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River ecological rehabilitation, as a river training scheme combining with water conservancy projects, is used to protect biological diversity, ecological environment and increase the ecological functions of rivers. Recently years, there have been a lot of studies and some examples of successful projects on the river ecosystem rehabilitation in China. And the current ecological rehabilitation study with selecting characteristics of the river in south China (Cixi Urban River, Zhejiang), endows with important theoretical and practical significance. A reach of Cixi Urban River located in Zone A of Zhishan culture square, which has been dredged, was selected as the experimental unit for controlling eutrophication and ecological rehabilitation of Urban River. By combination with these measures, including dredging, interception of sewage, algae inhabitation by slurry, different plant communities and aquatic animals silver carp and Bellamya sp were stocked in southern zone, eastern zone, northern zone and western zone of Cixi Urban River for rehabilitation of advanced aquatic plants producers and higher aquatic animals consumers in water ecology system food chain, the inhibition of algae, the increase of organic matter, nutrients etc in the migration, transformation, output channel and output quantity, and the multi-channel control and reduction of the input channels and the amount of pollutants. The original ecological imbalance has been gradually replaced by ecological balance, thereby realizing the improvement of water quality and the control of eutrophication.The results showed that the water ecosystem rehabilitation, water quality improvement, eutrophication control were related to the rehabilitation conditions of aquatic plants as intrinsic factors, source pollution load and external interference etc as extrinsic factors. In the south of Cixi Urban River, the plant communities composed mainly of sunken plants including Elodea, Vallisneria, Potamogeton crispus, Ceratophyllum demersum, Hydrilla verticillata, Trapabispinosa etc were repaired. After purified by aquatic plants and animals within the northern and the western of this river, the pollution loads of nitrogen, phosphorus, organic matter and other pollutants were controlled to some extent. The eutrophication of this reach was controlled completely with the change from high eutrophication level to medium eutrophication level. Water quality categories changed from the inferior classⅤto classⅡ, coupled with the water quality condition changing from severely pollution to quality good. At the same time, the water ecosystem structure and function have been well improved and external interference was adjusted with strong regulation and compensation, thereby maintaining the stability of ecosystems. In the east of this river, the pioneer plant communities composed mainly of pioneer plant water spinach were primarily repaired, and then sunken plant communities were repaired followed by the improvement of water quality. However, the external interferences caused by fishing, catching spiral shell, and even stocking with grass carp that exceeded the self-regulation, so the pioneer plant communities appeared again. After purified by aquatic plants and animals within the northern of the river, the pollution loads of nitrogen, phosphorus, organic matter and other pollutants were also controlled to some extent. The eutrophication was well controlled with the change from high eutrophication level to light eutrophication level. Water quality categories changed from class V to class III, coupled with the water quality condition changing from medium pollution to quality better. Usually, dissolved oxygen is a limiting factor in the improvement of water quality category, and water spinach can’t reproduce on their own due to lack of the nature through the winter during the floating plant water spinach communities dominated stage, so the improvement and stability of water ecosystem structure and function was not as well as the south, west and north of the river. Closed to the pollution sources, the input loads of nitrogen and phosphorus pollution were greater than the output loads, which exceeded its self-purification capacity of aquatic ecosystems and then resulted in eco-stranded. In the whole process of rehabilitation of aquatic plants, sunken plants were difficult to settle in the west and north of the river and the floating floor plants and the floating plants mainly dominated ecosystems. In the west of the Urban River, the eutrophication level was alleviated. Integrated nutrition state index decreased from 70 to 63. The water quality categories changed from the inferior class V to classⅤ, coupled with the water quality condition changing from severely pollution to moderately pollution. Similarly in the north of Urban River, the eutrophication level was controlled with the change of integrated nutrition state index that decreased from 77 to 68. The water quality conditions changed from high eutrophication level to medium eutrophication level. Water quality category was still in inferior class V as an indication of severe pollution, but the main pollution factors decreased in our study.Therefore, to realize the goals of the ecosystem rehabilitation of Cixi Urban River, the complete control of eutrophication and the improvement of water environment function, the following works needed to be completed. Firstly, pollution loads should be controlled to avoid ecological retention or depletion of metabolic resources in time and space scales. Secondly, the Cixi Urban River should be a kind of special society- economy-nature co-ecosystem affected by human activities and social behavior with the combinations of the hardware, software and heartware, the government, science and technology units, enterprises and the masses in preventing or reducing human activities and social behavior on the interference of aquatic ecosystems rehabilitation. Finally, the ecosystem rehabilitation should be carried out under water controlled conditions by combining sunken plants composed of Vallisneria, Ceratophyllum demersum, Hydrilla verticillata, Elodea, and Trapabispinosa etc, which growth alternately in warm and cold season, to aquatic animals composed of silver carp and spiral shell etc. However, under normal circumstances the ecosystem rehabilitation was carried out with a range of complex methods because the water level was inhibited by factors such as precipitation, and ecosystem rehabilitation carried out with sunken plants was restricted by water quantity. This ecological rehabilitation combining advantages of ecological, landscape and economic benefits endows with a good application prospect.
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