Environmental filtering dominates bacterioplankton community assembly in a highly urbanized estuarine ecosystem.

Estuaries are important in terms of biodiversity, biogeochemical function, and ecological balance due to their intense land-sea interactions. The sustainability of estuarine ecosystem function relies on a good understanding of the ecological processes related to microbial communities. However, microbial community assembly in such ecosystems is still not well understood. Here, based on 16S rRNA sequencing, we investigated the bacterioplankton community structure in the Pearl River Estuarine system during the wet and dry seasons. Results showed that there were significant seasonal and spatial variations in the bacterioplankton communities of the estuary, with seasonal variations being more remarkable. Multiple bacterioplankton with different abundances in the wet and dry seasons were observed, e.g., the class Actinobacteria and Oxyphotobacteria were enriched in the wet season, whereas Alphaproteobacteria and Saccharimonadia were more abundant in the dry season. Both variation partitioning and null model analysis revealed that environmental filtering dominated the bacterioplankton community assembly in the Pearl River Estuary. Water physical properties (e.g., salinity and temperature), nutrient content (e.g., nitrate), and upstream land use (e.g., urban land cover) together determined the distribution of the bacterioplankton composition in this highly urbanized estuarine ecosystem. These findings would help improve our understanding of the bacterioplankton communities in estuarine ecosystems and provide a theoretical foundation for estuarine ecological health management.