Authors: Horka, Petra; Musilova, Zuzana;Holubova, Katerina;Jandova, Katerina;Kukla, Jaroslav;

Although the concept of trophic interactions has been used for long time, there are still considerable gaps in our understanding of the trophic interactions of river fish communities. Stable isotope analysis can be used to infer ecological niche use in fish. δ13C and δ15N of 20 species of fish belonging to both eurytopic and rheophilic ecological groups from a large temperate river were used to evaluate trophic niche use, and to find out how environmental variability affects individual and community-wide aspects of trophic structure. Of the 11 environmental variables tested, nutrients had a significant effect on trophic niche area of species, indicating that eutrophication is of critical importance for fish communities. Isotopic niche area of species was found to be positively influenced by total phosphorus, and negatively affected by concentrations of N-NO3- and ammonia (N-NH4+). Niche diversification and total area of niche space (TA) were highest at sites with high heterogenity of both in-river and adjacent habitat, reflecting high resource diversity in less channelised reaches. An observed pattern where nutrient loading influenced both individual species and the overall trophic structure of the fish assemblage provides strong support for an anthropogenic influence on riverine food webs.