Watershed characteristics and chemical properties govern methyl mercury concentrations within headwater streams of boreal forests in Ontario, Canada.

Methyl mercury (MeHg) concentrations in boreal headwater streams are influenced by complex natural processes and disturbances such as forestry management. Understanding drivers of MeHg within boreal streams in Ontario, Canada, is of particular interest as there are legacy MeHg concerns. However, models accounting for the complexity of underlying processes have not yet been developed. We assessed how catchment characteristics and stream water chemistry influence MeHg concentrations within 19 watersheds of the Dryden - Wabigoon Forest in Ontario, Canada, using a structural equation modelling (SEM) approach. Despite the study area encompassing a large variation of boreal forest watersheds in the Canadian Shield, our SEM had substantial explanatory power across the region (χ251 = 45.37, p-value = 0.70, R2 = 0.75). Nitrate concentrations (p-value <0.001), water temperature (p-value = 0.002), and the latent watershed characteristic (p-value <0.001) had a positive influence on MeHg concentrations once variable interactions were accounted. Due to the inherent strengths of applying an SEM approach, we describe two plausible pathways driving MeHg concentrations: 1) indirect effect of forest-derived nutrients increases in-situ MeHg production in Dryden - Wabigoon Forest streams, and 2) direct supply of MeHg from inundated soils following consistent precipitation and inundation events (i.e., fill, sit, and spill).

Role of Ester Sulfate and Organic Disulfide in Mercury Methylation in Peatland Soils.

We examined the composition and spatial correlation of sulfur and mercury pools in peatland soil profiles by measuring sulfur speciation by 1s X-ray absorption near-edge structure spectrocopy and mercury concentrations by cold vapor atomic fluorescence spectroscopy. Also investigated were the methylation/demethylation rate constants and the presence of hgcAB genes with depth. Methylmercury (MeHg) concentration and organic disulfide were spatially correlated and had a significant positive correlation (p < 0.05). This finding is consistent with these species being products of dissimilatory sulfate reduction. Conversely, a significant negative correlation between organic monosulfides and MeHg was observed, which is consistent with a reduction in Hg(II) bioavailability via complexation reactions. Finally, a significant positive correlation between ester sulfate and instantaneous methylation rate constants was observed, which is consistent with ester sulfate being a substrate for mercury methylation via dissimilatory sulfate reduction. Our findings point to the importance of organic sulfur species in mercury methylation processes, as substrates and products, as well as potential inhibitors of Hg(II) bioavailability. For a peatland system with sub-µmol L-1 porewater concentrations of sulfate and hydrogen sulfide, our findings indicate that the solid-phase sulfur pools, which have a much larger sulfur concentration range, may be accessible to microbial activity or exchanging with the porewater.

Long-Term Experimental Manipulation of Atmospheric Sulfate Deposition to a Peatland: Response of Methylmercury and Related Solute Export in Streamwater.

Changes in sulfate (SO42-) deposition have been linked to changes in mercury (Hg) methylation in peatlands and water quality in freshwater catchments. There is little empirical evidence, however, of how quickly methyl-Hg (MeHg, a bioaccumulative neurotoxin) export from catchments might change with declining SO42- deposition. Here, we present responses in total Hg (THg), MeHg, total organic carbon, pH, and SO42- export from a peatland-dominated catchment as a function of changing SO42- deposition in a long-term (1998-2011), whole-ecosystem, control-impact experiment. Annual SO42- deposition to half of a 2-ha peatland was experimentally increased 6-fold over natural levels and then returned to ambient levels in two phases. Sulfate additions led to a 5-fold increase in monthly flow-weighted MeHg concentrations and yields relative to a reference catchment. Once SO42- additions ceased, MeHg concentrations in the outflow streamwater returned to pre-SO42- addition levels within 2 years. The decline in streamwater MeHg was proportional to the change in the peatland area no longer receiving experimental SO42- inputs. Importantly, net demethylation and increased sorption to peat hastened the return of MeHg to baseline levels beyond purely hydrological flushing. Overall, we present clear empirical evidence of rapid and proportionate declines in MeHg export from a peatland-dominated catchment when SO42- deposition declines.

