This study aimed to evaluate the isotopic fractionation and trophic position of three zooplankton species (Notodiaptomus amazonicus, Moina minuta and Bosmina hagmanni) in the Upper Paraná River floodplain. We predict that phytoplankton is the main food resource used by these species. Three zooplankton samples and three phytoplankton samples were taken from each sampling site, with three to four samples collected for each species. The number of individuals for samples varied according to the body size: from 100 to 130 individuals for Notodiaptomus amazonicus; 150 to 200 for Moina minuta; and from 250 to 300 for Bosmina hagmanni. The isotopic values for δ13C and δ15N were determined using mass spectrophotometer. The isotopic fractionation of 13C was performed according to the relationship Δ = δ13Czooplankton - δ13C phytoplankton. To determine the possible trophic position of these species, we used the expression TL = (δ15N zooplankton - δ15N phytoplankton)/Δ+ 1. The species showed high variation in isotopic fractionation and in trophic position in the different environments. We verified that the species use other food resources in addition to phytoplankton. The elucidation and understanding of the trophic position of the organisms based on stable isotopic analysis offers complementary information to traditional techniques. This analysis helps explain the flow of matter and energy in the food chain of floodplain aquatic environments as well as trace the trophic relationships involved in the ecological roles and strategies of distinct species.
stable isotopes; trophic level; zooplankton; food chain
