Diurnal variations of surface seawater pCO2 in contrasting coastal environments

Abstract

We examined diurnal variations of surface seawater pCO 2 (partial pressure of CO 2 ) 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 pCO 2 , ranging from 1.0 Pa to 1.6 Pa (10-16 matm) 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 pCO 2 diurnal variations were temperature, tide or current, and biological controls. Temperature was a major driver of the pCO 2 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 CO 2 fluxes, which may result in an uncertainty of 60.48-0.77 mmol C m 22 d 21 for the offshore sites in the SCS. Such uncertainties were larger in nearshore settings.

Coral reef ecosystems in the South China Sea as a source of atmospheric CO2 in summer

“…where T is the SST in degrees Celsius, and the subscripts "mean" and "in situ" indicate the averaged and the observed values, respectively [20,35,36]. The calculated NpCO 2 values show that similar variations in measured pCO 2 occurred in all three study areas, suggesting that local SST variations over the observation period did not affect observed diurnal cycles or overall trends of seawater pCO 2 .…”

“…The observed diurnal cycles (i.e. surface seawater pCO 2 increasing at night and decreasing during the daytime) are significantly correlated with reef metabolic processes, suggesting that biological metabolism dominates seawater pCO 2 variations in coral reef waters [17,20,37].…”

“…For example, Table 1 demonstrates that the seawater pCO 2 value decreased between the shallow-water reef flat to the deep-water site just outside of the main reef at Luhuitou. Dai et al [20] proposed that the daily seawater mass exchanged between the inner and outer sections of a given reef may be roughly estimated by determining the ratio of the average tidal range versus the average water depth. Given that the average water depths were ~10 m for the reef around Yongxing Island, ~4 m for the Luhuitou Fringe Reef, and ~25 m for the Yongshu Reef lagoon, and their average tidal ranges were ~1 m, ~0.8 m, and ~1.2 m, respectively, the estimated daily exchange amount of water mass between the inside and outside of the reefs were ~10%, ~20%, and ~4.8% of the total water mass in the reefs, respectively.…”

“…In open ocean ecosystems, seawater pCO 2 is controlled by a variety of environmental conditions, including SST, hydrodynamics, and biological activities [20]. In order to eliminate SST effects on observed pCO 2 , the measured pCO 2 values were normalized to the mean SST during the observation period, using the following equation:…”

“…However, little information is available on the contribution of coral reefs in the SCS to global atmospheric CO 2 . After a brief investigation at Yongxing Island, in the Xisha Islands, in the spring of 2006, Dai et al [20] found that surface seawater pCO 2 shows significant regular diurnal variations, and that the studied area was a minor source of atmospheric CO 2 . In order to fully understand the relationship between SCS coral reefs and atmospheric CO 2 , we carried out detailed measurements of air-sea CO 2 exchange at Yongshu Reef (atoll), Nansha Islands, southern SCS; Yongxing Reef (platform reef with a sand cay), Xisha Islands, north-central SCS; and Luhuitou Fringing Reef at Sanya of Hainan Island, northern SCS.…”