| Summary: | The mineral resource potential of Canada’s North has been recognized since the early 20th century with mines operating across all three territories. However, the long-term biological consequences of Northern mining operations and associated anthropogenic activities on surrounding freshwaters have not been investigated extensively. The sub-Arctic lakes around the City of Yellowknife (Northwest Territories) provide an important opportunity to fill this knowledge gap by examining the long-term cumulative impacts of past gold mining operations, land-use changes related to urbanization, and regional climatic changes on aquatic biota. In this thesis, diatom-based (Bacillariophyceae) paleolimnological approaches were used to assess the long-term biological responses to the multiple environmental stressors mentioned above through space and time. A spatial survey of diatom assemblages from the surface sediment of 33 lakes around Yellowknife indicated that assemblage composition was strongly influenced by variables related to lake-water nutrient concentrations and ionic composition in the water column, while metal(loid) contaminants (i.e. antimony, arsenic) associated with gold mining activities played a secondary role. Analyses of diatom assemblages from ten well-dated sediment cores from lakes along a gradient of anthropogenic activities (i.e. mining and urbanization) recorded notable shifts over the last ~200 years. The high species turnover in the lakes closest to the mines and the city were indicative of metal(loid) pollution from past gold mining activities (e.g. roaster stack emissions, accidental spills from tailings pond) and eutrophication from land-use changes (e.g. sewage disposal, developments in the catchment). The impacts of climate-mediated changes were also apparent in both the urban and remote lakes. The synergistic effects of warming and nutrient enrichment led to striking changes at Niven and Jackfish lakes. Recent diatom assemblage composition in all lakes was substantially different from pre-disturbance periods, which suggests that the sub-Arctic lakes around Yellowknife have crossed critical limnological thresholds in response to both anthropogenic activities and climatic changes. The long-term perspectives gained from this research contributes to the growing body of knowledge on biological responses to Northern mining contamination and urbanization in a warming world. PhD
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