Abstract: The status of summer microphytoplankton phosphorus stress in the East China Sea (ECS) was evaluated by alkaline phosphatase activity (APA) and the maximum quantum efficiency of photosynthesis (Fv/Fm). During the study period in 2005–2008, values of APA ranged 0.19–1414 pmol PO43− (µg Chl. a)−1 min−1 and those of Fv/Fm ranged 0.22–0.57. In general, low APA and high Fv/Fm occurred in the coastal zone as well as in the Changjiang (Yangtze River) plume, and high APA occurred in the mid-shelf region of the northern ECS. Based on regression analyses of APA and Fv/Fm, 2 mixing series that influenced the microphytoplankton phosphorus status were identified. Changjiang discharge with high nutrient concentrations and a high dissolved N:P ratio marked the starting point of the plume-driven mixing series. Along this mixing series, the river discharge gradually mixed with offshore waters, and microphytoplankton produced high APA values when the phosphate concentration dropped below 0.2 µmol P L−1. Coastal upwelling was observed to initiate a unique upwelling-driven mixing series in summer. As upwelled water mixed across the continental shelf, the utilization of nutrients caused low Fv/Fm values without a corresponding increase in the APA. The area of microphytoplankton showing symptoms of phosphorus stress in summer varied from year to year, and was closely related to the size of Changjiang plume and the strength of coastal upwelling.