Impacts of water level fluctuations on mercury concentrations in hydropower reservoirs: A microcosm experiment.

Hydropower generation, a renewable source of electricity, has been linked to elevated methylmercury (MeHg) concentrations in impoundments and aquatic biota. This study investigates the impact of water level fluctuations (WLF) on MeHg concentrations in water, sediment, and fish. Using a set of controlled microcosm experiments emulating the drawdown/refill dynamics and subsequent sediment exposure to air experienced in reservoirs, we demonstrate that less frequent WLFs, and/or increased exposure of sediment to air, can lead to elevated MeHg concentrations in sediment, and total mercury (THg) and MeHg concentrations in water. In examining the effects of WLF frequency (two-day, weekly, and monthly), the monthly treatment displayed the highest THg and MeHg water levels, while the weekly treatment was characterized by the highest MeHg levels in the sediment. Our work supports emerging evidence that longer duration between WLF creates a larger surface area of sediment exposed to air leading to conditions conducive to higher MeHg concentrations in sediments and water. In contrast, THg, MeHg, and fatty acid trends in fish were largely inconclusive characterized by similar among-treatment effects and minimal temporal variability over the course of our experiment. This result could partly be attributed to overall low mercury levels and simple "worm-forage fish" food web in our experiment. To elucidate the broader impacts of water fluctuations on aquatic chemistry and biota, other factors (e.g., longer WLF cycles, dissolved organic matter, temperature, more complex food webs) which modulate both methylation rates and food web dynamics must be considered.

Diverse Communities of hgcAB+ Microorganisms Methylate Mercury in Freshwater Sediments Subjected to Experimental Sulfate Loading.

Methylmercury (MeHg) is a bioaccumulative neurotoxin produced by certain sulfate-reducing bacteria and other anaerobic microorganisms. Because microorganisms differ in their capacity to methylate mercury, the abundance and distribution of methylating populations may determine MeHg production in the environment. We compared rates of MeHg production and the distribution of hgcAB genes in epilimnetic sediments from a freshwater lake that were experimentally amended with sulfate levels from 7 to 300 mg L-1. The most abundant hgcAB sequences were associated with clades of Methanomicrobia, sulfate-reducing Deltaproteobacteria, Spirochaetes, and unknown environmental sequences. The hgcAB+ communities from higher sulfate amendments were less diverse and had relatively more Deltaproteobacteria, whereas the communities from lower amendments were more diverse with a larger proportion of hgcAB sequences affiliated with other clades. Potential methylation rate constants varied 52-fold across the experiment. Both potential methylation rate constants and % MeHg were the highest in sediments from the lowest sulfate amendments, which had the most diverse hgcAB+ communities and relatively fewer hgcAB genes from clades associated with sulfate reduction. Although pore water sulfide concentration covaried with hgcAB diversity across our experimental sulfate gradient, major changes in the community of hgcAB+ organisms occurred prior to a significant buildup of sulfide in pore waters. Our results indicate that methylating communities dominated by diverse anaerobic microorganisms that do not reduce sulfate can produce MeHg as effectively as communities dominated by sulfate-reducing populations.

Isotopic Characterization of Atmospheric Gaseous Elemental Mercury by Passive Air Sampling.

Tracing emission sources and transformations of atmospheric mercury with Hg stable isotopes depends on the ability to collect amounts sufficient for reliable quantification. Commonly employed active sampling methods require power and long pumping times, which limits the ability to deploy in remote locations and at high spatial resolution and can lead to compromised traps. In order to overcome these limitations, we conducted field and laboratory experiments to assess the preservation of isotopic composition during sampling of gaseous elemental mercury (GEM) with a passive air sampler (PAS) that uses a sulfur-impregnated carbon sorbent and a diffusive barrier. Whereas no mass independent fractionation (MIF) was observed during sampling, the mass dependent fractionation (MDF, δ202Hg) of GEM taken up by the PAS was lower than that of actively pumped samples by 1.14 ± 0.24‰ (2SD). Because the MDF offset was consistent across field studies and laboratory experiments conducted at 5, 20, and 30 °C, the PAS can be used for reliable isotopic characterization of GEM (±0.3‰ for MDF, ±0.05‰ for MIF, 2SD). The MDF offset occurred more during the sorption of GEM rather than during diffusion. PAS field deployments confirm the ability to record differences in the isotopic composition of GEM (i) with distance from point sources and (ii) sampled at different background locations globally.

