Food chain length in a large floodplain river: planktonic or benthic reliance as a limiting factor

Abstract

Food chain length (FCL) is a key integrative variable describing ecosystem functioning. The aim of thepresent study was to test the hypothesis that the relative importance of planktonic and benthic energy pathways is a majorfactor affecting FCL in the Middle Parana´ River. Samples were obtained from in eight waterbodies, measuringchlorophyll-a concentrations and the abundance of benthic invertebrates and the trophic position of top predators by5 stable isotope analysis. There was no evidence that resource availability, disturbances or ecosystem size limited FCL.Similarly, the body size and trophic position of predators were not correlated. However, the relative abundance ofplanktonic and benthic resources was correlated with FCL. In addition, stable isotopes analysis showed that the benthicreliance of top predators is correlated with their trophic position. The results of the present study indicate that because themajor benthic primary consumer is a large fish (Prochilodus lineatus), the size structure of individual food chains is an10 important factor determining FCL. Whereas in floodplain rivers large detritivorous fishes are targets of commercialfishing, overfishing in the Middle Parana´ River could be expected to increase FCL, the opposite effect to that seen inmarine environments.