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

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.

Climate and industrial pollution determine the seasonal and spatial mercury variations in the China's Weihe River

Natural processes and human activities impact mercury (Hg) pollution in rivers. Investigating the individual contributions and interactions of factors affecting variations in Hg concentrations, particularly under climate change, is crucial for safeguarding watershed ecosystems and human health. We collected 381 water samples from China's Weihe River Basin (WRB) during dry and wet seasons to assess the total Hg (THg) concentration. Results revealed high Hg concentrations in the WRB (0.1-2200.9 ng/L, mean 126.2 ± 335.5 ng/L), with higher levels during the wet season (wet season: 249.1 ± 453.5 ng/L, dry season: 12.7 ± 14.0 ng/L), particularly in the mainstream and southern tributaries of the Weihe River. Industrial pollution (contributing 26.2 %) and precipitation (contributing 33.5 %) drove spatial heterogeneity in THg concentrations during the dry and wet seasons, respectively. Notably, combined explanatory power increased to 47.9 % when interaction was considered, highlighting the amplifying effect of climate change, particularly precipitation, on the impact of industrial pollution. The middle and downstream of the Weihe River, especially the Guanzhong urban agglomeration, were identified as high-risk regions for Hg pollution. With ongoing climate change the risk of Hg exposure in the WRB is expected to escalate. This study lays a robust scientific foundation for the effective management of Hg pollution in analogous river systems worldwide.

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.

Variation of Hg concentration and accumulation in the soil of maritime pine plantations along a coast-inland transect in SW Europe

Climatic conditions have been shown as a major driver of the fate of Hg in forest ecosystems at a global scale, but less is known about climatic effects at shorter scales. This study assesses whether the concentration and pools of Hg in soils collected from seventeen Pinus pinaster stands describing a coastal-inland transect in SW Europe vary along a regional climatic gradient. In each stand, samples of the organic subhorizons (OL, OF + OH) and the mineral soil (up to 40 cm) were collected and some general physico-chemical properties and total Hg (THg) were analyzed. Total Hg was significantly higher in the OF + OH than in the OL subhorizons (98 and 38 μg kg^-1, respectively), favored by a greater organic matter humification in the former. In the mineral soil, mean THg values decreased with depth, ranging from 96 μg kg^-1 in the 0-5 cm layers to 54 μg kg^-1 in the deepest layers (30-40 cm), respectively. The average Hg pool (PHg) was 0.30 mg m^-2 in the organic horizons (92% accumulated in the OF + OH subhorizons), and 27.4 mg m^-2 in the mineral soil. Changes in climatic factors, mainly precipitation, along the coast-inland transect resulted in a remarkable variation of THg in the OL subhorizons, consistent with their role as the first receiver of atmospheric Hg inputs. The high precipitation rate and the occurrence of fogs in coastal areas characterized by the oceanic influence would explain the higher THg found in the uppermost soil layers of pine stands located close to the coastline. The regional climate is key to the fate of mercury in forest ecosystems by influencing the plant growth and subsequent atmospheric Hg uptake, the atmospheric Hg transference to the soil surface (wet and dry deposition and litterfall) and the dynamics that determine net Hg accumulation in the forest floor.

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.

The role of afforestation species as a driver of Hg accumulation in organic horizons of forest soils from a Mediterranean mountain area in SW Europe

Forest areas are a primary sink of atmospheric mercury (Hg) within terrestrial ecosystems, whereas forest vegetation plays a key role in atmospheric Hg transfer to soil horizons. This study assessed variations in total Hg contents (Hg_T) and accumulation (Hg_Res) in the soil organic horizons of a forest area in NE Portugal, where post-wildfire afforestation led to the substitution of the native deciduous species (Quercus pyrenaica) by fast-growing coniferous species (Pseudotsuga menziesii and Pinus nigra). The study also evaluated, for each species, the links between Hg contents and other biophilic elements of soil organic matter (C, N, S) present in organic subhorizons (OL, OF, OH). Mean Hg_T in the organic horizons of the different tree species follow the sequence: P. nigra (88 μg kg^-1) < Q.pyrenaica (101 μg kg^-1) <P. menziesii (141 μg kg^-1). The highest Hg_Res for the entire organic horizon was found under P. menziesii (471 μg m^-2), followed by P. nigra (253 μg m^-2) and Q. pyrenaica (189 μg m^-2). Among the organic subhorizons, values of Hg_T and Hg_Res follow the sequence OL < OF < OH, which is consistent with the degree of organic matter humification. Indeed, Hg_T and Hg_Res correlated significantly with the C/N and C/S ratios for all species and organic subhorizons, suggesting that the quality of organic matter may influence strongly the Hg fate in these forest soils. Soils from P. menziesii plots have shown an Hg_Res 2.5 times higher than in plots dominated by the native Q. pyrenaica. Hg accumulation in the organic horizons, promoted in the coniferous species, may increase the risk of Hg mobilization due to wildfires and forest management practices. Therefore, forest management plans should select cautiously the tree species for afforestation in order to minimize adverse environmental effects caused by changes in the biogeochemical cycle of contaminants such as Hg.

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.

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.

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.

