ABSTRACT: Phosphatases play a crucial role in recycling organic phosphorus and determining primary production and phytoplankton communities in seawater, especially in phosphate-depleted coastal waters. The present study analyzed spatiotemporal variation in phosphomonoesterase (PMEase) activity (PMEA) and phosphodiesterase (PDEase) activity (PDEA) during 2 dinoflagellate blooms occurring in 2 coastal areas of Fujian Province, East China Sea, which differed in external phosphate conditions. Together with environmental variables and with a specific focus on the availability of different phosphorus forms in the seawater, the regulation of both phosphatases was studied. The results showed that dissolved organic phosphorus (DOP) was the major phosphorus source during blooms, especially in phosphate-depleted environments. Labile DOP accounted for more than 50% of DOP, of which phosphomonoester (PME) and phosphodiester (PDE) accounted for 75-94% and 6-25%, respectively. PMEA and PDEA were highly correlated, both increasing during blooms with a fixed PMEA:PDEA ratio of 2.5 under external phosphate conditions at both sites. Both PMEA and PDEA were negatively correlated with dissolved inorganic phosphorus (DIP) when phosphate was depleted, but positively correlated with DOP regardless of external phosphate conditions. During dinoflagellate blooms, temperature and phytoplankton biomass were the dominant variables determining both phosphatase activities under phosphate-depleted conditions, whereas the availability of DOP was the dominant variable determining both phosphatase activities under phosphate-replete conditions. This study suggests the importance of phosphatases in recycling DOP, and indicates similar regulation of PMEA and PDEA during dinoflagellate blooms.