Hydrological and thermal regime shifts in a sub-Arctic river under climate change: implications of potential changes in thermal habitat for Arctic char and Atlantic salmon

Graphical Abstract. Graph showing the work flow of the study. Legend: Graphical Abstract Climate change-driven effects are significant socio-environmental concerns in the Arctic and sub-Arctic regions. This study assesses the impact of climate change on the hydrological and thermal regimes of the Aux-Mélèzes River, an important sub-Arctic watershed in northeastern Canada. Using the process-based CEQUEAU model, we projected future (2040–2100) thermal regime shifts, validating against historical data (1980–2020) with strong performance metrics for summer flows and water temperature. Historically, the Aux-Mélèzes River has provided suitable summer habitat for Arctic char and Atlantic salmon, but projections indicate statistically significant increases in mean summer water temperature across all climate scenarios. Temperature duration curve analysis reveals a shift towards higher temperatures, with the probability of exceeding the 23 °C thermal stress threshold for key fish species increasing significantly by the century’s end. Furthermore, maximum summer water temperature projections highlight tributaries as critical warming hotspots, posing risks to fish habitat. The onset of positive water temperature is expected to decline by 45–55 days, significantly changing seasonal thermal timing. These hydrological and temperature changes represent a socio-ecological threat, emphasizing the need for adaptive management and enhanced predictive capabilities in these rapidly changing environments.