system; however, the extent to which heavy metal fluxes to the RDR have impacted biota still remains an important question.

Mercury (Hg) deposition and subsequent methylmercury (MeHg) concentrations within the biota of the Everglades (USA) are some of the highest in North America, leading to fish consumption advisories and wildlife toxicological concerns. However, there are spatial and temporal variations with respect to total Hg (THg) and MeHg in water, soil, and biota. Heretofore, the research conducted towards explaining this variability has emphasized the well known pathway of Hg methylation during sulfate reduction by sulfate-reducing bacteria. Although dissolved organic carbon (DOC) has been shown to influence the solubility and bioavailability of THg and MeHg in environments outside the Everglades, DOC has not been a major focus of research in the Everglades.

Eight locations, varying in chemical and biological characteristics, within the Everglades Protection Area (EvPA) were periodically monitored from 2011-2017 for water, floc, and biotic concentrations of THg and MeHg, along with ancillary chemical parameters in the surface waters (SUVA254 [specific UV absorbance at 254 nm], spectral slope between 275 and 295 nm [S275-295], and DOC, sulfate, sulfide, iron, phosphorus concentrations). Bivariate linear regression and multivariate (Principal Component Analysis) statistics indicated that DOC (range of 13 to 57 mg/L) and dissolved THg concentrations were most closely associated with the dissolved MeHg concentrations in water. With one exception, water MeHg concentrations increased with aqueous DOC concentrations across multiple sites; concomitantly, bioconcentration factors for periphyton decreased. The SUVA254 (a measure of the aromaticity of the DOC) was not related to aqueous MeHg concentrations across sites, but SUVA254 did show a positive linear relationship at one site dominated by invasive cattail (Typha) and Chara. Importantly, the slopes of trophic MeHg magnification throughout the food chain showed no relationship with DOC, or variation among sites. This suggests that spatial and temporal Hg variations in the upper trophic levels, such as in Gambusia, are primarily defined at the base of the food web, and that DOC may limit uptake of Hg particularly by basal food organisms.

Our data demonstrate that MeHg concentrations in the water column and biota are frequently higher at locations where the sulfate levels are ≤ 0.2 mg/L than in areas where sulfate consistently average between 15 and 32 mg/L. This suggests that sulfate reduction is not the only Hg methylation process operating in the Everglades, and that DOC acts to mediate the bioavailability of Hg and MeHg. Because DOC plays such a critical role in the uptake and bioaccumulation of MeHg in the food chain, investigations into the specific DOC structure and composition that influence the production of MeHg and its biological uptake should be pursued. This will assist in explaining why some areas of the Everglades are consistent Hg “hotspots”.

Bioaccumulation of methylmercury in aquatic food chains has led to national and state consumption advisories for many fish species. Atmospheric inorganic mercury deposited in bodies of water is converted to methylmercury by anaerobic sulfur-reducing bacteria. Through bioaccumulation, methylmercury levels trend higher in fish that are larger, at a high trophic level, with greater longevity. Mercury sequestration by plants may decrease availability for methylation and thereby reduce rates of bioaccumulation.

Here we report mercury levels in largemouth bass collected from 4 farm ponds in Daviess County, in the northwest region of Missouri, USA. The county is primarily agricultural and does not contain large urban areas; however, the significant number of coal-fired power plants in eastern Kansas and western Missouri may influence mercury deposition in this region. Fish lengths and weights were recorded and muscle samples frozen for determination of total mercury on a Teledyne Leeman Labs Hydra IIC Hg Analyzer. Fish lengths (cm) varied from 18.1 to 39.9. Total mercury (ng/g, wet weight) varied from 182 to 567. Additionally, DOC, alkalinity, cation concentration, anion concentration, and mercury concentration were measured in the associated water samples. Plant samples were separated into root, rhizome, and leaf sections, dried, and homogenized samples were analyzed for total mercury content.

The results show length and weight are highly correlated and thus either measure can be used when looking for gross trends in mercury accumulation. Largemouth bass above 10 inches in length (25 cm) contained mercury concentrations above the EPA limit of 300 ng/g. Based on a linear regression of data from all four ponds, variation in length explains most of the variation in mercury concentration. Pooling largemouth bass data from this year with similar work from previous years suggested that submerged aquatic vegetation (Potamogeton epiydrus - pondweed) might play a role in reducing bioaccumulation through mercury sequestration. Mercury concentration in the pondweed leaves was several orders of magnitude greater than in corresponding water samples, supporting sequestration by the plant material.

Waters in the Amazon Basin can be classified by its optical characteristics as “black”, “clear” and “white” with distinct physic-chemical properties defining aquatic environments. We studied total Hg (THg) methyl-Hg (MeHg) in water (W), suspended sediment (SSed), sediment (Sed), phytoplankton (P-plankton), Zooplankton (Z-plankton) and macro-invertebrates (Mac-Inv) in the three types of water found in the Madeira River Basin. The highest average concentrations of THg and MeHg were respectively 2.4ng.L-1, 0.43 ng.L-1 (black-W); 2.23 ng.L-1, 0.33 ng.L-1 (white-W); 2.2 ng.L-1, 0.31 ng.L-1 (clear-W). Average concentrations of THg in SSed were respectively 159.27 μg.Kg-1 (white-W); 122.39 μg.kg-1 (clear-W) and 107.16 μg.kg-1 (black-W). The average concentrations of THg and MeHg in Sed were respectively 77.5 μg.Kg-1, 0.44 μg.Kg-1 (black-W); 76.41 μg.kg-1, 0.33 μg.kg-1 (clear-W); 55.33 μg.kg-1, 0.24 μg.kg-1 (white-W). For P-plankton, average concentrations were respectively 322.35 μgKg-1, 19.2 μgKg-1 (black-W); 274.6 μg.kg -1, 12.9 μg.kg-1 (clear-W); 211.6 μg.kg-1, 9.4 μg.kg-1 (white-W). Z-plankton average of THg and MeHg concentrations were respectively 388.15 μgKg-1, 24.6 μgKg-1 (black-W); 333.87 μg.kg-1, 15.94 μg.kg-1 (clear-W); 246.34 μg.kg-1, 11.43 μg.kg-1 (white-W). The mean THg and MeHg concentrations in Mac-Inv were 701.39 μg kg-1, 65.8 μg.kg-1 (black-W); 636.91 μg.kg-1, 44.38 μg.kg-1 (light-W) and 645.73 μg.kg-1, 36.74 μg.kg-1 (white-W). Mean THg concentrations of Mac-Inv were not significantly different between the analyzed matrices. However, MeHg was significantly different between the analyzed matrices. The correlations between variables (W, SSed, Sed, P-plankton, Z-plankton and Mac-Inv) were more significant in the white-W, suggesting a greater connectivity of aquatic communities with geological environment. Correlations among biological variables (Phy, Zoo and Mic-Inv) were statistically significant in all three types of water suggesting an undiscernible pattern of biomagnification in these water ecosystems.

