*2.2. Sampling and Experiment*

In this study, the main rivers in Wuxi city were selected, and the sampling points were concentrated in eight flood control stations. Point 1 was at Beixingtang River, point 2 was at Bodugang River, point 3 was at the Dongting Maritime Section of Xibei Canal, point 4 was at the Ancient Canal, point 5 was at Jiuli River, point 6 was at Liangtang River, point 7 was at the Water Conservancy Bureau section of the Huancheng River, and point 8 was at the Canal Park section of the Huancheng River. Points 1–8 include two points upstream and downstream, respectively. For example, point 1 includes 1u and 1d, and 1u means point 1 upstream, and 1d means point 1 downstream (Figure 1). The water samples were collected and analyzed for the upstream and downstream areas of the eight flood control stations (Figure 1). In addition, sampling site No. 9 is located in Taihu Lake, and No. 10 and No. 11 are located in the Beijing–Hangzhou Grand Canal. These three water samples were used to compare and analyze the water quality differences between the urban rivers and Taihu Lake and the Beijing–Hangzhou Canal when the sluices were closed.

This study was conducted from 2018 to 2019, including the flat-water season, wet season, and dry season. A suitable temperature was required to reduce the interference from external factors, such as rainstorms, on water quality. Meanwhile, when collecting the water samples, the water samples were taken 0.5–1.0 m below the water surface and far away from the river shore to reduce the impact of edge effects due to the shallow rivers. The sluices were all closed when the samples were collected.

Before sampling, the river water was taken to clean and moisten the water extractor. Then, the polyethylene storage bottle was washed more than 3 times by the water in the water extractor, and the water sample was immediately taken full and put into the cryogenic storage box for preservation. In order to reduce errors and to simulate the real situation as much as possible, the number of water samples was six at each point, and all water samples were measured three times. After sampling, all of the water samples were placed in the laboratory refrigerator at 4 ◦C, and all of the water quality data were measured within 24 h.

TN and TP are significant indicators for evaluating the water quality of the Taihu Lake Basin to meet WEQSC (GB3838-2002) [26]. Therefore, TN and TP were selected as the water quality indicators in this study. According to the national standards of HJ636-2012 and HJ671-2013, the detection of TN concentration adopts the "Alkaline potassium persulfate ablation UV spectrophotometric method" [27], and TP adopts the "Ammonium molybdate spectrophotometric method" [28].
