[Diversity and Function Prediction of Bacterioplankton Under Human Disturbance in the Main Stream of the Laoguan River Before and After the Flood Season]

The Laoguan River is the tributary of Danjiangkou Reservoir located nearest to the water diversion outlet, and water quality here directly affects the safety of the diverted water. To explore the community composition and functional change of bacterioplankton in the Laoguan River before and after the flood season, four representative sites were sampled in the main stream before (May) and after (October) the 2018 and 2019 flood seasons. Water quality was assessed and high-throughput sequencing of bacterioplankton was performed. Yanghe (YH) was slightly disturbed, Xixiabei (XX) was moderately disturbed, Dangziling (DZL) was heavily disturbed, and Zhangying (ZY) was moderately disturbed. In total, 599 genera from 40 phyla were collected. The diversity of bacterioplankton before the flood season was higher than afterwards, and moderate levels of disturbance increased the Shannon-Wiener diversity index. LEfSe analysis indicated that significant differences existed in some dominant phyla; Armatimonadete in Yanghe, Epsilonbacteraeota and Firmicutes in Xixiabei, and Cyanophyta in Dangziling dominated before flood season, while significant differences only occurred in Firmicutes and Cyanophyta in Dangziling after the flood season. The PCoA of the bacterioplankton community showed that the sampling points were distinct before and after the flood season, and that differences among samples were greater before the flood season. The RDA and CCA indicated that total nitrogen and ammonia nitrogen were the main environmental factors causing declines in bacterioplankton diversity. PICRUSt showed a total of ten dominant categories of COG function genes. A total of 30 KO function genes related to nitrogen metabolism group also different between the two periods, with greater differences before flood season. The genes cynt and can in Yanghe were significant different from the other three sampling points before the flood season. Overall, nitrogen and phosphorus were the main factors regulating the bacterioplankton community structure and eutrophication in the Laoguan River. Changes in nitrogen inputs will result in changes in microbial nitrogen metabolic function in different regions of the river.