Impact of forestry on total and methyl-mercury in surface waters: distinguishing effects of logging and site preparation

Catchment land use effect on mercury concentrations in lake sediments: A high-resolution study of Qinghai Lake

Mercury (Hg) contamination in aquatic environments presents a significant ecological and human health concern. This study explored the relationship between catchment land use and Hg concentrations within Qinghai Lake sediment, the largest lake in China, situated on the Qinghai-Tibet plateau. The study entailed detailed mapping of Hg sediment concentrations and a subsequent environmental risk assessment. Considering the complex nature of the plateau landform and surface vegetation, the study area was delineated at a 100 km radius centered on Qinghai Lake, which was divided into 30 sectors to quantify relationships between land use and the sediment Hg concentration. The results revealed a mean sediment Hg concentration of 29.91 μg/kg, which was elevated above the background level. Kendall's correlation analysis revealed significant but weak associations between sediment Hg concentrations and three land use types: grassland (rangeland and trees) (rs = 0.27, p < 0.05), crops (rs = -0.37, p < 0.05), and bare ground (rs = -0.25, p < 0.1), suggesting that growing areas of grassland correlated with higher Hg levels in the lake sediment, in contrast to bare ground or crops area, which correlated with lower Hg concentrations. Multiple linear regression models also observed weak negative relationships between bare ground and crops with sediment Hg concentration. This research methodology enhances our understanding of the impact of land use on Hg accumulation in lake sediments and underscores the need for integrated watershed management strategies to mitigate Hg pollution in Qinghai Lake.

Mercury concentrations and export from small central Canadian boreal forest catchments before, during, and after forest harvest

Northern boreal forests are a strong sink for mercury (Hg), a global contaminant of significant concern to wildlife and human health. Mercury stored in forest soils can be mobilized via runoff and erosion, and under suitable conditions can be methylated to its much more bioaccumulative form, methylmercury. Forest harvesting can affect the mobilization and methylation of Hg, though the direction and magnitude of the impact is unclear or conflicting across previous studies. This study examined 5 harvested and 2 reference watersheds in northwestern Ontario, Canada, before, during, and after harvest to quantify changes in stream total and methylmercury concentration and loads and identified potential landscape and management factors that contribute to differences in stream response. In watersheds where streams were buffered by natural vegetation (≥30 m), no significant changes in total Hg or methylmercury concentrations or loads were observed. Significant increases in methylmercury concentrations and loads were observed downstream of a stream crossing in a watershed where the relatively small stream was unmapped and therefore only buffered by a 3 m machine exclusion zone. These results show that when current best management practices that minimize soil and water disturbance are followed, harvest can have a minimal impact on total and methylmercury loads, even in extensively harvested watersheds. However, there is a need for improved mapping of small streams to ensure best management practices are applied adequately across the landscape.

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.

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).

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.

Are fur farms a potential source of persistent organic pollutants or mercury to nearby freshwater ecosystems?

Farming of carnivorous animals for pelts potentially contaminates nearby ecosystems because animal feed and waste may contain persistent organic pollutants (POPs) and metals. Mink farms in Nova Scotia (NS), Canada, provide mink with feed partially composed of marine fish meal. To test whether mink farms potentially contribute contaminants to nearby lakes, we quantified organochlorine pesticides (OCP), polychlorinated biphenyls (PCB), and total mercury (THg) in mink/aquaculture feed, waste, and sediment collected from 14 lakes within rural southwest NS where mink farms are abundant and have operated for decades. Mercury, PCBs, dichlorodiphenyltrichloroethane (DDT), hexachlorocyclohexane (HCH), and dieldrin were present in mink/aquaculture feed and mink waste, indicating they are potential contaminant sources. Lakes with mink farms in their catchment exhibited significantly higher THg_flux than lakes downstream of mink farming activity and reference lakes (p < 0.0001) after the intensification of mink farming in 1980, indicating mink farming activity is likely associated with increased lacustrine THg_flux. Sedimentary ƩPCB_flux was elevated in lakes with mink farms in their catchments, suggesting possible PCB contributions from mink farming, local agriculture, and atmospheric deposition. Elevated ƩDDT in lakes near mink farms relative to reference lakes suggests a possible enrichment related to mink farming, although mixed land use and historical DDT usage related to forestry in the region complicates DDT source attribution. Maximum dieldrin_flux and HCH_flux in lake sediment occurred coeval with peak worldwide usage in the 1970s and are unlikely to be associated with local mink farming. Lakes with mink farming activities in their catchments were associated with increased THg_flux, ƩPCB_flux, and possibly ƩDDT_flux, suggesting a possible connection between marine fish meal, fur farms, and aquatic ecosystems in NS.

Are There Longitudinal Effects of Forest Harvesting on Carbon Quality and Flow and Methylmercury Bioaccumulation in Primary Consumers of Temperate Stream Networks?

