Long-Term Patterns of Temperature and Oxygen Conditions in Reservoirs: Implications of a Warming Climate For Reservoir Fisheries

Future management plans for reservoir fisheries should evaluate how climate change is reshaping abiotic conditions in these managed ecosystems. We use long-term data from Illinois reservoirs to characterize inter-annual changes in thermal structure and the availability of dissolved oxygen under different atmospheric temperatures. Warmer years tend to be associated with earlier, stronger, and longer stratification and greater epilimnetic temperatures. Early onset of stratification, increased resistance to mixing because of greater density differences between top and bottom water, and longer duration of stratification during warm years is also associated with longer periods of oxygen depletion in hypolimnetic waters, resulting in lower oxygen concentrations and increased volume of oxygen-depleted water. These changes to thermal stratification and oxygen availability resulting from a warming climate have the potential to restrict the availability of suitable habitat in the summer for many important sportfish species. Stressful conditions during the summer may also spill over into the fall if the increased volume of anoxic water from the hypolimnion is of a sufficient volume during fall mixing. Rapid turnover of oxygen-depleted water would reduce oxygen concentration at all depths, resulting in a period of reduced growth, and potentially, high stress and mortality. Sensitivity of reservoirs to atmospheric temperature anomalies was not consistent, however, as some reservoirs exhibited greater resilience to temperature anomalies than others. These patterns and their exceptions all have implications for the future of reservoir fisheries management.