A highly connected river network continuum is not resilient to non-point source pollution from anthropogenic activity

Urbanization significantly impacts the community structure of aquatic organisms and poses a major threat to river biodiversity. However, the extent to which urbanization is linked to the homogenization of microbial communities and the underlying mechanisms remains poorly understood. In this study, we investigated the bacterial and archaeal communities from cities and neighboring natural rivers across river network located in the Qinling Mountains, Northwest China, and further investigated the alpha and beta diversity patterns and the mechanisms influenced by urbanization. We found that the influx of urban non-point source pollutants created a eutrophic condition, and enhanced the urban river microbial populations. Meanwhile, the rapid urbanization tends to decrease the overall habitat heterogeneity, and imposed stronger homogeneous selection and caused microbial communities biotic homogenization. The mechanisms of biotic homogenization can be attributed to modulating generalist/specialist species and invasion of nonnative species. For instance, the urban river had a greater proportion of fast-growing bacteria, algae, nitrifiers, PAH-degrading bacteria, pathogens, fecal bacteria and antibiotic-resistant bacteria than natural river ecosystems. Overall, urbanization leads to a more uniform river biosphere, causing the extinction of unique local species and a subsequent decrease in the regional species pool.