Diurnal variations of surface seawater pCO2 in contrasting coastal environments

Dai, Minhan, Zhongming Lu, Weidong Zhai, Baoshan Chen, Zhimian Cao, Kuanbo Zhou, Wei-Jun Cai, Chen-Tung Arthur Chen

Limnol. Oceanogr., 54(3), 2009, 735-745 | DOI: 10.4319/lo.2009.54.3.0735

ABSTRACT: We examined diurnal variations of surface seawater pCO2 (partial pressure of CO2) in a suite of coastal marine environmental systems in the vicinity of the South China Sea (SCS) from inshore and nearshore settings in Xiamen Bay, Shenhu Bay, and the southwestern Taiwan Strait, to offshore sites in the basin and on the slope of the northern South China Sea as well as in a coral reef system at Xisha Islands in the middle of the SCS. There were significant diurnal changes of surface pCO2, ranging from 1.0 Pa to 1.6 Pa (10-16 µatm) in the offshore and oligotrophic sites, ~4.1 Pa in the Taiwan Strait, 5.1-15.2 Pa in Xiamen Bay and Shenhu Bay, to as high as 60.8 Pa in the coral reef system at Xisha Islands. Processes that modulate these pCO2 diurnal variations were temperature, tide or current, and biological controls. Temperature was a major driver of the pCO2 diurnal variability in the oligotrophic regions, while tidal effects were important in the nearshore. In the coral reef system, biological metabolism dominated variability. Diurnal variability could have a potentially important implication on the estimate of air-sea CO2 fluxes, which may result in an uncertainty of ±0.48-0.77 mmol C m-2 d-1 for the offshore sites in the SCS. Such uncertainties were larger in nearshore settings.

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