Impact of the riverbed morphology on the source of organic material and the trophic structure of fish community along the upper reaches of the Neve #382;is River, Lithuania

Background. The catchment of the Nevežis River, the sixth longest river in Lithuania, is among the most affected by agricultural activities. Although seriously disturbed by land reclamation and agricultural pollution, the Nevežis River still has natural, anthropogenically unaffected stretches. Fish species were studied at two sites of the upper part of the river with different hydrological regime, exhibiting clear differences in the trophic status of fish. The combined effect of hydrogeomorphic and water-chemistry variability on the stable isotope composition of fish assemblages in Lithuania has not been investigated. The presently reported study was intended to explore the possible effects of the riverbed morphology and agricultural activities on the chemistry and stable isotope composition of the three main fish species. Materials and methods. This study covered three species of freshwater fishes: the European perch, Perca fluviatilis Linnaeus, 1758; the roach, Rutilus rutilus (Linnaeus, 1758); and the northern pike, Esox lucius Linnaeus, 1758. The sampling was performed in both natural and regulated river stretches using backpack electrofishing equipment. Stable isotope composition of the samples was revealed by applying stable isotope analyses (SIA) with an elemental analyser connected to an isotope ratio mass spectrometer. To obtain more information about environmental variability and variation of stable isotopes δ13C and δ15N in fish, the entire data matrix was analysed using Principal Component Analysis (PCA). Results. In the study sites on the Nevežis River, algal carbon isotope ratios were more 13C- depleted than those from terrestrial sources. The fishes were clearly dependent on terrestrial carbon sources in natural locations. However, the fishes collected from regulated sites had carbon isotope signatures more similar to algal than to terrestrial sources. Mean values of δ15N of the fishes from the natural sites were lower than those from the regulated ones. Conclusion. The presently reported study revealed impact of abiotic habitat variables indicating the importance of physicochemical and morphological characteristics on δ13C and δ15N isotopic values variance of fish at natural and regulated river sites. Our findings would help to evaluate and predict changes on river food webs that occur due to hydromorphological alterations determined by human activity and climate change. The results could be usef