Diurnal Variations Of Sea Surface PCO₂ And The Controls In Coastal Seas

The constraint of air-sea CO₂ fluxes and their variability at various time and spatial scales remain a central task in the global carbon and climate studies. Coastal ocean may represent one of the hot spots, which typically exerts poorly constrained and debated air-sea CO₂ exchange rates. The extreme heterogeneity and dynamic nature of coastal oceans critically affect the reliability of reported CO₂ flux data.To understand how and to what extent the pCO₂ diurnal variations may influence the estimation of air-sea CO₂ fluxes, it is very important to clearly identify the mechanisms that control the surface seawater pCO₂ diurnal variability, high resolution time-series observation on diurnal scale is the most direct and effective means to study it.This study attempts to examine diurnal variations of surface seawater pCO₂ and its potential implications on the estimate of air-sea CO₂ fluxes. Experiments were conducted in a variety of environmental settings such as at Xiamen Bay (Xiamen, China), Shenhu Bay (Shishi, China), Xisha Islands (A coral reef system in the South China Sea) as well as in the slope of the Northern South China Sea. Experiments were conducted with an autonomous fiber optical chemical pCO-2 sensor and an infrared CO₂ analyzer equipped with a shower head equilibrator.A sensor system suitable for monitoring changes in pCO-2 in surface seawater or in the atmosphere has been developed. Surface seawater samples are pumped into a PVC tube enclosing an inner Teflon AF tube, which served as a long pathlength gas-permeable liquid-core waveguide for spectrophotometry. The Teflon cell contains a pH sensitive indicator-buffer solution consisting of bromothymol blue (BTB) and sodium carbonate. Carbon dioxide in the sample diffuses into the indicator-buffer solution to reach equilibrium, resulting in pH changes, which are detected by changes in the absorbance of BTB at wavelengths of 620 and 434 nm. The pCO₂ in the sample is then derived from the pH change. The sensor has a response time of 2 mins at the 95% equilibrium value and a measurement precision of 0.26%-0.37% in the range 200 to 800μatm pCO₂. This chemical sensor takes advantage of a combination of long pathlength, multiple wavelength detection, indicator solution renewal, and in situ automatic control technology, and has the feature of low power consumption (the average being -4 W with a peak of -8 W).Through seawater pCO₂ time series observations along with the surface water chemistry, we observed significant diurnal changes of surface pCO₂ ranging from -10μatm in the oligotrophic SCS, 50-150 uatm in Xiamen Bay and Shenhu Bay, up to as high as 600μatm in the coral reef system at Xisha Islands. Processes that modulate the pCO₂ diurnal variations are significantly different between sites, which can be grouped into three categories: temperature control, tide / current control and biological control. This diurnal pCO₂ variation may cause significant bias in the estimate of CO₂ air-sea fluxes if not properly considered. A first order and upper limit estimate points towards uncertainties of -±0.48 mmol C m^-2 d^-1 at the slope, which account for nearly 50% of the flux uncertainties for open areas in the marginal seas. The uncertainties caused by diurnal variations in estuary / bay areas is±2.19 - 6.58 mmol C m^-2 d^-1, account for 9-27% of the flux in the regions of this nature. In coral reefs, the uncertainties can be as high as±8.77-26.31 mmol C m^-2 d^-1, much higher than its net flux.