What Drives the Changing Characteristics of Plankton in Floodplain Lakes: Season, Environmental Factors, or Lake Characteristics?

The floodplain lake ecosystem is one of the most biodiverse ecosystems on Earth. Its water level changes with seasonal floods. These periodic water level fluctuations have far-reaching impacts on ecosystem structure and function. Systematic understanding of the structural characteristics of plankton communities in floodplain lakes, the long-term coupled effects of environmental variables, and multi-pathway action mechanisms will facilitate the accurate revelation of plankton adaptation strategies in dynamic hydrological environments. This study investigated the structure, taxonomic and functional diversity, community assembly, and niche differentiation of plankton communities in floodplain lakes in the middle and lower reaches of the Yangtze River during dry and flood seasons to identify key driving factors and coupled environmental effects. The results showed that plankton communities were consistently dominated by Cyanobacteria, Chlorophyta, Bacillariophyta, Cryptophyta, and Rotifera across four lake types, although their relative contributions varied markedly with hydrological season and lake type. Plankton density and taxonomic α-diversity were significantly higher in the flood season, whereas both taxonomic and functional β-diversity were lower, indicating enhanced community homogenization under high hydrological connectivity. Taxonomic β-diversity was primarily driven by species turnover, while functional β-diversity was dominated by nestedness, suggesting substantial functional redundancy among lakes. Neutral community models showed poor fit (R 2 < 0), and niche breadth analyses revealed significant seasonal contraction from flood to dry seasons, indicating that deterministic processes governed plankton community assembly. Environmental filtering associated with water temperature, water depth, turbidity, and nutrient availability, together with niche differentiation, played dominant roles in shaping plankton communities. Structural equation modeling further demonstrated that land use and lake type indirectly regulated plankton density and diversity through their coupled effects on water quality and nutrient conditions, with hydrological seasonality exerting the strongest overall influence. These findings provide a scientific basis for ecological assessment and management of floodplain lakes.