Hydrological characteristics strongly dominate the spatiotemporal variation of bacterioplankton sub-communities in the Yangtze River Estuary

Seasonal variations in river discharge in estuarine ecosystems drive dynamic interactions between freshwater and seawater, significantly impacting the structure and functionality of bacterioplankton sub-communities. Seasonal saltwater intrusion during the dry season intensifies the dynamic variations in water quality and salinity between the northern and southern branches of the Yangtze River Estuary, further complicating the ecological processes governing bacterioplankton communities. Nevertheless, the mechanisms underlying these processes and their effects on distinct bacterioplankton sub-communities remain insufficiently explored. In this study, the composition, functions, and assembly mechanisms of bacterioplankton communities in the Yangtze River Estuary during different hydrological periods were examined using molecular technique and various statistical analysis methods. The results showed that rare sub-communities exhibited the highest α-diversity, abundant and transient sub-communities primarily contributed to the diversity differences across hydrological periods. Saltwater intrusion into the northern branch altered bacterioplankton community and weakened the distance-decay pattern of the transient and rare sub-communities. Additionally, stochastic processes governed the assembly of rare sub-communities, and saltwater intrusion disrupted their migration and dispersal patterns. In contrast, abundant and transient sub-communities maintain their stability by adjusting their ecological strategies in response to salinity changes. Functional analysis indicated that external nutrient inputs and hydrodynamic changes in the wet season promoted community functional diversity and activity, while abundant sub-communities in the dry season were more effective at occupying resources and performing specific functions related to carbon cycling. These findings highlight the relationships between bacterioplankton sub-communities and environmental changes in estuarine ecosystems, underscoring their key roles in biogeochemical cycling.