Abstract not available.

This study investigated the photo-degradation kinetics of MeHg in the presence of various size fractions of dissolved organic matter (DOM) with MW<3.5 kDa, 3.510 kDa. The DOM fraction with MW<3.5 kDa was most effective in MeHg photo- degradation. Increasing UV intensity resulted in the increase of photodegradation rate of the MeHg in all size of DOM fractions. Higher rates of MeHg degradation was observed at higher pH values. For the portion of MW<3.5 kDa, the photo-degradation rate of MeHg increased with increasing DOM concentration, indicating that radicals such as singlet oxygen (1O2) radicals can be effectively produced by DOM. At higher portion of MW>3.5 kDa, the inhibition of MeHg degradation was observed due to the photo-attenuation effect. Our result indicates that radical mediated reaction is the main mechanism of photodegradation of MeHg especially in the presence of MW<3.5 kDa. Our results imply that the smallest molecular weight fractions (MW<3.5 kDa) of DOM mainly increased the photodegradation rate of MeHg.This study investigated the photo-degradation kinetics of MeHg in the presence of various size fractions of dissolved organic matter (DOM) with MW<3.5 kDa, 3.510 kDa. The DOM fraction with MW<3.5 kDa was most effective in MeHg photo- degradation. Increasing UV intensity resulted in the increase of photodegradation rate of the MeHg in all size of DOM fractions. Higher rates of MeHg degradation was observed at higher pH values. For the portion of MW<3.5 kDa, the photo-degradation rate of MeHg increased with increasing DOM concentration, indicating that radicals such as singlet oxygen (1O2) radicals can be effectively produced by DOM. At higher portion of MW>3.5 kDa, the inhibition of MeHg degradation was observed due to the photo-attenuation effect. Our result indicates that radical mediated reaction is the main mechanism of photodegradation of MeHg especially in the presence of MW<3.5 kDa. Our results imply that the smallest molecular weight fractions (MW<3.5 kDa) of DOM mainly increased the photodegradation rate of MeHg.

The METAALICUS study, a whole-watershed mercury (Hg) addition experiment, was designed to evaluate the response of the Lake 658 ecosystem to changing Hg loads. In this paper, we examine the availability of Hg for methylation in sediments over 12 years, following 7 years of a whole-lake enriched stable isotope spike and 5 years of recovery. We compare MeHg production from the lake spike with MeHg formed from native Hg in lake sediments. In surface (0-2 cm) sediments, the bioavailability of Hg for methylation declined rapidly after deposition to sediments. In the first year of lake spike additions, roughly 20% of Hgspike newly deposited to sediments accumulated as MeHg, compared to 3% of native Hg (which had an average age of about 14 years). The spike %MeHg dropped exponentially over the first 3 years of lake Hg additions, to an asymptote of about roughly 3-4%, similar to native MeHg. Sediment:water partitioning of spike Hg was an order of magnitude below that for native Hg in year one, rising to match native Hg after about 3 years. We conclude that the early sediment diagenesis into more recalcitrant phases (rather than burial) reduces the availability of sediment Hg for methylation with a decay half time of a few months. However, it did not decline to zero with the time period measured.

The absolute concentrations of spike Hg and MeHg in surface (0-2 cm) sediments rose during the 7 years of spiking, following the accumulation of loading. After spiking stopped, spike Hg and MeHg concentrations in surface sediments and pore waters declined, lessening the contribution of the spike Hg to overall MeHg production. However, there were low levels of ongoing spike Hg methylation for a decade, perhaps due to recycling of sediment Hg. These were reflected in low levels of lake spike MeHg in anoxic bottom waters, and in the lower food web, even several years after the spike stopped. These results strengthen our original METAALICUS conclusion that lakes can respond rapidly to decreases in direct Hg loading. However, the rate of response among lakes may vary with the rate of sedimentation. For example, if Hg deposited to sediments within the last year or two is a small fraction of the overall Hg pool in microbially-active surface sediments, the contribution of older, less-available Hg to overall MeHg production could be significant.

During a period of mercury use in the 1950s and 1960s, large quantities of mercury were released to the headwaters of East Fork Poplar Creek (EFPC), a freshwater stream in Oak Ridge, Tennessee, resulting in elevated mercury concentrations in downstream soils, sediments, and biota. Mercury-contaminated soils are eroding into the stream, providing a potential source for conversion to methylmercury and bioaccumulation in the food chain. Our goal was to understand the extent, rate, and mechanisms of geochemical releases of mercury from EFPC bank soils. A series of equilibrium and kinetic batch experiments were conducted using EFPC water, water from an uncontaminated analogue stream, artificial creek water (ACW) with solution chemistry similar to EFPC, and ACW with 1 mg/L Sewannee River natural organic matter. Experiments were conducted with bulk soils at varying solid:water ratios (w/vol 1:5, 1:10, 1:30, 1:50, 1:100, 1:500, 1:1000) in ACW using a reaction time of 6 hours. Experiments were conducted using different soil particle size fractions (<0.053, 0.053-0.125, 0.125-1.00, 1.00-2.36, >2.36 mm) and all four water chemistries. High resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) were used to determine the form of mercury in the soils. In the near future, selective extraction will be used to estimate the extent of mercury release using 5 increasingly strong reagents.

The extent of mercury release increased until 12 hours, then decreased, and finally plateaued until the end of the experiment at 170 hours. Solid:water ratios up to 1:50 increased the extent of mercury release, but above this threshold, mercury release was invariant. At a 1:30 ratio, after 6 hours reaction time, and with size fraction 0.125-1.00 mm, the soils released from 0.001% to 0.05% of the original concentration of mercury, which ranged from 0.002 to 0.55 μg/L. SEM/HRTEM/EDS analyses suggested that mercury was present as mercury sulfides. After the solutions were reacted with the soils, the solutions were analyzed for major anions, cations, pH, specific UV absorbance at 254 nm, and dissolved organic carbon. Geochemical modeling will be used to predict the extent of dissolution using the equilibrium solution chemistry. Preliminary SEM/HRTEM/TEM results suggest that mercury was present as mercury sulfide, indicating the major mechanism of release is dissolution. Our results suggest that eroding bank soils could contribute substantial mass of Hg to EFPC waters, and thereby provide a source for mercury methylation in the creek.

