Publications

The mobilization of rare earth elements (REEs) in aquatic ecosystems is expected to rise significantly due to intensified exploitation, erosion, and climate change. As a result, more attention has been brought to study their environmental fate. However, our ability to assess contamination risks in freshwater organisms remains limited due to scarce data on the composition and accumulation of REEs. Understanding how organisms bioaccumulate REEs requires knowledge of their environmental conditions, exposure pathways, and ecological characteristics—areas few studies have explored. In this study, we examined the fate of REEs across abiotic (water, suspended sediments, and sediments) and biotic (invertebrates and fishes) compartments in the St. Lawrence River (Canada), identifying the main drivers of their accumulation and relative composition. The results were consistent with REE biodilution along the food chain, with concentrations greater in suspended (REEs = 76.1–241.4 μg g⁻¹) and bulk sediments (REEs = 4.2–204.2 μg g⁻¹). Higher concentrations were found in fine-grained sediments, with a relative enrichment in middle REEs, likely due to REE adsorption onto Fe- or Mn-bearing minerals. Nonpredatory invertebrates ingesting suspended sediments, such as Ephemeroptera and Diptera larvae, exhibited higher concentrations of REEs than both filter-feeding species (i.e., mussels, polychaetes) and fish. Additionally, some amphipods displayed anomalous concentrations of gadolinium (Gd/Gd∗ = 5.7, 2.6, and 2.0), possibly originating from anthropogenic activities near Montreal Island. While fish bioaccumulated only light REEs in their liver, multiple regression models revealed how their length and the concentration of REEs in surrounding water—in dissolved form or as free ions—influenced their concentrations. Finally, benthivorous species like Moxostomaspp. and Ictalurus punctatus accumulated more REEs compared to piscivorous Sanderspp., reflecting differences in feeding behavior and trophic level. Overall, these findings provide insights into how REE concentrations and compositions varied among organisms, likely due to differences in environmental conditions and ecological characteristics.

A notable greening and warming of the Arctic and Subarctic due to climate change has uncertain implications for the global cycling of mercury (Hg). Snowpacks are dynamic reservoirs for Hg susceptible to solar radiation and wind pumping, with vegetative cover potentially altering Hg photochemistry. However, the impact of northern greening on the transformation of major Hg species and on Hg stocks remain poorly understood. Temporal surface snow and snowpit sampling was conducted under tree canopies and open tundra sites at the boreal-tundra ecotone in Nunavik, Canada. Maximum (mean) concentrations of 69.1 ng/L (8.8 ng/L) total mercury (HgT) and 46.9 ng/L (5.5 ng/L) reactive mercury (HgR) were measured in forest surface snow, with maximums attributed to rapid atmospheric oxidation events. Significant post-depositional reductions were recorded in the bay, tundra, and forest (67–99% HgR) and suggested greater Hg sequestration may occur under tree canopies. Increasing methylmercury (MeHg), HgT, and dissolved organic carbon (DOC) concentrations were detected across a vegetation gradient shifting towards humic-like organic matter. Notably, springtime depth profiles presented an approximate 12-fold greater accumulation of HgT under tree canopies compared to open tundra (p < 0.01), with up to 16-times higher stocks (HgT, MeHg, DOC) at elevated vegetation density (p < 0.05). In the North, increasing vegetation cover and surface warming may favor Hg accumulation and methylation in snowpacks, facilitated by interactions with organic matter, and further enriched by the reduced wind and solar exposure experienced under forest canopies.

Fish consumption is the main exposure pathway of the neurotoxicant methylmercury (MeHg) in humans. The risk associated with exposure to MeHg may be modified by its interactions with selenium (Se) and arsenic (As). In vitro bioaccessibility studies have demonstrated that cooking the fish muscle decreases MeHg solubility markedly and, as a consequence, its potential absorption by the consumer. However, this phenomenon has yet to be validated by in vivo models. Our study aimed to test whether MeHg bioaccessibility can be used as a surrogate to assess the effect of cooking on MeHg in vivo availability. We fed pigs raw and cooked tuna meals and collected blood samples from catheters in the portal vein and carotid artery at: 0, 30, 60, 90, 120, 180, 240, 300, 360, 420, 480 and 540 min post-meal. In contrast to in vitro models, pig oral bioavailability of MeHg was not affected by cooking, although the MeHg kinetics of absorption was faster for the cooked meal than for the raw meal. We conclude that bioaccessibility should not be readily used as a direct surrogate for in vivo studies and that, in contrast with the in vitro results, the cooking of fish muscle did not decrease the exposure of the consumer to MeHg

Unlike large dams which favor methylation of Hg in flooded soils over long periods, run-of-river dams are designed to flood a limited area of soils and are therefore not expected to significantly affect mercury (Hg) cycling or carbon processing. We studied the Hg and carbon cycles within food webs from several sectors along the Saint-Maurice River, Quebec, Canada, that differ in how they are influenced by two run-of-river dams and other watershed disturbances. We observed peak Hg concentrations in fish five-year postimpoundment, but these levels were reduced three years after this peak. Methylmercury concentrations in low trophic level fish and invertebrates were related to their carbon source (δ¹³C) rather than their trophic positions (δ¹⁵N). Biomagnification, measured by trophic magnification slopes, was driven mainly by methylmercury concentrations in low-trophic level organisms and environmental factors related to organic matter degradation and Hg-methylation. River sectors, δ¹³C and δ¹⁵N, predicted up to 80% of the variability in food web methylmercury concentrations. The installation of run-of-river dams and the related pondages, in association with other watershed disturbances, altered carbon processing, promoted Hg-methylation and its accumulation at the base of the food web, and led to a temporary increase in Hg levels in fish.

Monomethylmercury (MMHg) is a biomagnifiable neurotoxin of global concern with risks to human health mostly associated with fish consumption. Hydroelectric reservoirs are known to be sources of MMHg many years after their impoundment. Little is known, however, on run-of-river dams flooding smaller terrestrial areas, although their numbers are expected to increase considerably worldwide in decades to come. Production of MMHg is associated mostly with anaerobic processes, but Hg methylation has been shown to occur in periphytic biofilms located in oxic zones of the water column. Therefore, in this study, we investigated in situ production of MMHg by periphytic communities in habitats impacted by the construction of a run-of-river dam by combining transformation rate measurements with genomic approaches targeting hgcAB genes, responsible for mercury methylation. These results provide extended knowledge on mercury methylators in river ecosystems impacted by run-of-river dams in temperate habitats.

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 ratiosuggesting 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.

The redox transformations of arsenic (As) in lakes are thought to be mainly controlled by algal/microbial processes. We tested if photochemical oxidation could also be a significant redox process in northern freshwater systems. We exposed filtered natural water samples from four northern mine-impacted lakes (Northwest Territories, Canada) and three unimpacted ponds (Nunavik, Canada) to simulated solar radiation, and followed As(III) oxidation. During short-term incubation experiments (less than 8 h), all samples amended with As(III) displayed a rapid photooxidation (ca. 0.06-0.27 h⁻¹) whereas no significant oxidation was observed in the dark. This photooxidation process was controlled by UV radiation, was influenced by short-lived oxidants and was partly driven by the concentrations of organic matter. Natural As complexes in unamended water samples were more rapidly photooxidized than in samples with As(III) amendments. Addition of goethite or simulation of snowmelt did not alter photooxidation rates significantly. Overall, these results imply that photooxidation of As should be considered as a potentially significant mechanism controlling inorganic As speciation in aquatic systems. This is particularly true for northern systems affected by mining activities and by changes of inputs of organic matter related to climate change.