Forest harvesting affects dissolved organic matter (DOM) and aqueous mercury inputs as well as the food web structure in small-headwater streams, but how these upstream changes manifest downstream is unclear. To address this uncertainty, we examined DOM quality, autochthony in the caddisfly Hydropsychidae (using δ² H), and methylmercury (MeHg) concentrations in stream water and the caddisfly along a longitudinal gradient (first- to fourth-order streams, subcatchments of 50-1900 ha) in paired partially harvested and reference catchments in central Ontario, Canada. Although measures of DOM quality (specific ultraviolet absorbance at 254 nm 2.20-11.62) and autochthony in caddisflies (4.9%-34.0%) varied among sites, no upstream-to-downstream differences in these measures were observed between the paired harvested and reference catchments. In contrast, MeHg levels in stream water (0.06-0.35 ng/L) and caddisflies (29.7-192 µg/kg dry wt) were significantly higher in the upstream sites but not the farthest downstream sites in the harvested catchments compared to the reference catchments. This suggests that while current mitigation measures used by forestry companies did not prevent elevated MeHg in water and invertebrates at smaller spatial scales (subcatchments of 50-400 ha), these upstream impacts did not manifest at larger spatial scales (subcatchments of 800-1900 ha). The present study advances our understanding of spatially cumulative impacts within harvested catchments, which is critical to help forest managers maintain healthy forest streams and their provisioning of aquatic ecosystem services. Environ Toxicol Chem 2022;41:1490-1507. © 2022 SETAC.

Role of organic matter and microbial communities in mercury retention and methylation in sediments near run-of-river hydroelectric dams

Run-of-river power plants (RoRs) are expected to triple in number over the next decades in Canada. These structures are not anticipated to considerably promote the mobilization and transport of mercury (Hg) and its subsequent microbial transformation to methylmercury (MeHg), a neurotoxin able to biomagnify in food webs up to humans. To test whether construction of RoRs had an effect on Hg transport and transformation, we studied Hg and MeHg concentrations, organic matter contents and methylating microbial community abundance and composition in the sediments of a section of the St. Maurice River (Quebec, Canada). This river section has been affected by the construction of two RoR dams and its watershed has been disturbed by a forest fire, logging, and the construction of wetlands. Higher total Hg (THg) and MeHg concentrations were observed in the surface sediments of the flooded sites upstream of the RoRs. These peaks in THg and MeHg were correlated with organic matter proportions in the sediments (r² = 0.87 and 0.82, respectively). In contrast, the proportion of MeHg, a proxy for methylation potential, was best explained by the carbon to nitrogen ratio suggesting the importance of terrigenous organic matter as labile substrate for Hg methylation in this system. Metagenomic analysis of Hg-methylating communities based on the hgcA functional gene marker indicated an abundance of methanogens, sulfate reducers and fermenters, suggesting that these metabolic guilds may be primary Hg methylators in these surface sediments. We propose that RoR pondages act as traps for sediments, organic matter and Hg, and that this retention can be amplified by other disturbances of the watershed such as forest fire and logging. RoR flooded sites can be conducive to Hg methylation in sediments and may act as gateways for bioaccumulation and biomagnification of MeHg along food webs, particularly in disturbed watersheds.

Surface-air mercury fluxes and a watershed mass balance in forested and harvested catchments

Forest soils are among the world's largest repositories for long-term accumulation of atmospherically deposited mercury (Hg), and understanding the potential for remobilization through gaseous emissions, aqueous dissolution and runoff, or erosive particulate transport to down-gradient aquatic ecosystems is critically important for projecting ecosystem recovery. Forestry operations, especially clear-cut logging where most of the vegetaiton is removed, can influence Hg mobility/fluxes, foodweb dynamics, and bioaccumulation processes. This paper measured surface-air Hg fluxes from catchments in the Pacific Northwest, USA, to determine if there is a difference between forested and logged catchments. These measurements were conducted as part of a larger project on the impact of forestry operations on Hg cycling which include measurements of water fluxes as well as impacts on biota. Surface-air Hg fluxes were measured using a commonly applied dynamic flux chamber (DFC) method that incorporated diel and seasonal variability in elemental Hg (Hg⁰) fluxes at multiple forested and harvested catchments. The results showed that the forested ecosystem had depositional Hg⁰ fluxes throughout most of the year (annual mean: -0.26 ng/m²/h). In contrast, the harvested catchments showed mostly emission of Hg⁰ (annual mean: 0.63 ng/m²/h). Differences in solar radiation reaching the soil was the primary driver resulting in a shift from net deposition to emission in harvested catchments. The surface-air Hg fluxes were larger than the fluxes to water as runoff and accounted for 97% of the differences in Hg sequestered in forested versus harvested catchments.