Lake sediments are an important source of the neurotoxic monomethylmercury (MMHg) for surface water, in which trophic transfer occurs in both benthic and pelagic organisms. Geochemical processes involved in mercury (Hg) methylation generally occur in surface sediments and imply various micro-organisms amongst which the most documented are sulfate reducing (SRB), methanogens and iron-reducing (IRB) bacteria. The goal of the present study was to improve our understanding regarding the fate and behavior of post-depositional Hg in the sediments of the high-altitude Lake Titicaca (3809 m a.s.l., Bolivia). Vertical profiles from six cores representative of the various sediment facies and ecological repartition were analyzed in high-resolution (5 to 10 mm-scale) for filtered and particulate Hg species (Hg and MMHg) together with redox-sensitive elements (N-species, Mn, Fe and SO42-) to evaluate the influence of early diagenetic processes on Hg interconversion and partitioning between porewater (PW) and sediment.

Our results highlight that the highest MMHg concentrations are found in surface sediment PW located in shallow (0 to 10m) and very shallow (0 to 2m) sites (5.8 ± 5.6 ng L-1) with MMHgPW rising up to 12.2 ng L-1 in top sediment covered by epibenthic biofilms while lowest MMHgPW were encountered in the deep (40m) lake sediments (0.84 ± 0.23 ng L-1). In particular, highest MMHg is encountered in the shallow carbonate facies sites where the low sediment OM and the high sulfate contents in both sediment and water (~ 250 mg L-1) are likely the major fuel for Hg methylation by SRB - the main Hg methylators in such environments. A striking feature of our results is the very elevated percentage MMHg encountered in almost all sites standing at 47 ± 26 % in very shallow, 39 ± 27 % in shallow sites and 23 ± 15 % in deeper sites.

Finally, calculated diffusive fluxes to the overlying water (e.g., 104 ± 16 ng.m-2.d-1 and 12 ± 8 ng.m-2.d-1 for carbonate facies and organic facies sediments, respectively) are compared with benthic chamber flux measurements performed in situ to evaluate diurnal and site-specific variability.

A mathematical model was developed to predict mercury (Hg) concentrations for lakes in the Upper Peninsula (UP) of Michigan to help further understand the cycling of Hg. Several Hg models have been published but few have been evaluated for uncertainty in predictions. Models are often over-parameterized with up to 70 coefficients, few being constrained by measurements. These parameters are often lake-specific and cannot be generalized for all lakes. To address this problem, the UP is used as the location for this analysis because this region has many lakes, but many variables remain constant (e.g., latitude, land cover, deposition).

The model was developed in R and incorporates a mass balance for three Hg species (elemental, divalent, and methyl) in three compartments (epilimnion, hypolimnion, and sediments). It considers seasonality such as temperature, stratification, photolysis, ice cover, hydrology, and solids concentrations. Kinetics integrated into the mass balances include redox reactions, methylation, demethylation, photodemethylation, and partitioning. The model was calibrated, and sensitivity and uncertainty analyses were performed. Calibration consisted of comparing model predictions with lake measurements. For sensitivity analysis, model parameters were changed by a fixed percentage and changes in model predictions were compared. This presentation focuses on uncertainty analysis using a Markov Chain Monte Carlo (MCMC) Bayesian approach. This approach is based on Bayes Rule, where the posterior distribution of a parameter, given the dependent variable and fixed model data, is proportional to the likelihood function multiplied by the prior distribution. The prior distribution of parameters is obtained by compiling values cited in literature. In the MCMC approach, the model is run thousands of times to converge to the posterior distribution, and the estimate for each parameter is defined by the posterior mean.

The goal for this project is to use the model to understand sources of variability among lakes and to predict regional Hg storage and emission from lakes. Multivariate statistics and model scenario analysis are used to examine the sources of variability. Results from this statistical approach have shown the importance of the pH and DOC on Hg cycling in lakes; model scenarios suggest that seasonal ice cover influences methylmercury concentrations. Regional-scale estimation applies posterior distributions of parameters from the uncertainty analysis to groups of lakes having similar characteristics (size, mixing regime, trophic state). The combination of these approaches reveals some fallacies in common assumptions.

Mercury (Hg) is a global pollutant that severely impacts ecosystem and human health when it bioaccumulates as methylmercury (MeHg). MeHg production is mediated by microbes that thrive in the absence of oxygen, as such, there is a growing need to understand the processes that affect Hg availability in anaerobic environments. Hg redox cycling is a key pathway that determines the amount of substrate HgII or Hg0 available for methylation. To date, the majority of work has focussed on Hg redox cycling performed by chemotrophic bacteria in aerobic environments, with the role of anaerobes and phototrophs having gone largely overlooked. Recently, weve shown that anoxygenic phototrophic purple bacteria contribute to Hg redox cycling when excess reducing power is available (in the presence of light and reduced carbon), however, whether such pathways occur in other anaerobes and/or phototrophs remains elusive. Our objective was to test whether similar Hg redox cycling pathways were present in a representative of the family Heliobacteria, Heliobacterium modesticaldum Ice1, capable of phototrophic and fermentative growth. We found that H. modesticaldum can perform Hg redox cycling during phototrophic growth, but discovered that it can also do so during fermentatively. We determined that Hg redox cycling during fermentation depends on the availability of reduced cofactors and demonstrate that this pathway may be widespread in other anaerobes possessing a similar chemotrophic metabolism.

There has been a rising trend in the amount of mercury (Hg) found in freshwater fish in northern lake ecosystems. In particular, climate change may lead to increased mobility of Hg by increased fluxes of dissolved organic carbon (DOC) associated with Hg to surface waters. The concentration of mercury in fish is thought to be strongly driven by processes at the base of the food web, where there are knowledge gaps and a great deal of variability in the degree of Hg accumulation.

Here, we present the result of a detailed seasonal study of Hg in the lower food web of two boreal lakes in southern Norway that have contrasting levels of DOC. Water and food web samples were taken from Røysjø (clear water lake) and Store Øyvannet (brown water lake). We tested out possible explanatory driving factors for Hg in aquatic food webs, in order to better be able to make predictions on concentrations and uptake and transfer of Hg through the food web. Analysis was carried out for a comprehensive set of samples taken during spring, summer and fall of 2016. Measurements of water quality, DOC and Hg were paired with the characterization of the base of the food web and trophic interactions based on stable isotope analysis in order to describe Hg trophodynamics.

We found that Hg concentrations in water from Store Øyvannet (the brown water lake) were approximately three-fold higher compared to Røysjø, highlighting the importance of organic carbon content as a factor for the Hg concentration in water. We saw seasonal patterns in Hg in both water and zooplankton. Methyl mercury (MeHg) concentrations tended to increase with increasing zooplankton size (the zooplankton was size fractionated). There was a significant difference in MeHg concentration in zooplankton (in the 200-500 micrometer size range) between lakes, with concentrations in Store Øyvannet approximately five-fold higher than in Røysjø. These data provide important insight into seasonal MeHg cycling in the lower food web of two contrasting lakes, and the effects of water chemistry and trophic interactions on MeHg concentrations in zooplankton.

