Food web structure and resource use by fish in the upper part of the Vltava river, Czech Republic

Understanding trophic niche structure is essential for describing the organization and functioning of fish communities in river ecosystems. Stable isotope analysis of carbon (δ13C) and nitrogen (δ1⁵N) was used to characterize trophic niches and food web structure of eight species of fish inhabiting the upper reaches of the Vltava river (Czech Republic). Eight species were analysed together with six categories of food resources. The isotopic composition of muscle tissue and basal food resources was measured using a Delta V mass spectrometer, and trophic metrics were calculated using Stable Isotope Bayesian Ellipses in R (SIBER) and MixSIAR models. Results revealed distinct isotopic separation among species, indicating variation in resource use and trophic position. The highest δ1⁵N enrichment and thus the highest trophic position within the community were recorded for dace Leuciscus leuciscus, which is likely to be because it mainly feeds on terrestrial invertebrates. In contrast, nase, Chondrostoma nasus occupied the lowest trophic level and was the most enriched in δ13C, due to feeding on periphyton and sediment-associated organic matter. Chub (Squalius cephalus), roach (Rutilus rutilus) and bleak (Alburnus alburnus) occupy broad isotopic niches. Species with more specific habitat requirements, such as barbel (Barbus barbus) and bullhead (Cottus gobio), had narrower trophic niches, reflecting their feeding specialization on aquatic invertebrates. This study highlights the value of stable isotopes for understanding community trophic structure and supports their use in riverine ecosystem management and river community ecology.