Investigating the Sources and Transport of Benzotriazole UV Stabilizers during Rainfall and Snowmelt across an Urbanization Gradient.

Benzotriazole UV stabilizers (BT-UVs) have attracted increasing attention due to their bioaccumulative nature and ubiquitous presence in surface waters. We apply high-frequency sampling in paired watersheds to describe, for the first time, the behavior of BT-UVs in stream channels during snowmelt and rainfall. Relative to a largely agricultural watershed, concentrations of BT-UVs in an urban watershed were 4-90 times greater during rainfall and 3-21 times greater during snowmelt. During rainfall, a decrease in BT-UV concentrations on particles with increasing suspended sediments and streamflow occurred at all urban sites due to input of relatively clean sediments, while both decreases and increases were observed at rural sites. Where increases occurred in the rural watershed, road sediments were consistently suggested as the source. Contrasts between the urban and rural sites were also observed during snowmelt. While BT-UV concentrations on particles peaked with peak suspended sediment levels at urban stream sites, the opposite was true at rural stream sites. This appeared to be driven partially by different snowpack melt rates in the two watersheds, with earlier melt and presumably higher streamflow facilitating suspension or erosion of more contaminated sediment in the urban stream. In general, it appears that relatively high, consistent emissions in the form of informal (plastic) debris disposal by consumers or industrial releases have likely led to more homogeneous BT-UV profiles and temporal behavior in the urban watershed. In the rural watershed, low emissions instead entail that emissions variability is more likely to translate to variability in chemical profiles and temporal behavior.

Assessing the Source-to-Stream Transport of Benzotriazoles during Rainfall and Snowmelt in Urban and Agricultural Watersheds.

While benzotriazoles (BTs) are ubiquitous in urban waters, their sources and transport remain poorly characterized. We aimed to elucidate the origin and hydrological pathways of BTs in Toronto, Canada, by quantifying three BTs, electrical conductivity, and δ18O in high-frequency streamwater samples taken during two rainfall and one snowmelt event in two watersheds with contrasting levels of urbanization. Average concentrations of total BTs (∑BT) were 1.3 to 110 times higher in the more urbanized Mimico Creek watershed relative to the primarily agricultural and suburban Little Rouge Creek. Strong correlations between upstream density of major roads and total BT concentrations or BT composition within all events implicate vehicle fluids as the key source of BTs in both watersheds. Sustained historical releases of BTs within the Mimico Creek watershed have likely led to elevated ∑BT in groundwater, with elevated concentrations observed during baseflow that are diluted by rainfall and surface runoff. In contrast, relatively constant concentrations, caused by mixing of equally contaminated baseflow and rainfall/surface runoff, are observed in the Little Rouge Creek throughout storm hydrographs, with an occasional first flush occurring at a subsite draining suburban land. During snowmelt, buildup of BTs in roadside snowpiles and preferential partitioning of BTs to the liquid phase of a melting snowpack leads to early peaks in ∑BT in both streams, except the sites in the Little Rouge Creek with low levels of vehicle traffic. Overall, a history of BT release and land use associated with urbanization have led to higher levels of BTs in urban areas and provide a glimpse into future BT dynamics in mixed use, (sub)urbanizing areas.

Accumulation of Methylmercury in Invertebrates and Masked Shrews (Sorex cinereus) at an Upland Forest-Peatland Interface in Northern Minnesota, USA.

