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An important goal of environmental and comparative physiology research is to identify species or populations that may be susceptible to environmental change such as heat wave events that are predicted to become more frequent and intense in the future. This study tested the hypothesis that fishes inhabiting alkaline lakes face significant physiological challenges, which results in reduced thermal tolerance. Brook stickleback (Culaea inconstans) were collected from an alkaline lake (pH 9.3) in Alberta, Canada and held under neutral conditions in the laboratory. Subsequently, fish were acutely exposed (4 d) to neutral (pH 7) or alkaline (pH 9.5) waters at 10 or 25°C. Exposure to alkaline water reduced critical thermal maximum (CTmax) in stickleback by approximately 1°C, but thermal acclimation capacity ("thermal plasticity") was unaffected by alkaline exposure. Alkaline conditions resulted in physiological disturbances characteristic of exposure to high pH including elevated whole-body ammonia and lactate concentrations. Acute warming to CTmax in alkaline-exposed fish resulted in reductions in whole-body sodium and chloride concentrations. In addition, alkaline exposure compromised recovery from exercise at elevated temperatures. Overall, these results suggest that the physiological disturbances observed in response to alkaline exposure may render fish more susceptible to acute warming, reducing thermal tolerance. [ABSTRACT FROM AUTHOR]