## 2 Materials and Methods  
### Study Area  
The River Beiyun is an important river in North China with 56% urban land cover, supporting 72% of Beijing's population [77]. The air temperatures range from \(-\)5-26 \({}^{\circ}\)C throughout the year and the mean annual precipitation is 440 mm [78]. Three urbanized tributaries were selected in this study (River Liangshui, River Tonghui and River Wenyu) which are WWTP discharge channels for Beijing ([77]; Figure 1). The effluent standard is based on the \"Pollutant Discharge Standards for Urban Wastewater Treatment Plants in China (DB11/890-2012)\", including TN at 15 mg/L, TP at 0.3 mg/L, ammonium (NH\({}_{4}\)-N) at 1.5 mg/L, biological oxygen demand (BOD) at 6 mg/L, chemical oxygen demand (COD) at 30 mg/L and solids suspended (SS) at 5 mg/L ([[http://sthji.beijing.gov.cn](http://sthji.beijing.gov.cn)]([http://sthji.beijing.gov.cn](http://sthji.beijing.gov.cn)), accessed on 29 June 2012). Two study sites were selected on each river, and and their locations were determined by sampling accessibility. These sites were named according to the initials of the river name (i.e., L1, L2, T1, T2, W1, W2), where 1 and 2 represent upstream and downstream sites (relative location towards to the River Beiyun) respectively.  
The River Liangshui is 68 km long with a catchment area of 605.7 km\({}^{2}\)[79]. The distance between the two sites L1 and L2 was 10 km, of which L1 is located at the effluent outlet from the Xiaohongmen WWTP, which serves 2,415,000 people with a mean daily wastewater treatment of 616,000 m\({}^{3}\) ([[https://www.dowater.com](https://www.dowater.com)]([https://www.dowater.com](https://www.dowater.com)), accessed on 24 September  
Figure 1: Sampling sites on the River Liangshui, the River Tonghui, the River Wenyu, three urban tributaries of the River Beiyun, Beijing. Site numbers 1 and 2 represent upstream and downstream sites (relative locations of the two sites towards the River Beiyun) respectively.  
2014). The water depth at L1 is 0.2-0.4 m and the channel width is 15 m. The water depth at L2 is 0.3-0.6 m and the channel width is 30 m.  
The River Tonghui is 20 km long with a catchment area of 258.26 km\({}^{2}\)[80]. The distance between the two sampling sites T1 and T2 is 6 km, of which T1 is located in an urban reservoir formed by the effluent from the Gaobeidian WWTP, which serves 2,400,000 people with a mean daily wastewater treatment of 1,000,000 m\({}^{3}\) ([[https://www.dowater.com](https://www.dowater.com)]([https://www.dowater.com](https://www.dowater.com)), accessed on 7 June 2008) and is used for industrial and agricultural irrigation water. The water depth at T1 is 0.2-0.3 m and the channel width is 100 m. The water depth at T2 is 0.4-0.7 m and the channel width is 10 m. As a reservoir, T1 is bottomed with cement attaching a large amount of aquatic vegetation, a small amount of sediment and smaller phytoplankton; abundant larger phytoplankton is observed in the channel at T2.  
The River Wenyu is 48 km long with a catchment area of 4300 km\({}^{2}\)[81]. The two sites W1 and W2 with a distance of 2 km are located in the downstream area of the River Wenyu, of which W2 is only 1.5 km away from the main river--River Beiyun. Such a close distance from the main river causes the water quality of this reach to have a direct impact on the water environment of the River Beiyun. The River Ba, a tributary of the River Wenyu, flows into the river between the two sites, containing TN at 2 mg/L, TP at 0.4 mg/L, NH4-N at 2 mg/L, BOD at 10 mg/L, COD at 40 mg/L [82], resulting in the difference of water quality between the two sites. The water depth at W1 is 0.3-0.6 m and the channel width is 200 m. The water depth at W2 is 0.4-0.8 m and the channel width is 150 m. Higher water depths were often observed in the rainy season (i.e., summer).