Acknowledgements: Norwegian Research Council (CLIMER project, project number 243644), Per-Johan Færøvig, Marthe Torunn Solhaug Jenssen and Morten Jartun.

Methylmercury (MeHg) bioaccumulation in biota is a serious concern for some remote ecosystems in Southwestern Nova Scotia. Sources of mercury to this area are both natural and anthropogenic, including bedrock and atmospheric deposition. While much is known about mercury methylation in freshwater, much less is known about demethylation. The loss of MeHg from water columns of lakes is dependent on several processes including adsorption, deposition to sediments, and demethylation. Microbial demethylation in lake water columns is very slow. Therefore, daytime photodemethylation facilitated by solar radiation can dominate MeHg removal from the water column. To better quantify the photodemethylation potential within lakes we must determine a) the variation in photoreactive compounds such as dissolved organic matter (DOM) and iron (Fe) and b) the availability of solar radiation with depth in water columns. Freshwater lakes were chosen in Kejimkujik National Park (44.23°N, 64.13°W) to include a wide range in dissolved organic carbon (DOC) and Fe concentrations. Water samples were collected over 3 years and analyzed for ultraviolet (UV)-visible absorbance, DOC, dissolved ions, total mercury, and MeHg concentrations. Floating sensors for UV, photosynthetically active radiation (PAR), and temperature were installed in two lakes of contrasting DOC concentrations. The depth of 95% UV attenuation was 40-50 cm in the lower carbon lake compared to 10-20 cm in the higher carbon lake. The effect of rainfall on UV attenuation was also less in the lower carbon lake compared to the higher carbon lake, most likely due to a difference in catchment area. Seasonal alterations to the solar angle of incident radiation strongly controlled the amount of solar radiation entering lake water surfaces and therefore outlined a possible photoreactive season or period within each year. These observations suggest that photodemethylation of methylmercury in lake water columns may be limited to a short period of approximately 4 months a year at 44°N. Overall, these field observations provide fine resolution solar radiation data and excellent temporal resolution over 3 years for mercury and carbon cycling.

Methylmercury (MeHg) toxicity is of particular interest in remote Canadian environments, far from point sources of contamination, where high levels of MeHg are accumulating in top predators. The ecosystem variables that control this sensitivity of food webs are not well known. MeHg concentration in water and uptake into the base of the food web is one key factor controlling mercury entry into food webs. Few studies have directly considered photodemethylation reactions in combination with physical attributes of aquatic ecosystems to predict where and when dissolved MeHg may be available. To address this research gap we have used numerous controlled and semi-controlled experiments that focused primarily on the quantification of the relationships between solar radiation exposures, dissolved organic matter (DOM), and MeHg within six freshwater lake systems in Kejimkujik National Park and National Historic Site in southwestern Nova Scotia. To better quantify the photodemethylation potential within these lakes we determined a) the behaviour of photoreactive compounds and b) the availability of solar radiation with depth in water columns. Experimental treatments were 1-week long in summer and fall and were exposed to natural solar radiation over that time period. Using these experimental outcomes coupled with field measurements of solar radiation availability within lake water columns, we have developed a model for predicting photodemethylation potential and efficiency within the top 1 m3 in oligotrophic dystrophic temperate lakes. These predictive results were then scaled up and used to calculate the overall photodemethylation potential in each of our six study lakes for comparison with MeHg concentrations in the corresponding food webs. This model may be appropriate for other aquatic ecosystems by simple standardization techniques depending on water quality characteristics such as DOM photoreactivity (structure), pH, and dissolved ionic species. Overall, this body of work yielded a method for predicting mercury availability to food webs depending on environmental and physicochemical factors. Climate change in temperate and boreal regions of Atlantic Canada is projected to increase rainfall amounts and occurrences and thus lead to browning of freshwaters and further inhibition to the photodemethylation pathway of MeHg reduction.

The St. Louis River Estuary (SLRE) is a highly utilized recreational shallow (1-3m) freshwater estuary at the mouth of the largest tributary to Lake Superior. Fish and other aquatic biota in the estuary often contain high levels of methylmercury (MeHg) compared to the upstream river or Lake Superior. The upstream river is heavily impacted by mining sulfate loads in a watershed dominated by atmospheric mercury depositions from regional and global sources. The downstream estuarine area contains some high mercury legacy sediment contamination from previous industrial activities, contributing to the listing of the SLRE by the EPA as one of 27 Areas of Concern in the Great Lakes region. Legacy contamination, water and sediment chemistry, and wetland area abundance in the estuary have all been presented as possible contributors to differences in mercury mobility and bioavailability as well as methylmercury production. In this study, we focus on the impact of sediment chemistry and solid-phase mercury speciation on inorganic- and methyl-mercury bioavailability and partitioning in relation methylmercury production in the estuary. Sediment cores and porewater were collected from five geochemically distinct locations within the estuary and analyzed for total and methylmercury, carbon (solid and porewater), and AVS/SEM or dissolved sulfide. In addition, selective extractions were used to separate solid phase mercury compounds into behavioral classes. Initial results indicate legacy mercury contamination does exist in some locations (at an order of magnitude higher than unaffected sediments), primarily in the lower harbor. Methylmercury production was highest in sediments with intermediate organic carbon content (around 10% LOI), however porewater total mercury was negatively correlated to organic carbon. Also, methylmercury production and porewater methylmercury was lowest in sediment with the highest sulfide, suggesting both inorganic and methylmercury availability are partially controlled by solid phase organic carbon and sulfate. Selective extraction results help to elucidate the solid phases involved in binding mercury, separating the solid-phase mercury present into water soluble, stomach acid soluble, organo-chelated, elemental, and mercuric sulfide classes. Coupled with total and methylmercury data, selective extraction results allow for the assessment of bioavailable mercury pools throughout the SLRE.