Sequestration of Mercury in Soils under Scots Pine and Silver Fir Stands Located in the Proximity to a Roadway

Forest soils are the main source of mercury (Hg) in stream water. Stocks of Hg in forest soils are related to several factors, including forest species composition. In this study, the potential source of Hg pollution was a relatively new roadway traversing forested areas. We compared Hg accumulation in soils of two coniferous species: Scots pine (Pinus sylvestris L.) and silver fir (Abies alba Mill.). The experimental plots were located near the S7 expressway in Central Poland. The stands differed in the length of time they had been exposed to Hg, because different parts of the roadway were built and opened to traffic at different times. We analyzed 480 soil samples from organic horizons (O) and the top 10 cm of mineral soil (A) sampled from six plots. The overall average Hg concentrations (irrespective of forest stand, n = 240) was 0.225 mg kg⁻¹ in the O horizons and 0.075 mg kg⁻¹ in the mineral horizons. The Hg concentration in the O horizons was more than three times greater in fir stands than that in pine stands. The average Hg:C ratios in the O and A horizons were 1.0 and 2.3 mg Hg kg⁻¹ C, respectively. Our data does not clearly show the effect of road on Hg accumulation near the road. The concentrations of Hg in investigated soils adjacent to the roadway were only slightly higher than ranges reported for unpolluted areas, and no clearly affected by the vicinity of roadway. In contrast to the other reports, our data indicate a significant impact of tree species on Hg concentrations in both the O and A horizons. Moreover, the average Hg:C ratio was strongly dependent on the tree species.

Litterfall mercury reduction on a subtropical evergreen broadleaf forest floor revealed by multi-element isotopes

Litterfall mercury (Hg) deposition is the dominant source of soil Hg in forests. Identifying reduction processes and tracking the fate of legacy Hg on forest floor are challenging tasks. Interplays between isotopes of carbon (C) and nitrogen (N) may shed some lights on Hg biogeochemical processes because their biogeochemical cycling closely links with organic matters. Isotope measurements at the evergreen broadleaf forest floor at Mt. Ailao (Mountain Ailao) display that δ²⁰²Hg and Δ¹⁹⁹Hg both significantly correlate with δ¹³C and δ¹⁵N in soil profiles. Data analysis results show that microbial reduction is the dominant process for the distinct δ²⁰²Hg shift (up to ∼1.0‰) between Oi and 0-10 cm surface mineral soil, and dark abiotic organic matter reduction is the main cause for the Δ¹⁹⁹Hg shift (∼-0.18‰). Higher N in foliage leads to greater Hg concentration, and Hg⁰ re-emission via microbial reduction on forest floor is likely linked to N release and immobilization on forest floor. We thus suggest that the enhanced N deposition in global forest ecosystems can potentially influence Hg uptake by vegetation and litter Hg sequestration on forest floor.

Recent advances in understanding and measurement of mercury in the environment: Terrestrial Hg cycling

This review documents recent advances in terrestrial mercury cycling. Terrestrial mercury (Hg) research has matured in some areas, and is developing rapidly in others. We summarize the state of the science circa 2010 as a starting point, and then present the advances during the last decade in three areas: land use, sulfate deposition, and climate change. The advances are presented in the framework of three Hg "gateways" to the terrestrial environment: inputs from the atmosphere, uptake in food, and runoff with surface water. Among the most notable advances: These and other advances reported here are of value in evaluating the effectiveness of the Minamata Convention on reducing environmental Hg exposure to humans and wildlife.

Sources and transport of methylmercury in the Yangtze River and the impact of the Three Gorges Dam

The magnitude of environmental change due to anthropogenic impacts might greatly exceed that of natural disturbances. In this work, we quantitatively examine the impacts of river damming, soil erosion, and point-source release on the transport of methylmercury (MeHg) throughout the Yangtze River, the third longest river in the world. Based on seasonal observations and the subsequent material flow analysis, we found that in 2016, the Yangtze River discharged 470 ± 200 kg MeHg to the coastal and shelf areas, a value at least ten-fold larger than existing observations in other large rivers around the world. The construction of the Three Gorges Dam (TGD), the world's largest hydropower dam, induced a substantial amount of MeHg (at least 250 ± 220 kg) accumulation in the reservoir and a relatively small amount of MeHg (150 ± 37 kg) discharge to the downstream region in 2016. The reservoir itself is not expected to be more contaminated by MeHg than the downstream areas of the river after the TGD, and the TGD has an additive effect on downstream MeHg transport. The riverine MeHg flux in the river mouth was 3-fold that discharged from the TGD mainly due to TGD-induced resuspension of MeHg from the downstream riverbed, as well as MeHg imports to the downstream area from tributaries, soil erosion, municipal wastewater, and in situ production. Our analysis offers new evidence that in future decades, the increase in estuarine MeHg contamination resulting from the increasing construction of large dams might pose a challenge for global coastal fisheries.

