Long-term changes in freshwater molluscan communities in the middle stretch of the Danube River (Slovakia) over a 23-year period

This study, as a part of complex environmental monitoring initiated by the construction of the Gabcikovo-Nagymaros waterworks system, was designed to assess long-term temporal trends (from 1991 to 2013) in freshwater molluscan communities and their ecological indices in different habitats of the Danube floodplain in Slovakia. Mollusc samples were taken seasonally at 6 sites that represent various habitat types in terms of their hydrological connectivity and position of dam impact. Multivariate techniques were used to detect variability and trends in composition of the molluscan communities over the study period, and change-point analyses were performed to identify the time points of significant change in the ecological indices of communities (e.g. species richness, abundance, diversity). A total of 40 (32 gastropods and 8 bivalves) species were identified in the investigated floodplain area. The multivariate analysis (nMDS and two-way crossed ANOSIM), based on both presence/absence and quantitative data, discerned four significantly different communities inhabiting the main channel and side arms. At the main channel sites, nMDS analysis clearly demonstrated a one-way shift in community composition and four main breaks were identified in the values of the univariate community indices over the study period. Reversible community changes were found in the side arms, especially in the plesiopotamal side arm type. This was also associated with a smaller overall modification of community metrics there, the extent of which was evidently influenced by the hydrological connectivity of these habitats to the main Danube channel.