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 (pCO₂) showed regular diurnal cycles. Minimum values occurred in the evening and maximum values in the morning. Air pCO₂ in each of the three study areas showed small diurnal variations, while large diurnal variations were observed in seawater pCO₂. The diurnal variation amplitude of seawater pCO₂ 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 CO₂ flux across the South China Sea (e.g. ∼0.4 mmol CO₂ m⁻² d⁻¹ at Yongshu Reef, ∼4.7 mmol CO₂ m⁻² d⁻¹ at Yongxing Island, and ∼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 pCO₂ 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 pCO₂ variations in coral reef ecosystems.