Seasonal variations of seawater pCO₂ and sea‐air CO₂ fluxes in a fringing coral reef, northern South China Sea

Abstract

Evidence based on four field surveys conducted between July 2009 and April 2011 indicates that both sea surface partial pressures of CO 2 (pCO 2 ) and sea-air CO 2 fluxes at Luhuitou fringing reef in Sanya, Hainan Island, northern South China Sea (SCS) are subject to significant seasonal variations. The diurnal variation of seawater pCO 2 ranges from 264 to 579 latm in summer, which is much larger than that in autumn (152-335 latm), in winter (84-260 latm), and in spring (114-228 latm). The sea-air CO 2 flux in summer (9.6 mmol CO 2 m 22 d 21 ) is also larger than that in other seasons (i.e.,3 mmol CO 2 m 22 d 21 in spring, 3.5 mmol CO 2 m 22 d 21 in autumn, and 2.7 mmol CO 2 m 22 d 21 in winter). The atmospheric pCO 2 in this reef shows small diurnal and seasonal variations. The integration of the time-series pCO 2 data shows that the reef area is a weak source of atmospheric CO 2 at 0.54 mol CO 2 m 22 yr 21 . Further analyses indicate that the seasonal variations of the surface seawater pCO 2 in Luhuitou fringing reef are mainly affected by seasonally-dependent biological metabolic processes (organic processe and inorganic process), and that the organic process play a more important role than the inorganic process. Seasonal sea surface temperature (SST) variations and hydrodynamic processes may also have some influence on seawater pCO 2 variation.

Links Between the Coral δ13C Record of Primary Productivity Variations in the Northern South China Sea and the East Asian Winter Monsoon

“…Our result, however, indicated that the enhanced PP in winter has a stronger influence to draw down the seawater 12 CO 2 than the influx of the DIC with lower δ 13 C values associated with the vertical mixing at our study site. This interpretation is further supported by the field surveys on the coral reef systems in the NSCS, which suggested that the seawater carbonate chemistry of the coral reef was primarily controlled by biological processes (Chen et al, 2015;Dai et al, 2009;Yan et al, 2011Yan et al, , 2016. These results suggest that the decadal variability of the coral Δδ 13 C can be used as a reliable proxy for the PP in the NSCS.…”

Changes in microbial communities, photosynthesis and calcification of the coral Acropora gemmifera in response to ocean acidification

“…The variation in skeletal δ 18 O values of A. gemmifera was consistent with the findings of a previous study demonstrating an enrichment of δ 18 O in the coral skeleton in response to elevated p CO 2 49. The coral calcification rate decreases under reduced pH conditions9, which corresponds to heavier skeletal δ 18 O, whereas low p CO 2 and higher pH lead to species with lighter δ 18 O because HCO 3 − is isotopically heavier than CO 3 2345678910111213141516171819202122232425262728293031323334353637383940414243444546474849. Consequently, the significantly enriched δ 18 O and more depleted δ 13 C in A. gemmifera observed herein may reflect slight reductions in photosynthesis and calcification at the high p CO 2 .…”

“…Although there is limited evidence for biological adaptation to climate change in coral microbial symbionts, the adaptive power to climate change has been well documented in the photosymbiotic Symbiodinium 101144. The shallow habitat of the coral A. gemmifera sampled in the present study has been experiencing regular large diurnal and seasonal variations in pH/ p CO 2 (see Supplementary Table S1), which are mainly driven by biological activities of the reef282930. Therefore, it is most likely that microbial communities harbored by the natural population of A. gemmifera are resistant to the increased p CO 2 , due to long-term acclimatization/adaptation to the highly dynamic pH conditions within the reef flat.…”

“…S1) and used to have a high coverage of living coral, which has declined by 80% since the 1960s27. The diurnal and seasonal variation of the reef flat seawater pH/ p CO 2 is high282930, and the recorded extreme level is even lower than the value currently predicted at the end of this century (see Supplementary Table S1). The rapidly-growing branching coral Acropora , which is distributed worldwide, is an ecologically important genus in this reef flat.…”

Air‐Sea Gas Exchange and CO2 Fluxes in a Tropical Coral Reef Lagoon

“…For example, a parameterization that includes current velocity and depth, along with wind speed, would be more appropriate for such environments. For previous experiments conducted in coral reef ecosystems shallower than 5 to 10 m (e.g., Cyronak et al, 2014;Frankignoulle et al, 1996;Gattuso et al, 1993;Longhini et al, 2015;Yan et al, 2016), using an ocean wind speed/gas exchange parameterization will likely produce k(600) and CO 2 fluxes that are too low (Table 1).…”