Seasonal variability of carbon chemistry at the SEATS time-series site, northern South China Sea between 2002 and 2003

Analyses of total carbon dioxide (TCO2) and titration alkalinity (TA) in the mixed-layer was performed approximately bimonthly at the SEATS. time series site (18 degrees 15'N, 115 degrees 35'E) in the northern South China Sea (SCS) from March 2002 to April 2003. These measurements and the calculated-fCO(2) were then used to document their seasonal variations and to estimate the seasonal air-sea flux of CO2 during the observed period at the site. Results show that the normalized TCO2 (NTCO2 = TCO2 x 35/S) fluctuates seasonally between similar to 1972 and similar to 1997 mu mol kg(-1), with the highest value in winter. The decline of NTCO2 in spring-summer mainly results from in situ biological utilization, while the resurgence of NTCO2 in fall-winter is due to entrainment of the TCO2-rich subsurface waters from below. TA varies from similar to 2190 to similar to 2220 mu mol kg(-1) in tandem with salinity, suggesting the prime control of physical processes. fCO(2) increases progressively from spring to summer, reaches a maximum in July (similar to 382 mu atm), then decreases from fall to winter to a minimum (similar to 347 mu atm) in January with an amplitude of similar to 35 mu atm. The seasonal variability of fCO(2) is in phase with temperature changes but is inversely correlated with the fluctuation of NTCO2, suggesting that the fCO(2) seasonality is primarily controlled by temperature changes, though other factors have compensated partially to yield the observed low amplitude of its variability. The sea-to-air. CO2 fluxes for spring, summer, fall and winter are estimated to range from 0.00 +/- 0.01 to -0.02 +/- 0.05, +0.03 +/- 0.01 to +0.23 +/- 0.06, +0.18 +/- 0.10 to +0.45 +/- 0.25, and -0.62 +/- 0.20 to -1.42 +/- 0.46 molC m(-2) year(-1), respectively. Throughout the year, the annual flux is calculated to be -0.11 +/- 0.08 similar to -0.23 +/- 0.18 molC m(-2) year(-1) during the observed period. Furthermore, although there is a drawdown of NTCO2 of similar to 25 mu mol kg(-1) from winter to summer, which implies a net community production of 6.80 +/- 0.77 mmolC m(-2) year(-1) in the mixed layer at the SEATS site, there is no corresponding change of nitrate observed, suggesting other sources of nitrogen required to sustain the new production.