Using watershed-scale hydrological models to predict the impacts of increasing urbanization on freshwater fish assemblages

Natural flow regimes are important predictors of variation in freshwater biodiversity, yet urbanization can significantly impact the volume and variability of water moving through a watershed. Moreover, there is limited quantitative understanding of spatial variation in the effects of urbanization on watershed hydrology and the impacts of this variation on aquatic species distributions and assemblage composition. We used a physically based watershed model (MIKE-SHE), a hydrodynamic river model (MIKE-11), and current landcover classifications to characterize flows at sites across the Big River watershed in eastern Missouri. Estimates of streamflow from the MIKE-SHE model based on current landcover classifications were then used to predict variation in fish species richness at 44 sites across the watershed. We also quantified the potential effects of five urbanization scenarios on streamflows throughout the watershed and used flow estimates from the urbanization hydrological models to predict changes in fish species richness as urban areas increase. Responses varied among sites, with some sites predicted to increase in species richness whereas others are predicted to decrease in species richness. This mosaic of responses is likely due to the inconsistent impact of urbanization on streamflows throughout the watershed. Results also differed among taxonomic groups. The number of species of Cyprinidae (minnows) is generally expected to increase within sites across the watershed, whereas the number of species of Centrachidae (bass and sunfish) is expected to decrease. This differential shift in cyprinid and centrarchid richness is predicted to have a compensatory effect, with overall richness remaining, on average, relatively constant among sites.