Abstract: Based on the data of the cruise that was carried out in Dapeng Cove, mainly including thermal discharge area from nuclear power plant, artificial reef area, aquaculture area and three main estuarine areas into Dapeng Cove, Nanyong River, Longqi River and Wangmu River in Aug., 2014 (summer), we investigated the urea concentration (expressed by nitrogen, Urea-N) and urease activity of phytoplankton together with the other environmental parameters to discuss the bioavailability of urea for phytoplankton. Obvious difference between the bay and the estuary with average urea concentrations ranging from 0.28 to 1.21 μmol·L^-1 and 0.38 to 3.50 μmol·L^-1, respectively, and the urea concentration was 6.30%~24.31% of DIN in the bay and 3.13%~6.77% of DIN in the estuary, resulting in inorganic nitrogen source for phytoplankton in Dapeng Cove. Urea concentration reached the maximum at Longqi estuary and Wangmu estuary, followed by aquaculture area. The urease activity of phytoplankton in the bay and the estuary ranged from 0.61 to 1.03 μmol·(L·h)^-1 and 0.82 to 1.07 μmol·(L·h)^-1, respectively. Urease activity was positively related with urea concentration, and was also affected by inorganic nutrient and phytoplankton biomass. In the artificial reef area, aquaculture area and Nanyong estuary without N and P limitation, high Chl-a concentration could promote urease activity; in the aquaculture area, urea was the main nitrogen source for phytoplankton, and Chl-a had very significant positive correlation with urease activity; in the thermal discharge area (Longqi estuary and Wangmu estuary), P limitation resulted in inhibitory regulation of urease activity.