Tree species affects the concentration of total mercury (Hg) in forest soils: Evidence from a forest soil inventory in Poland

This study was performed to test the hypothesis that tree species significantly affects mercury (Hg) sequestration in forest soils. We analyzed the effect of seven dominant tree species (Scots pine, black alder, Norway spruce, silver birch, deciduous oak, silver fir, and European beech on the concentrations and pools of Hg in a range of forest soils in Poland. We set up 277 sample plots representing dominant tree species in Poland. Soil samples were taken and analyzed for total Hg content, soil texture, and soil C and nitrogen (N) content. Concentrations of total Hg in forest soil (organic and mineral horizons) varied by several orders of magnitude as a result of natural variations in organic matter, sand content, and altitude. Spatial analysis revealed that maximum concentrations (mg kg^-1) and stocks (mg m^-2) of Hg were related to mountain stands at higher elevations with loamy soils and greater accumulation of soil organic matter. The stocks of Hg in the investigated soil profiles increased in the order of: pine (12 mg m^-2) ≈ birch (15 mg m^-2) < oak (21 mg m^-2) ≈ alder (24 mg m^-2) < beech (45 mg m^-2) ≈ spruce (50 mg m^-2) < fir (66 mg m^-2). Simple analysis of variance suggested an important effect of dominant tree species on Hg concentrations and stocks in entire soil profiles, but multiple regression analysis showed that dominant tree species had a significant effect on accumulation of Hg in soil, but only in the organic horizon; in mineral soil the Hg was content was related to C content, soil texture and altitude. The organic horizon had greater accumulation of Hg under coniferous tree species (Scots pine, silver fir and Norway spruce) and European beech when compared with deciduous oak, black alder, and silver birch.

Climate and productivity affect total mercury concentration and bioaccumulation rate of fish along a spatial gradient of subarctic lakes

Climate change is resulting in increased temperatures and precipitation in subarctic regions of Europe. These changes are extending tree lines to higher altitudes and latitudes, and enhancing tree growth enabling intensification of forestry into previously inhospitable subarctic regions. The combined effects of climate change and land-use intensification extend the warm, open-water season in subarctic lakes and increase lake productivity and may also increase leaching and methylation activity of mercury within the lakes. To assess the joint effects of climate and productivity on total mercury (THg) bioaccumulation in fish, we conducted a space-for-time substitution study in 18 tributary lakes of a subarctic watercourse forming a gradient from cold pristine oligotrophic lakes in the northern headwaters to warmer and increasingly human-altered mesotrophic and eutrophic systems in the southern lower reaches. Increasing temperature, precipitation, and lake productivity were predicted to elevate length- and age-adjusted THg concentrations, as well as THg bioaccumulation rate (the rate of THg bioaccumulation relative to length or age) in muscle tissue of European whitefish (Coregonus lavaretus), vendace (Coregonus albula), perch (Perca fluviatilis), pike (Esox lucius), roach (Rutilus rutilus) and ruffe (Gymnocephalus cernua). A significant positive relationship was observed between age-adjusted THg concentration and lake climate-productivity in vendace (r² = 0.50), perch (r² = 0.51), pike (r² = 0.55) and roach (r² = 0.61). Higher climate-productivity values of the lakes also had a positive linear (pike; r² = 0.40 and whitefish; r² = 0.72) or u-shaped (perch; r² = 0.64 and ruffe; r² = 0.50) relationship with THg bioaccumulation rate. Our findings of increased adjusted THg concentrations in planktivores and piscivores reveal adverse effects of warming climate and increasing productivity on these subarctic fishes, whereas less distinct trends in THg bioaccumulation rate suggest more complex underlying processes. Joint environmental stressors such as climate and productivity should be considered in ongoing and future monitoring of mercury concentrations.

Does forest harvest increase the mercury concentrations in fish? Evidence from Swedish lakes

A number of studies have evaluated the effects of forest harvest on mercury (Hg) concentrations and exports in surface waters, but few studies have tested the effect from forest harvest on the change in fish Hg concentrations over the course of several years after harvest. To address this question, mercury (Hg) concentrations in perch (Perca fluviatilis) muscle tissue from five lakes were analyzed for two years before (2010-2011) and three years after (2013-2015) forest harvest conducted in 2012. Fish Hg concentrations in the clear-cut catchments (n=1373 fish specimens) were related to temporal changes of fish Hg in reference lakes (n=1099 fish specimen) from 19 lakes in the Swedish National Environmental Monitoring Programme. Small (length<100mm) and large perch (length≥100mm) were analyzed separately, due to changing feeding habitats of fish over growing size. There was considerable year-to-year and lake-to-lake variation in fish Hg concentrations (-14%-121%) after forest harvest in the clearcut lakes, according to our first statistical model that count for fish Hg changes. While the effect ascribed to forest harvest varied between years, after three years (in 2015), a significant increase of 26% (p<0.0001) in Hg concentrations of large fish was identified in our second statistical model that pooled all 5 clearcut lakes. The large fish Hg concentrations in the 19 reference lakes also varied, and in 2015 had decreased by 7% (p=0.03) relative to the concentrations in 2010-2011. The majority of the annual changes in fish Hg concentrations in the clearcut lakes after harvest were in the lower range of earlier predictions for high-latitude lakes extrapolated primarily from the effects of forest harvest operations on Hg concentrations in water. Since the risk of forest harvest impacts on Hg extends to fish and not just surface water concentrations, there is even more reason to consider Hg effects in forestry planning, alongside other ecosystem effects.

