Environmental variables
At each site, the annual mean temperature (T ) was used to characterize the climate, and the dataset (unit: 0.1 °C, year: 2015, resolution: 1 km) was obtained from the Resource and Environment Science and Data Center (http://www.resdc.cn/). A water sample was collected, and the total nitrogen (TN ), total phosphorus (TP ), and chemical oxygen demand (COD ) were determined using a Palintest 7500 Photometer (Palintest, UK). Trophic state indices were calculated by the following equations: TLI(TP ) = 10(9.436+1.624 lnTP ), TLI(TN ) = 10(5.453+1.694 lnTN ), TLI(COD ) = 10(0.109+2.661 lnCOD ). Water nutrient status (Nwater ) was the mean value of TLI(TP ), TLI(TN ), and TLI(COD ) (Cao et al. 2021). The water depth (Hquadrat ) in each quadrat was measured by a pole and ruler, average water depth at each site (Hsite ) was the mean value ofHquadrat s of all quadrats. In addition, the linear distance between the site location and the nearest town (Lsite-town ) was determined to measure the anthropogenic disturbance level (Gosper et al. 2015).