| Summary: | Water diversion project (WDP) plays an increasingly important role in improving the water quality of urban rivers along the Yangtze Delta megalopolis of China. However, their effects on the water ecological environment has generated extensive debates. Water quality variables and phytoplankton community dynamics were tracked and monitored during the summer–winter water diversions to explore the comprehensive effects of WDP on the urban aquatic ecological environment. The multi-index comprehensive evaluation model (MCEM) was adopted for further quantitative assessment, together with a newly proposed progressive degree calculation model (PDCM), which broke the limitations of the traditional ways using Markov chains. The grade evaluation was based on the Environmental Quality Standards for Surface Water (GB 3838–2002) in China. The comprehensive water quality evaluated by MCEM was improved with the largest improvement of two grades after WDP in summer, which on the contrary was worsened because of water transfers in winter by a maximum decrement of one grade. The PDCM evaluation on water quality index indicated that WDP has improved the water quality to some extent but without grade changes. Also, the proportion of Cyanobacteria and Chlorophyta decreased in the phytoplankton community of the three rivers after WDP, whereas the proportion of Bacillariophyta increased, indicating that the community structure was more inclined to the Yangtze River. The comprehensive water pollution situation evaluated by MCEM was improved with the largest improvement from middle pollution to lower pollution (one grade) because of water transfers. The changes in aquatic biological indexes evaluated by PDCM were more obvious than those of water quality indexes, which were improved by one grade. © 2021 The Authors
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