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Abstract

Cultus Lake, British Columbia is critical freshwater lake habitat for two species-at-risk, the Cultus Lake Sockeye Salmon (Oncorhynchus nerka, Cultus population) and Cultus Lake Pygmy Sculpin (Coastrange Sculpin, Cottus aleuticus, Cultus population). This lake ecosystem is degrading and threatened by epilimnetic warming and decreasing hypolimnetic dissolved oxygen concentrations, largely the result of the effects and interactions of ongoing lake eutrophication and climate change. Exploration of lake modeling and monitoring data (2009-2018) revealed a correlation between Cultus Lake, BC hypolimnetic temperatures and the volume of hypoxic (17ºC; -25% of lake volume currently; -30% (-34%) under RCP4.5 (RCP8.5) by 2100), and hypoxic hypolimnetic waters (< 6mg/L DO; -20% of lake volume currently; -41% (-58%) under RCP4.5 (RCP8.5). With warming temperatures, the predicted temperature-oxygen squeeze (and volumetric optimal habitat loss) is likely to progressively intensify through time, reducing suitable oxythermic habitat conditions for stenothermic, hypoxia-intolerant fish species in Cultus Lake. If nutrient loading to Cultus Lake remains unabated, our model predicts the likely elimination of optimal critical habitat for both the Cultus Lake Sockeye Salmon and Cultus Pygmy Sculpin, with intensified warming and/or nutrient loading likely to significantly advance such exclusion. Reducing the known nutrient loadings to Cultus Lake is essential in order to lower lake primary productivity and associated seasonal hypolimnetic oxygen deficits, particularly before significant internal loading of nutrients from sediment stores enhances and more permanently reinforces the eutrophied lake state.