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

Hongqiang Yan,Qi Shi,Lingying Huang,Pinxian Wang,Yehui Tan,Shu Li,Tianran Chen,Meixia Zhao,Kefu Yu,Huiling Zhang

doi:10.1007/s11434-011-4372-8

Abstract

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.

Highlights

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 showed regular diurnal cycles
The diurnal amplitude of seawater pressures of CO2 (pCO2) observed at Yongshu Reef lagoon was much lower than that of the reef flats at Yongxing Island and Luhuitou Fringing Reef
We concluded that: (1) Both air and surface seawater pCO2 of the three studied coral reef areas showed significant diurnal cycling, with a decreasing phase during the daytime and an increasing phase during the night. This cyclic variation was clear in areas that had high proportions of coral cover, such as the reef flats of Luhuitou Fringing Reef and Yongxing Island

Summary

Introduction

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. ~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) These positive values suggest that coral reef ecosystems of the SCS may be a net source of CO2 to the atmosphere. After a brief investigation at Yongxing Island, in the Xisha Islands, in the spring of 2006, Dai et al [20] found that surface seawater pCO2 shows significant regular diurnal variations, and that the studied area was a minor source of atmospheric CO2. In order to fully understand the relationship between SCS coral reefs and atmospheric CO2, we carried out detailed measurements of air-sea CO2 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. From May to August, the area is under the influence of SE-SW monsoons with an average wind speed of 5–6 m s–1 [25]

Methods

Results

Conclusion