Co-occurrence patterns and assembly processes of abundant and rare bacterioplankton in plain river network areas of eastern China

In the context of anthropogenic impacts on riverine ecosystems globally, understanding the response of bac-terioplankton to anthropogenic stress is important for human and environmental health. Bacterioplankton communities are critical for maintaining ecosystem stability, but little is known about their co-occurrence net-works and assembly processes in intense human-impacted plains river networks. By applying cooccurrence networks, variance partitioning, and null model analysis, we investigated the mechanisms of interaction and assembly of abundant (>1% relative abundance) and rare (<0.01%) taxa at 32 sites in river networks along an urbanization gradient (urban, suburban, and agricultural areas). Our results show that interactions between the bacterioplankton communities were more complex in urban areas than in other areas, and that environmental factors (primarily fluoride, nitrate nitrogen, and dissolved organic carbon) as well as spatial factors (i.e., Moran's Eigenvector Maps) explained most of the variation in bacterioplankton communities. Abundant taxa showed stronger spatial turnover than rare taxa, indicating that spatial factors played a greater role in the assemblage of abundant taxa. Land use types, especially impervious surfaces, had a unique influence on rare taxa, but contributed less to community change than other factors. In addition, rare taxa were mainly influenced by deterministic processes, whereas abundant taxa were more influenced by stochastic processes, especially in agricultural areas. These findings suggest that the shares of abundant and rare taxa mediate disturbances in local environmental conditions in anthropogenic river network areas, while deterministic processes play a greater role in shaping bacterioplankton communities. Overall, our study provides important insights for environmental monitoring and management in areas of intense human activity, emphasizing the need to consider both abundant and rare taxa when assessing the impact of human activities on riverine ecosystems.