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The degradation of freshwater quality induced by cyanobacterial blooms is a major global environmental concern. Microbially driven nitrogen removal could alleviate eutrophication to some degree in freshwater ecosystems. However, the response of anaerobic ammonium oxidizing (anammox) bacterial communities to cyanobacterial blooms remains poorly understood, especially in reservoir ecosystems. Here we compared the dynamics of anammox bacterial communities during and after a cyanobacterial bloom in a subtropical reservoir. Our data showed that a cyanobacterial bloom triggered a significant increase in bottom anammox bacterial abundance. During the bloom period, anammox bacterial abundance in bottom waters was 9-fold and 52-fold higher compared with non-bloom stratification and mixing periods, respectively. The community composition of anammox bacteria in surface waters changed substantially accompanied by the disappearance of the cyanobacterial bloom, and a shift of dominance from unidentified anammox genera to Ca. Brocadia was observed. Although Ca. Brocadia was always predominant in both middle and bottom waters, the non-bloom period had more unique taxa than the bloom period. Cyanobacterial bloom-related changes in environmental conditions (e.g. NH4-N and total organic carbon) and water stratification together influenced the distribution and dynamics of anammox bacteria. Altogether, our study lays the basis for a better understanding of the breakdown of cyanobacterial blooms in a stratified reservoir.

Both phytoplankton and bacteria are key and abundant components of aquatic ecosystems and play pivotal roles in maintaining ecosystem structure and function. However, the extent to which phytoplankton community succession influences changes in bacterial community composition (BCC) is largely unknown. In this study, we evaluated the correlations between bacteria and phytoplankton communities and determined the relative contribution of phytoplankton community succession to temporal variation of BCC in a subtropical drinking water reservoir (Tingxi Reservoir, southeast China). Bacterial communities were investigated by quantitative PCR and 454 pyrosequencing of 16S rRNA genes, while phytoplankton communities were analyzed by light microscopy. A remarkable seasonal succession from Cyanophyta to Bacillariophyta was observed during the study period, and this succession can accurately predict the distribution and abundance of the bacterial OTUs based on the discriminant function analysis. Association networks revealed that 38 of the 46 abundant bacterial OTUs exhibited significant correlations with phytoplankton. More interestingly, the positive correlations dominated the associated network, which may suggest that facilitative correlations between phytoplankton and bacteria are more important than inhibitory correlations in the Tingxi Reservoir. In addition, some bacterial OTUs were closely correlated with the dynamics of Microcystis, and they were affiliated with the divisions Acidobacteria, Actinobacteria, and Proteobacteria. Structural equation model showed that succession of phytoplankton community explained the largest part of temporal variation in BCC. Therefore, our data suggest that the distinct succession of phytoplankton community may mediate the temporal dynamics of bacterial community in the Tingxi Reservoir.