Crucial role of conditionally rare taxa in preserving bacterioplankton community stability in a large reservoir during seasonal water level fluctuations

Bacterioplankton are core drivers of biogeochemical cycles in reservoir ecosystems, playing a vital role in maintaining water quality and ecosystem functioning. However, the response mechanisms and functional logic of sub-communities with varying abundances under intense hydrological fluctuations remain poorly understood. This study investigated the community dynamics and assembly processes of rare taxa (RT), conditionally rare taxa (CRT), and conditionally rare and abundant taxa (CRAT) across different hydrological periods in the Danjiangkou Reservoir. Results showed that microbial species richness and evenness peaked during high-water periods but decreased significantly during drawdown and low-water phases due to hydrological shifts. Network topology analysis revealed that rare taxa preserved latent diversity during stable water levels, while conditionally rare taxa (CRT) consistently served as the core topological backbone, facilitating community structural reorganization through species replacement after disturbances. Functional annotation confirmed that CRT possess remarkable metabolic plasticity, ensuring the continuity of key nutrient cycles through functional compensation under environmental stress. Hydrological fluctuations primarily regulated the succession pathways and functional homeostasis of sub-communities by altering the vertical distribution of physicochemical factors and nutrient availability. These findings highlight the important ecological role of CRT in maintaining microbial network robustness and community organization under hydrological fluctuations, providing scientific support for adaptive reservoir management and ecological assessment of strategic water resources under climate change.