Food Web Structure in the Xingu River Rapids Prior to Operation of the Amazon’S Largest Hydropower Plant

Abstract

Recent theoretical studies suggest that food webs are size-structured with top predators coupling different energy sources, but evidence for this hypothesis is still scarce, especially in highly diverse tropical rivers. Here, we explored the association between body size, trophic position, and the use of allochthonous and autochthonous basal production sources in the Volta Grande rapids of the Xingu River, a major clearwater tributary of the Amazon River, during the period prior to operation of the Belo Monte Hydropower Plant (BMHP). This section of the river contains a maze of channels with rocky shoals that support dozens of endemic fishes, mollusks and other aquatic taxa that may be impacted by flow regulation by the Pimental Dam located upstream. During the low-water season, we surveyed fish, crustaceans, mollusks, sponges, aquatic and terrestrial insects, zooplankton, and basal production sources to obtain tissue samples for analysis of stable isotope ratios of carbon (δ13C) and nitrogen (δ15N). Biomass of most aquatic organisms appears to be largely supported by riparian vegetation, highlighting the importance of the lateral connectivity between aquatic and terrestrial habitats. Unlike expected, we did not observe a gradual increase in coupling of energy pathways with increasing body size and trophic position. Findings provide a baseline for trophic ecology of this river under the natural flow regime for future impact assessments, and also indicated that more complex food web models, potentially including additional functional traits (e.g., gut length), are needed to describe resource and habitat use in highly diverse tropical ecosystems.