the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Apr 2026
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.
Status: open (extended)
- RC1: 'Comment on egusphere-2026-2052', Anonymous Referee #1, 04 Jun 2026 reply
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General comments:
The manuscript From cold-water refuge to thermal stress: projected warming of Czech rivers and implications for brown trout is a generally well written manuscript that clearly identifies its research objectives to simulate potential future stream temperatures in Czech rivers, and to identify potential changes in thermal stresses for the fish species of concern, brown trout. The authors provide a clear and straight-forward analysis using a common approach to relate stream temperature to air temperature. The regional nature of the analysis permits nationwide assessment of potential thermal habitat changes through the future decades.
Specific comments:
I have two main suggestions to strengthen the manuscript, provided below, and additional secondary comments and technical corrections in the attached PDF.
Given that air temperature – stream temperature regressions are simplifications of the energy exchanges ultimately controlling stream temperature responses to environmental / climate change, the manuscript would benefit from further discussion of the potential of a model fitted to current conditions and processes to be less representative of future conditions. For example, there are studies that suggest that such empirical relationships fitted to current data will underestimate future stream temperatures due to altered subsurface heat storage (loss of snow melt, warming soil and groundwater temperature, e.g., https://doi.org/10.1029/2018WR024236, https://doi.org/10.1890/14-1354.1).
Additionally, several stream temperature studies have highlighted the need to use physically based models to robustly predict stream temperatures under future conditions, rather than empirical air temperature – stream temperature regressions (e.g., https://doi.org/10.1002/hyp.70033) or have demonstrated that empirical regressions may not produce reliable estimates of future stream temperature (Arismendi et al., 2014, as cited in the authors’ manuscript). It is difficult to implement process-based models at regional scales due to the data requirements of such models, but in their absence, predictions from empirical regressions require careful consideration and interpretation due to (1) the real potential for shifting future hydrological processes and environmental conditions which alter the air-stream temperature relationships, and (2) the uncertainty attributed to model prediction errors which may be greater than the projected changes in stream temperature.
With this context in mind, I suggest that the authors provide additional information in the Results specifically regarding the projected future stream temperatures. Given the model prediction errors (RMSE) under validation ranged from 1.5 – 3.1ºC, it would be very beneficial to include either a table or figure(s) providing the site-specific increases in projected stream temperatures through the future decades. This would enable readers to be aware of and to be able to gauge the uncertainty in the projections. As the results currently stand, they focus on exceedances and temperature thresholds. This is understandable given the ecological perspective of the manuscript, but inclusion of projected stream temperatures strike me as necessary to have confidence in the use and interpretation of the thermal implications of the study.
Secondly, Section 4.4 in the Discussion would benefit from expansion to include the methodological context discussed above. Discussion of other factors that may be require consideration for interpretation of the results would also be beneficial.