Abstract: As sensitive indicators of environmental change, bacterial communities provide valuable insights into the healthy status of estuarine ecosystems through their spatial distribution patterns. The Pinglu Canal, a vital infrastructure project designed to achieve land-sea connectivity, is scheduled to commence for navigation in 2026. However, its potential ecological impacts remain uncertain, underscoring the importance of establishing baseline ecological data. In this study, seawater samples were collected in August 2024 along a transect extending from the mouth of the Pinglu Canal to the offshore zone, which was classified into three regions of estuarine, transition, and offshore. Integrated by diversity and the index of biotic integrity assessments, co-occurrence network construction, and null model simulation, high-throughput sequencing of the 16S rRNA gene was employed to investigate the distribution patterns of bacterioplankton communities and explore the relationships between community heterogeneity and environmental constraints. Results revealed that the dominant phyla were Proteobacteria, Cyanobacteriota, Bacteroidota, and Actinobacteria, accounting for 40.1%, 24.0%, 16.0%, and 10.4% of the total sequences, respectively. Rare taxa were more enriched in the transition and offshore zones. Alpha diversity followed the trend of transition zone > estuarine zone > offshore zone. Significant differences in community structure were observed across the three regions (p<0.01), with NO₂^- concentration identified as a key factor driving structural heterogeneity. The bacteria-based index of biotic integrity, developed using a random forest model, showed the lowest value in the estuarine zone, indicating reduced stability in both community structure and functional integrity compared to the transitional and offshore zones. Bacterial communities were primarily assembled through stochastic processes, which accounted for a relative contribution of 58.0%, with hydrodynamic interference acting as a primary factor shaping these mechanisms. By establishing a preliminary bacteria-based index of biotic integrity and elucidating the distribution patterns of bacterioplankton communities for the coastal zone of the Pinglu Canal, this study provides foundational data and scientific insights to guide future ecological management and conservation efforts.

Key words: bacterioplankton, diversity, index of biotic integrity, assembly mechanism, Beibu Gulf