From cold-water refuge to thermal stress: projected warming of Czech rivers and implications for brown trout

Abstract. Water temperature is a fundamental driver of freshwater ecosystem functioning, governing physical, chemical, and biological processes across riverine environments. Despite growing evidence of climate-driven thermal change in Central Europe, region-specific projections linking continuous thermal shifts to ecologically meaningful thresholds for cold-water species remain scarce. Here, we developed a linear air–water temperature regression model (calibrated and validated using observed daily water temperatures from 35 river profiles across the Czech Republic (2002–2022) and forced with an ensemble of seven global circulation models (GCMs) and one regional climate model (RCM) under four SSP scenarios) to analyse observed thermal regimes through 2025 and project future conditions through 2085. Rather than focusing solely on mean temperature trends, we quantified changes in biologically relevant threshold exceedance days for brown trout (Salmo trutta), including cold-water persistence (<17 °C), sub-lethal stress (17–19 °C, 19–21 °C, and 21–23 °C), and extreme heat (>23 °C). Results show that thermally suitable habitat – defined as river reaches maintaining water temperatures below 17 °C for at least 350 days per year – declined from nearly 100 % of the Czech river network in 1975 to approximately 77 % by 2010, and is projected to fall below 5 % by 2085 under median projections. This contraction is driven primarily by chronic sub-lethal warming rather than by rare extreme events, and is accompanied by a pronounced redistribution of thermal exposure toward the 19–21 °C and 21–23 °C stress bands in warm and intermediately warm rivers. Cold-season and early spring warming further threatens spawning success and early development. High-altitude headwater systems emerge as the last persistent thermal refugia, underscoring the urgency of protecting longitudinal connectivity and riparian shading as climate adaptation priorities.