Abstract: The phytoplankton growth has a tight relationship with the carbon fixation, hence the study of mechanism can discover the process of marine carbon cycle. It is believed that dissolved organic carbon (DOC, measured by High Temperature Combustion Method) is actively released by phytoplankton to compensate for the luxury carbon uptake in the bloom zone. The study results in several bloom zones in Changjiang estuary and Pearl River estuary mid-high salinity zone (salinity ranged from 10 to 30), however, showed the similar DOC concentration in these blooms and non-bloom zones. In the meantime, the total organic carbon (TOC) discovered increased significantly, TOCs were 60~140 μmol/L and >110 μmol/L higher than DOCs in the two estuaries, respectively, indicating most of the fixed carbon existing as particular organic carbon (POC). The incubation data indicated that only 10% of fixed carbon existing as DOC (i.e., 90% of the fixed carbon existing as POC) in non-sterilized seawater; on the contrary, >80% of the fixed-carbon existing as DOC (i.e., ~20% of the fixed carbon existing as POC) in sterilized seawater, suggesting bacteria take an important role in the decomposition of the phytoplankton produced DOC in the bloom zones in the two estuaries.