Palsas and lithalsas are permafrost landforms that are ubiquitous parts of the subarctic landscape, commonly found in areas of discontinuous permafrost. Their degradation can lead to the formation of thermokarst lakes, releasing contaminants previously stored in permafrost, such as mercury (Hg). In this study, two thermokarst lakes (KANG1 and KANG2) were sampled in the winter and summer of 2023 in the Canadian subarctic (Kangiqsualujjuaq, Nunavik). The thermokarst lakes differ in origin, with KANG1 formed through lithalsa degradation and KANG2 through palsa degradation. Water column analyses included total Hg (THg) and monomethylmercury (MMHg). Bottom lake sediments were also analyzed, and incubation assays conducted to evaluate methylation and demethylation potential, as well as the contributions of key microbial methylators. Winter conditions appeared to favor Hg methylation and accumulation in the water column. In KANG1, the fraction of Hg that is present in the form of MMHg (%MMHg) ranged from 56.8 ± 2.9% to 100% (measured as 102.7 ± 1.8%) in winter, with lower summer values averaged 19.8 ± 1.0%, suggesting potential transport to adjacent freshwater systems during spring thaw. Sediment THg concentrations were similar across seasons, but the %MMHg and methylation rates were higher in winter exhibiting the same pattern observed in the water column. Conversely, KANG2 exhibited higher THg, %MMHg and methylation rates, reaching 0.42 ± 0.02 day-1 during summer. Incubation results highlighted the important role of sulfate-reducing bacteria (SRB) and methanogens in MMHg production in these systems and revealed distinct seasonal patterns in methylation and demethylation rates that may be linked to shifts in microbial guilds responsible for these processes.

Mercury (Hg) is a natural occurring element but is often emitted from anthropogenic sources and reaches the Arctic via long-range atmospheric transport. Organic matter (OM)-rich thermokarst lakes are characteristic features of the permafrost landscape in this region, where monomethylmercury (MMHg) production can be enhanced, as this process is mainly carried out by prokaryotes. To better understand the complex Hg biogeochemical cycle, two distinct thermokarst lakes (SAS 1A and SAS 2A) in sporadic permafrost in the Sasapimakwananistikw (SAS) River Valley, Canadian Subarctic, were sampled during winter and summer of 2022. Water column analysis showed no seasonal variation in total Hg (THg) and MMHg concentrations in SAS 2A but significantly higher THg and MMHg in winter in SAS 1A. Biogeochemical parameters affecting the activity of known methylating communities drive both inter-lake and seasonal variations in the water column. Strong correlations between MMHg and dissolved organic carbon (DOC) were found, with SAS 1A showing almost seven times more MMHg variability with DOC than SAS 2A. This difference is potentially linked to variations in OM composition between the sites. The lakes showed high THg seasonality in sediments, with higher concentrations in winter. In contrast, no seasonal variation was observed in MMHg concentrations with SAS 1A exhibiting higher values. Different divalent mercury (Hg(II)) bioavailability might explain these differences. By conducting incubation experiments with isotope-enriched Hg in the sediment, the important role of sulfate-reducing bacteria in the methylation process was revealed. This study highlights the complexity of thermokarst lakes which are increasing in Northern landscapes and might be hotspots for MMHg formation.