Biotic methylation of Hg appears to depend on factors such as microbial activity and the concentration and bioavailability of Hg to the methylating organisms. Recently, the possible relationship between cyanobacteria and the production of methylmercury (MeHg) has been suggested. In this sense, this work aims to test in microcosms the net MeHg production in relation to the concentrations of periphytic cyanobacteria, DOC, phosphorus and primary periphytic production rates. We collected water and samples of periphyton for cultivation and isolation of cyanobacteria in floodplain environments of the Guapor River Wetland (Amazonia, Brazil). Periphyton was cultivated in microcosms with different concentrations of wild local cyanobacteria (n=30), total phosphorus (n=15) and natural local DOC (n=15). Tests of net mercury methylation potentials were done in incubations with local water and samples of the microcosms, using 203HgCl2 as a tracer. The primary periphytic production was evaluated by the oxigen production in light and dark bottles. Incubations with commercial cyanobacterial crops were also conducted. The highest net MeHg production occurred in the microcosm with the addition of wild cyanobacteria (7 to 25%), followed by microcosms with phosphorus addition (6 and 11%) and DOC (6 and 9%). There was no significant Hg methylation in incubations with commercial crops. In our experiments, there was a positive correlation between the production of MeHg, additions of cyanobacteria (R2= 0.694; p=0.001), phosphorus (R2= 0.521; p=0.04), DOC (r2= 0.611; p=0.001) and periphytic primary productivity. Our results point to two new possible predictors of Hg net methylation potentials: i) periphytic primary productivity, and ii) cyanobacterial concentrations in the periphyton. We hypothesized that the participation of these variables are due to the increase in energy resources (mainly H) for communities of Hg-methylating bacteria.

The Newberry volcano crater lakes near Bend, Oregon, are small, seasonally mixed twin lakes filled with carbonate-rich, meteoric waters at ambient temperatures. East Lake (EL) has a pH of 6.5-7, 65 ppm SO4, and an active geothermal-volcanic CO2 + H2S input at its lake bottom. The diffusive CO2 flux into the atmosphere of ~50 tonnes CO2 /day is presumably similar to the geothermal gas input rate. EL fish have Hg contents 0.5-3 ppm Hg, and the sediment, with ~ 10% Corg, has up to 4 ppm Hg. Elemental Hg gas enters at the lake bottom with the geothermal gas flows, and bottom waters have 1.0 - 3.3 picomolar Hg(o). The Hg(o) is oxidized to Hg2+ in the water column, which then is probably methylated by sulfate-reducing bacteria. This methylated Hg is then taken up by diatoms and other algae, and accumulates in the sediment at a rate of ~5 kg Hg/year. Elemental Hg gas may leave the lake surface as well, where we find ~ 1.8 picomolar Hg (o) in the water. The combined input of Hg and sulfur makes for a perfect Hg methylation storm in EL. Paulina Lake (PL) is separated by a < 1 mile wide volcanic ridge from EL, and has no free CO2 bubbles, has more carbonate-rich waters devoid of SO4, and has a pH ~8. PL sediment has <50 ppb Hg and its fish carry <0.1 ppm Hg. PL has up to 250 ppm As and up to 14 % Fe2O3 in its sediment, and ~ 15 ppb As in the water column. EL on the other hand is virtually free of As, and its sediment has < 2% Fe2O3. We conclude that a phase separation at depth in the geothermal fluids leads to a largely gaseous input with Hg into EL; the residual degassed geothermal fluids enter PL, carrying abundant Si, As, P, and Fe. Some PL sediment contains 10 % of the diagenetic mineral vivianite (Fe-phosphate), a potential host for the As. The sediment of both lakes consists largely of biogenic silica (diatoms), ~ 10 % silicic volcanic ash, Corg, with an additional Fe-rich hydrothermal component at PL. The two lakes have existed ~ 8000 years suggesting a lifetime accumulation of 40 tonnes Hg in the EL sediment column. Future work involves a study of the methylation process in EL and potential Hg fluxing from its surface.

To reduce mercury in fish, Minnesotas Statewide Mercury Total Maximum Daily Load study set a goal for Hg emissions reductions. To potentially inform additional mitigation, studies are underway to better understand processes governing mercury bioavailability in aquatic food webs. Here we present initial results of load monitoring of Hg and MeHg, along with ancillary analytes, for seven northern Minnesota rivers. Two years of load monitoring are presented for four of the rivers and one year for the others. In addition to the most downstream station on all rivers, four rivers had a sub-watershed station, and one river was monitored at ten sub-watershed stations. Flow gages provided mean daily discharges and were maintained by the interagency Cooperative Stream Gaging Program. Water sampling usually began after ice out (March-April) and continued through October, acquiring 15-25 samples during that period. The rivers are distinctly different in their mix of land cover (ranging from 5 to 89 percent forest/wetlands) and water chemistry (e.g., sulfate, total organic carbon, and suspended solids). Total organic carbon concentrations ranged from 5.7 to 33 mg/L (medians: 9.3 21.3mg/L). Sulfate concentrations ranged from 0.57 to 1800 mg/L (medians: 1.1 697.3 mg/L). Rivers with the highest MeHg concentrations showed a stronger response (steeper slope) to water temperature. Annual yields ranged from 0.327 2.59 g/km2 for total Hg and 0.011 0.206 g/km2 for MeHg. Yields for Hg and MeHg were strongly correlated to TOC yields (R2: 0.85 and 0.84). The positive relationships between TOC and Hg and MeHg were much stronger in terms of yield than concentration. There were no significant relationships between yields of Hg, MeHg, sulfate, and suspended solids. An additional year of load monitoring for the rivers with only one year will complete this part of a larger study of mercury bioavailability. The final product of this study (ca 2018) will compare results from co-occurring studies of mercury methylation and food web biomagnification.

Through water quality and fish monitoring across Minnesota watersheds, five rivers have been identified to have particularly elevated mercury concentrations in fish despite relatively low total mercury concentrations in water and sediment. One hypothesis for these “high-five” rivers is that methylmercury production in riparian areas and the hydrological connectivity of riparian areas to streams in these five watersheds are important contributors to river methylmercury loads and bioaccumulation. To test this hypothesis, we conducted methylation (k-meth) and demethylation (k-demeth) assays, using enriched mercury isotopes, across two geomorphically distinct riparian zones in each of the five watersheds and across seasons from 2015 through 2016. Early analysis suggests that mercury methylation is enhanced in riparian areas within both wetland and agricultural landscapes versus riparian areas within forested landscapes. K-meth in riparian soils within agricultural and wetland environments are 0.014/d ± 0.011 and 0.013/d ± 0.010, respectively. Except for one study location, k-meth in riparian soils of forested landscapes are lower, 0.009/d ± 0.006. Methylmercury demethylation is similarly greatest in the wetland and agricultural riparian environments compared to forested riparian zones, although significant variability is observed. K-demeth values in soils from the agricultural, wetland, and forested riparian landscapes are 1.20/d ± 0.93, 1.51/d ± 1.37, 1.08/d ± 0.58, respectively. Our results suggest that both higher methylation and demethylation are positively related to areas that are more frequently inundated, such as in the agricultural and wetland landscapes. Riparian areas within forested regions may not contribute as much to riverine methylmercury because of lower inundation frequency and extent across these watersheds.

