Carl P. J. Mitchell

Wetlands are critical sites for methylmercury (MeHg) production, a neurotoxin that bioaccumulates in organisms and biomagnifies in aquatic food webs. Net MeHg production in wetland soil depends on the balance between microbially mediated MeHg production and the degradation of MeHg through both microbial and abiotic pathways. Given that microbial activity is temperature sensitive, climate warming is expected to alter MeHg production in wetland soil. The temperature dependence of both mercury methylation and demethylation processes in boreal wetland soil remains unclear, thus inhibiting our ability to predict future changes. To address this research gap, we conducted a controlled, growth chamber‐based experiment in a closed flow‐through system incubating wetland soils across ecologically relevant temperatures (5°–25°C). Using additions of enriched mercury isotopes, we simultaneously measured first order rate constants for methylation (k _m ) and demethylation (k _d ). We found that temperature had a stronger effect on k _m than k _d , with no significant response from k _d to warming temperatures. Methylation peaked at 20°C (∼5x higher than the lowest temperature (5°C)), with changes in k _m being significantly related to wetland soil respiration, as measured by CO ₂ and CH ₄ production. Our study shows that mercury methylation becomes sulfate limited at higher temperatures (25°C), revealing a possible substrate availability negative feedback to mercury methylation. Thus, climate driven MeHg risks in boreal wetlands will depend on both warming and substrate supply interactions, which can feed back into methylation processes.

Beaver reservoirs and forestry are landscape disturbances that can increase the transfer of mercury (Hg) to stream food webs through increased mobilization from forest soils and methylation within the landscape, and through changes to food web structure. Both disturbances are widespread and co-occur throughout Canada’s boreal region, yet their combined effects on Hg bioaccumulation have not been studied. We sampled upstream and downstream of beaver reservoirs in harvested ( n = 3) and non-harvested ( n = 3) watersheds in Northwestern Ontario, Canada, for water, food sources (biofilm, leaves, and detritus), macroinvertebrate consumers (herbivorous and predatory), and top predators (fishes). When only harvesting effects were considered (upstream of reservoirs), Hg concentrations ([Hg]) of water, aquatic food sources and consumers, and trophic magnification slopes were highest in harvested compared to non-harvested watersheds; such effects were mostly absent downstream, indicating that effects of beaver reservoirs and harvest were not cumulative. In non-harvested landscapes, significantly higher biotic [Hg] was observed downstream of one reservoir only, potentially due to shifts in consumer diets from terrestrial to aquatic food sources and higher downstream availability of Hg. The most extensively harvested site displayed significant downstream decreases in biotic [Hg] and dissolved organic carbon, while the other two harvested sites had either increases in biotic [Hg], or no changes, indicating that reservoirs vary in their contribution to downstream Hg transfer, but may buffer the overall effects of extensive harvest on downstream Hg dynamics. Results provide new knowledge that helps forest managers incorporate natural disturbances (beaver impoundments) into evaluations and mitigations of forest management impacts on Hg in aquatic food webs.

Wetlands are critical sites for methylmercury (MeHg) production, a neurotoxin that bioaccumulates in organisms and biomagnifies in aquatic food webs. Net MeHg production in wetland soil depends on the balance between microbially-mediated MeHg production and the degradation of MeHg through both microbial and abiotic pathways. Given that microbial activity is temperature sensitive, climate warming is expected to alter MeHg production in wetland soil. The temperature dependence of both mercury methylation and demethylation processes in boreal wetland soil remains unclear, thus inhibiting our ability to predict future changes. To address this research gap, we conducted a controlled, growth chamber-based experiment in a closed flow-through system incubating wetland soils across ecologically relevant temperatures (5–25 °C). Using additions of enriched mercury isotopes, we simultaneously measured first order rate constants for methylation (km) and demethylation (kd). We found that temperature had a stronger effect on km than kd, with km peaking at 20 °C (~5x higher than the lowest temperature (5 °C)), with changes in km being significantly related to wetland soil respiration, as measured by CO2 and CH4 production. Our study shows that mercury methylation becomes sulfate limited at higher temperatures (25 °C), revealing a substrate availability negative feedback to methylation activity. Thus, climate driven MeHg risks in boreal wetlands will depend on both warming and substrate supply interactions, which can feed back into methylation processes.

