Assessing the impact of climate-induced heatwaves on habitat conditions for reservoir fish communities

• A new methodology is developed to assess fish habitat volume for growth under heatwave impacts. • Heatwaves' intensity and frequency are projected to rise significantly by the century's end. • Temperature rises during heatwaves reduce dissolved oxygen, limiting fish habitat zones. • Even tolerant species may experience stress from heatwaves, raising risks of fish mortality. Global climate change is intensifying the frequency, duration, and severity of heatwaves (HW), posing significant threats to aquatic ecosystems, particularly freshwater reservoirs. This study evaluates the impact of future climate-induced HW on the habitats of five fish species in a Portuguese reservoir using the CE-QUAL-W2 2D hydrodynamic model. Climate projections for the end of the century were based on three bias-adjusted EURO CORDEX simulations - MOHC HadGEM2-ES/RACMO22E, MOHC HadGEM2-ES/CCLM4–8–17, and MPI-MPI-ESM-LR/REMO2009 - all under the RCP 8.5 high-emission scenario. The model effectively simulated the dynamics of water temperature and dissolved oxygen within the reservoir. The results indicate that heatwave-induced temperature increases lead to significantly reduced dissolved oxygen levels. The most significant increase in reservoir water temperature was 9.2 °C (μ = 7.2; σ = 1.2) from July to August. Meanwhile, dissolved oxygen levels decreased by 2.1 mg/L (μ = -1.4; σ = 0.5) during the same period, thereby restricting the areas available for fish growth. These changes were primarily driven by increases in water temperature, especially when it exceeded 30ºC. In one of the scenarios the Barbus spp. habitat volume decreased 80 % for 7 days. Even species with a high tolerance for extreme temperatures and low oxygen levels are projected to experience physiological stress, potentially leading to increased mortality. Conversely, more heat-tolerant species may see population growth in certain years, leading to ecological imbalances, such as prey depletion and reduced species diversity. These findings highlight the value of such modeling approaches for guiding water management and conservation strategies in the face of climate change.