Coupling of surface <i>p</i>CO<sub>2</sub> and dissolved oxygen in the northern South China Sea: impacts of contrasting coastal processes

Abstract

Abstract. We examined the relationship between CO 2 partial pressure (pCO 2 ) 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 pCO 2 and DO saturation were identified in different regimes. In coastal upwelling areas and the Pearl River estuary, biological drawdown of pCO 2 and production of O 2 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 pCO 2 and DO were mainly influenced by airsea exchange during water mixing. Given the fact that airsea re-equilibration of O 2 is much faster than that of CO 2 , the observed pCO 2 -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 pCO 2 and DO measurements, which may have applicability in many coastal systems with a large gradient of changes in their physical and biogeochemical conditions.

Seasonal variations of sea–air CO<sub>2</sub> fluxes in the largest tropical marginal sea (South China Sea) based on multiple-year underway measurements

“…The Weiss and Price (1980) saturated water vapor pressure and the Takahashi et al (1993) temperature effect coefficient of 4.23 % • C −1 were used to calculate in situ pCO 2 . The overall uncertainty of the xCO 2 measurements and pCO 2 data processing is less than 1 % (Zhai et al, 2005a;Zhai and Dai, 2009). CO 2 concentration in the air was typically determined every 1 to 3 h in the day and every 4 h in the night.…”

“…Thus far, limited data sets with different spatial coverage have suggested that the SCS proper serves as a weak or moderate source of atmospheric CO 2 in warm seasons (Rehder and Suess, 2001;Zhai et al, 2005a;Zhai et al, 2009;Dai et al, 2013). In shaping this source term, at least in the basin area of the SCS, the inflow of the CO 2 -enriched North Pacific deep water through the Luzon Strait and its subsequent upward transport into the thermocline through vertical mixing and upwelling may have played a critically important role .…”

“…2). Among the four domains, A could absorb CO 2 from the atmosphere in the spring/summer bloom periods (Dai et al, 2008;Zhai et al, 2009;Cao et al, 2011), while during the northeast monsoon seasons it was influenced by the cooling effect and intrusion of the Kuroshio (Fig. 1).…”

“…We must point out that beyond the four domains that the present study is examining, other physicalbiogeochemical processes may dominate. For example, summer coastal upwelling-induced CO 2 dynamics must be considered in the southwestern part of the Taiwan Strait, in the eastern coast off Hainan Island, and in the eastern coast off Vietnam (e.g., Zhai et al, 2009;Cao et al, 2011).…”

“…During our April 2008 cruise, an automated flowing pCO 2 measuring system (GO8050, General Oceanics Inc., USA) was used ). In these systems, a Li-Cor ® nondispersive infrared spectrometer (Li-6252 during the October 2003 and May 2004 surveys, Li-7000 during the other cruises) was used to measure dried CO 2 fractions (xCO 2 ) in the equilibrator and in the air (Zhai et al, 2005b;Zhai and Dai, 2009). For calibration purposes, a series of CO 2 gas standards with xCO 2 values from 138 to 967 ppmv (parts per million volumes in dry air; the same hereafter) were applied.…”