Abstract: A bloom of Mesodinium rubrum was found in the investigation of the bloom of Phaeocystis globosa on December 14th, 2009.Then, three sampling stations were set up for monitoring the bloom of Mesodinium rubrum. During the bloom of Mesodinium rubrum, the alkaline phosphatase activity of a single phytoplankton was characterized using the methodology of enzyme-labeled fluorescence.It was found that:(1) the overall labeled rate of alkaline phosphatase activity in phytoplankton was 51%, in which dinoflagellate had a strong expression of alkaline phosphatase with a labeled rate higher than 99%; while diatoms had a very weak expression of alkaline phosphatase with a labeled rate of only 3.4%; (2) dinoflagellate was easier to be induced to produce alkaline phosphatase with respect to diatoms; (3) phytoplankton was at a disadvantage in competing for nutrients, compared to Mesodinium rubrum.