Aquatic and Terrestrial Invertebrates Support Fish Assemblages of Amazonian Streams

Basilio, Pedro H. S. and Leal, Cecília G. and de Carvalho, Débora R. and Brejão, Gabriel L. and Drager, Dennys H. C. and Salvador, Gilberto N. and Alves‐Filho, Marcos A. and Costa, Vladimir E. and Ferraz, Gabriel O. and Ferreira, Gustavo S. and Leitão, Rafael P. and Montag, Luciano F. A. and Santos, Jady V. A. S and Silva, Everton C. and Pompeu, Paulo S. (2026) Aquatic and Terrestrial Invertebrates Support Fish Assemblages of Amazonian Streams. Freshwater Biology, 71 (5): e70235. ISSN 0046-5070

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Abstract

The Amazonian stream network is remarkable not only for its extent but also for its rich fish diversity. These streams are interconnected with the surrounding forests. However, key ecological aspects of Amazonian freshwater ecosystems, such as energy flows and carbon sources sustaining fish assemblages, remain poorly understood. In this study, we used carbon (δ13C) and nitrogen (δ15N) stable isotopes to evaluate, for the first time, the trophic structure of Amazonian stream fish and to identify the food resources sustaining their assemblages. We sampled fish and a broad array of potential food resources, including macrophytes, riparian vegetation, filamentous algae, aquatic and terrestrial invertebrates, and periphyton. We calculated diversity metrics, trophic niches, trophic positions, and assimilated resource proportions. Fish biomass was primarily supported by terrestrial invertebrates (31%–55%) and benthic invertebrates (22%–40%). Shrimps were the third most important resource contributing to fish biomass (9%–22%). Collectively, invertebrates accounted for 79%–90% of the total fish biomass across the studied streams. Trophic diversity metrics showed little variation among stream sites, with most pairwise Bayesian comparisons indicating substantial overlap in posterior distributions. Our findings underscore the critical role of both aquatic and terrestrial invertebrates in supporting the trophic dynamics of stream fish in the Amazon, demonstrating the strong ecological linkages between streams and their surrounding landscapes. Moreover, the results highlight the importance of riparian forests in providing essential food resources for fish assemblages of Amazonian aquatic ecosystems. Our study provides novel insights into the carbon flow and isotopic ecology in small Amazonian streams, contributing to understanding ecosystem functioning and informing conservation strategies for one of the most species‐rich freshwater systems on Earth.