Stream temperature in the Basque Mountains during winter: thermal regimes and sensitivity to air warming

Winter stream temperature variability is important for detritus decomposition and over-wintering of aquatic organisms. However, winter temperatures are rarely studied. We studied water temperatures in 33 streams of the Basque Mountains in Northern Spain. High coherence of stream temperature within the study area and high correlation between stream and air temperature indicate a similar thermal response of streams to daily changes in atmospheric conditions at regional scale. Linear regression models of air temperature against water temperature successfully predict daily stream temperature in watersheds between 1 and 1000 km(2) in Northern Spain during winter, but it is not suitable for making long-term predictions because it failed to account for inter-annual trends in stream temperature. Averaging temperature observations collected in different streams to form regional stream temperature indicators enhanced the relationship between air and water temperature and improved the performance of the daily stream temperature model. Four thermal classes (lowland, pluvial, nival and geo streams) defined as a function of the mean water temperature of the coldest day and the winter and daily temperature ranges showed different sensitivity to changes in weather conditions. Therefore, spatial differences in the thermal sensitivity of streams at local scale should be addressed for making predictions about the effects of global warming.