Study area and sampling
The study area covers the Weihe River, Hanjiang River, and their tributaries in the Qinling Mountains in Southern Shaanxi province, China (Fig. 1). The Weihe and Hanjiang Rivers run on the north and south sides of the Qinling Mountains, respectively. Weihe River passes through Baoji, Xianyang, and Xi’an, which are densely populated cities. Hanjiang River passes through the cities of Hanzhong and Ankang. The rivers have numerous tributaries at high elevations, which are characterized by low anthropogenic disturbance in the Qinling Mountains.
In October 2017 (autumn) and April 2018 (spring), water and sediment samples were collected from 52 sites in 12 rivers (Fig. 1, Table S1). The sampling sites span the north mainstem (Weihe River), south mainstem (Hanjiang River), and 10 tributaries (Luofu River, Bahe River, Shidi River, Shitou River, Heihe River, Jinqian River, Jinshui River, Xuhe River, Xushui River, and Yuehe River) in Qinling Mountains. At each sampling site, water samples were collected from 0.5-m depths with aseptic polyethylene bottles. The samples for chemical analysis (2 L each) were preserved at 4°C before being transported to the laboratory. The samples for DNA-based analysis (4 L each) were filtered through 0.22-µm mixed cellulose ester membranes (Millipore, Bedford, MA, USA) within 24 h. The membranes were stored at –80°C until DNA extraction. Each sediment sample was collected for microbial analysis.