Assembly processes and source tracking of planktonic and benthic bacterial communities in the Yellow River estuary.

Estuaries connect rivers with the ocean and are considered transition regions due to the continuous inputs from rivers. Microbiota from different sources converge and undergo succession in these transition regions, but their assembly mechanisms along environmental gradients remain unclear. Here, we found that salinity had a stronger effect on planktonic than on benthic microbial communities, and the dominant planktonic bacteria changed more distinctly than the dominant benthic bacteria with changes in salinity. The planktonic bacteria in the brackish water came mainly from seawater, which was confirmed in the laboratory, whereas the benthic bacteria were weakly affected by salinity, which appeared to be a mixture of the bacteria from riverine and oceanic sediments. Benthic bacterial community assembly in the sediments was mainly controlled by homogeneous selection and almost unaffected by changes in salinity, the dominant assemblage processes for planktonic bacteria changed dramatically along the salinity gradient, from homogeneous selection in freshwater to drift in seawater. Our results highlight that salinity is the key driver of estuarine microbial succession and that salinity is more important in shaping planktonic than benthic bacterial communities in the Yellow River estuary.