Mercury (Hg) methylation is often elevated at the terrestrial-peatland interface, but methylmercury (MeHg) production at this "hot spot" has not been linked with in situ biotic accumulation. We examined total Hg and MeHg levels in peat, invertebrates and tissues of the insectivore Sorex cinereus (masked shrew), inhabiting a terrestrial-peatland ecotone in northern Minnesota, USA. Mean MeHg concentrations in S. cinereus (71 ng g-1) fell between concentrations measured in spiders (mean 70-140 ng g-1), and ground beetles and millipedes (mean 29-42 ng g-1). Methylmercury concentrations in S. cinereus increased with age and differed among tissues, with highest concentrations in kidneys and muscle, followed by liver and brain. Nearly all Hg in S. cinereus was in the methylated form. Overall, the high proportional accumulation of MeHg in peat at the site (3.5% total Hg as MeHg) did not lead to particularly elevated concentrations in invertebrates or shrews, which are below values considered a toxicological risk.

Susceptibility of Soil Bound Mercury to Gaseous Emission As a Function of Source Depth: An Enriched Isotope Tracer Investigation.

Soil mercury (Hg) emissions are an important component of the global Hg cycle. Sunlight induced photoreduction of oxidized Hg to gaseous elemental Hg is an important mechanism controlling emissions from the soil surface, however we currently understand little about how subsurface Hg stores participate in gaseous Hg cycling. Our study objective was to investigate the ability of Hg at deeper soil depths to participate in emissions. Soil fluxes were measured under controlled laboratory conditions utilizing an enriched stable Hg isotope tracer buried at 0, 1, 2, and 5 cm below the surface. Under dry and low-light conditions, the Hg isotope tracer buried at the different depths participated similarly in surface emissions (median flux: 7.5 ng m(-2) h(-1)). When the soils were wetted, Hg isotope tracer emissions increased significantly (up to 285 ng m(-2) h(-1)), with the highest fluxes (76% of emissions) originating from the surface 1 cm amended soils and decreasing with depth. Mercury associated with sandy soil up to 6 cm below the surface can be emitted, clearly demonstrating that volatilization can occur via processes unrelated to sunlight. These results have important implications for considering how long older, legacy soil Hg contamination continues to cycle between soil and atmosphere.

Impacts of forest harvesting on mercury concentrations and methylmercury production in boreal forest soils and stream sediment.

Methylmercury (MeHg) is the most neurotoxic and bioaccumulative form of mercury (Hg) present in the terrestrial and aquatic food sources of boreal ecosystems, posing potential risks to wildlife and human health. Harvesting impacts on Hg methylation and MeHg concentrations in forest soils and stream sediment are not fully understood. In this study, a field investigation was carried out in 4 harvested and 2 unharvested boreal forest watersheds, before and after harvest, to better understand impacts on Hg methylation and MeHg concentration in soils and stream sediment, including their responses to different forest management practices. Changes in total Hg (THg) and MeHg concentrations, first-order potential rate constants for Hg methylation and MeHg demethylation (Kmeth and Kdemeth) as well as total carbon content and carbon-to-nitrogen ratio post-harvest in upland, wetland and riparian soils and stream sediment were assessed and compared. Increases in MeHg production were minimal in upland, wetland or riparian soils after harvest. Sediment in streams with minor buffer protection (∼3 m), greater fractions (>75%) of harvested watershed area and more road construction had significantly increased THg and MeHg concentrations, %-MeHg, Kmeth and total carbon content post-harvest. From these patterns, we infer that inputs of carbon and inorganic Hg into harvest-impacted stream sediment are likely sourced from the harvested upland areas and stimulate in situ MeHg production in stream sediment. These findings indicate the importance of stream sediment as potential MeHg pools in harvested forest watersheds. The findings also demonstrate that forest management practices aiming to mitigate organic matter and Hg inputs to streams can effectively alleviate harvesting impacts on Hg methylation and MeHg concentrations in stream sediment.

Methylmercury photodegradation in paddy water: An overlooked process mitigating methylmercury risks.

