Author: Rana  El-Sabaawi,  Associate Professor (University of Victoria)

Description:
Tropical river ecosystems often host a high diversity of vertebrate primary consumers. Among grazing and detritivorous fishes, a spectrum of feeding strategies allows utilization of both algae and detritus to varying degrees. Characterizing trophic differences among species is critical for both ecological and evolutionary inferences, but results of gut content and stable isotope analyses are often ambiguous due to differences in assimilation of basal resources or inability to distinguish them visually or isotopically. We used a combination of stable isotopes and fatty acids to characterize the diets of primary-consumer fishes in a Neotropical river. We found that fatty acid fingerprints are far more powerful for separating allochthonous and autochthonous resources than carbon and nitrogen isotopes.  Specifically, algae were enriched in 18:3n3, while wood was enriched in 18:2n-6, and this difference was evident in specialized algivorous and wood-eating species. In contrast, stable isotopes provided poor differentiation among resources or fish species. Our data also revealed assimilation of microbial resources associated with other terrestrial detritus. Bayesian cluster analysis suggests considerable niche partitioning among these fish species, perhaps contributing to coexistence of numerous primary consumers in the same microhabitats. Our study demonstrates the power of multi-tracer approaches in characterizing energy flow and trophic relationships in complex food webs.

Category: Scientific Program Abstract > Special Session > SS13 Getting to the Bottom of Freshwater Food Webs: A Global Perspective on the Role of Terrestrial Resource Subsidies