Estimated net community production during the summertime at the SEATS time-series study site, northern South China Sea: Implications for nitrogen fixation

摘要

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 (N-bio) from atmospheric deposition, vertical diffusion, and N-2-fixation by Trichodesmium and Richelia together could only account for similar to 55% of N-bio required for the calculated NCP. Unicellular cyanobacteria, which are identified unambiguously by the presence of nitrogenase genes ( nifH) in 1 - 10 mm fraction of planktons collected at SEATS, may account for the N-bio deficit. Our results, thus, support the potential importance of unicellular cyanobacteria in providing an additional N-bio source in oligotrophic waters such as the SCS.