Photodegradation is critical to reduce the potent neurotoxic methylmercury (MeHg) in water and its subsequent accumulation along food chains. However, this process has been largely ignored in rice paddies, which are hotspots of MeHg production and receive about a quarter of the world's developed freshwater resources. Here, we reported that significant MeHg photodegradation, primarily mediated by hydroxyl radicals, occurs in the overlying water during rice growth. By incorporating field-measured light interception into a rice paddy biogeochemistry model, as well as photodegradation rates obtained from 42 paddy soils stretching ∼3500 km across China, we estimated that photodegradation reduced MeHg concentrations in paddy water and rice by 82 % and 11 %, respectively. Without photodegradation, paddy water could be a significant MeHg source for downstream ecosystems, with an annual export of 178 - 856 kg MeHg to downstream waters in China, the largest rice producer. These findings suggest that photodegradation in paddy water is critical for preventing greater quantities of MeHg entering human food webs.

A hidden demethylation pathway removes mercury from rice plants and mitigates mercury flux to food chains.

Dietary exposure to methylmercury (MeHg) causes irreversible damage to human cognition and is mitigated by photolysis and microbial demethylation of MeHg. Rice (Oryza sativa L.) has been identified as a major dietary source of MeHg. However, it remains unknown what drives the process within plants for MeHg to make its way from soils to rice and the subsequent human dietary exposure to Hg. Here we report a hidden pathway of MeHg demethylation independent of light and microorganisms in rice plants. This natural pathway is driven by reactive oxygen species generated in vivo, rapidly transforming MeHg to inorganic Hg and then eliminating Hg from plants as gaseous Hg°. MeHg concentrations in rice grains would increase by 2.4- to 4.7-fold without this pathway, which equates to intelligence quotient losses of 0.01-0.51 points per newborn in major rice-consuming countries, corresponding to annual economic losses of US$30.7-84.2 billion globally. This discovered pathway effectively removes Hg from human food webs, playing an important role in exposure mitigation and global Hg cycling.

Spatial and seasonal patterns of mercury concentrations, methylation and demethylation in central Canadian boreal soils and stream sediment.

Terrestrial ecosystems store large amounts of mercury (Hg), which may be subject to methylation, mobilization and uptake into downstream aquatic ecosystems. Mercury concentrations, methylation and demethylation potentials are not well characterized simultaneously across different habitats in boreal forest ecosystems, particularly not so in stream sediment, leading to uncertainties about the importance of various habitats as primary production areas of the bioaccumulative neurotoxin methylmercury (MeHg). In this study, we collected soil and sediment samples from 17 undisturbed, central Canadian boreal forested watersheds during spring, summer and fall to robustly characterize the spatial (upland and riparian/wetland soils, and stream sediment) and seasonal patterns of total Hg (THg) and MeHg concentrations. Mercury methylation and MeHg demethylation potentials (Kmeth and Kdemeth) in the soils and sediment were also assessed using enriched stable Hg isotope assays. We found the highest Kmeth and %-MeHg in stream sediment. In both riparian and wetland soils, Hg methylation was lower and less seasonally variable compared to stream sediment, but had comparable MeHg concentrations, suggesting longer-term storage of MeHg produced in these soils. Soil and sediment carbon content, and THg and MeHg concentrations were strong covariates across habitats. Additionally, sediment carbon content was important for delineating between stream sediment with relatively high vs. relatively low Hg methylation potential, which generally separated between different landscape physiographies. Broadly, this large and spatiotemporally diverse dataset is an important baseline for understanding Hg biogeochemistry in boreal forests both in Canada and possibly in many other boreal systems globally. This work is particularly important with respect to future possible impacts from natural and anthropogenic perturbations, which are increasingly straining boreal ecosystems in various parts of the world.

Singular and combined effects of blowdown, salvage logging, and wildfire on forest floor and soil mercury pools.

