Seasonal Variability of Carbon Chemistry at the SEATS Site, Northern South China Sea Between 2002 and 2003

Abstract

2 ) and titration alkalinity (TA) in the mixed-layer was performed approximately bimonthly at the SEATS time series site (1815'N, 11535'E) in the northern South China Sea (SCS) from March 2002 to April 2003. These measurements and the calculatedf CO 2 were then used to document their seasonal variations and to estimate the seasonal air-sea flux of CO 2 during the observed period at the site. Results show that the normalized TCO 2 ( NTCO 2 = TCO 2 x 35/S) fluctuates seasonally between ~1972 and ~1997 mol kg 1 -, with the highest value in winter. The decline of NTCO 2 in spring-summer mainly results from in situ biological utilization, while the resurgence of NTCO 2 in fall-winter is due to entrainment of the TCO 2 -rich subsurface waters from below. TA varies from ~2190 to ~2220 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 (~382 atm), then decreases from fall to winter to a minimum (~347 atm) in January with an ampli- tude of ~35 atm. The seasonal variability of fCO 2 is in phase with tem- perature changes but is inversely correlated with the fluctuation of NTCO 2 , suggesting that the f CO 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 CO 2 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 year 2 1 --, respectively. Throughout the year, the annual flux is calculated to be -0.11 0.08 ~ -0.23 0.18 molC m year 2 1 --during the observed period. Furthermore, although there is a drawdown of NTCO 2 of ~25 mol kg 1 -from winter to summer, which implies a net community production of 6.80 0.77 mmolC m year 2 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.