Mercury transport, transformation and mass balance on a perspective of hydrological processes in a subtropical forest of China

Forest ecosystem has long been suggested as a vital component in the global mercury (Hg) biogeochemical cycling. However, there remains large uncertainties in understanding total Hg (THg) and methylmercury (MeHg) variations and their controlling factors during the whole hydrological processes in forest ecosystems. Here, we quantified Hg mass flow along hydrological processes of wet deposition, throughfall, stemflow, litter leachate, soil leachate, surface runoff, and stream, and litterfall Hg deposition, and air-forest floor elemental Hg (Hg⁰) exchange flux to set up a Hg mass balance in a subtropical forest of China. Results showed that THg concentration in stream was lower than that in wet deposition, while an opposite characteristic for MeHg concentration, and both THg and MeHg fluxes of stream were lower than those of wet deposition. Variations of THg and MeHg in throughfall and litter leachate had strong direct and indirect effects on controlling variations of THg and MeHg in surface runoff, soil leachate and stream, respectively. Especially, the net Hg methylation was suggested in the forest canopy and forest floor layers, and significant particulate bound Hg (PBM) filtration was observed in soil layers. The Hg mass balance showed that the litterfall Hg deposition was the main Hg input for forest floor Hg, and the elemental Hg vapor (Hg⁰) re-emission from forest floor was the dominant Hg output. Overall, we estimated the net THg input flux of 13.8 μg m^-2 yr^-1 and net MeHg input flux of 0.6 μg m^-2 yr^-1 within the forest ecosystem. Our results highlighted the important roles of forest canopy and forest floor to shape Hg in output flow, and the forest floor is a distinct sink of MeHg.

Timber harvest alters mercury bioaccumulation and food web structure in headwater streams

Timber harvest has many effects on aquatic ecosystems, including changes in hydrological, biogeochemical, and ecological processes that can influence mercury (Hg) cycling. Although timber harvest's influence on aqueous Hg transformation and transport are well studied, the effects on Hg bioaccumulation are not. We evaluated Hg bioaccumulation, biomagnification, and food web structure in 10 paired catchments that were either clear-cut in their entirety, clear-cut except for an 8-m wide riparian buffer, or left unharvested. Average mercury concentrations in aquatic biota from clear-cut catchments were 50% higher than in reference catchments and 165% higher than in catchments with a riparian buffer. Mercury concentrations in aquatic invertebrates and salamanders were not correlated with aqueous THg or MeHg concentrations, but rather treatment effects appeared to correspond with differences in the utilization of terrestrial and aquatic basal resources in the stream food webs. Carbon and nitrogen isotope data suggest that a diminished shredder niche in the clear-cut catchments contributed to lower basal resource diversity compared with the reference of buffered treatments, and that elevated Hg concentrations in the clear-cut catchments reflect an increased reliance on aquatic resources in clear-cut catchments. In contrast, catchments with riparian buffers had higher basal resource diversity than the reference catchments, indicative of more balanced utilization of terrestrial and aquatic resources. Further, following timber harvest THg concentrations in riparian songbirds were elevated, suggesting an influence of timber harvest on Hg export to riparian food webs. These data, coupled with comparisons of individual feeding guilds, indicate that changes in organic matter sources and associated effects on stream food web structure are important mechanisms by which timber harvest modifies Hg bioaccumulation in headwater streams and riparian consumers.

Transport Dynamics of Mercury in Rainfall-runoff After Summer Drought in a Small Agricultural Watershed in the Three Gorges Reservoir Region

The objective of this study was to investigate the transport dynamics of total mercury (THg) and methylmercury (MeHg) in the first rainfall-runoff event after summer drought, to understand flushing effects (FFEs) and to quantificationally estimate contributions to the annual outputs of Hg. The results showed that both THg and MeHg in rainfall-runoff predominated by particulate fraction peaked at the beginning of the monitoring period. On average, more than 80% of THg and MeHg loadings were transported during the initial runoff (≤ 6 h). Simultaneously, significant FFEs were observed for both THg and MeHg, with a larger effect for MeHg. More importantly, the estimated output fluxes of THg and MeHg in runoff produced by this rainfall event contributed 3.0% (THg) and 7.8% (MeHg) to the annual output fluxes, respectively, suggesting the importance of the first-rainfall on the Hg loss (especially for MeHg).

eDNA as a tool for identifying freshwater species in sustainable forestry: A critical review and potential future applications

Environmental DNA (eDNA) is an emerging biological monitoring tool that can aid in assessing the effects of forestry and forest manufacturing activities on biota. Monitoring taxa across broad spatial and temporal scales is necessary to ensure forest management and forest manufacturing activities meet their environmental goals of maintaining biodiversity. Our objectives are to describe potential applications of eDNA across the wood products supply chain extending from regenerating forests, harvesting, and wood transport, to manufacturing facilities, and to review the current state of the science in this context. To meet our second objective, we summarize the taxa examined with targeted (PCR, qPCR or ddPCR) or metagenomic eDNA methods (eDNA metabarcoding), evaluate how estimated species richness compares between traditional field sampling and eDNA metabarcoding approaches, and compare the geographical representation of prior eDNA studies in freshwater ecosystems to global wood baskets. Potential applications of eDNA include evaluating the effects of forestry and forest manufacturing activities on aquatic biota, delineating fish-bearing versus non fish-bearing reaches, evaluating effectiveness of constructed road crossings for freshwater organism passage, and determining the presence of at-risk species. Studies using targeted eDNA approaches focused on fish, amphibians, and invertebrates, while metagenomic studies focused on fish, invertebrates, and microorganisms. Rare, threatened, or endangered species received the least attention in targeted eDNA research, but are arguably of greatest interest to sustainable forestry and forest manufacturing that seek to preserve freshwater biodiversity. Ultimately, using eDNA methods will enable forestry and forest manufacturing managers to have data-driven prioritization for conservation actions for all freshwater species.