Trophic structure of fish assemblages from oligotrophic tropical rivers: evidence of growing assimilation of autochthonous primary producers with the increase in river dimensions

In this study, we analyzed the trophic structure and carbon sources supporting fish assemblages of five low-productivity rivers with different dimensions. Using carbon (delta C-13) and nitrogen (delta N-15) isotope ratios, we identified the major carbon sources to fish species and estimated fish trophic positions in each river. We observed a similar trophic structure in almost all low-productivity rivers. Detritus (allochthonous resources derived from riparian zone-CPOM and processed organic matter from upstream sources-FPOM) were important to sustain the fish assemblages, especially in smaller rivers. We also observed a growing consumption of autochthonous primary producers, algae and periphyton, with the increase in river dimensions and especially for primary consumers. Fishes from the smallest river occupied lower mean trophic positions; however, the assemblage exhibited wider food-chain length than those from larger rivers. Our results indicate an important role of autochthonous primary producers even in rivers with low nutrient concentrations. These results are important to increase the knowledge about medium-size oligotrophic rivers, especially because tropical rivers can support highly diverse fish assemblages and productive fisheries even exhibiting low in situ primary productivity.