Dissolved organic carbon (DOC) concentrations have increased over the past few decades in surface waters across Europe and North America. This has drawn a lot of attention, given the key role of DOC in the global carbon cycle and in surface water biogeochemistry and ecology. While many reports have focused on DOC response to environmental changes in headwater streams and lakes taken separately, there is a lack of studies that combines streams and lakes with varying catchment characteristics in a network-scale perspective. Here, long-term (1987-2018) trends were analyzed and environmental drivers of year-to-year variations in DOC concentrations were examined in headwater streams, lakes and lake outflows at the Turkey Lakes Watershed (TLW) in Ontario, Canada. Results indicated significant increasing of DOC trends in ten out of 12 headwater streams and in four out of 12 lakes and lake outflows over the study period. In addition, piecewise regression analysis detected breakpoints in the 2000 s for DOC time series data in some stations. Multivariate analysis showed that variations in hydro-climatic conditions and the chemistry of atmospheric precipitations explained 13 % to 99 % of year-toyear variations in DOC concentrations. Air temperature emerged as the most influential factor for lakes and lake outflows while precipitation chemistry was the main driver of inter-annual DOC variation in headwater streams. For the latter, the rate of DOC increase and the proportion of explained variance were mainly dependent on catchment characteristics, notably wetland cover which was related to mean catchment slope and total relief. In the context of global change, further research is needed to better understand how changes in climate and atmospheric deposition may be modulated by catchment attributes and ecosystem types for determining future DOC fate and behaviour in surface waters.

Permafrost predominates in polar and high mountain regions, encompassing nearly 15 % of the exposed land in the Northern Hemisphere. It denotes soil or rock that remains at or below 0 °C for the duration of at least two consecutive years. These frozen soils serve as a barrier to contaminants that are stored and accumulated in permafrost over extended periods of time. One of these chemical compounds is mercury (Hg), a heavy metal well recognized for its severe toxic effects. Mercury presents a major risk worldwide to ecosystems, biota and human health and is strengthened by the Minamata Convention on Mercury. The International Panel on Climate Change (IPCC) scientific group monitors and assesses the science related to climate change and highlights the significant impacts of global warming. The phenomenon known as Arctic amplification has accentuated warming of the Arctic in recent years and has led to the degradation and rapid thawing of permafrost. This process has significant implications in hydrology of the ecosystems and for the mobility of previously sequestered carbon and trace metals, such as Hg, with possible adverse environmental and human health impacts. In this article, we provide a comprehensive review of the current understanding of the Hg cycle in permafrost regions, exploring the effects of global warming on these intricate processes. Additionally, we highlight existing research gaps and propose directions for future investigations.

Mercury (Hg) is a toxic metal that presents a major risk to ecosystems, biota, human health, and remains a priority concern. In temperate and boreal lakes Hg and methylmercury (MMHg) are expected to vary as a function of atmospheric Hg deposition, lake water chemistry, catchment characteristics and climate variables. The aim of this study was to quantify Hg and MMHg in unperturbed oligotrophic lakes and to identify the factors controlling their distribution. We first hypothesized that lake Hg (and MMHg to lesser extent) spatial variations are linked to atmospheric deposition, catchment characteristics, and terrestrial exportation of dissolved organic carbon (DOC). We secondly examined if lake Hg concentrations have followed the decrease in atmospheric Hg emission observed between the mid-1990s to the end-2010s. We found that overall, atmospheric Hg has little impact on lake Hg and MMHg concentrations, which are both primarily influenced by DOC input originating from the forest catchment. The relationship between DOC and Hg differed between the spring and the fall, with a Hg-to-DOC ratio twice as high in spring. This seems related to snowmelt input of Hg (with a relatively reduced input of DOC) or the internal lake build-up of Hg during the ice-covered period. Of the 10 lakes intensively visited over a 20-year period, only 3 showed significant lake Hg decreases despite significant negative trends in atmospheric Hg concentrations, suggesting a lag between atmospheric and surface water temporal trends. Overall, terrestrial catchments retain around 80% of atmospheric Hg implying that large Hg pools have been built up in soils in the last decades. As such, the reduction of atmospheric Hg alone will not necessarily result in Hg decreases in lakes, since the Hg concentrations may be modulated by DOC export trends and catchment characteristics. This stresses the need to improve our understanding of the processes governing Hg transfers from catchments into lakes.

