Effect of a series of reservoirs on the environmental conditions and non-insect benthic communities in Slovakia's longest river

The number of studies assessing the downstream effects of multiple reservoirs on benthic communities over a large spatial scale has been disproportionally less compared with studies of the effects of a single reservoir, while even fewer studies have examined the responses of non-insect macroinvertebrates on the impact of river discontinuity. The main objective here is to present the effect of seven large reservoirs in a 364-km long stretch of the Vah River, which is the longest river in Slovakia. We assessed the longitudinal changes in environmental variables to identify their possible cumulative effect and recovery distance. Moreover, the spatial variability in the taxonomical and functional structure of the non-insect benthic invertebrate community was evaluated. We do not detect the true cumulative effect of any variable, because of the apparent recovery before reaching the next reservoir. Although the environmental variables closely match with the division of the river based on the river network development, the non-insect community indicates a more extensive disruption of the river environment. On the basis of species data clustering, three groups of sites (stretches) are found to reflect their longitudinal position in the river rather than their position relative to the reservoir (upstream, downstream and in-between sites). The difference between these groups also mirrors the functional structure of the benthic community. Water temperature, conductivity and chemical oxygen demand create a significant gradient causing shifts in taxonomical, as well as functional community composition. The canonical correspondence analysis shows that all reservoirs cause a shift in the structure of benthic communities as they are similar to communities at more distant sites downstream. Moreover, the reservoirs situated in the upper and middle stretch of the river were detected to have the most impact on the ecological attributes of the community. The multi-level pattern analysis reveals 31 non-insect taxa and four functional traits associated with one or two river stretches in a longitudinal profile of the Vah River. Of the indicator species, Stylaria lacustris, Nais christinae, Ophidonais serpentina and Limnodrilus claparedeianus are found to be associated with the severe downstream modification (potamalization) of the river environment induced by reservoirs.