Planktonic microorganisms play an important role in urban aquatic ecosystems; however, environmental changes significantly affect their role in the degradation and transformation of pollutants. The highly artificial North Canal River was chosen as the research area in this study. Seasonal changes in planktonic microbial community structure were studied using 16S rRNA high-throughput sequencing. The seasonal change mechanism of planktonic microbial diversity in urban rivers supplied with reclaimed water and its response relationship with environmental parameters were examined. The results showed that there were significant seasonal changes in the diversity and structure of the planktonic microbial community. The alpha diversity in summer was significantly higher than that in spring, owing to the enhancement of water diffusion capacity caused by seasonal rainfall and physical disturbance of the reclaimed water supply. The beta diversity of the planktonic microbial community in summer was weakened compared to that in spring, also owing to the enhancement of water diffusion capacity. Seasonal runoff and temperature were the main driving factors of the seasonal variation in hydrology and water quality in the highly artificial urban river. The changes in NO2--N and TP caused by seasonal runoff and temperature change were the main reason for planktonic microbial diversity changes in the river. The reductive environment of the river was caused by static and discontinuous flow in the spring. Anaerobic bacteria such as Bacteroidetes related to the degradation of dissolved organic matter and Gracilibacteria related to the denitrification process were dominant in the river. Seasonal runoff and frequent rainfall in summer, as well as the increase in the opening and closing frequency of river sluice gates, enhanced the reoxygenation capacity of the river. This significantly alleviated nutrient pollution in the North River Cannel. Additionally, aerobic bacteria and facultative anaerobic bacteria were dominant species in the river during spring. Cyanobacteria with high temperature characteristics, Chloroflexi and other autotrophic microorganisms, as well as Acidobacteria and Gemmatimonadetes played an important role in the degradation and transformation of pollutants. The results of this study have practical significance for urban river pollution control and ecological restoration with reclaimed water as the recharge water source.
Keywords: environmental interpretation; microbial diversity; reclaimed water; seasonal changes; urban river.