Seasonal effects of river flow on microbial community coalescence and diversity in a riverine network

Terrestrial microbial communities may take advantage of running waters and runoff to enter rivers and mix with aquatic microorganisms. However, the environmental factors governing the interchange of the microbial community within a watercourse and its surrounding environment and the composition of the resulting community are often underestimated. The present study investigated the effect of flow rate on the mixing of water, soil, sediment and biofilm at four sites along the Lancang River and one branch of the river in winter and summer and, in turn, the resultant changes in the microbial community within each habitat. 16S rRNA gene-based Illumina high-throughput sequencing illustrated that bacterial communities were apparently distinct among biofilm, water, soil and sediment. Biofilms had the lowest richness, Shannon diversity and evenness indices compared with other habitats, and those three indices in all habitats increased significantly from winter to summer. SourceTracker analysis showed a significant coalescence between the bacterial communities of sediment, water and biofilm samples at lower flow rates. Additionally, the proportion of Betaproteobacteria in sediment and biofilms increased with a decrease in flow rate, suggesting the flow rate had a strong impact on microbial community composition and exchange among aquatic habitats. These results were further confirmed by a Mantel test and linear regression analysis. Microbial communities in all samples exhibited a significant but very weak distance-decay relationship (r = 0.093, P = 0.024). Turbidity played a much more important role on water bacterial community structure in summer (i.e. rainy season) (BIOENV, r = 0.92). Together, these results suggest that dispersal is an important factor affecting bacterial community structure in this system.