Trophic structure of fish assemblages varies across a Mesoamerican river network with contrasting climate and flow conditions

Abstract Assessments of trophic diversity are critical for evaluating ecological integrity of habitats, but interpretations of such assessments require an understanding of variation across natural environmental gradients. This can be problematic when comparing structure of assemblages in under-studied regions or habitats, such as watersheds in dry tropical forests. Here, we compared assemblage-wide trophic metrics and intraspecific variation for a subset of consumer traits across rivers and among different ecosystem types within the Grijalva and Usumacinta River basins of Mexico. The two rivers differ with respect to flow-regime alteration and climate: the Grijalva River has been hydrologically altered by a series of dams and has wet and dry tropical forests in its watershed, whereas the Usumacinta River remains unimpounded with a watershed dominated by tropical wet forest. Use of allochthonous resources was pervasive in Usumacinta basin tributaries, with stable isotope signatures suggesting that many fishes directly consumed riparian plants and detritus. In contrast, fish assemblages in Grijalva basin tributaries were supported by higher proportions of in-stream production. Food-chain length was highest in a Grijalva River reservoir fish assemblage, although trophic diversity was low in the impounded system compared to the mainstem Usumacinta River, where fishes consumed the broadest variety of food resources. We also observed differences in trophic ecology and body nutrient content within taxa across habitat types and basins. The differences we observed suggest that even in relatively intact watersheds, expectations for trophic structure in tropical streams should be adjusted based on factors such as discharge, climate, and riparian forest cover.