Following the rapid development and urbanization in Yangtze River basin, in particular the operation of the Three Gorges Reservoir, the hydrological regime, sediment transport and water quality at the Yangtze outlet have changed dramatically, which significantly affected the estuarine ecosystem. Water quality deterioration, eutrophication and algal bloom become serious problems of the Yangtze estuary. The impoundment of the Three Gorges Reservoir has three stages: the first stage is 135 m in 2003, and the second stage is 156 m in 2006 and the final stage is 175 m in 2010. The impoundment of 175 m in 2010 makes the operation of the reservoir reach to the designed level. It is therefore valuable to investigate the consequent effects on the ecosystem of the downstream, especially the estuary. Based on the survey of hydro鄄 environmental and phytoplankton data in the Yangtze estuary and its adjacent areas in August and November 2010 and May 2011, the paper analyzed the spatial鄄temporal features of dissolved oxygen (DO) and chlorophyll a (Chla) , as well as the affecting mechanism after the impoundment level of the Three Gorges Reservoir reached to the final stage of 175 m. The results showed that in summer, the low DO area is in the turbid belt in surface layer, and the bottom layer exists oxygen depletion zone. In autumn and spring, the vertical profile of DO concentration is rather uniform from surface to bottom. To investigate the effect of stratification in summer, two typical points A3 and A5 are selected. In A3, DO and temperature
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