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

Field measurements of air-sea CO2 exchange in three coral reef areas of the South China Sea (i.e. the Yongshu Reef atoll of the Nansha Islands, southern South China Sea (SCS); Yongxing Island of Xisha Islands, north-central SCS; and Luhuitou Fringing Reef in Sanya of Hainan Island, northern SCS) during the summers of 2008 and 2009 revealed that both air and surface seawater partial pressures of CO2 (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 similar to 70 mu mol mol(-1) at the Yongshu Reef lagoon, 420-619 mu mol mol(-1) on the Yongxing Island reef flat, and 264-579 mu mol mol(-1) on the reef flat of the Luhuitou Fringing Reef, and 324-492 mu mol mol(-1) in an adjacent area just outside of this fringing reef. With respect to spatial relations, there were large differences in air-sea CO2 flux across the South China Sea (e.g. similar to 0.4 mmol CO2 m(-2) d(-1) at Yongshu Reef, similar to 4.7 mmol CO2 m(-2) d(-1) at Yongxing Island, and similar to 9.8 mmol CO2 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 CO2 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.