Present and Future Thermal–Oxygen Habitat for Glacial‐Relict Fish at Southern Range Margins

Climate change and eutrophication may alter the distribution of species through their effects on freshwater thermal–oxygen regimes. We quantified the deep‐water thermal–oxygen niche of cold‐adapted glacial‐relict fish ( Salvelinus alpinus and Coregonus spp.) at the southern range to assess their vulnerability to a changing environment. Using data from 104 lakes across the UK and Ireland, we developed a logistic regression model that identified temperature in summer at the 7 mg L −1 oxygen isoline (TDO7) as a significant predictor of species presence. The model pooling S. alpinus and Coregonus species presence–absence estimated a TDO7 central niche of < 11.5°C (CI 10.9°C–12.1°C) and an upper niche of 17.5°C (CI 16.2°C–19.2°C), whereas the model for S. alpinus alone estimated central and upper thresholds of < 10.7°C (CI 10.2°C–11.5°C) and 16.6°C (CI 15.2°C–18.3°C), respectively. Under a worst‐case climate scenario for 2061–2080, modelled TDO7 across 29 lakes supporting cold‐adapted fish increased by an average of 3.25°C in response to a projected 3.43°C average rise in maximum air temperature. Under these conditions, 56% of lakes were predicted to remain below the upper niche range; however, none remained within the central niche. Lake maximum depth was the strongest predictor of TDO7, and future TDO7 remained below the upper niche range only in stratified lakes with a maximum depth > 27.4 m and spring total phosphorus < 19.1 μg L −1 . Deep, stratified, oligo‐mesotrophic lakes therefore represent critical climate refugia, highlighting the importance of protecting these systems from non‐climatic stressors to maximise species persistence under climate warming.