[Effect of a Comprehensive Improvement Project on Water Quality in Urban Lakes: A Case Study of Water Quality Variation in Lihu Lake Over the Past 30 Years].

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.

[Temporal-spatial Distribution of Nitrogen and Phosphorus Nutrients in Lake Taihu Based on Geostatistical Analysis].

It is of great significance to investigate the spatio-temporal variation in nitrogen and phosphorus nutrients as a mechanism for controlling the sudden increase of algae in eutrophic water. Based on the geostatistical analysis, we studied the sources and occurring forms of nitrogen and phosphorus in different areas of Lake Taihu-a large shallow and eutrophic lake in China. We also examined the spatial distribution of the type of nutrient restriction and its reason by monitoring the sites seasonally from August 2014 to May 2015. The results showed that:① The concentrations of soluble nitrogen and phosphorus were higher in the winter than in other seasons, and they gradually decreased spatially from the northwest to southeast. The concentrations of particulate nitrogen, phosphorus, and chlorophyll a were highest in the summer, and the high-concentration regions in the winter and other seasons were located in the southern and northwestern parts of the lake, respectively. ② The composition of nitrogen and phosphorus in algae-and macrophyte-dominated regions changed substantially with the seasonal change. The algae-dominated regions were dominated by nitrate nitrogen and organic phosphorus in the winter and particulate nitrogen and phosphorus during the other seasons. However, the macrophyte-dominated region was dominated by particulate nitrogen and phosphorus in the winter and ammonia, organic nitrogen, and phosphorus during the other seasons. ③ The ratios of total nitrogen to total phosphorus in the algae-dominated region were greater than 16:1 in the autumn and winter but less than 16:1 in the summer and spring. Meanwhile, the ratios of total nitrogen to total phosphorus in the macrophyte-dominated region increased from less than 16:1 in the autumn and winter to more than 16:1 in the summer and spring. In the algae-dominated region, the spatial variation in the dissolved total nitrogen to total phosphorus ratio was consistent with the ratio of total nitrogen to total phosphorus. In the macrophyte-dominated region, the dissolved total nitrogen to total phosphorus ratio increased from less than 16:1 in the autumn to more than 16:1 in the summer, winter, and spring. The temporal and spatial variation in the ratio of particulate nitrogen to phosphorus was not significant (P>0.05), and this ratio was less than and greater than 16 in algae-and macrophyte-dominated regions, respectively.

Long-term moderate wind induced sediment resuspension meeting phosphorus demand of phytoplankton in the large shallow eutrophic Lake Taihu.

The objective of this study was to investigate the impact of sediment resuspension and phosphorus (P) release on phytoplankton growth under different kinds of wind-wave disturbance conditions in the large and shallow eutrophic Lake Taihu in China. Short-term strong wind (STSW) conditions, long-term moderate wind (LTMW) conditions, and static/calm conditions were investigated. To address this objective, we (1) monitored changes in surface water P composition during field-based sediment resuspension caused by STSW conditions in Lake Taihu, and also conducted (2) a series of laboratory-based sediment resuspension experiments to simulate LTMW and calm conditions. The results showed that under both strong and moderate wind-wave conditions, suspended solids (SS) and total phosphorus (TP) in the water column increased significantly, but total dissolved phosphorus (TDP) and soluble reactive phosphorus (SRP) remained low throughout the experiments, indicating that the P released from sediments mainly existed in particulate forms. In STSW conditions, alkaline phosphatase activity (APA) and enzymatically hydrolysable phosphorus (EHP) increased rapidly, with the peak value occurring following the peak value of wind speed for 1-2 days, and then rapidly decreased after the wind stopped. Under LTMW conditions, APA and EHP increased steadily, and by the end of the laboratory experiments, APA increased by 11 times and EHP increased by 5 times. Chlorophyll a (Chl-a) in LTMW conditions increased significantly, but remained low under STSW conditions, demonstrating that the former type of sediment P release promoted phytoplankton growth more effectively, and the latter type did not. Despite the fact that STSW conditions resulted in the release of more TP, TP settled to the bottom rapidly with SS after the wind stopped, and did not promote algal growth. Under LTMW conditions, suspended particulate P was hydrolyzed to SRP by phosphatase and promoted algae growth. Algal growth in turn secreted more phosphatase and accelerated particulate P regeneration, which may be the main mechanism of sediment bio-available P release that promotes phytoplankton growth in shallow lakes.