In order to improve water quality, many urban lakes in China have undergone environmental restoration since the introduction of China's tenth five-year plan. To understand the effectiveness of improvement projects on eutrophic urban lakes, we analyze the changes in water quality of Lihu Lake over the past 30 years, i.e., before and after comprehensive remediation. We use long-term monitoring data from TLLER and from two regional investigations undertaken in 2017. The results were as follows. ① Concentrations of total nitrogen (TN) and total phosphorus (TP), the permanganate index, and chlorophyll-a (Chl-a) in Lihu Lake all increased dramatically since the 1990s and reached the worst levels during the period from 1997 to 2003. After comprehensive improvement measures for the lake undertaken by the local government in 2003, the water quality improved remarkably year by year, but reduced slightly in the past two years assessed here. There was no obvious improvement in water transparency when comparing data from before to after the remediation. ② Before the improvement measures, the water quality fluctuated greatly with season, however, water quality sampled during the winter post remediation was significantly better than during the summer. ③ Spatially, the water quality in the western region of Lihu Lake was significantly better than of that in the eastern region. When comparing government measures in different eutrophic urban lakes, we found that engineering management measures can improve the water quality of urban lakes over a relatively short time period. However, after the water quality has been improved, it is necessary to restore the macrophyte-dominated ecosystem, which should be supplemented by ecological restoration based on biological regulation. By improving species diversity, the aquatic ecosystem can be successfully transformed from being phytoplankton-dominated to macrophyte-dominated, thereby enabling the service functions of a lake ecosystem to be truly restored.
Keywords: Lihu Lake; eutrophication; urban lake; water environment improvement; water quality improvement.