Prokaryotes represent the largest biodiversity pool and drive the biogeochemical cycles in fluvial environments. However, the mechanisms underlying the assembly of prokaryotic communities are largely unexplored at taxonomic and functional levels, simultaneously. Here, we investigated the spatio-seasonal distribution of prokaryoplankton communities in a salinity-influenced watershed, China using 16S rRNA gene amplicon sequencing. The OTUs were divided into core and satellite, based on a statistical approach. Our results indicated that the core taxa accounted for 87.4% of all the communities, predominantly including Betaproteobacteria, Actinobacteria and Thaumarchaeota. Whereas 10 of 22 metabolic phenotypes annotated using METAGENassist had significantly higher relative abundances in the core than satellite communities. Temporal variations in the composition of ammonia-oxidizing prokaryotes could be attributed to the seasonal nutrient concentrations. Multivariate statistical analysis indicated that environmental filtering rather than dispersal limitation is the primary mechanism driving the assembly of the whole, core and satellite communities at both taxonomic and functional levels. Network analysis revealed that the mutualistic interactions might also have certain roles in the assembly of the core taxonomic community. However, taxonomic community fluctuations can only partially reflect the functional community variations. Collectively, this study significantly advanced our understanding of the biogeographic pattern of the fluvial prokaryotic communities.