Optical Properties of Dissolved Organic Matter and Their Relation to Mercury Concentrations in Water and Biota Across a Remote Freshwater Drainage Basin

Dissolved organic matter (DOM) includes an array of carbon-based compounds that vary in size and structure and have complex interactions with mercury (Hg) cycling in aquatic systems. While many studies have examined the relationship between dissolved organic carbon concentrations ([DOC]) and methyl Hg bioaccumulation, few studies have considered the effects of DOM composition (e.g., protein-content, aromaticity). The goal of this study was to explore the relationships between total and methyl [Hg] in water, invertebrates, and fish and optically derived measures of DOM composition from 47 lake and river sites across a boreal watershed. Results showed higher aqueous total [Hg] in systems with more aromatic DOM and higher [DOC], potentially due to enhanced transport from upstream or riparian areas. Methyl [Hg] in biota were all positively related to the amount of microbial-based DOM and, in some cases, to the proportions of labile and protein-like DOM. These results suggest that increased Hg bioaccumulation is related to the availability of labile DOM, potentially due to enhanced Hg methylation. DOM composition explained 68% and 54% more variability in [Hg] in surface waters and large-bodied fish, respectively, than [DOC] alone. These results show that optical measures of DOM characteristics are a valuable tool for understanding DOM-Hg biogeochemistry.

Challenges and opportunities for managing aquatic mercury pollution in altered landscapes

The environmental cycling of mercury (Hg) can be affected by natural and anthropogenic perturbations. Of particular concern is how these disruptions increase mobilization of Hg from sites and alter the formation of monomethylmercury (MeHg), a bioaccumulative form of Hg for humans and wildlife. The scientific community has made significant advances in recent years in understanding the processes contributing to the risk of MeHg in the environment. The objective of this paper is to synthesize the scientific understanding of how Hg cycling in the aquatic environment is influenced by landscape perturbations at the local scale, perturbations that include watershed loadings, deforestation, reservoir and wetland creation, rice production, urbanization, mining and industrial point source pollution, and remediation. We focus on the major challenges associated with each type of alteration, as well as management opportunities that could lessen both MeHg levels in biota and exposure to humans. For example, our understanding of approximate response times to changes in Hg inputs from various sources or landscape alterations could lead to policies that prioritize the avoidance of certain activities in the most vulnerable systems and sequestration of Hg in deep soil and sediment pools. The remediation of Hg pollution from historical mining and other industries is shifting towards in situ technologies that could be less disruptive and less costly than conventional approaches. Contemporary artisanal gold mining has well-documented impacts with respect to Hg; however, significant social and political challenges remain in implementing effective policies to minimize Hg use. Much remains to be learned as we strive towards the meaningful application of our understanding for stakeholders, including communities living near Hg-polluted sites, environmental policy makers, and scientists and engineers tasked with developing watershed management solutions. Site-specific assessments of MeHg exposure risk will require new methods to predict the impacts of anthropogenic perturbations and an understanding of the complexity of Hg cycling at the local scale.

Stream Mercury Export in Response to Contemporary Timber Harvesting Methods (Pacific Coastal Mountains, Oregon, USA)

Land-use activities can alter hydrological and biogeochemical processes that can affect the fate, transformation, and transport of mercury (Hg). Previous studies in boreal forests have shown that forestry operations can have profound but variable effects on Hg export and methylmercury (MeHg) formation. The Pacific Northwest is an important timber producing region that receives large atmospheric Hg loads, but the impact of forest harvesting on Hg mobilization has not been directly studied and was the focus of our investigation. Stream discharge was measured continuously, and Hg and MeHg concentrations were measured monthly for 1.5 years following logging in three paired harvested and unharvested (control) catchments. There was no significant difference in particulate-bound Hg concentrations or loads in the harvested and unharvested catchments which may have resulted from forestry practices aimed at minimizing erosion. However, the harvested catchments had significantly higher discharge (32%), filtered Hg concentrations (28%), filtered Hg loads (80%), and dissolved organic carbon (DOC) loads (40%) compared to forested catchments. MeHg concentrations were low (mostly <0.05 ng L^-1) in harvested, unharvested, and downstream samples due to well-drained/unsaturated soil conditions and steep slopes with high energy eroding stream channels that were not conducive to the development of anoxic conditions that support methylation. These results have important implications for the role forestry operations have in affecting catchment retention and export of Hg pollution.

Formation of mercury methylation hotspots as a consequence of forestry operations

Earlier studies have shown that boreal forest logging can increase the concentration and export of methylmercury (MeHg) in stream runoff. Here we test whether forestry operations create soil environments of high MeHg net formation associated with distinct microbial communities. Furthermore, we test the hypothesis that Hg methylation hotspots are more prone to form after stump harvest than stem-only harvest, because of more severe soil compaction and soil disturbance. Concentrations of MeHg, percent MeHg of total Hg (THg), and bacterial community composition were determined at 200 soil sampling positions distributed across eight catchments. Each catchment was either stem-only harvested (n=3), stem- and stump-harvested (n=2) or left undisturbed (n=3). In support of our hypothesis, higher MeHg to THg ratios was observed in one of the stump-harvested catchments. While the effects of natural variation could not be ruled out, we noted that most of the highest % MeHg was observed in water-filled cavities created by stump removal or driving damage. This catchment also featured the highest bacterial diversity and highest relative abundance of bacterial families known to include Hg methylators. We propose that water-logged and disturbed soil environments associated with stump harvest can favor methylating microorganisms, which also enhance MeHg formation.

