Evidence of a temperature–oxygen squeeze within floodplain thermal refuge habitats

Abstract Vertical heterogeneity in lakes and estuaries can present cold‐adapted fishes with a temperature–oxygen squeeze, such that the epilimnion is stressfully warm and the cooler hypolimnion is hypoxic, thereby restricting fishes to the metalimnion. In temperate floodplain rivers, patches of lentic habitat (e.g., alcoves) have the potential to provide thermal refuge for cold‐water fishes during summer, but little is known about whether these smaller habitat features present fish with temperature–oxygen constraints. In this study, we measured temperature and oxygen profiles in six cold‐water alcoves on the Willamette River floodplain (Oregon, U.S.A.) to characterise potential trade‐offs in temperature and oxygen for a cold‐water fish, coastal cutthroat trout ( Oncorhynchus clarkii clarkii ). To evaluate how fish responded to trade‐offs, we used synergistic methods (needle probe thermocouple, surgically implanted iButtons, and external temperature transmitting radio tags) to monitor fish body temperatures at different temporal scales and compare them to temperature–depth profiles. The cold‐water alcoves displayed a linear relationship between dissolved oxygen and temperature, where cooler temperatures came at the expense of reduced oxygen. Fish body temperatures were intermediate to temperatures recorded at the surface and bottom of the alcove. The span of depths selected by these fish represents less than 20% of the available vertical habitat in these alcoves. These results demonstrate that refuges formed by cool hyporheic upwelling can generate a temperature–oxygen squeeze where fish use only a subset of the refuge habitat feature. Oxygen constraints on thermal refuge use may be a blind spot for climate adaptation planning for cold‐water fishes, because dissolved oxygen data are limited compared to water temperature data, and many cool refuges are fed by subsurface flows which are potentially hypoxic.