Spatial-temporal dynamic changes of nitrogen and phosphorus and difference analysis in water body of Lihu Lake

Lihu Lake is a typical shallow lake in the transitional period from unshiftable state of algae-dominated turbid water to unshiftable state of macrophytes-dominated clear water. The distribution characteristics, change rule and key impacting factors of nitrogen and phosphorus in overlying water were discussed by the field survey data investigation from 2012-2013 and the historical monitoring date collection. And the spatial-temporal difference of nitrogen and phosphorus forms in overlying water and the corresponding control measures were also studied on focus. The eutrophication of Lihu Lake was still not fundamentally solved, and the concentration of nitrogen and phosphorus in overlying was still in an unstable state. The concentration of nitrogen and phosphorus was in the range of 0.74~4.93mg/L and 0.03~0.31mg/L, with the mean value of 1.35 and 0.073mg/L, in respectively. Concentrations of total nitrogen(TN) and total phosphorus(TP) decreased from the east district of Lihu Lake to the west, and lakeside areas was higher than lake center. Concentrations of TN and TP were higher in summer and autumn, but lower in winter and spring. Dissolved nitrogen was the main nitrogen state in water, accounting for 35%~99% of TN, with the mean value of 77.98%.While phosphorus mainly existed in particulate state, and particulate phosphorus accounted for 11%~90% of TP, and the mean value was 59%. Results of multivariate statistics analysis showed that TN was positively correlated with DTN and TSS, but with smaller correlation coefficient with TSS. However, TP was significantly positive with both DTP and TSS. Therefore, two methods should be used to reduce the concentration of nitrogen and phosphorus in water body, one is cutting down the dry and wet deposition into lake, and the other is reducing sediment resuspension and restraining the release of nitrogen and phosphorus from sediments.