Predicting heat mortality in freshwater communities: temperature and oxygen effects

Freshwater organisms face a dual challenge in warming waters due to increased metabolic demands and declining oxygen levels. However, the relationship between temperature, oxygen, and heat-induced mortality in natural systems remains poorly understood. Here, we combine heat tolerance measurements in the laboratory with high-resolution water temperature and oxygen records in the field to predict mortality of different species under natural conditions. We demonstrate this approach in three species, one crab and two fish, of the Mapocho River in central Chile, and validated predictions with a capture-recapture experiment under natural settings. Our work unequivocally shows that current water temperatures can trigger mortality in some areas of the river and provides a robust approach to predict when heat-induced mortality is likely to occur. This approach offers a valuable tool for evaluating water quality and when river conditions may be stressful or restrictive for monitoring purposes, and to design possible mitigation strategies.