Thermokarst ponds (thaw lakes) are ubiquitous in northern landscapes. They are hotspots for the biogeochemical processing of elements, such as carbon (C), nitrogen (N), sulfur (S), iron (Fe) and manganese (Mn). In turn, those elementary cycles may control the mobility of selenium (Se), an essential micronutrient. To unravel these coupled biogeochemical cycles and identify processes controlling Se mobility, we studied four thermokarst ponds in a subarctic peatland valley influenced by permafrost thaw. The data set comprises of water column and sediment porewater concentration profiles collected during both summer and winter. Physicochemical parameters and dissolved concentrations of major elements, nutrients, and Se were measured and used to model fluxes at the sediment-water interface and to calculate Se speciation. The results suggest that the proximity of the pond from the permafrost structures influenced their biogeochemical dynamics. In the ponds close to permafrost, Se concentrations are 2-fold higher in winter compared to summer, accompanied by an increase in sediment fluxes from 13 to 149 pmol cm(-2) yr(-1) between summer and winter. The combination of comparatively older dissolved organic matter and of oxygenated conditions explain the seasonal variation in Se concentrations. In the ponds further from the permafrost, Se concentrations are higher, remain unchanged in the water column across seasons, and are linearly correlated with both DOC (R-2 = 0.64, p < 0.01, n = 50) and Fe (R-2 = 0.60) concentrations. Thermodynamic calculations show that Se(IV) dominates Se speciation in the porewater at all sites, while the water column reaches saturation with respect to elemental zerovalent Se, suggesting that precipitation of elemental Se could mediate dissolved Se concentrations. Collectively, our results point to the strong control that redox conditions exert on Se mobility, via DOC and Fe, and to the linkages between landscape features, pond physicochemistry, and Se dynamics.

This study deployed 59 passive air samplers (PAS) across the province of Ontario, Canada (and eight additional deployments in bordering states of northern USA) to assess the influence of local/regional sources, temporal differences, and possible transformations in the atmospheric gaseous elemental mercury (GEM) pool. The spatial mapping achieved with the PASs allowed for the observation of differences in GEM and its isotopic composition over the region. Concentrations of GEM were higher with more negative δ 202 Hg values near urban/industrial areas and suspected emission sources in southern Ontario. In northern Ontario, far from industrial influences, lower concentrations of GEM with isotopically more positive δ 202 Hg values were found at the boreal forest sites compared to sites within the Hudson Bay Lowlands. The differences in northern Ontario may be from greater uptake of GEM by the boreal forest removing isotopically light Hg and lowering concentrations. Lower GEM and a shift toward more positive δ 202 Hg were also observed during the summer relative to other times of year supporting vegetation uptake as an important sink for GEM. PASs were also deployed along a transect of increasing distance from a Hg recycling facility. The Hg emissions from the facility had unique positive Δ 199 Hg and Δ 200 Hg signatures, but GEM concentrations and isotopic compositions returned to background within 400 m of the facility. The province‐wide variations in atmospheric GEM and isotopic compositions show that both sources (e.g., urban/industrial emissions) and sinks (e.g., vegetation uptake) contribute to the spatial and temporal patterns of the residual atmosphere GEM pool.

Mercury (Hg) is a pollutant of concern across Canada and transboundary anthropogenic Hg sources presently account for over 95% of national anthropogenic Hg deposition. This study applies novel statistical analyses of 82 high-resolution dated lake sediment cores collected from 19 regions across Canada, including nearby point sources and in remote regions and spanning a full west-east geographical range of ∼4900 km (south of 60°N and between 132 and 64°W) to quantify the recent (1990-2018) spatial and temporal trends in anthropogenic atmospheric Hg deposition. Temporal trend analysis shows significant synchronous decreasing trends in post-1990 anthropogenic Hg fluxes in western Canada in contrast to increasing trends in the east, with spatial patterns largely driven by longitude and proximity to known point source(s). Recent sediment-derived Hg fluxes agreed well with the available wet deposition monitoring. Sediment-derived atmospheric Hg deposition rates also compared well to the modeled values derived from the Hg model, when lake sites located nearby (<100 km) point sources were omitted due to difficulties in comparison between the sediment-derived and modeled values at deposition "hot spots". This highlights the applicability of multi-core approaches to quantify spatio-temporal changes in Hg deposition over broad geographic ranges and assess the effectiveness of regional and global Hg emission reductions to address global Hg pollution concerns.

