Variation in Fish and Macroinvertebrate Assemblage Structure Along the Longitudinal Fluvial Gradient of the Pecos River, New Mexico and Texas

The taxonomic and functional structure of stream assemblages is expected to vary with changes in multi-scale environmental variables along longitudinal fluvial gradients. In some cases, fishes and stream macroinvertebrates have been shown to respond to environmental factors at different scales. We sought to examine variation in both assemblages across the Pecos River, where habitat conditions change along with substantial shifts in physiography, water availability, salinity, and land use. We collected benthic macroinvertebrates, fishes, and environmental variables at twelve sites along the main stem of the Pecos River. Reaches with less degraded habitats supported greater taxonomic and functional richness of fishes and invertebrates. Ordination analyses revealed different patterns of similarity among sites based on fish and invertebrate assemblages. This lack of concordance suggests that different environmental variables influence the structure of the two types of assemblages. Patterns of fish assemblage structure were highly correlated with regional physiographic variation, whereas macroinvertebrates appeared to be influenced more by riparian characteristics. Structure of both assemblages was strongly correlated with salinity and substrate type. Our results suggest that in addition to natural longitudinal variation, environmental variables associated with land use and water withdrawal potentially shape taxonomic and functional structure in this region.