Determination of vertical and horizontal assemblage drivers of bacterial community in a heavily polluted urban river

Abstract Identifying vertical and horizontal assemblage drivers of bacterial community is important for improving the efficacies of ecological evaluation and remediation for a huge contaminated river (e.g. black-odor urban river). However, little is known about the effect of stochastic vs. deterministic processes on the reliability of the identification processes. Here, a comprehensive analysis was performed to reveal vertical and horizontal assemblage drivers of bacterial community in a heavily polluted urban river (total area of 4.23 km 2 and total length of 9.3 km), considering the relative importance of stochastic and deterministic processes. Heterogeneous bacterial community assemblages were observed in both vertical and horizontal profiles and the differences in the bacterial community between depths were relatively significant at genus level. The higher values for the Simpson dissimilarity index (horizontal β SIM  = 0.59 ± 0.02; vertical β SIM  = 0.48 ± 0.03) compared to the nestedness-resultant dissimilarity index (horizontal β NES  = 0.05 ± 0.02; vertical β NES  = 0.05 ± 0.05) showed that species replacement explained both the vertical and horizontal beta-diversity patterns. Comparison of horizontal and vertical Sorensen dissimilarity indices further indicated that the biodiversity of vertical community deserved more attention due to the shorter geographical distance with similar beta-diversity patterns compared to horizontal assemblages. Various traditional analysis without consideration for phylogenetic turnover revealed that TN, TP, NH 4 + -N, DO, ORP, Conductivity and COD Mn were all the related environmental factors that influenced bacterial community. However, after taking stochastic vs. deterministic processes into account, only NH 4 + -N and ORP were identified as the main driving forces of trends in the vertical and in the horizontal assembly of bacterial community in the polluted urban river, respectively. This study is helpful for improving ecological assessment methodology and remediation strategy for contaminated urban rivers.