The intense mining extraction of oil sand (OS) has increased over the last few decades, raising concerns about the release of OS contaminants and toxicity in resident aquatic organisms in the Athabasca River (Alberta, Canada). To address this, endemic Pyganodon grandis mussels were caged for 6 weeks at various upstream and downstream sites of industrial OS mining activities. Post-exposure mussels were then analyzed for light/medium/heavy polyaromatic hydrocarbons (PAHs) in tissues, general health (weight to length ratio, growth rate, air survival time), biotransformation (cytochrome P4501A and 3A and glutathione S-transferase activities), oxidative stress/inflammation (lipid peroxidation-LPO and arachidonate cyclooxygenase-COX), genotoxicity (DNA strand breaks), and gonad status (triglycerides, GSI and vitellogenin-like proteins). The following effects significantly differed between OS mining area and natural/background sites: health condition, growth rate, air survival time, COX (immune/inflammation) activity, P4501A/GST activity, LPO and DNA breaks in the digestive gland and vitellogenin-like proteins in the gonad. Correlation analysis revealed that the biochemical responses were scaled to at least one of the following impacts at the individual level: air survival time, weight to length ratio, growth rate and vitellogenin-like proteins. These indices were therefore identified as key adverse outcome pathways of mussels impacted by OS mining activities. Based on the relative levels of light/medium/heavy PAHs in tissues, the observed effects appears to be associated rather to the disturbance of OS in this area than contamination from OS tailing ponds leaching into the aquatic environment.

Greenhouse gas emissions from permafrost organic carbon decomposition in lakes and rivers can accelerate global warming. We used radiocarbon (14C) measurements to determine the predominant sources of dissolved organic carbon (DOC), particulate organic carbon (POC), dissolved inorganic carbon (DIC), and methane (CH4) in five thermokarst lakes and three rivers in an area of widespread permafrost degradation in Northern Quebec to assess contributions from thawing permafrost and other old carbon (fixed before CE 1950) reservoirs. We compared emission pathways (dissolved gas and ebullition), seasons (summer and winter), and surface soil type (mineral and peat soils). Modern carbon (fixed after CE 1950) was the dominant source of DOC, DIC, and CH4 of non‐peatland aquatic systems, while POC and sediment carbon were predominantly fixed in the last millennia. In the peatland systems, modern and permafrost carbon were important sources of DOC, lake DIC, lake ebullition CO2, and lake dissolved CH4. In contrast, POC, lake ebullition CH4, and river DIC were dominated by millennial‐old carbon. In winter, the 14C age of DOC, DIC, and POC in the peatland lakes increased, but the 14C age of dissolved CH4 did not change. Our results point to a clearly older overall carbon source for ebullition CH4 relative to dissolved CH4 in the peatland lakes but not the non‐peatland lakes. The younger ages of dissolved CH4 and DIC relative to DOC and POC in all lakes suggest that recent primary productivity strongly influences the large total lake CH4 and CO2 emissions in this area, as diffusion fluxes greatly exceed ebullition fluxes.

Thermokarst lakes result from the thawing of ice-rich permafrost and are widespread across northern landscapes. These waters are strong emitters of methane, especially in permafrost peatland regions, where they are stained black by high concentrations of dissolved organic matter (DOM). In the present study, we aimed to structurally characterize the DOM from a set of peatland thermokarst lakes that are known to be intense sites of microbial decomposition and methane emission. Samples were collected at different depths from three thermokarst lakes in the Sasapimakwananisikw (SAS) River valley near the eastern Hudson Bay community of Kuujjuarapik–Whapmagoostui (Nunavik, Canada). Samples were analyzed by spectrofluorometry, Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), and elemental analysis. Fluorescence analyses indicated considerable amounts of autochthonous DOM in the surface waters of one of SAS 1A, indicating a strong bioavailability of labile DOM, and consequently a greater methanogenic potential. The three lakes differed in their chemical composition and diversity, suggesting various DOM transformations phenomena. The usefulness of complementary analytical approaches to characterize the complex mixture of DOM in permafrost peatland waters cannot be overlooked, representing a first step towards greater comprehension of the organic geochemical properties of these permafrost-derived systems.

