Effects of long-term floodplain disconnection on multiple facets of lake fish biodiversity: Decline of alpha diversity leads to a regional differentiation through time.

Hydrological disconnection is increasingly threatening biodiversity in river floodplain ecosystems worldwide, but studies reporting long-term change of aquatic biodiversity in relation to floodplain disconnection are seldom, especially from multifaceted biodiversity perspectives. Here, we examined how loss of river-lake connectivity affected multifaceted (taxonomic, functional and phylogenetic) alpha and beta diversity of fish assemblages in 11 Yangtze River floodplain lakes over the past 70 years. We found that all three facets of alpha diversity significantly decreased through time, but the decrease rate was highest (31.4%) in taxonomic richness, second in functional (26.4%) and lowest in phylogenetic facet (4.7%). Nevertheless, taxonomic, functional and phylogenetic structures of fish fauna all exhibited differentiation. The taxonomic and phylogenetic differentiations were due to the joint increases in their turnover and nestedness-resultant component, whereas the functional differentiation was mainly driven by the increase in its turnover component. Such distinct results were because of the imbalanced extirpations of fish species (especially from species-poor orders and families) in disconnected lakes and connected lakes. With few exceptions of strong correlations between changes in taxonomic dissimilarities and phylogenetic dissimilarities, we generally found weak correlations between changes in different facets of both alpha and beta diversity. This discrepancy highlights that measuring different biodiversity facets offer distinct information about biodiversity dynamics and can enhance our ability to detect and evaluate the impacts of floodplain disconnection on biodiversity. We therefore recommend an integrative approach embracing taxonomic, functional and phylogenetic diversity is essential to effective biodiversity assessment and conservation in large river floodplains.