Organic horizon and mineral soil mercury along three clear-cut forest chronosequences across the northeastern USA

Mercury (Hg) is a globally distributed pollutant trace metal that has been increasing in terrestrial environments due to rising anthropogenic emissions. Vegetation plays an important role in Hg sequestration in forested environments, but increasing tree removal for biofuels and wood products may affect this process. The long-term effect of clear-cutting on forest soil Hg remains uncertain, since most studies are limited to measuring changes for < 10 years following a single harvest event. The chronosequence approach, which substitutes space for time using forest stands of different ages since clear-cutting, allows for investigation of processes occurring over decades to centuries. Here, we utilized three clear-cut forest soil chronosequences across the northeastern USA to understand Hg accumulation and retention over several decades. Total Hg concentrations and pools were quantified for five soil depth increments along three chronosequences. Our results showed Hg concentrations and pools decreased in the initial 20 years following clear-cutting. Mineral soil Hg pools decreased 21-53% (7-14 mg m^-2) between 1-5-year-old stands and 15-25-year-old stands but mineral soil Hg pools recovered in 55-140-year-old stands to similar values as measured in 1-5-year-old stands. Our study is one of the first to demonstrate a decrease and recovery in Hg pool size. These changes in Hg did not correspond with changes in bulk density, soil C, or pH. We utilized a simple two-box model to determine how different Hg fluxes affected organic and mineral soil horizon Hg pools. Our simple model suggests that changes in litterfall and volatilization rates could have caused the observed changes in organic horizon Hg pools. However, only increases in leaching could reproduce observed decreases to mineral soil Hg pools. Further studies are needed to determine the mechanism of Hg loss from forest soils following clear-cutting.

Source tracing of natural organic matter bound mercury in boreal forest runoff with mercury stable isotopes

Terrestrial runoff represents a major source of mercury (Hg) to aquatic ecosystems. In boreal forest catchments, such as the one in northern Sweden studied here, mercury bound to natural organic matter (NOM) represents a large fraction of mercury in the runoff. We present a method to measure Hg stable isotope signatures of colloidal Hg, mainly complexed by high molecular weight or colloidal natural organic matter (NOM) in natural waters based on pre-enrichment by ultrafiltration, followed by freeze-drying and combustion. We report that Hg associated with high molecular weight NOM in the boreal forest runoff has very similar Hg isotope signatures as compared to the organic soil horizons of the catchment area. The mass-independent fractionation (MIF) signatures (Δ¹⁹⁹Hg and Δ²⁰⁰Hg) measured in soils and runoff were in agreement with typical values reported for atmospheric gaseous elemental mercury (Hg⁰) and distinctly different from reported Hg isotope signatures in precipitation. We therefore suggest that most Hg in the boreal terrestrial ecosystem originated from the deposition of Hg⁰ through foliar uptake rather than precipitation. Using a mixing model we calculated the contribution of soil horizons to the Hg in the runoff. At moderate to high flow runoff conditions, that prevailed during sampling, the uppermost part of the organic horizon (Oe/He) contributed 50-70% of the Hg in the runoff, while the underlying more humified organic Oa/Ha and the mineral soil horizons displayed a lower mobility of Hg. The good agreement of the Hg isotope results with other source tracing approaches using radiocarbon signatures and Hg : C ratios provides additional support for the strong coupling between Hg and NOM. The exploratory results from this study illustrate the potential of Hg stable isotopes to trace the source of Hg from atmospheric deposition through the terrestrial ecosystem to soil runoff, and provide a basis for more in-depth studies investigating the mobility of Hg in terrestrial ecosystems using Hg isotope signatures.

Mercury critical concentrations to Enchytraeus crypticus (Annelida: Oligochaeta) under normal and extreme conditions of moisture in tropical soils - Reproduction and survival

Soil provides many ecosystem services that are essential to maintain its quality and healthy development of the flora, fauna and human well-being. Environmental mercury levels may harm the survival and diversity of the soil fauna. In this respect, efforts have been made to establish limit values of mercury (Hg) in soils to terrestrial fauna. Soil organisms such as earthworms and enchytraeids have intimate contact with trace metals in soil by their oral and dermal routes, reflecting the potentially adverse effects of this contaminant. The main goal of this study was to obtain Hg critical concentrations under normal and extreme conditions of moisture in tropical soils to Enchytraeus crypticus to order to assess if climate change may potentiate their acute and chronic toxicity effects. Tropical soils were sampled from of two Forest Conservation Units of the Rio de Janeiro State - Brazil, which has been contaminated by Hg atmospheric depositions. Worms were exposed to three moisture conditions, at 20%, 50% and 80% of water holding capacity, respectively, and in combination with different Hg (HgCl₂) concentrations spiked in three types of tropical soil (two natural soils and one artificial soil). The tested concentrations ranged from 0 to 512mg Hg kg^-1 dry weight. Results indicate that the Hg toxicity is higher under increased conditions of moisture, significantly affecting survival and reproduction rate.