The simultaneous presence of natural and anthropogenic stressors in aquatic ecosystems can challenge the identification of factors causing decline in fish populations. These stressors include chemical mixtures and natural abiotic and biotic factors such as water temperature and parasitism. Effects of cumulative stressors may vary from antagonism to synergism at the organismal or population levels and may not be predicted from exposure to individual stressors. This study aimed to evaluate the combined effects of chronic exposure to cadmium (Cd) and elevated water temperature (23 degrees C) or parasite infection in juvenile rainbow trout (Oncorhynchus mykiss) using a multi-level biological approach, including RNA-sequencing. Fish were exposed to diet-borne Cd (6 mu g Cd/g wet feed), individually and in combination with thermal (23 degrees C) or parasitic stressors, for 28 days. The parasite challenge consisted of a single exposure to glochidia (larvae) of the freshwater mussel (Strophitus undulatus), which encysts in fish gills, fins and skin. Results indicated lower fish length, weight, and relative growth rate in fish exposed to a higher water temperature (23 degrees C). Body condition and hepatosomatic index of trout were, however, higher in the 23 degrees C temperature treatment compared to the control fish kept at 15 degrees C. Exposure to thermal stress or parasitism did not influence tissue Cd bioaccumulation. More than 700 genes were differentially transcribed in fish exposed to the individual thermal stress treatment. However, neither Cd exposure nor parasite infection affected the number of differentially transcribed genes, compared to controls. The highest number of differentially transcribed genes (969 genes) was observed in trout exposed to combined Cd and high temperature stressors; these genes were mainly related to stress response, protein folding, calcium metabolism, bone growth, energy metabolism, and immune system; functions overlapped with responses found in fish solely exposed to higher water temperature. Only 40 genes were differentially transcribed when fish were exposed to Cd and glochidia and were related to the immune system, apoptosis process, energy metabolism and malignant tumor. These results suggest that dietary Cd may exacerbate the temperature stress and, to a lesser extent, parasitic infection stress on trout transcriptomic responses. Changes in the concentrations of liver ethoxyresorufin-o-deethylase, heat shock protein 70 and thiobarbituric acid reactive substances coupled to changes in the activities of cellular glutathione S-transferase and glucose-6-phosphate dehydrogenase were also observed at the cellular level. This study may help understand effects of freshwater fish exposure to cumulative stressors in a changing environment.

In 2015, five billion liters of untreated urban wastewater (UWW) were released into the St. Lawrence River (Quebec, Canada) over the course of four days in order to repair the Montreal’s sewer interceptor network related to the city’s primary wastewater treatment plant. The UWW discharge originated mainly from household, industrial, and hospital sources. The objective of this study was to investigate the toxicological effects of this unprecedented punctual UWW release on aquatic invertebrates to gather information that could help understand the potential impacts to the receiving environment of overflow episodes occurring during heavy rain events. Water samples were collected at four impacted and non-impacted sites during and four weeks after the release. The freshwater crustacean Daphnia magna were experimentally exposed to surface water collected from UWW-impacted sites for 13 days and analyzed for life-history endpoints and suitable biomarkers related to oxidative stress (i.e., catalase, superoxide dismutase, lipid peroxidation, and glutathione-s-transferase) and reproduction (chitinase). Results indicated that D. magna growth and reproduction were significantly increased by exposure to UWWs. These effects were correlated with an increase in chitinase activity, which is primarily controlled by reproductive hormones and involved in growth, suggesting potential impacts on these processes. Results also indicated that some UWW samples might have caused oxidative stress during the release but that it was overcome by antioxidant defenses and did not lead to cellular damage. Overall, current results contribute to a better understanding of the biological impacts of UWW to aquatic invertebrates for a better stormwater management.