Forest harvesting can lead to mercury (Hg) mobilization from soils to aquatic habitats and promote the transformation of inorganic Hg to highly neurotoxic and bioaccumulative methyl-Hg (MeHg). Multiple past studies reveal broad variation of stream water MeHg and total Hg (THg) concentration responses to forest harvesting, which has confounded messaging to forest and resource managers. To advance beyond divergent and sometimes contradictory findings, we synthesized information for 23 previously studied catchments in North America and Fennoscandia and compiled a uniform set of soil, landscape, and harvesting properties to identify forest management, riparian, and hillslope factors that influence responses of stream water MeHg and THg concentrations. From this synthesis, we found catchments with high soil moisture and organic soil layers >100 cm to be at highest risk for disturbance-induced increases in MeHg formation after harvest but not necessarily affecting concentrations of MeHg in stream waters. Instead, the combination of MeHg formation in soils along with factors that affect mobilization with runoff to streams most influenced how forest harvest affects MeHg concentrations in stream waters.

Artisanal and small-scale gold mining (ASGM) is one of the largest primary sources of mercury (Hg) pollution in the atmosphere globally; however, there is a paucity of atmospheric Hg data in ASGM areas. We measured atmospheric gaseous elemental mercury (GEM) concentrations and stable Hg isotopes at fine spatial resolution in the Madre de Dios region of Peru, where ASGM is a major source of Hg. This study employed new passive air samplers that overcome logistical challenges in measuring atmospheric Hg in remote locations. Regional GEM concentrations were elevated (∼1.3 to 11 ng m^-3) compared to the background (5000 ng m^-3) associated with mining areas and gold shops. Because ASGM-derived GEM is isotopically distinct, its contribution to regional and local atmospheric Hg was estimated using an isotope mixing model and found to be generally over 70%. We also show that vegetation is taking up ASGM-derived GEM, affecting both the concentrations and isotope compositions of GEM as well as in foliage and litter samples. This supports vegetation uptake as a key removal process of GEM from the atmosphere and therefore a major source of Hg to terrestrial ecosystems and soils, which is heightened in ASGM regions.

Beaver ponds and forest harvest are common disturbances in the Canadian boreal forest that result in major changes to catchment hydrology and thus also influence the mobilization and methylation of mercury (Hg). Though both beaver ponds and forest harvest frequently occur in the same watersheds, the possible interactive effects are not well understood. To evaluate the comparative effects of these two disturbances, this study examined in-stream total mercury and methylmercury (MeHg) across 7 stream reaches in the central Canadian boreal forest. Results showed that downstream-to-upstream MeHg concentration ratios were more highly correlated to the presence of beaver ponds than to the presence of forest harvest. However, MeHg concentrations upstream of ponds were higher in streams within harvested watersheds; these streams demonstrated a weaker correlation between beaver pond presence and downstream-to-upstream MeHg concentration ratios. Understanding these comparative and cumulative effects of beaver ponds and forest harvest will allow forest managers to consider how harvest activity could affect downstream MeHg in areas with high beaver activity.

The export of neurotoxic mercury (Hg) and its bioavailable form, methylmercury (MeHg), from wetland-dominated catchments is common throughout boreal regions. Wetlands vary significantly, however, in their degree of minerotrophy, which affects wetland Hg cycling and is related to their hydrogeomorphic setting. In this field study, we highlight how hydrogeomorphic setting, expressed as degree of minerotrophy, impacts streamwater total-Hg (THg) and MeHg export from Canadian Western Boreal Plain, fen-dominated catchments along a series of increasing minerotrophy (poor fen < moderate fen < channel fen < rich fen). Streamwater from the catchments dominated by less minerotrophic poor and moderate fens had the highest study period MeHg yields (13 and 19 mg km⁻², respectively) and percent MeHg in exported streamwater, while THg yield was greatest from the rich fen headwaters (260 mg km⁻²). MeHg and THg yields decreased within the rich fen. Decreases in the rich fen MeHg concentrations and yields coincides with greater total manganese, suggesting manganese redox chemistry may be important in regulating MeHg cycling and/or mobility in more minerotrophic wetlands. We extend previous studies showing some swamps to be net MeHg importers to include rich fens as another wetland type that removes MeHg from streamwaters and provides another possible mechanism, manganese reduction, that influences MeHg cycling.