Regional Response to Global Warming: Water Temperature Trends in Semi-Natural Mountain River Systems

River water temperature (TW) is a key environmental factor that determines the quality of the fluvial environment and its suitability for aquatic organisms. Atmospheric warming, accompanied by more frequent extreme weather phenomena, especially heat waves and prolonged drought, may pose a serious threat to the river environment and native river ecosystems. Therefore, reliable and up-to-date information on current and anticipated changes in river flow and thermal conditions is necessary for adaptive water resource management and planning. This study focuses on semi-natural mountain river systems to reliably assess the magnitude of water temperature change in the Polish Carpathians in response to climatic warming. The Mann-Kendall test was used to detect trends in water temperature series covering the last 35 years (1984-2018). Significant, rising trends in annual water temperature were found for all studied sites, with differences in intensity (0.33-0.92 degrees C per decade). Trends in TW were strongest in summer and autumn (0.75-1.17 and 0.51-1.08 degrees C per decade), strong trends were found in spring (0.82-0.95 degrees C per decade), and weaker in winter (0.25-0.29 degrees C per decade). Simultaneous air temperature trends were broadly consistent with water temperature trends. This indicates the urgent need for adaptive management strategies to counteract thermal degradation of the fluvial environment under study.