In recent years, due to unsustainable production methods and the demands of daily life, the water quality of the Yangtze River has deteriorated. In response to Yangtze River protection policy, and to protect and restore the ecological environment of the river, a two-dimensional model of the Jiangsu section was established to study the water environmental capacity (WEC) of 90 water environment functional zones. The WEC of the river in each city was calculated based on the results of the water environment functional zones. The results indicated that the total WECs of the study area for chemical oxygen demand (COD), ammonia nitrogen (NH3-N), and total phosphorus (TP) were 251,198 t/year, 24,751 t/year, and 3251 t/year, respectively. Among the eight cities studied, Nanjing accounted for the largest proportion (25%) of pollutants discharged into the Yangtze River; Suzhou (11%) and Zhenjiang (12%) followed, and Wuxi contributed the least (0.4%). The results may help the government to control the discharge of pollutants by enterprises and sewage treatment plants, which would improve the water environment and effectively maintain the water ecological function. This research on the WEC of the Yangtze River may serve as a basis for pollution control and water quality management, and exemplifies WEC calculations of the world’s largest rivers.

Total nitrogen in Taihu Lake, China has gradually decreased since 2015 while the total phosphorus concentration has exhibited an increasing trend, indicating an asynchronous change. The dominant nitrogen removal process in freshwater ecosystems is denitri cation which primarily occurs at the sediment-water interface. In this study, 15 N isotope incubation experiments were attempted to analyze the effect of water temperature on denitri cation, to construct the regional denitri cation Arrhenius equations considering water temperature, and to identify the nitrate source of denitri cation in Lake Taihu sediments. The results indicated that the potential N 2 production rates and denitri cation rates generally decreased in the west to east direction, which was signi cantly positively correlated with the nitrate concentration of overlying water by Pearson correlation coe cient analysis (P < 0.05). In addition, when the water temperature was lower than 30°C, the rates of the potential N 2 production and denitri cation were higher with an increase in water temperature, but when the water temperature was overhigh, denitri cation was inhibited. The ratio of the total denitri cation rate of nitrate from the water column in the sediment to the total denitri cation rate during the incubation experiment was above 0.5 at each sampling site. This indicated that the denitri cation in the Lake Taihu sediment primarily occurred at the expense of nitrate from the water column. The research results of Arrhenius equation construction and nitrate source identi cation of denitization can be applied to improve the accuracy of water quality model of Taihu Lake, which is of great signi cance to improve Taihu Lake water quality, and can act as a reference for the water environment treatment of other shallow eutrophic lakes in China and abroad.

Total nitrogen in Taihu Lake, China has gradually decreased since 2015 while the total phosphorus concentration has exhibited an increasing trend, indicating an asynchronous change. The dominant nitrogen removal process in freshwater ecosystems is denitrification which primarily occurs at the sediment–water interface. In this study, 15N isotope incubation experiments were attempted to analyze the effect of water temperature on denitrification, to construct the regional denitrification Arrhenius equations considering water temperature, and to identify the nitrate source of denitrification in Lake Taihu sediments. The results indicated that the potential N2 production rates and denitrification rates generally decreased in the west to east direction, which was significantly positively correlated with the nitrate concentration of overlying water by Pearson correlation coefficient analysis (P < 0.05). In addition, when the water temperature was lower than 30°C, the rates of the potential N2 production and denitrification were higher with an increase in water temperature, but when the water temperature was overhigh, denitrification was inhibited. The ratio of the total denitrification rate of nitrate from the water column in the sediment to the total denitrification rate during the incubation experiment was above 0.5 at each sampling site. This indicated that the denitrification in the Lake Taihu sediment primarily occurred at the expense of nitrate from the water column. The research results of Arrhenius equation construction and nitrate source identification of denitization can be applied to improve the accuracy of water quality model of Taihu Lake, which is of great significance to improve Taihu Lake water quality, and can act as a reference for the water environment treatment of other shallow eutrophic lakes in China and abroad.

Total nitrogen in Taihu Lake, China has gradually decreased since 2015 while the total phosphorus concentration has exhibited an increasing trend, indicating an asynchronous change. The dominant nitrogen removal process in freshwater ecosystems is denitrification which primarily occurs at the sediment–water interface. In this study, 15N isotope incubation experiments were attempted to analyze the effect of water temperature on denitrification, to construct the regional denitrification Arrhenius equations considering water temperature, and to identify the nitrate source of denitrification in Lake Taihu sediments. The results indicated that the potential N2 production rates and denitrification rates generally decreased in the west to east direction, which was significantly positively correlated with the nitrate concentration of overlying water by Pearson correlation coefficient analysis (P<0.05). In addition, when the water temperature was lower than 30°C, the rates of the potential N2 production and denitrification were higher with an increase in water temperature, but when the water temperature was overhigh, denitrification was inhibited. The ratio of the total denitrification rate of nitrate from the water column in the sediment to the total denitrification rate during the incubation experiment was above 0.5 at each sampling site. This indicated that the denitrification in the Lake Taihu sediment primarily occurred at the expense of nitrate from the water column. Research on denitrification has important implications for improving the water quality of Lake Taihu, and the findings of the study act as a reference for the water environment treatment of other shallow eutrophic lakes in China and globally.