Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 6 11 2009 10.5194/bg-6-2589-2009 http://www.biogeosciences.net/6/2589/2009/ http://www.biogeosciences.net/6/2589/2009/bg-6-2589-2009.html http://www.biogeosciences.net/6/2589/2009/bg-6-2589-2009.pdf 2589 2598 2009-11-17 Coupling of surface <i>p</i>CO₂ and dissolved oxygen in the northern South China Sea: impacts of contrasting coastal processes W. D. Zhai M. Dai mdai@xmu.edu.cn W.-J. Cai State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China Department of Marine Sciences, University of Georgia, Athens, GA 30602–3636, USA We examined the relationship between CO₂ partial pressure (<i>p</i>CO₂) and dissolved oxygen (DO) based on a cruise conducted in July 2004 to the northern South China Sea (111&deg;–118&deg; E 18&deg;–23&deg; N), spanning from estuarine plume, coastal upwelling and deep basin areas. Distinct relationships between <i>p</i>CO₂ and DO saturation were identified in different regimes. In coastal upwelling areas and the Pearl River estuary, biological drawdown of <i>p</i>CO₂ 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 <i>p</i>CO₂ 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 <i>p</i>CO₂-DO relationship deviated from that of the theoretical prediction based on the Redfield relationship in the offshore areas. 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