Analysis of trophic pathways in freshwater ecosystems using stable isotope signatures

Analysis of stable isotope ratios has proven to be a powerful approach to food web studies, whereby community trophic patterns can be detected rapidly and can highlight processes that influence consumer production across explicit temporal and spatial boundaries. In some cases, isotopes are partitioned in a predictable fashion in biota so that their isotopic ratios provide both information about sources of organic carbon important to consumers, and insights about how materials are processed with trophic transfers. The application of isotope analyses to examine river food webs has received far less attention than take, stream, and marine systems. Several hypotheses were examined regarding the influence of riverine geochemistry and vegetative attributes on trophic structure and energy pathways supporting consumers. Over 300 tissue samples from fishes, invertebrates, and autotroph sources were collected from four geochemically-distinct river systems in Venezuela, and analyzed for stable carbon and nitrogen ratios. Fishes in nutrient-poor black waters were lighter in delta13C and heavier in delta15N than fishes in productive savanna rivers. Carbon ratios in black water fishes indicated dependence on allochthonous detritus or a very isotopically light algal source. The heavier delta13C values for fishes in more productive rivers were partially attributed to some assimilation of C4 grass. In all four rivers, there was significant enrichment in 15N from the base of the food web to top piscivores. Contrary to the hypothesis of more productive systems supporting more trophic links, fishes in the least productive system had the most enriched delta15N values.;Freshwater wetlands contain multiple primary production sources that often have overlapping delta13C values. To improve the resolution of which of these sources was being transferred up the food chain, samples of plants, invertebrates, and fishes were sampled seasonally from a temperate oxbow lake in Texas. Both invertebrate and fish delta13C values tracked seasonal changes in algal delta13C, indicating consumer dependence on this source. In contrast, delta13C in terrestrial sources was less seasonally variable, and was not considered a major energy source for fast growing consumers.