Evaluating Historical Changes in Timing and Synchrony of Two Biologically-Relevant Annual Hydro-Climatic Events in Streams

Although flow and temperature are two strongly linked environmental stressors limiting key ecological processes in streams they are often studied separately as single factors in space and time. Increases in synchrony and magnitude of extreme events of these two stressors influenced by the recent warming climate may lead to intensified effects on aquatic organisms.  Here, we examined the inter annual variability and regional temporal trends on timing and magnitude of two annual events, the stream flow minima (Qmin) and stream temperature maxima (Tmax_w). These two events represent periods of maximum stress for aquatic organisms in terms of extent of suitable habitats and thermal experience. In light of the recent warming climate, we tested recent trends in magnitude and timing of Qmin and Tmax_w that potential might increase their synchrony. We used long-term available information of temperature and flow available for the period 1950-2010 in 22 minimally human-influenced streams in western U.S. Using both 1-day and 7-day events we found strong evidence of increasing synchrony between Qmin and Tmax_w. During years of extreme low Qmin the Tmax_w increase and vice versa. Over time, there are decreasing and increasing trends in the magnitude of Qmin and Tmax_w respectively. Collectively, our results suggest that a recent warming climate experienced in this region is affecting streams by altering the timing and magnitude of both Qmin and Tmax_w. This study highlights the use of multiple environmental drivers to understand the effects of the recent climate variability on freshwater ecosystems.