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

Abstract

during the summers of 2008 and 2009 revealed that both air and surface seawater partial pressures of CO 2 (pCO 2 ) showed regular diurnal cycles. Minimum values occurred in the evening and maximum values in the morning. Air pCO 2 in each of the three study areas showed small diurnal variations, while large diurnal variations were observed in seawater pCO 2 . The diurnal variation amplitude of seawater pCO 2 was ~70 μmol mol -1 at the Yongshu Reef lagoon, 420-619 μmol mol -1 on the Yongxing Island reef flat, and 264-579 μmol mol -1 on the reef flat of the Luhuitou Fringing Reef, and 324-492 μmol mol -1 in an adjacent area just outside of this fringing reef. With respect to spatial relations, there were large differences in air-sea CO 2 flux across the South China Sea (e.g. ~0.4 mmol CO 2 m -2 d -1 at Yongshu Reef, ~4.7 mmol CO 2 m -2 d -1 at Yongxing Island, and ~9.8 mmol CO 2 m -2 d -1 at Luhuitou Fringing Reef). However, these positive values suggest that coral reef ecosystems of the SCS may be a net source of CO 2 to the atmosphere. Additional analyses indicated that diurnal variations of surface seawater pCO 2 in the shallow water reef flat are controlled mainly by biological metabolic processes, while those of deeper water lagoons and outer reef areas are regulated by both biological metabolism and hydrodynamic factors. Unlike the open ocean, inorganic metabolism plays a significant role in influencing seawater pCO 2 variations in coral reef ecosystems. coral reefs, pCO 2 , carbon cycle, summer, South China Sea Citation:

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, ; Dai et al, ; Yan et al, , ). These results suggest that the decadal variability of the coral ∆ δ 13 C can be used as a reliable proxy for the PP in the NSCS.…”

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

“…These results made clear that photosynthesis/respiration caused the major changes of p CO 2 between different seasons. These results also support those obtained in Yongxing and Yongshu Reef (∼1:1.6, ∼1:3.3) [ Yan et al ., ], which are much higher than the ratio of the SCS basin (∼1:14) [ Cao et al ., ], suggesting CaCO 3 production is much more prominent in the coral reef system. The Luhuitou fringing reef flat may therefore function as other typical coral systems in terms of the metabolic processes.…”

“…Observed results, e.g., ∼0.4 mmol C m −2 d −1 at Yongshu reef and ∼4.7 mmol C m −2 d −1 at Yongxing reef of SCS by Yan et al . [], showed that these two reefs act as sources of atmospheric CO 2 in summer. The Yongxing reef acts as a weak source of atmospheric CO 2 at ∼1.48 mmol C m −2 d −1 in winter [ Dai et al ., ].…”

“…The tides at Luhuitou fringing reef range from irregular diurnal‐mixed tides to diurnal tides. The mean tidal level is ∼1.02 m, and the mean tidal range is ∼0.79 m, with the maximum reaching ∼1.89 m [ Yan et al ., ]. Yu et al .…”

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

“…However, regionally averaged primary production and sea surface chlorophyll concentrations vary within limited ranges (Tan and Shi, 2009;Sasai et al, 2013). On the other hand, due to abundant coral reefs on Nansha, Xisha, and Zhongsha islands, this domain was potentially influenced by CaCO 3 formation, which releases CO 2 into the atmosphere Yan et al, 2011). Domain D was located between 18 and 21 • N and 118 and 121 • E, where the Kuroshio intrusions generate mesoscale cyclonic eddies in the northeast monsoon season (Sheu et al, 2010a) and thus induce pumping/entrainment of nutrients and CO 2 from the depths (e.g., Xu et al, 2009;Shang et al, 2012;Sasai et al, 2013).…”