Spatiotemporal variation in functional and taxonomic organization of stream-fish assemblages in central Texas

Stream ecosystems across the world are dynamic and complex. Biogeographic and historical constraints cause many of these systems to be unique and create difficulty in predicting compositional changes in fish assemblages. However, each system appears to comprise similar functional groupings despite phylogenetic differences. I hypothesized that assemblages within a river would be more similar to one another, regardless of season, than assemblages from other localities based on taxonomic organization; however, assemblages from different rivers but within the same season would be more similar to one another based on functional organization. I assessed functional and taxonomic organizations of fish assemblages from three tributaries of the Colorado River in Texas (Pedernales River, San Saba River, and South Llano River) across all four seasons. Direct ordination methods resulted in assemblages from the same river, regardless of season, clustering together. More specifically, the functional and taxonomic organizations of assemblages from the South Llano River, regardless of season, were noticeably different than that from both the Pedernales River and the San Saba River, at least with respect to the first ordination axis. Current velocity, habitat availability, and temperature were influential in structuring the functional organization of assemblages in central Texas, while current velocity, substrate availability, and stream width were influential in structuring the taxonomic organization.