A number of factors influence the amount of mercury (Hg) in forest floors and soils, including deposition, volatile emission, leaching, and disturbances such as fire. Currently the impact on soil Hg pools from other widespread forest disturbances such as blowdown and management practices like salvage logging are unknown. Moreover, ecological and biogeochemical responses to disturbances are generally investigated within a single-disturbance context, with little currently known about the impact of multiple disturbances occurring in rapid succession. In this study we capitalize on a combination of blowdown, salvage logging and fire events in the sub-boreal region of northern Minnesota to assess both the singular and combined effects of these disturbances on forest floor and soil total Hg concentrations and pools. Although none of the disturbance combinations affected Hg in mineral soil, we did observe significant effects on both Hg concentrations and pools in the forest floor. Blowdown increased the mean Hg pool in the forest floor by 0.76 mg Hg m(-2) (223%). Salvage logging following blowdown created conditions leading to a significantly more severe forest floor burn during wildfire, which significantly enhanced Hg emission. This sequence of combined events resulted in a mean loss of approximately 0.42 mg Hg m(-2) (68% of pool) from the forest floor, after conservatively accounting for potential losses via enhanced soil leaching and volatile emissions between the disturbance and sampling dates. Fire alone or blowdown followed by fire did not significantly affect the total Hg concentrations or pools in the forest floor. Overall, unexpected consequences for soil Hg accumulation and by extension, atmospheric Hg emission and risk to aquatic biota, may result when combined impacts are considered in addition to singular forest floor and soil disturbances.

Methylmercury declines in a boreal peatland when experimental sulfate deposition decreases.

Between 2001 and 2008 we experimentally manipulated atmospheric sulfate-loading to a small boreal peatland and monitored the resulting short and long-term changes in methylmercury (MeHg) production. MeHg concentrations and %MeHg (fraction of total-Hg (Hg(T)) present as MeHg) in the porewaters of the experimental treatment reached peak values within a week of sulfate addition and then declined as the added sulfate disappeared. MeHg increased cumulatively over time in the solid-phase peat, which acted as a sink for newly produced MeHg. In 2006 a "recovery" treatment was created by discontinuing sulfate addition to a portion of the experimentally treated section to assess how MeHg production might respond to decreased sulfate loads. Four years after sulfate additions ceased, MeHg concentrations and %MeHg had declined significantly from 2006 values in porewaters and peat, but remained elevated relative to control levels. Mosquito larvae collected from each treatment at the end of the experiment exhibited Hg(T) concentrations reflective of MeHg levels in the peat and porewaters where they were collected. The proportional responses of invertebrate Hg(T) to sulfate deposition rates demonstrate that further controls on sulfur emissions may represent an additional means of mitigating Hg contamination in fish and wildlife across low-sulfur landscapes.

Inter-annual and spatial variability in hillslope runoff and mercury flux during spring snowmelt.

Spring snowmelt is an important period of mercury (Hg) export from watersheds. Limited research has investigated the potential effects of climate variability on hydrologic and Hg fluxes during spring snowmelt. The purpose of this research was to assess the potential impacts of inter-annual climate variability on Hg mobility in forested uplands, as well as spatial variability in hillslope hydrology and Hg fluxes. We compared hydrological flows, Hg and solute mobility from three adjacent hillslopes in the S7 watershed of the Marcell Experimental Forest, Minnesota during two very different spring snowmelt periods: one following a winter (2009-2010) with severely diminished snow accumulation (snow water equivalent (SWE) = 48 mm) with an early melt, and a second (2010-2011) with significantly greater winter snow accumulation (SWE = 98 mm) with average to late melt timing. Observed inter-annual differences in total Hg (THg) and dissolved organic carbon (DOC) yields were predominantly flow-driven, as the proportion by which solute yields increased was the same as the increase in runoff. Accounting for inter-annual differences in flow, there was no significant difference in THg and DOC export between the two snowmelt periods. The spring 2010 snowmelt highlighted the important contribution of melting soil frost in the timing of a considerable portion of THg exported from the hillslope, accounting for nearly 30% of the THg mobilized. Differences in slope morphology and soil depths to the confining till layer were important in controlling the large observed spatial variability in hydrological flowpaths, transmissivity feedback responses, and Hg flux trends across the adjacent hillslopes.

Elemental composition of sediments in Lake Jinzai, Japan: assessment of sources and pollution.