With the establishment of the Great Lakes Restoration Initiative in 2010 and an understanding that fish consumption advisories for mercury exist for all five of the Laurentian Great Lakes, the U.S. Geological Survey (USGS) and the U.S. Environmental Protection Agency (USEPA) initiated an effort to better understand mercury sources, cycling, and bioaccumulation. A significant motivation for this effort stemmed from the observation that while elevated fish mercury levels are well known, mercury monitoring and assessment efforts in the Great Lakes are relatively few, and no previous efforts have been conducted to systematically sample and analyze water, sediment, and lower trophic-level biota across the ecosystem. Starting in August, 2010, biannual (April and August) sampling surveys of water, seston, sediment, and benthos were conducted onboard the USEPA research vessel Lake Guardian. Seston (>63 um) was collected vertically with a plankton net towed through the epilimnion (20 m deep at most sites) to the surface. Surface water was simultaneously collected using Teflon-lined Niskin bottles mounted on a sampling rosette that were remotely triggered to collect water at key depths. Surficial sediments (top 2 cm) and Quagga mussels were collected with a Ponar dredge. Samples were analyzed for MeHg and dissolved organic carbon (DOC, waters only) at the USGS Mercury Research Laboratory.

Concentrations of MeHg (1.5 9.0 pg/L) and DOC (1.2 2.4 mg/L) in filter-passing water were extremely low, and increased from the upper to the lower Great Lakes following a general increasing trend of productivity and anthropogenic influence (SuperiorHuron>Michigan>Ontario>Erie) as productivity decreases and was negatively correlated with water and sediment MeHg. Clear seasonal differences for seston MeHg were also observed. April MeHg concentrations were approximately two times higher than August in all lakes except Erie, where April concentrations were half of August levels. Methylmercury concentrations in Quagga Mussels and Lake Trout (6.5 19.0 and 92 208 ng/g dry weight, respectively) were positively correlated with seston MeHg, suggesting that seston mercury drives foodweb concentrations. Seston-water MeHg partitioning (log-Kd) ranged from 5.4 to 7.0 L/kg and reflected both seasonal and between-lake trends observed for seston MeHg concentrations. We observed a negative relationship between water-seston MeHg partitioning and DOC, indicating that MeHg partitioning to DOC may limit MeHg uptake into the foodweb.

Mercury (Hg) methylation and monomethylmercury (MMHg) demethylation activity of periphyton biofilms from East Fork Poplar Creek, a Hg impacted stream in East Tennessee, were measured throughout 2016 using stable Hg isotopic rate assays. To evaluate methylation/demethylation rate potentials, ²⁰¹Hg(II) and MM202Hg were added to structurally intact periphyton samples in ambient streamwater while the formation of MM²⁰¹Hg and loss of MM202Hg were monitored over time.

The data were modeled by simultaneously fitting first-order production (MM²⁰¹Hg) and decay (MM202Hg) equations to estimate methylation and demethylation potentials, respectively. Preliminary results indicated methylation rates were non-first order, resulting in poor model fits to the MMHg production data. This was hypothesized to be caused by the decreasing bioavailability of the ²⁰¹Hg(II) during the assay. Additional experiments approximated the change in ²⁰¹Hg(II) bioavailability during the assays by measuring both ²⁰¹Hg(II) sorption onto periphyton and the conversion of ²⁰¹Hg(II) to ²⁰¹Hg(0) over time after its addition to periphyton samples. Hg(II) sorption was well described by two-site first-order kinetic sorption. The data also suggested decreasing bioavailability of the MM202Hg over time which was approximated by measuring the rate of MM202Hg sorption. The MM202Hg sorption data were also well described by two-site first-order kinetic sorption. The model was extended to include the (i) Hg(II) sorption, (ii) Hg(0) production, and (iii) MMHg sorption as mechanisms contributing to the temporal decline in bioavailability. This new model yielded improved first-order fits to the methylation and demethylation data. Comparisons between our modelling method and more commonly used fitting techniques (e.g., the use of single time points to determine methylation and demethylation and fitting production and decay equations to methylation and demethylation data separately) are made.

Catskill region of New York State, USA, was identified as one of the hotspots for mercury (Hg) bioaccumulation in gamefish in a 2008 study of 131 waterbodies within New York State (NYS). We reinvestigated from May to November 2016 13 waterbodies within the Catskills in collaboration with Syracuse University (SU) and the NYS Department of Environmental Conservation as part of the statewide monitoring program commissioned by the NYS Energy Research and Development Authority. A total of 277 individual fish were sampled from a pool of four frequently consumed gamefish species: Sander vitreus (Walleye), Perca flavescens (Yellow Perch), Micropterus dolomieu (Smallmouth Bass) and Micropterus salmoides (Largemouth Bass). Each fish was identified by species and measured for total length (mm) and weight (g), and a tissue sample was taken using an Acuderm^® inc. 7mm acu-punch. Tissue samples were analyzed at SU for total mercury in solids using a Milestone DMA-80 in compliance with EPA method 7473. Nearly all (> 95%) of total Hg (THg) in fish is known to be methylmercury (MeHg); therefore THg served as a reasonable proxy for MeHg in fish. The highest THg concentrations were detected in Walleye (mean = 0.526 µg g^-1 THg, standard deviation = 0.514), ranging 0.054 to 1.785 µg THg g^-1 wet weight while yellow perch had the lowest mercury concentrations (mean = 0.187 µg g^-1, standard deviation = 0.174) and ranged from 0.016 to 0.911. Using Akaike’s information criterion (AIC) model selection, we concluded that out of the three morphometric parameters and all possible combinations, the additive effects of species and length best explained the observed patterns in Hg concentrations in fish, while weight had minimal predictive utility. Further data analysis to evaluate the utility of water chemistry and geographical parameters in prediction of fish Hg is ongoing.

Mercury (Hg) assimilation by microbes in aquatic environments is an important step in bioaccumulation and bioconcentration in the food web. The inorganic dissolved mercury concentration available to microbes is strongly influenced by other ligands in solution, notably sulfur species and dissolved organic matter (DOM). In this work, we used the bacterial bioreporter, Escherichia coli HMS174 (pRB28/27) in an experimental study to assess the concentration of Hg2+ in the presence of 3 different DOM samples.

The three DOM samples that were used, include: Suwanee River NOM, primarily from a terrestrial blackwater source; Upper Mississippi River NOM, mainly a terrestrial-derived DOM; and Pony Lake fulvic acids isolated from an entirely microbial source. The patterns of mercury bioavailability were assessed in three different Hg and dissolved organic carbon (DOC) concentrations and statistically analyzed using linear regression and correlations to the DOM characteristics, including elemental composition, molecular structure, and optical properties through fluorescence 3D excitation emission matrices. Results indicated Hg bioavailability was most strongly correlated to the sulfur content of DOM, more specifically, the Hg:S ratios between 90 and 50 ng Hg mg-1 S (r = -0.94, p = 0.22). As the relative sulfur content of DOM increased, so did Hg binding to the DOM, likely attributable to the strong affinity Hg2+ has to sulfur functional groups.

