Persistence, loss and appearance of bacteria upstream and downstream of a river system

Bacterial taxa shape microbial community composition and influence aquatic ecosystem dynamics. Studies on bacterial persistence in rivers have primarily focussed on microbial-source tracking as an indicator for faecal-source contamination, whereas archetypal freshwater species have received minimal attention. The present study describes the river microbial communities upstream and 3.3km downstream of a small rural town. By 16S rDNA sequencing, we report three patterns in microbial community composition, namely, persistence, loss and appearance. Persistence was observed as 46% inter-site similarity, perhaps owing to generalists that have information lengths that exceed 3.3km and are capable of adapting to system fluctuations. Loss was observed as 10% site exclusivity upstream, perhaps owing to removal processes such as predation and lysis during transport downstream. Last, appearance was observed as 44% site exclusivity downstream, indicating potential anthropogenic impacts from land run-off on bacterial community composition. High multivariate dispersion among downstream samples, as well as overall sample dissimilarity, present as microscale hotspots of discrete Firmicutes and Cyanobacteria species, indicated higher heterogeneity downstream, and therefore increased patchiness from downstream transport and inputs of bacterial genotypes. These findings suggest relativities among three fates for bacterial species of fluvial systems, persistence, loss and appearance, with each having different effects on system dynamics.