Damming Reshapes Beta Diversity in Planktonic Ciliate Communities in a River-Floodplain System

This study evaluated temporal patterns of beta diversity in planktonic ciliate communities over a 14-year period (2010–2024) in aquatic environments with different levels of dam impacts in the Upper Paraná River floodplain, Brazil. Two systems were compared: one directly impacted by dams (EID) and another located in a protected area without direct dams (ENID). We tested four predictions related to temporal variation in richness, abundance, composition, and community structure. Our results indicated declines in ciliate richness and density in both systems, with greater intensity in EID. Temporal beta diversity was significantly higher in the impacted system, reflecting more pronounced changes in community composition over time. Elevated values for local contributions to beta diversity (LCBD) were observed primarily in EID environments, contradicting expectations of greater ecological uniqueness in non-impacted areas. Analysis of species contributions (SCBD) revealed that abundant and widely distributed species contributed more to beta diversity than rare species. Finally, variance partitioning analysis showed that abiotic factors were the primary drivers of communities in both systems, while temporal influence was relevant but similar between systems. Our findings, even when derived from communities of small, short-life cycle organisms such as ciliates, provide evidence that territorial protection offered by conservation units alone is insufficient to ensure the ecological integrity of floodplain systems. Maintaining natural flood pulse dynamics is essential in preserving ecological integrity and aquatic community diversity in regulated systems. The results also indicate that the continuity of ecological processes depends on the maintenance of natural hydrological regimes, particularly flood pulses, which promote connectivity, organism dispersal, and community renewal, even in environments with territorial protection (e.g., non-dammed areas).