Spatio-temporal dynamics of stream temperature and its interactions with environmental variables on the instream- and catchment scale

Stream ecosystems are among the most dynamic and sensitive components of the freshwater environment, providing critical habitats, regulating water quality, and supporting biodiversity. However, these systems are increasingly threatened by multiple stressors. In multiple stressor research, stream temperature is often referred to as a key stressor influencing ecological processes such as species’ tolerances and distribution, metabolic rates, dissolved oxygen (DO) levels, and overall water quality. With the intensification of global warming and human impacts on stream ecosystems, understanding and predicting thermal dynamics in freshwater environments has become increasingly important.

The aim of this thesis is to apply a combination of observational data and modelling approaches to improve both mechanistic understanding of stream temperature and predictive capacity. Underlining the importance of scale, this thesis integrates global-scale data synthesis, instream field observations using fiber-optic distributed temperature sensing (FO-DTS), process-based catchment-scale modelling with the ecohydrological catchment-scale model Soil and Water Assessment Tool (SWAT+), and its integration in ecological analyses.

The FO-DTS observations effectively detected localized hyporheic exchange flows and thermal buffering zones, giving impulses for improved modelling of stream temperature. Stream temperature simulations were improved at the spatial and temporal scale by integrating hydrological and heat exchange processes into SWAT+. Based on this improvement, the first SWAT+ DO model was proposed and improved by considering spatially heterogeneous DO processes.

This thesis demonstrates the relevance of stream temperature in freshwater research by linking hydrology, ecology and multiple stressor research, while pointing to future research needs, particularly regarding model scaling, anthropogenic stressors and interdisciplinary integration.

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