Channel morphology, water temperature, and assemblage structure of stream insects

Temperature is known to be an important mechanism affecting the growth and distribution of stream insects. However, little information exists that describes how variable temperatures are among streams of similar size, especially in physically heterogeneous landscapes. We measured summer daytime temperature and the structure of riffle benthic insect assemblages from 45 montane streams in California. Summer stream temperature was nearly randomly distributed across large-scale geographic gradients of latitude (6 degrees) and elevation (2000 m). The lack of geographic trends in summertime stream temperature appeared to be caused by the strong relationship between local channel morphology and summer water temperature. Mean daytime water temperature was most strongly related to the % of the channel present as pools, which did not vary;systematically with either latitude or elevation. We used multiple multivariate regression analysis, non-metric multidimensional scaling (NMDS), and graphical techniques to both quantify differences in insect assemblage structure among streams and to determine the degree to which assemblage structure was related to temperature. NMDS analyses were conducted on 3 similarity matrices based on: 1) presence and absence of all aquatic insect taxa encountered during the study, 2) densities of the 16 most numerically abundant taxa, and 3) population biomasses of the 16 most common taxa. All 3 analyses showed that variation in assemblage structure among streams was significantly related to temperature although assemblage structure was most strongly related to sampling date-a consequence of sampling over a 98-d period. Temperature probably influenced assemblage structure in 2 ways: 1) by influencing developmental rates of individual taxa and overall assemblage phenology, thus affecting the relative abundances of tara found on a specific sampling date, and 2) by excluding taxa unable to tolerate certain temperature ranges. Because of the strong dependency of assemblage structure on temperature and the lack of strong geographic trends in temperature among these streams, much of the measured variation in assemblage structure appeared to be unrelated to latitude or elevation. These results have important implications for both our understanding of natural biogeographic patterns of lotic organisms and our ability to detect and model the effects of climate change and other thermal alterations on stream ecosystems.