In microbial biogeography, it is crucial to link spatial patterns with underlying drivers in natural ecosystems. Bacterial communities driving key biogeochemical processes in coastal zones, which are important interfaces between terrestrial and marine ecosystems, are affected by perturbations due to both natural and anthropogenic factors. However, the assembly of bacterioplankton communities, either free-living (FL) or particle-associated, in coastal ecosystems is still poorly understood. In this study, we investigated FL, nanoparticle-associated (NA) and microparticle-associated (MA) bacterial communities in the SCSPR, using environmental DNA metabarcoding based on the V4 region of the 16S rRNA gene. We assessed the relative importance of ecological processes using null model analyses based on a two-step framework. We found that the observed amplicon sequence variants (ASVs) increased from FL, NA to MA communities, and a remarkably pervasive core set of ASVs closely belonged to potential hydrocarbonoclastic bacteria with wide habitat niche breadths. Analyses of similarity tests revealed that FL, NA and MA communities differed significantly but weakly, based on unweighted (R = 0.27, p < 0.001) and weighted (R = 0.18, p < 0.001) dissimilarities. Fundamental regulation of bacterial communities via homogeneous selection was most prominent in FL (90%), followed by NA (86.8%) and MA (73.3%), whereas the relative importance of dispersal limitation was most pronounced in MA (13.2%), followed by NA (11.7%) and FL (8.6%). In summary, bacterial communities in the SCSPR can be predominantly influenced by human activities, as indicated by the core ASVs closely related to hydrocarbonoclastic bacteria. Importantly, anthropogenic influences can temporally overwhelm the large environmental heterogeneities in coastal ecosystems. This study fills knowledge gaps in bacterial community assembly, which may facilitate future studies regarding anthropogenic influences on coastal ecosystems.

community assembly,environmental DNA,homogeneous selection,hydrocarbonoclastic bacteria,metabarcoding