The dispersal processes shape the spatial turnover of fish communities across a natural channel ecotone in a large riverine floodplain

Ecotones drive the spatial turnover of biodiversity between heterogeneous habitats. In riverine ecosystems, however, the ecological influence of ecotones on fish assemblages remains unpredictable due to highly dynamic hydrological regimes and strong species dispersal capacities, leaving the community assembly processes within these transition zones poorly understood. Here, we utilized an extensive dataset covering 20 sampling sites and six surveys over 2 years from China's large riverine floodplain ecosystem (the Yangtze River-Poyang Lake Riverine Floodplain). We aimed to characterize the spatial turnover of fish communities across a natural 45-km channel ecotone and its connected river and lake habitats, and to explore the underlying mechanisms driving these patterns. Using a split-moving window dissimilarity analysis (SMWDA), we detected an abrupt, non-linear breakpoint in community composition along the distance gradient from river to lake. Within the channel ecotone itself, we observed a localized decline in community diversity, indicating a sharp spatial turnover rather than a uniform distance-decay transition. Furthermore, threshold indicator taxa analysis (TITAN) revealed that this ecotonal transition significantly influenced rheophilic and lake-river migratory species, spanning an influence distance of approximately 92 km. The neutral community model (NCM) demonstrated that dispersal processes played a dominant role in shaping fish communities across all habitats, highlighting the water channel as a vital connectivity corridor for floodplain fish expansion. Overall, our study reveals that the channel in the Yangtze River-Poyang Lake Riverine Floodplain does not function as a classic high-diversity ecotone, but rather emphasizes the critical importance of water channels in supporting regional metacommunity dispersal.