Downscaling climate projections to forecast ecological changes in river systems

Freshwater ecosystems are experiencing accelerating degradation due to climate change, yet predictive assessments that integrate thermal stress and habitat fragmentation remain limited. This study evaluates the combined impacts of rising maximum summer temperatures (Tmax) and precipitation variability on Atlantic salmon (Salmo salar) in Newfoundland and Labrador by integrating downscaled climate projections with river connectivity assessments. High-resolution climate models (ClimGen-derived model outputs) were used to project Tmax and precipitation shifts, while the Dynamic Connectivity Assessment Tool (D-CAT), used for assessing river connectivity, quantified how climate-induced thermal stress and migration barriers exacerbate habitat fragmentation. Findings indicate that by 2090-2094, over 70 % of rivers will exceed the physiological stress threshold of 22 °C, with some interior watersheds surpassing 25 °C, reaching lethal limits for salmon. Increasing precipitation variability-marked by alternating drought-induced habitat desiccation and high-intensity rainfall-driven sedimentation-is projected to amplify hydrological instability. The overlap between thermal stress zones and in-stream barriers will severely restrict migration routes, with 70 % of barriers located in regions exceeding critical Tmax thresholds, limiting access to essential spawning and rearing habitats. A detailed assessment of 13 case study rivers reveals significant geographic variability in climate vulnerability, with interior rivers exhibiting the highest Tmax increases, while coastal systems face more volatile precipitation patterns. These results emphasize the urgent need for integrated climate adaptation strategies that prioritize barrier mitigation, adaptive hydrological management, and riparian restoration to improve freshwater connectivity. While downscaling enhances regional climate impact assessment, uncertainties remain regarding short-term climate extremes (heatwaves, flash floods) and species-level adaptation responses.