The evaluation index system of urban aquatic ecosystem health is of great significance for the assessment and management of urban river networks, and for urban development planning. In this paper, the concept of urban aquatic ecosystem health was analyzed by the relationship between human, city and aquatic ecosystem, and its evaluation index system was established from environmental conditions, ecological construction, and social service. In addition, the weight value of each index was calculated by the analytic hierarchy process, and the grading standard of each index was set. Jiading New City, a typical city of the river network plain area in Yangtze River delta, was selected as the aquatic ecosystem health evaluation sample. The fuzzy comprehensive method was used to evaluate the aquatic ecosystem health of Jiading New City. The results indicated that the water ecosystem health of Jiading New City reached the “good” level. For the criterion level, environmental conditions and ecological construction reached the “good” level, and social services reached the “excellent” level. For the indicator level, most indicators reached “good” and “excellent” levels, but the river complexity and benthic macroinvertebrate diversity are still in the “poor” state, which indicates that the aquatic environment has greatly improved, but the aquatic ecosystem has not been fully restored. Results suggested that river complexity and biodiversity should be increased in urban construction planning. The evaluation index system established in this paper can be used to reflect the urban aquatic ecosystem health conditions in river network plain areas.

The plain river network area has a developed economy, a large population, a complex water system, and weak hydrodynamics. In order to improve the hydrodynamic conditions of the plain river network area, clean water sources are introduced through water resources regulation to supplement the ecological water quantity of the river and effectively improve the water environment of the river. In this paper, the Qingsong area in Shanghai is used as the research object. Prototype observation, numerical simulation, and other methods are used to build a hydrodynamic mathematical model of the river network in the Qingsong area, evaluate the water resources regulation effect of the Qingsong area, and put forward two optimization schemes. The results indicate that the overall flow of the Qingsong area is from north to south and west to east, with a maximum flow of 10–68 m³/s in the north–south backbone, exchange cycle between the backbone river, and the peripheral water bodies in the Qingsong area is 4.3 days. The largest amount of water is diverted along the Wusong River, accounting for 53% of the total water diversion; the drainage mainly relies on the gates along the Huangpu River and the Diandong Pumping Gate, accounting for 43% and 42%, respectively. In view of the lowering of the water level in the slice during the low tide period, an optimal regulation scheme has been developed for more diversion and less drainage along the Huangpu River, which can raise the water level in the slice by 0.05–0.10 m. Assessment and optimization of water resources scheduling in the Qingsong area can provide a reference for hydrodynamic enhancement in the river network area of the tidal plain.

Urban plain river networks often suffer from ecosystem impairment due to sluggish hydrodynamics and insufficient water replenishment. This study assessed the aquatic ecosystem of the Yunxi area in Hangzhou, a river network characterized by insufficient hydrodynamics. A systematic ecological survey of phytoplankton, zooplankton, and macrobenthic communities was conducted at 11 key sites, and their potential driving mechanisms were explored by integrating physicochemical water analysis with redundancy analysis (RDA). The results indicate generally weak hydrodynamic conditions and elevated nitrogen concentrations at several sampling sites. The aquatic biological communities were characterized by relatively high abundances of several pollution-tolerant taxa, mainly including Bacillariophyta, Rotifera, Oligochaeta, and Gastropoda. Furthermore, local dominance of Cyanophyta and Cryptophyta at specific sites indicate spatial variation in community composition. RDA further indicated that organic pollution and nutrient enrichment were closely associated with variations in aquatic community composition. According to the observed community patterns, the weak water exchange combined with nutrient and organic-matter enrichment may be associated with variations in the aquatic community composition. Therefore, targeted ecological water supplementation might be useful to enhance the regional hydrodynamic conditions, increase water exchange and reduce pollutant accumulation.

As a representative large shallow freshwater lake in China, Lake Taihu has suffered from persistent cyanobacterial blooms for a long time. Although intensive restoration actions have been carried out and caused visible improvements, the long-term evolution path and inner driving mechanisms of its ecosystem are still not fully made clear. Based on long-term monitoring data during 2011 to 2024, this study aims to characterize temporal dynamics of the aquatic environment, find out key drivers that shape community succession, and offer a scientific foundation for effective lake management. A series of data about hydrometeorological factors, physicochemical water quality indexes, and biological community data was analyzed by using the Mann–Kendall trend test, Pettitt change-point test, Redundancy Analysis, and correlation heatmaps. The results show that the Taihu ecosystem has experienced a notable regime shift in the past 14 years. First, nitrogenous nutrients reacted quickly to external emission reductions, showing a notable monotonic decline; in comparison, Total Phosphorus and Cyanobacterial Density followed a non-linear “U-shaped” path, with a notable shift happening in 2020, which marks the change from a “deterioration phase” to a “recovery phase.” Second, correlation analysis has confirmed that the lake is mainly phosphorus-limited, and a clear “decoupling” between nitrogen levels and algal outbreaks has taken place. Third, the “10-year Fishing Ban” (initiated in 2020), together with sustained phosphorus control, reduced the competitive exclusion of phytoplankton by cyanobacteria, promoting the recent rebound in biodiversity. This study points out that Lake Taihu has passed a tipping point of ecological restoration, shifting from a turbid “algae-dominated state” to a stable state with higher biodiversity. Future management strategies should put first the mitigation of internal phosphorus loading and adaptive management against extreme climatic events.