Toward an Assessment of the Global Inventory of Present-Day Mercury Releases to Freshwater Environments

Aquatic ecosystems are an essential component of the biogeochemical cycle of mercury (Hg), as inorganic Hg can be converted to toxic methylmercury (MeHg) in these environments and reemissions of elemental Hg rival anthropogenic Hg releases on a global scale. Quantification of effluent Hg releases to aquatic systems globally has focused on discharges to the global oceans, rather than contributions to freshwater systems that affect local exposures and risks associated with MeHg. Here we produce a first-estimate of sector-specific, spatially resolved global aquatic Hg discharges to freshwater systems. We compare our release estimates to atmospheric sources that have been quantified elsewhere. By analyzing available quantitative and qualitative information, we estimate that present-day global Hg releases to freshwater environments (rivers and lakes) associated with anthropogenic activities have a lower bound of ~1000 Mg· a-1. Artisanal and small-scale gold mining (ASGM) represents the single largest source, followed by disposal of mercury-containing products and domestic waste water, metal production, and releases from industrial installations such as chlor-alkali plants and oil refineries. In addition to these direct anthropogenic inputs, diffuse inputs from land management activities and remobilization of Hg previously accumulated in terrestrial ecosystems are likely comparable in magnitude. Aquatic discharges of Hg are greatly understudied and further constraining associated data gaps is crucial for reducing the uncertainties in the global biogeochemical Hg budget.

High mercury accumulation in two subtropical evergreen forests in South China and potential determinants

Forests play an important role in global mercury (Hg) cycling. To explain the high Hg accumulation in subtropical forest ecosystems, we studied temporal dynamics of Hg, carbon (C), nitrogen (N), and sulfur (S) in forest soil profiles, as well as litterfall flux and precipitation, in an old-growth moist evergreen broadleaf (EB) forest and a mossy coppice (MC) forest from South China over seven years. The mean soil Hg concentration was 257 ± 14 ng g^-1 in the O-horizon and 248 ± 15 ng g^-1 in the A-horizon for the EB forest, and 94 ± 27 ng g^-1 in the O-horizon and 70 ± 11 ng g^-1 in the A-horizon for the MC forest. Annual variations in Hg concentration were suggested to be associated with variations in precipitation and litterfall biomass. Significant vertical Hg transport was only observed in the MC forest, which was attributed to its lower organic matter content. Correlation and stoichiometry analyses further suggested that the dynamics in Hg concentration in the forest floor was also closely linked to the variation in S concentration. Additionally, the difference in the soil Hg pool between these two forests was attributed to different litterfall biomass fluxes.

Methyl Mercury Formation in Hillslope Soils of Boreal Forests: The Role of Forest Harvest and Anaerobic Microbes

Final harvest (clear-cutting) of coniferous boreal forests has been shown to increase streamwater concentrations and export of the neurotoxin methyl mercury (MeHg) to freshwater ecosystems. Here, the spatial distribution of inorganic Hg and MeHg in soil as a consequence of clear-cutting is reported. A comparison of soils at similar positions along hillslopes in four 80 years old Norway spruce (Picea abies) stands (REFs) with those in four similar stands subjected to clear-cutting (CCs) revealed significantly (p < 0.05) enhanced MeHg concentrations (ng g(-1)), MeHg areal masses (g ha(-1)), and percent MeHg of HgTOT in O horizons of CCs located between 1 and 41 m from streams. Inorganic Hg measures did not differ between REFs and CCs at any position. The O horizon thickness did not differ between CCs and REFs, but the groundwater table and soil water content were significantly higher at CCs than at REFs. The largest difference in percent MeHg of HgTOT (12 times higher at CCs compared to REFs, p = 0.003) was observed in concert with a significant enhancement in soil water content (p = 0.0003) at intermediate hillslope positions (20-38 m from stream), outside the stream riparian zone. Incubation experiments demonstrated that soils having significantly enhanced soil pools of MeHg after clear-cutting also showed significantly enhanced methylation potential as compared with similarly positioned soils in mature reference stands. The addition of inhibitors demonstrated that sulfate-reducing bacteria (SRB) and methanogens were key methylators. Rates of demethylation did not differ between CCs and REFs. Our results suggest that enhanced water saturation of organic soils providing readily available electron donors stimulate Hg-methylating microbes to net formation and buildup of MeHg in O horizons after forest harvest.