Peatlands are hotspots of carbon and mercury (Hg) cycling, and have high concentrations of both solutes in outlet streams. Importantly, trends of increasing dissolved organic matter (DOM) concentrations have been found in streams for many northern catchments, which has been attributed to various drivers of environmental change. Since DOM and Hg cycling and transport are tightly coupled, it is important to determine if long-term increases in stream DOM concentrations are associated with changes to Hg transport in northern peatland ecosystems. The Marcell Experimental Forest (MEF) in forests of northern Minnesota (USA) was established during the 1960s to study the hydrology and ecology of catchments having upland soils that drain through central peatlands to streams that are headwaters of the Mississippi River and Hudson Bay. We show that annual volume-weighted concentrations of stream total organic carbon (TOC) and total mercury (THg) concentrations increased from 1993 to 2015. Over the same period, streamflow decreased. Stream THg concentration showed a weak positive correlation with TOC concentration and a weak negative correlation with streamflow. Likewise, interannual shifts in streamflow, TOC concentration, and THg concentration sometimes varied directions and relative magnitudes, suggesting complex underlying mechanisms. While suggestive of long-term changes in carbon cycling and Hg dynamics in peatland catchments, we urge caution in extrapolating these specific results for other sites where DOM concentrations have increased. Nonetheless, the long-term record may be the first documentation of THg concentrations that have increased in recent decades and that the increase has occurred over the same period as increases in TOC concentrations.

Since 1922, Manitoba has seen significant hydroelectric development with the opening of 15 hydroelectric reservoirs and the Churchill River Diversion which transports significant flow from the Churchill River to the developed Burntwood-Nelson River System, harnessing its power. Currently an average of 30 billion kilowatt hours per year are produced from the 15 operational hydroelectric reservoirs in Manitoba. Total mercury (THg) concentrations in fish are known to increase within 10 years of reservoir impoundment. Increases are thought to be temporary, with recovery to pre-impoundment concentrations generally occurring within a few decades. Over the last decade however, there has been an increasing trend in the fish mercury levels of both the regulated and unregulated reservoirs in Manitoba. In this study, we aim to quantify the contribution of hydroelectric regulation on the production of methyl mercury within an ecosystem with a rapidly changing climate.

As part of a larger interdisciplinary project to assess changes to the Hudson Bay System, we are addressing gaps in historical monitoring programs in the Hudson Bay watershed to incorporate controls on mercury cycling in aquatic systems. This is being achieved by providing meaningful seasonal results of sulfide, THg, methyl mercury and a more comprehensive characterization of the organic matter (OM) in the system. We have identified 4 key lakes, 2 regulated and 2 non-regulated, in the Churchill River Diversion and the Nelson River systems in northern Manitoba as field sites for determining the extent to which reservoir mercury cycling is impacted long-term by hydroelectric regulation.

Preliminary results from water column sulfide, THg, and methyl mercury concentrations in our field sites suggest that methylation in the systems in question is linked to sulfide production. During the studied seasons, THg, sulfide and MeHg concentrations differed amongst on system and off system reservoirs. Future analysis of OM and sediments will allow us to draw connections to mercury methylation and build upon relationships explored in experimental reservoirs.

The mercury (Hg) transport within watersheds is regulated by water flux and hydrological paths, and coupled with the movement of dissolved organic matter (DOM) and particulate material. In Brazo Rincn (BR), a branch of Lake Nahuel Huapi (Northwestern Patagonia, Argentina), lake biota bear high Hg levels after the precipitation season, which is apparently related to volcanic activity in combination with high seasonal precipitation in the area (3500 mm y-1). We propose that these factors together with low DOM concentrations favor high Hg levels in the aquatic biota. We examined the occurrence of different Hg species (Total Hg, THg; Methyl Hg, MeHg and dissolved gaseous Hg, DGM) in the freshwater network draining to BR, applying Cold Vapor Atomic Fluorescence Spectroscopy (CVAFS). We analyzed the effect of DOM (concentration and quality), and suspended load (SL) as factors potentially influencing Hg speciation and partitioning. DOM concentration was estimated by dissolved organic carbon [DOC] and its quality through absorbance and fluorescence parameters. Chlorophyll a (Chla) was determined in the depth profile of BR. Overall, THg ranged between 41-363 ng L-1 in streams while in BR was higher in upper (114-268 ng L-1) than in deeper (16-36 ng L-1) strata. Particulate-THg and Filtered-THg ranged between 7-67 ng L-1and 15-304 ng L-1 in streams and 0.3-77 ng L-1 and 17-202 ng L-1 in BR. Estimated Hg2+ was higher in streams (40-362 ng L-1) compared to BR (17-263 ng L-1) in which upper layers showed higher concentrations. MeHg ranged between 0.01-0.3 ng L-1 in streams, and varied between 0.01-0.16 ng L-1 in BR, with higher concentrations in the upper strata. DGM was lower in streams (0.12-1.8 ng L-1) than in the water column of BR (0.1-7 ng L-1), with higher values up to 60 m, decreasing sharply towards the bottom. In general, our results suggest that the high but variable levels of different Hg species in streams and their positive relationship with terrestrial signatures of the DOM, reflect coupled inputs from the catchment and low effect of internal processing. Conversely, in the lake, different conditions are observed in the depth profile, indicating that internal processing (autochthonous production, photo- and biodegradation) operate concomitantly on DOM and Hg species to favor the production of DGM in the illuminated upper layers and binding of Hg to biological particles (phytoplankton), as suggested by the positive relationship Hg-Chla. Interestingly, in the streams Hg was mostly bound to inorganic particles.

Production of methylmercury (MeHg) from ionic mercury (Hg2+) occurs largely in aquatic environments devoid of oxygen and nitrate (NO3-) such as, lake and estuarine sediments, wetlands, and anaerobic hypolimnia. Recent studies have found that methylation of Hg2+ is a result of the activity of hgcA and hgcB enzymes and confirmed the methylating capability of >70 microorganisms, thus unveiling potential new habitats for methylation.

We studied the transformation of mercury species and bacterial community structure in two meromictic lakes, Glacial Lake, Jamesville, NY and Green Lake, Fayetteville, NY. Meromictic lakes are stable anoxic basins with lower, euxinic stratum, which is permanently separated from an upper, well oxygenated stratum. The interface between the two strata (chemocline) is characterized with steep temperature, density, and geochemical gradients. Glacial Lake and Green Lake harbor rich microbial assemblages of phototrophic S bacteria (PSB), which likely process Hg but no phylogenetic or field study have been undertaken to confirm their involvement.

