Influence of Surface and Near-surface Geology on Fish Assemblages in the Colorado River Basin of Texas
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Fish communities are distributed heterogeneously within river basins. Heterogeneity is attributed to a number of physical, chemical, and biological processes. Among river basins that traverse a diversity of surficial geologies, physical and chemical properties of surficial geologies influence stream characteristics and regional aquatic communities. Likewise, stream characteristics and aquatic communities of distinct surficial geologies (i.e., georegions) can respond differently to anthropogenic stressors. Purposes of this study were to assess the influence of surficial geologies (i.e., georegions) on stream characteristics and fish communities in the Colorado River basin of Texas, a representative western gulf slope basin of southcentral USA, and determine if anthropogenic stressors differentially affect fish communities by georegion. Using measures of discrete (i.e., georegions, stream type) and continuous (e.g., stream order, distance from river mouth) community variation (i.e., spatial delineations), I found that georegion, stream type, stream order, and distance from mouth distinguished stream characteristic types within the basin, but only georegion explained a significant portion (41%) of the fish community variation. Using fish community changes between time periods (1933 to 1980; 1981 to 2018), which generally corresponds with pre- and post- dam constructions within the basin, I found that anthropogenic flow alterations had more of an effect on fish communities in some georegions than others. My findings support the concept of georegions having a hierarchical influence on stream characteristics and aquatic community heterogeneity within a basin, and that anthropogenic modifications can differentially affect aquatic communities, depending on factors associated with georegions and stream characteristics. Potential benefits of this work include understanding factors influencing the heterogeneity in aquatic communities and the role of anthropogenic stressors across georegions (e.g., prairie streams, karst terrains, lowland coastal rivers) within and outside the western gulf slope basins.