Spatial and temporal variation in aquatic food-web structure in the Pecos River in New Mexico and Texas, USA
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Understanding how aquatic communities change with natural variation along longitudinal gradients and how they respond to anthropogenic impacts is critical for effective riverine management. In addition to natural shifts along its continuum, the Pecos River is impacted by a variety of anthropogenic influences that affect discharge, salinity, and riparian and instream habitat structure. I assessed variation in main-stem river food webs by estimating carbon sources, trophic positions, niche breadth and resource use by consumers at twelve sites using stable isotope analysis (carbon and nitrogen). Across physiographic regions, fish species richness, food chain length, and assemblage-wide niche breadth were inversely related to specific conductivity. IsoError mixing models revealed differences in sources of dietary carbon supporting fish production along the longitudinal fluvial gradient, with a shift toward reliance on algal production in the middle reaches of the river. Trophic niche breadth was greatest for assemblages in less degraded sites with higher fish species richness. My analyses also suggest that anthropogenic inputs may be enriching baseline nitrogen isotope ratios at one study site. Across seasons, isotope signatures remained fairly constant in five common fish species. In general, my results suggest that anthropogenic influences in portions of the Pecos River have negative impacts on the diversity of consumers and trophic resources in local food webs. 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.