Trophic level, food chain length and omnivory in the Paraná River: a food web model approach in a floodplain river system

Abstract

To determine the mean trophic level and food chain length and to quantify omnivory in a floodplain lake of the Middle Paraná River, we constructed a food web model based on available data. Because proper selection of basal resources is essential, we performed stepwise model selection to decide which sources to include in the model. We estimated mean trophic level, food chain length and omnivory using a binary web (non-weighted trophic links) and a weighted web (trophic links weighted using stable isotopes). Model selection excluded macrophytes as relevant resources for the food web of this floodplain lake. In general, trophic links were relatively even, leading to similar estimations of mean trophic level and omnivory in both binary and weighted webs. Food chain length, however, was higher for the binary web, which was caused by the strong link of top predators with lower-trophic level prey in this ecosystem. Overall, the estimate of food chain length in the Paraná River was not particularly high,despite the high productivity and size of this ecosystem. We suggest that the periodical hydrological fluctuation of large rivers could be a major factor precluding the occurrence of long food chains.