Abstract: Monitoring of the partial pressure of CO₂ (p(CO₂))was conducted in the backwater area of Xiaojiang River in Three Gorges Reservoir.Water samples were taken from the five cross sections in the Quma-Heikou reach of Xiaojiang River during the spring phytoplankton bloom from April 10 through May 25,2010.Results show that there are obvious stratifications of chlorophyll a (Chla),dissolved oxygen (DO),and p(CO₂)in water columns.The value for p(CO₂)is low in surface water due to phytoplankton photosynthesis effects.The surface water can be the CO₂ sink if sufficient sunshine presents.A minimum value of 4.3 Pa for p (CO₂)was observed from the cross section of Shuangjiang on April 25,with an absorption flux of -0.28 mmol/m²/h.The value for p(CO₂)increases vertically with the water depth,and will be stabilized in the 10-15m layer of water.The partial least squares regression analy-sis reveals that the growth and metabolism of phytoplankton in the surface water are the key factors controlling p(CO₂).A negative relationship between the water temperature and p(CO₂)has been found,but its influence on p(CO₂)is minimum.Using the indices of Excess p(CO₂)(Ep(CO₂))and apparent oxygen utilization (ADO),a three-stage bloom of phytoplankton in the Xiaojiang River was observed,during which,the surface water had gone through a sink-source-sink cycle for CO₂.