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

Food chain length (FCL) is a key integrative variable describing ecosystem functioning. The aim of the present study was to test the hypothesis that the relative importance of planktonic and benthic energy pathways is a major factor affecting FCL in the Middle Paraná River. Samples were obtained from in eight waterbodies, measuring chlorophyll-a concentrations and the abundance of benthic invertebrates and the trophic position of top predators by 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 of planktonic and benthic resources was correlated with FCL. In addition, stable isotopes analysis showed that the benthic reliance of top predators is correlated with their trophic position. The results of the present study indicate that because the major benthic primary consumer is a large fish (Prochilodus lineatus), the size structure of individual food chains is an important factor determining FCL. Whereas in floodplain rivers large detritivorous fishes are targets of commercial fishing, overfishing in the Middle Paraná River could be expected to increase FCL, the opposite effect to that seen in marine environments.