The continuous summertime drawdown of normalized dissolved inorganic carbon has been observed repeatedly in the mixed-layer at the South East Asia Time-series Study (SEATS) site, northern South China Sea (SCS) between March 2002 and November 2004. Mass balance calculation reveals that a net community production (NCP) of -4.47 mmolC m-2 day-1 is responsible for this drawdown. Nonetheless, bioavailable nitrogen (Nbio) from atmospheric deposition, vertical diffusion, and N2-fixation by Trichodesmium and Richelia together could only account for ~55% of Nbio required for the calculated NCP. Unicellular cyanobacteria, which are identified unambiguously by the presence of nitrogenase genes (nifH) in 1--10 ¿m fraction of planktons collected at SEATS, may account for the Nbio deficit. Our results, thus, support the potential importance of unicellular cyanobacteria in providing an additional Nbio source in oligotrophic waters such as the SCS.