Coupling of surface pCO₂ and dissolved oxygen in the northern South China Sea: impacts of contrasting coastal processes

Abstract. We examined the relationship between CO₂ partial pressure (pCO₂) and dissolved oxygen (DO) based on a cruise conducted in July 2004 to the northern South China Sea (111°–118° E 18°–23° N), spanning from estuarine plume, coastal upwelling and deep basin areas. Distinct relationships between pCO₂ and DO saturation were identified in different regimes. In coastal upwelling areas and the Pearl River estuary, biological drawdown of pCO₂ and production of O₂ were simultaneously observed. The two properties were coupled with each other primarily via photosynthesis and respiration. The stoichiometric relationship of the two properties however, was quite different in these two environments due to different values of the Revelle factor. In the offshore areas, apart from the estuary and upwelling, the dynamics of pCO₂ and DO were mainly influenced by air-sea exchange during water mixing. Given the fact that air-sea re-equilibration of O₂ is much faster than that of CO₂, the observed pCO₂-DO relationship deviated from that of the theoretical prediction based on the Redfield relationship in the offshore areas. Although this study is subject to the limited temporal and spatial coverage of sampling, we have demonstrated a simple procedure to evaluate the community metabolic status based on a combination of high-resolution surface pCO₂ and DO measurements, which may have applicability in many coastal systems with a large gradient of changes in their physical and biogeochemical conditions.