Two stable isotopes δ13C and δ15N were used to identify the energy sources and trophic relationships of the main freshwater macroinvertebrates in a floodplain lake of the Beni River (Bolivian Amazonia). Four energy sources (seston, bottom sediment, periphyton, and aquatic macrophytes) and macroinvertebrate communities were collected during three periods of the river hydrological cycle. Macroinvertebrates showed greater temporal variation in isotope values than their food sources. Six trophic chains were identified: four were based on seston, periphyton, C3 macrophytes, and bottom sediments, and the last two chains on a combination of two carbon sources. One mixed seston and periphyton sources during the wet season while the other mixed periphyton and macrophytes sources during the wet and dry seasons. Periphyton was the most important energy source supporting the highest number of trophic levels and consumers. The macrophytic contribution was only significant during the dry season. Bottom sediments constituted a marginal energy source. As each season is associated with different physical and chemical conditions, processes organizing macroinvertebrate food web structure in the Beni floodplain seem strongly linked to hydrological seasonality.
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Acknowledgments We are grateful to Carmen Mendoza and Lucia Alanoca who collected most of the source samples, and to Patrica Miranda and Marianela García who began the work on the decapods. We thank the technical staff of IRD—La Paz: Marcelo Claure, Abdul Castillo and Julio Cesar Salinas as well as our guide and skipper in Rurrenabaque Carlos Manu. Valuable assistance in the Laboratory of Environmental Quality was provided by: Jaime Chincheros and Berenice Carazas. Adriana Sanjinez identified the aquatic plants. We also thank B. Hugueny and D. Achá for comments on the manuscript. Finally we are grateful to Danny Rejas for valuable discussions and suggestions during the analysis. This work received financial support from the Institut de Recherche pour le Développement (IRD) and from the World Wildlife Fund (WWF).