Long-Term Changes in Planktonic Ciliate Beta Diversity Under Contrasting Hydrological Regulation in A River-Floodplain System.

This study assessed temporal beta (β) diversity of planktonic ciliates over 14 years (2010-2024) in the Upper Paraná River floodplain, comparing a dam-impacted system (EID) and a non-impacted, protected system (ENID). We tested predictions regarding temporal variation in richness, abundance, composition, and community structure. Both systems exhibited declines in richness and density, more pronounced in EID. Temporal β diversity (TBI) was higher in the impacted system, indicating stronger compositional turnover over time. The local contribution to β diversity (LCBD) values were also elevated mainly in EID, suggesting greater local ecological uniqueness in impacted environments-contrary to expectations for protected areas. The species contribution to β diversity (SCBD) results showed that common, widespread taxa contributed more to β diversity than rare species, emphasising the role of dominant taxa in structuring temporal variation. Variation partitioning indicated that abiotic factors were the primary drivers of community dynamics in both systems, while temporal effects were secondary and similar between them. Overall, the findings demonstrate that conservation status alone does not ensure ecological integrity in floodplain systems. Instead, maintaining natural hydrological regimes-especially flood pulse dynamics-is critical, as these processes regulate connectivity, dispersal, and community renewal, ultimately sustaining biodiversity even in nominally protected environments.