Impacts of forest harvesting on mobilization of Hg and MeHg in drained peatland forests on black schist or felsic bedrock

Forest harvesting, especially when intensified harvesting method as whole-tree harvesting with stump lifting (WTHs) are used, may increase mercury (Hg) and methylmercury (MeHg) leaching to recipient water courses. The effect can be enhanced if the underlying bedrock and overburden soil contain Hg. The impact of stem-only harvesting (SOH) and WTHs on the concentrations of Hg and MeHg as well as several other variables in the ditch water was studied using a paired catchment approach in eight drained peatland-dominated catchments in Finland (2008-2012). Four of the catchments were on felsic bedrock and four on black schist bedrock containing heavy metals. Although both Hg and MeHg concentrations increased after harvesting in all treated sites according to the randomized intervention analyses (RIAs), there was only a weak indication of a harvest-induced mobilization of Hg and MeHg into the ditches. Furthermore, no clear differences between WTHs and SOH were found, although MeHg showed a nearly significant difference (p = 0.06) between the harvesting regimes. However, there was a clear bedrock effect, since the MeHg concentrations in the ditch water were higher at catchments on black schist than at those on felsic bedrock. The pH, suspended solid matter (SSM), dissolved organic carbon (DOC), and iron (Fe) concentrations increased after harvest while the sulfate (SO4-S) concentration decreased. The highest abundances of sulfate-reducing bacteria (SRB) were found on the sites with high MeHg concentrations. The biggest changes in ditch water concentrations occurred first 2 years after harvesting.

Managing Swedish forestry's impact on mercury in fish: Defining the impact and mitigation measures

Inputs of anthropogenic mercury (Hg) to the environment have led to accumulation of Hg in terrestrial and aquatic ecosystems, contributing to fish Hg concentrations well above the European Union standards in large parts of Fennoscandia. Forestry operations have been reported to increase the concentrations and loads of Hg to surface waters by mobilizing Hg from the soil. This summary of available forestry effect studies reveals considerable variation in treatment effects on total Hg (THg) and methylmercury (MeHg) at different sites, varying from no effect up to manifold concentration increases, especially for the bioavailable MeHg fraction. Since Hg biomagnification depends on trophic structures, forestry impacts on nutrient flows will also influence the Hg in fish. From this, we conclude that recommendations for best management practices in Swedish forestry operations are appropriate from the perspective of mercury contamination. However, the complexity of defining effective policies needs to be recognized.

Influence of catchment vegetation on mercury accumulation in lake sediments from a long-term perspective

Organic matter (OM) cycling has a large impact on the cycling of mercury (Hg) in the environment. Hence, it is important to have a thorough understanding on how changes in, e.g., catchment vegetation - through its effect on OM cycling - affect the behavior of Hg. To test whether shifts in vegetation had an effect on Hg-transport to lakes we investigated a sediment record from Herrenwieser See (Southern Germany). This lake has a well-defined Holocene vegetation history: at ~8700years BP Corylus avellana (hazel) was replaced by Quercus robur (oak), which was replaced by Abies alba (fir) and Fagus sylvatica (beech) ~5700years BP). We were particularly interested in testing if coniferous vegetation leads to a larger export of Hg to aquatic systems than deciduous vegetation. When hazel was replaced by oak, reduced soil erosion and increased transport of DOM-bound mercury from the catchment resulted in increases in both Hg-concentrations and accumulation rates (61ngg(-1) and 5.5ngcm(-2)yr.(-)(1) to 118ngg(-1) and 8.5ngcm(-2)yr.(-)(1)). However, even if Hg-concentrations increased also in association with the introduction of fir and beech (173ngg(-1)), as a result of higher Hg:C, there was no increase in Hg-accumulation rates (7.6ngcm(-2)yr.(-)(1)), because of a decreased input of OM. At around 2500years BP Hg-accumulation rates and Hg-concentration indicated an additional input of Hg to the sediment (316ngg(-1) and 10.3ngcm(-2)yr.(-)(1)), which might be due to increased human activities in the area, e.g., forest burning or mining. Our results contrast those of several paired-catchment studies that suggest a higher release of Hg from coniferous than deciduous forest, and there is a need for studies with a long-term perspective to increase our understanding of the effects of slow and gradual processes on mercury cycling.

Parsimonious Model for Simulating Total Mercury and Methylmercury in Boreal Streams Based on Riparian Flow Paths and Seasonality

The complexity of mercury (Hg) biogeochemistry has made it difficult to model surface water concentrations of both total Hg (THg) and especially methylmercury (MeHg), the species of Hg having the highest potential for bioaccumulation. To simulate THg and MeHg variation in low-order streams, we have adapted a conceptual modeling framework where a continuum of lateral flows through riparian soils determines streamflow concentrations. The model was applied to seven forest catchments located in two boreal regions in Sweden spanning a range of climatic, soil, and forest management conditions. Discharge, and simulated riparian soil water concentrations profiles, represented by two calibrated parameters, were able to explain much of the variability of THg and MeHg concentrations in the streams issuing from the catchments (Nash Sutcliffe (NS) up to 0.54 for THg and 0.58 for MeHg). Model performance for all catchments was improved (NS up to 0.76 for THg and 0.85 for MeHg) by adding two to four parameters to represent seasonality in riparian soil water THg and MeHg concentrations profiles. These results are consistent with the hypothesis that riparian flow-pathways and seasonality in riparian soil concentrations are the major controls on temporal variation of THg and MeHg concentrations in low-order streams.