Bottom sediments from Lake Jinzai in southwest Japan were analyzed to determine their chemical compositions and to assess the potential for ecological harm by comparison with sediment quality guidelines. The pollution status of lake sediments was evaluated by employing contamination factor (CF), pollution load index (PLI), and geoaccumulation index (I(geo)), focusing on a suite of elements in lakebed and core sediments. Elevated concentrations of As, Pb, Zn, Cu, TOC, N, and P were present in several layers of the upper core and other surface sediments. The elevated metal concentrations are likely related to the fine-grained nature of the sediments, reducing bottom conditions produced by abundant organic matter, and possibly minor non-point anthropogenic sources. Moreover, correlations between the concentrations of trace metals and organic carbon, nitrogen, phosphorus, and iron, suggest that these elements play a role in controlling abundances. Calculated CF, PLI, and I(geo) indicate that the sediments are strongly polluted with respect to As, moderately to strongly polluted with Zn, and moderately polluted with Pb and Cu. Metal concentrations exceed the New York State Department of Environmental Conservation (NYSDEC) lowest effect level and the Canadian Council of Ministers of the Environment (CCME) interim sediment quality guidelines that indicate moderate impact on aquatic organisms in the study area.

Mercury methylation and methylmercury demethylation in boreal lake sediment with legacy sulphate pollution.

Sulphate and dissolved organic matter (DOM) in freshwater systems may regulate the formation of methylmercury (MeHg), a potent neurotoxin that biomagnifies in aquatic ecosystems. While many boreal lakes continue to recover from decades of elevated atmospheric sulphate deposition, little research has examined whether historically high sulphate concentrations can result in persistently elevated MeHg production and accumulation in aquatic systems. This study used sediment from a historically sulphate-impacted lake and an adjacent reference lake in northwestern Ontario, Canada to investigate the legacy effects of sulphate pollution, as well as the effects of newly added sulphate, natural organic matter (NOM) of varying sulphur content and a sulphate reducing bacteria (SRB) inhibitor on enhancing or inhibiting the Hg methylation and demethylation activity (Kmeth and Kdemeth) in the sediment. We found that Kmeth and MeHg concentrations in sulphate-impacted lake sediment were significantly greater than in reference lake sediment. Further adding sulphate or NOM with different sulphur content to sediment of both lakes did not significantly change Kmeth. The addition of a SRB inhibitor resulted in lower Kmeth only in sulphate-impacted sediment, but methylation was not entirely depressed. Methylmercury demethylation potentials in sediment were consistent across lakes and experimental treatments, except for some impacts related to SRB inhibitor additions in the reference lake sediment. Overall, a broader community of microbes beyond SRB may be methylating Hg and demethylating MeHg in this system. This study reveals that legacies of sulphate pollution in boreal lakes may persist for decades in stimulating elevated Hg methylation in sediment.

Effects of forest management on mercury bioaccumulation and biomagnification along the river continuum.

Forest management can alter the mobilization of mercury (Hg) into headwater streams and its conversion to methylmercury (MeHg), the form that bioaccumulates in aquatic biota and biomagnifies through food webs. As headwater streams are important sources of organic materials and nutrients to larger systems, this connectivity may also increase MeHg in downstream biota through direct or indirect effects of forestry on water quality or food web structure. In this study, we collected water, seston, food sources (biofilm, leaves, organic matter), five macroinvertebrate taxa and fish (slimy sculpin; Cottus cognata) at 6 sites representing different stream orders (1-5) within three river basins with different total disturbances from forestry (both harvesting and silviculture). Methylmercury levels were highest in water and some food sources from the basin with moderate disturbance (greater clearcutting but less silviculture). Water, leaves, stoneflies and fish increased in MeHg or total Hg along the river continuum in the least disturbed basin, and there were some dissipative effects of forest management on these spatial patterns. Trophic level (δ15N) was a significant predictor of MeHg (and total Hg in fish) within food webs across all 18 sites, and biomagnification slopes were significantly lower in the basin with moderate total disturbance but not different in the other two basins. The elevated MeHg in lower trophic levels but its reduced trophic transfer in the basin with moderate disturbance was likely due to greater inputs of sediments and of dissolved organic carbon that is more humic, as these factors are known to both increase transport of Hg to streams and its uptake in primary producers but to also decrease MeHg bioaccumulation in consumers. Overall, these results suggest that the type of disturbance from forestry affects MeHg bioaccumulation and trophic transfer in stream food webs and some longitudinal patterns along a river continuum.