Dissolved inorganic carbon (DIC), titration alkalinity (TA), and nitrate + nitrite (N+N) are measured from seasonal cruises at the time-series site SEATS in the northern South China Sea (18¢XN 116¢XE) between March 2002 and November 2004. The most distinctive feature of the annual nDIC (DIC normalized to a constant salinity of 33.8) cycle is an increase in wintertime and a decrease in summertime (March¡VAugust). The nDIC drawdown (-3.15 ¡Ó 2.23 mmol m-2 day-1) at summertime is mainly attributed to biological uptake of DIC. The other terms in the DIC budget calculation, i.e. the carbon fluxes of air-sea CO2 gas exchange, horizontal advection and vertical diffusion, are estimated to be -0.48 ¡Ó 0.31, -0.70 ¡Ó 0.86, and 2.50 ¡Ó 0.59 mmol m-2 day-1, respectively. Accordingly, results from the DIC budget calculation reveals a net community production (NCP) of -4.47 ¡Ó 1.98 mmol m-2 day-1. This calculated NCP from our data is in good agreement with the export and new production previously reported in the South China Sea. The consistency demonstrates that carbon system is almost in a steady state during summertime at SEATS. According to the Refield C : N ratio of 106 : 16, a flux of 0.67 ¡Ó 0.30 mmol m-2 day-1 of bioavailable nitrogen (Nbio) is needed to sustain the calculated NCP. The source terms in the Nbio budget calculation, i.e. the nitrogen fluxes of vertical diffusion, wet deposition, dry deposition and the contribution from the putative nitrogen-fixing cyanobacteria Trichodesmium and Richelia intra., are estimated to be 0.20 ¡Ó 0.04, 0.03 ¡Ó 0.01, 0.04 ~ 0.08, and 0.02 ~ 0.13 mmol m-2 day-1, respectively. It thus seems that all the source terms can only collectively account for 50 ~ 70% Nbio needed to support the estimated NCP. With this regard, unicellular cyanobacteria, which have been reported as an important N2-fixer in the subtropical North Pacific and identified by the nitrogenase genes (nifH) in the small size (less than 10