À la chasse aux métaux traces dans un Nord canadien en évolution rapide : approches limnologiques, écologiques et collaborative = Hunting for trace metals in a rapidly changing North : limnological, ecological, and collaborative approaches

Climate change will have wide-ranging effects on Arctic ecosystems and communities. Accelerated warming combined with significant pressure to exploit natural resources has led to Arctic ecosystems vulnerable to both climatic and socio-environmental change. As the Arctic is an important indicator of future global changes, it is important to study the impact of these changes on human and ecosystem health. A key knowledge gap in our understanding is how rapid changes in the North will affect the transport and biogeochemical cycling of key contaminants, including trace metals. In this thesis, I used a large-scale ecological approach to study the environmental fate, bioaccumulation, and trophic transfer of trace metals in Canada’s northern ecosystems. In the first chapter of this thesis, I studied the potential impacts of a changing climate on mercury cycling within Arctic lakes and ponds. This large-scale, multi-year study investigated whether microbially-active permafrost thaw ponds were potential sources of mercury in the eastern Canadian Arctic. Our results showed that thaw ponds are small but abundant sources of methylmercury, with potentially significant downstream effects linked to permafrost thaw. The second chapter of this thesis examined the effects of aquatic productivity on the uptake of methylmercury in biota at the base of freshwater food webs. I sampled a gradient in ecosystem productivity to study the implications of climate-induced changes in productivity on mercury bioaccumulation. Our results suggested that indicators of productivity, such as algal biomass and nutrient stoichiometry, are not the main drivers of methylmercury accumulation within unproductive Arctic lakes. The third and fourth chapters of this thesis focused on the behaviour and environmental fate of rare earth elements in Arctic ecosystems. Few ecotoxicological studies exist for rare earth elements and our goal was to establish baseline data before the proliferation of rare earth mining projects at northern latitudes. Our results found that rare earth element bioaccumulation patterns appear to be species- and tissue-specific and they do not biomagnify in natural food webs. I also identified key environmental drivers of bioaccumulation in zooplankton, including, pH, dissolved organic carbon and the rare earth element free-ion concentrations. These studies highlight the utility of zooplankton as a biomonitor for rare earth elements in freshwater ecosystems. Lastly, my thesis highlights the importance of collaborative research with Indigenous communities by examining a successful community-based environmental monitoring program in Nunavik. I participated in a collaborative and land-based environmental monitoring and science education program with the aim of integrating Indigenous knowledge into ecological research. I also examined the role of peer-led workshops as an effective platform for early-career researchers to reflect on the benefits and challenges of conducting community-collaborative research in Indigenous communities. Overall, my thesis greatly improves our understanding of how rapid socio-environmental change may affect the bioavailability and trophic transfer of mercury and rare earth elements in the eastern Canadian Arctic. Understanding the impact of future changes on the environmental fate of metals is crucial to ensuring the health of Arctic ecosystems and communities.

Keyphrases: bioaccumulation, environment, mercury, methylmercury, rare earth elements, metals, arctic, subarctic, freshwater, food webs, bioaccumulation, zooplankton, lakes, ponds, community-based monitoring, community-collaborative research