Patterns of fish species distributions replicated across three parallel rivers suggest biotic zonation in response to a longitudinal temperature gradient

Environmental gradients determine the distributions of individual species, which, in turn, shape patterns of species assemblage across those gradients. We used species distribution models to study the assemblage of fish species along the three mainstem rivers in the South Saskatchewan River Basin (SSRB) in Alberta, which flow in parallel across an 800-km longitudinal span and down 1400 m from the Rocky Mountains to the Great Plains of North America. We estimated the similarity of species assemblages along each river to identify general patterns of species assemblage associated with temperature and five other physiochemical variables. Mean July water temperature, which ranged from <11 degrees C at high elevation to >21 degrees C at low elevation, was strongly associated with the presence-absence of most species in the SSRB. We found that high turnover occurred at two locations along the longitudinal gradient: where mean July water temperature was approximately 15 degrees C and where mean July water temperature was approximately 19 or 20 degrees C. There was also an increase in species richness at lower elevations where water temperatures were higher. Models incorporating forecasted changes in water temperature with climate change will likely provide accurate predictions of changes in the diversity and distribution of riverine fish communities across topographically heterogeneous landscapes.