Spatial Variation in Aquatic Food-Web Structure in an Anthropogenically Impacted Arid-Land River

Anthropogenic influences have been shown to disrupt natural longitudinal patterns in riverine food-web structure.  The Pecos River is impacted by threats common in semi-arid regions including increased salinity, diminished discharge, altered flow regime, and conversion of watershed land for human uses. We used stable isotope analysis to characterize trophic positions of species and assemblage-wide resource use at twelve sites along the main stem of the Pecos River in New Mexico and Texas. Stable isotope ratios revealed different sources of carbon supporting fish production along the longitudinal fluvial gradient. Nitrogen isotopes elucidated variation in trophic position among species. For example, the coastal nonnative killifish, Fundulus grandis appears to feed at a higher trophic level than the native killifishes, F. zebrinus and Lucania parva. In addition, it appears that anthropogenic inputs are enriching baseline nitrogen isotopic ratios in part of the river.  Furthermore, assemblage-wide trophic niche breadth was found to be greatest in less degraded sites with higher fish species richness. Characterizing changes in food-web structure in relation to natural and anthropogenic factors is important for habitat assessment, stream restoration, and management and conservation strategies.