Spatio-temporal distribution, influencing factors and control strategies of nitrogen of Nanhu Lake and its surrounding rivers

The total nitrogen (TN) concentrations in the water of Nanhu Lake in Jiaxing City maintain a high level for a long time, affecting the overall water quality and eutrophication status of the lake. The spatio-temporal distribution characteristics of various nitrogen species in Nanhu Lake and the surrounding rivers were analyzed, and the influencing factors were discussed based on historical investigation data and field observation. The results showed that TN concentrations in Nanhu Lake were between 3.81 and 4.99 mg/L, with the high concentrations observed near the river entering the lake, and TN concentrations were higher than Ⅴ level of Environmental Quality Standards for Surface Water(GB 3838-2002). The seasonal change of TN concentrations decreased in an order of winter > spring > summer > autumn. TN concentrations in the surrounding river waters were between 1.39 and 8.82 mg/L, with the high concentrations appearing in the western and northern parts, especially including Suzhou Pond, Hangzhou Pond, Changxian Pond and Xincheng Pond. TN concentrations in Nanhu Lake water were significantly higher than those in some lakes in the middle and lower reaches of the Yangtze River in China. The composition characteristics of nitrogen in Nanhu Lake and the surrounding waters showed that the dissolved nitrogen was the predominant species in the water. According to the results, the nitrogen contamination in Nanhu Lake should be controlled through the pollutant interception and abatement, remediation of the rivers into the lake, and ecological restoration in the lake area, in order to achieve the whole-processes control of source control - process to intercept - lake restoration and expansion for nitrogen pollution in Nanhu Lake, and to reduce nitrogen concentration in Nanhu Lake water.