Hypoxia in a warming world: performance and fitness consequences of dual stressors for inland fishes

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Freshwater organisms face multiple threats associated with land conversion, pollution, and eutrophication, in addition to overharvesting and species invasions. In addition, there is mounting evidence that freshwaters are highly sensitive to climate change. Ultimately, to understand and predict effects of human activities on aquatic biodiversity, we must explore both independent and interactive effects of multiple stressors. Hypoxia (low dissolved oxygen, DO) is a widespread stressor in aquatic systems driven by influxes of wastes and fertilizers that can accelerate eutrophication; and severe hypoxia can be a powerful limiting factor on fish performance. Climate warming is likely to exacerbate hypoxia because fish metabolism and therefore oxygen demand increase with water temperature, while hypoxia limits oxygen supply. Potential interactions between these dual stressors have been the focus of several recent studies; many of which have focused on short-term (days, weeks) exposures of fish to one or both stressors. In this research program, we will quantify performance and fitness consequences of both short-term and life-long (rearing studies) exposure of fish to different temperature and DO regimes; and we will explore how fish populations from different temperature and DO regimes differ in their response. We will build on our 30-yr foundation of studies on East African fish species that cross strong gradients of temperature and DO in nature. A combination of ecophysiological assays on field populations and lab acclimation and rearing studies will be used to (a) test for oxygen dependency of upper thermal limits; (b) quantify interactive effects of hypoxia and thermal stress on performance and fitness-related traits, and (c) test whether local adaptation to hypoxia (e.g., larger gills) improves tolerance to warming waters. We will study populations crossing natural gradients of temperature and DO, and those experiencing change in DO or temperature in response to human impacts, such as deforestation that can increase water temperature or eutrophication that can decrease DO. Disentangling effects of multiple stressors is a key challenge to conservation and management of fish populations across the globe. Our long-term field system in Africa allows us to test specific predictions on independent and interactive effects of hypoxia and thermal stress on fishes based on long-term data and integrative research. By blending interpopulational studies, long-term environmental data, and rearing experiments, we will provide a much needed link between effects of dual stressors under controlled lab conditions and in nature. Consistency in field versus lab effects with strengthen our ability to predict consequences of the global hypoxia crisis in the face of climate change, and knowledge gained will help us to study and manage systems locally and abroad.