Samples were collected between 2014-2016. In situ field measurements were made for dissolved oxygen (DO), NO3-, sulfide (HS-), temperature, pH, specific conductance, turbidity, and chlorophyll a. Additional samples were collected for chemical (total Hg, MeHg, dissolved organic carbon, and bacterial chlorophyll) and microbiological (DNA sequencing, droplet PCR, and qPCR) analysis. Redox parameters delineated a spatially-wide zones of anoxic and anaerobic metabolism in both lakes, which followed the theoretical thermodynamic sequence of electron acceptors. Vertical distribution of total Hg and MeHg had distinct profiles with noteworthy peaks at the chemocline, associated with high turbidity and peaks in bacterial chlorophyll. Anoxygenic phototrophic bacteria comprised a sizable portion of the microbial community at the chemocline of both lakes. Droplet PCR and phylogenetic sequencing showed high abundance of purple PSBs (>50 % of total biomass) in the chemocline of Green Lake, while predominance of green PSBs (> 52% of total microorganisms sequenced) in Glacial Lake. Concentrations of the hgcA and hgcB genes reached maximum at the chemocline as well. Multiple lines of evidence suggest that anoxygenic phototrophic bacteria contribute to the enhanced methylation of Hg2+ at the chemocline.

Previous studies have found elevated levels of MeHg in certain Arctic freshwater fish, which are a staple in northern Aboriginal peoples diets. This raises environmental and human health concerns, highlighting the importance of understanding the spatial and seasonal variations in MeHg sources to freshwater ecosystems. This research aims to determine where hotspots for production (methylation) and degradation (demethylation) of MeHg occur in High Arctic landscapes. To address these objectives a series of field based experiments to quantify Hg methylation and MeHg demethylation, as well as spatiotemporal surveys of MeHg and total Hg concentrations, were conducted in the Skeleton Lake sub-catchment of Ellesmere Island, Nunavut, Canada. This sub-catchment allows us to track MeHg concentrations and production during downstream transport along a continuum from the permafrost seep headwaters, through Skeleton Lake, a series of shallow ponds, a wetland stream, a sedge meadow wetland and finally at the inflow of the creek into Lake Hazen. Thus, we are able to quantify how MeHg is transformed during downstream transport through various compartments of the catchment, particularly at the terrestrial-aquatic interface. Sampling and experiments were conducted during the ice-free, summer growing season of 2016. Hg methylation and MeHg demethylation were quantified using enriched stable isotope tracers in 1) the Skeleton Lake water column; 2) Skeleton Lake sediments; 3) downstream pond sediments; and 4) wetland soils, both along the stream and in the sedge meadow. A spatial survey was also conducted along the entire length of the wetland stream and meadow to quantify Hg and MeHg in wetland soils across the terrestrial-aquatic interface, and aqueous concentrations of MeHg and total Hg were measured weekly throughout the continuum. Preliminary data suggests that while there is net production of MeHg in the lake and pond sites, wetland soils act as a very important sink for MeHg, reducing its export into Lake Hazen. In spring 2017, additional methylation/demethylation measurements will be carried out in snowpacks, and in Skeleton Lake water, at a time persistent ice cover has led to the development of bottom water anoxia, thus providing ideal conditions to support the activity of anaerobic methylating microorganisms. This research will result in a better understanding as to where MeHg is produced and decomposed on the Arctic landscape as well as provide insight into seasonal patterns and differences in MeHg sources and cycling.

The photochemical degradation of methylmercury (MeHg) by sunlight is an important process which can influence MeHg concentrations and cycling in surface waters. We compared MeHg photo-degradation rates from surface waters of three rivers (Sacramento, Mokelumne, San Joaquin) with surface waters from freshwater wetlands (Yolo Wildlife Area) in the San Francisco Bay Delta Estuary (Delta), California, U.S.A. Photo-degradation rates were determined using in situ bottle and bag incubations, with incubation periods from a few hours to several days. PAR and UV photo-degradation rate constants (k, ng L-1 E-1 m2) were obtained by regressing MeHg concentrations obtained at different time periods against cumulative light intensity or total light exposure. PAR and UV photo-degradation rate constants were -0.0139 and -0.202 ng L-1 E-1 m2 respectively for Delta rivers and -0.0047 and -0.116 ng L-1 E-1 m2 respectively for Delta freshwater wetlands. An ANCOVA was used to test for the equality of slopes and showed that the photo-degradation rates (slopes) for the rivers and wetlands are significantly different. This suggests that the type of water body (e.g. Rivers vs wetlands) has significant influence on MeHg photo-degradation rates and hence MeHg concentrations. Consideration of the water type is therefore critical when developing models of photo-degradation of MeHg across the Delta. Water body specific MeHg photo-degradation rates may be required for accurate predictions as the Deltas physical make up includes arrays of large rivers, small tributaries, marshes, diked islands, and flooded tracts. Preliminary findings reveal that when integrated temporally and spatially, photo-degradation of MeHg is a dominant loss term for dissolved MeHg in the Delta, exceeding riverine export losses by over a factor of 3.

The South River is a high gradient stream located in central Virginia, USA that seasonally generates increased concentrations of methylmercury (MeHg) in surface water despite lacking geomorphological characteristics thought to favor methylation (e.g., wetlands). Concentrations of MeHg in surface water range from <0.1 to ~3 ng/L in the unfiltered fraction, and have a strong seasonal component. Analysis of the previous decade of data collected seasonally in the South River reveals that the proportion of total mercury (THg) as MeHg in surface water is predicted (multiple R2 = 0.78) by three factors the concentration of inorganic mercury (IHg), the surface water discharge and surface water temperature. This relationship is significant (ANOVA; p<0.001) over a wide range of temperatures and the concentration of IHg in both the filter-passing and particulate fractions; the relationship is observed in both upstream reference areas and downstream of the historical source, indicating that methylation rates are responding to increases in temperature in the river channel.

This finding has significant implications for the South River food web and responses to remediation. Larval aquatic organisms are exposed to MeHg on particles and in the colloidal (i.e., filter-passing) phases of surface water. The concentration of IHg in surface water, which is correlated with the concentration in sediment is therefore a critical indicator of MeHg exposure to the food web. Remedial solutions should focus on reducing the loading of new IHg to the system, which is likely methylated more rapidly than older IHg in the system based on results from other systems. Climate change and changes to the watershed (e.g., urbanization) that may affect surface water temperature could also affect microbial methylation rates. However, the relationship between methylation and temperature is not completely linear the highest MeHg concentrations are not necessarily observed during maximal surface water temperature. Future monitoring data on surface water will allow for more insight into how the South River might respond to perturbations including remediation, climate change and urbanization.