Sampling and environmental parameters
The study area was located in the Fenhe River of Shanxi province, northern China (latitude 35°–39° N; longitude 110°–114° E) (Fig. 1). We collected 144 samples from January 2019 to December 2019 at 6 sampling stations. A 2 L sampler was used to collect water samples at the depth of 0.5 m, then filtered through 0.22 μm mixed fiber microporous membranes. And the fraction >0.22 μm was used for DNA extraction, the corresponding water samples were used to determine the physical and chemical parameters. The nitrate (NO3), nitrite (NO2), and phosphate (PO43−) were measured following the methods in Wang, Lou, Sun, and Song (2008). The dissolved organic carbon (DOC) concentration was determined according to the method described by Bolan, Baskaran, and Thiagarajan (1996). The Water and Wastewater Monitoring Analysis Method was used to measure Chlorophyll a (Chl a) concentrations (Zhu et al., 2019).