Once released in the environment, engineered nanoparticles (NPs) can undergo important transformation resulting in changed properties under natural conditions. This study investigated the fate, the bioavailability and the immunotoxicity of cerium oxide (CeO2) nanoparticles in fish exposed to CeO2 in representative surface waters differing in pH, organic matter content and conductivity (green and brown waters). Following an incubation period of NP CeO2 in different surface waters, particle size distribution and shape were determined by ultrafiltration and ICP-mass spectrometry, electronic microscopy and dynamic light scattering (DSL). Bioaccumulation and effect biomarkers focusing on the immune system responses (viability of immune cells and phagocytic activity) were also determined. Particle size distributions significantly changed under all types of surface waters where aggregation of NPs was commonly observed. Indeed, >90% of NPs CeO2 were found as aggregates (>450 nm) and large colloids (>100 nm). Less than 1% cerium (Ce) was found in the truly dissolved fraction (<1 kDa) suggesting no evidence of degradation for NP CeO2 in the water samples after 96 h. The NPs CeO2 were preferably accumulated in fish gills and accumulation was the highest in green waters which contained less total organic carbon (TOC), higher conductivity (218 μS/cm) and higher pH (7.8-8.0) than brown waters. The toxic properties (induced phagocytosis) of NP CeO2 also differed when dispersed in brown, green and tap waters. NPs CeO2 induced fish mortality at initial concentration of 10 μg/L Ce in both tap and green waters but not in brown waters which have different and high organic matter sources, lower pH and conductivity values. In conclusion, NPs CeO2 tends aggregate in representative freshwater, adsorb on gills and the immunotoxic potential is reduced in the presence of high natural organic matter, mildly acidic pH and low conductivity as found in brown waters.

The Athabasca oil sands deposit is the world's largest known reservoir of crude bitumen and the third-largest proven crude oil reserve. Mining activity is known to release contaminants, including metals, and to potentially impact the aquatic environment. The purpose of this study was to determine the impacts of oil sands mining on water quality and metal bioaccumulation in mussels from the Fort McMurray area in northern Alberta, Canada. The study presents two consecutive years of contrasting mussel exposure conditions (low and high flows). Native freshwater mussels (Pyganodon grandis) were placed in cages and exposed in situ in the Athabasca River for four weeks. Metals and inorganic elements were then analyzed in water and in mussel gills and digestive glands to evaluate bioaccumulation, estimate the bioconcentration factor (BCF), and determine the effects of exposure by measuring stress biomarkers. This study shows a potential environmental risk to aquatic life from metal exposure associated with oil sands development along with the release of wastewater from a municipal treatment plant nearby. Increased bioaccumulation of Be, V, Ni and Pb was observed in mussel digestive glands in the Steepbank River, which flows directly through the oil sands mining area. Increased bioaccumulation of Al, V, Cr, Co, Ni, Mo and Ni was also observed in mussel gills from the Steepbank River. These metals are naturally present in oil sands and generally concentrate and increase with the extraction process. The results also showed different pathways of exposure (particulate or dissolved forms) for V and Ni resulting from different river water flows, distribution coefficient (Kd) and BCF. Increasing metal exposure downstream of the oil sands mining area had an impact on metallothionein and lipid peroxidation in mussels, posing a potential environmental risk to aquatic life. These results confirm the bioavailability of some metals in mussel tissues associated with detoxification of metals (metallothionein levels), and oxidative stress in mussels located downstream of the oil sands mining area. These results highlight a potential ecotoxicological risk to biota and to the aquatic environment downstream of the oil sands mining area, even at low metal exposure levels.