Abstract: Cruises were carried out to survey urea concentration and urease activity together with other environmental parameters in the Daya Bay. The bioavailability of urea for succession in the phytoplankton community was analyzed based on these cruise data. The results showed that the average concentration of urea varied from 0.81 to 8.54 μmol N·L^-1 in the four seasons. The urea concentration was the highest in summer, up to 4.32±1.65μmol N·L^-1, and the lowest in winter, up to 1.57±0.49 μmol N·L^-1. There were many stations where urea concentrations were 1-3 times higher than dissolved inorganic nitrogen (DIN) in summer and autumn, which suggested that urea as one important nitrogen source should not be ignored in the growth and productivity of phytoplankton in the Daya Bay. Higher levels of urea were distributed in the aquaculture area and the nearshore waters where sewage drains were located. Plankton urease activities were the highest in autumn, up to 61.0 ± 33.6 nmol N ·L^-1·h^-1; however, in winter and spring the urease activities were often below the detection limit. Dinoflagellates densities were closely related to urease activities in autumn. Urease activity has been shown to be the most important way in the hydrolysis of urea by of phytoplankton in the oceans, so urea may affect the growth of dinoflagellates and other phytoplankton in the Daya Bay.