Air-sea CO2 fluxes and spatial distribution of seawater pCO2 in Yongle Atoll, northern-central South China Sea

Abstract Yongle Atoll is a complex atoll composed of 13 separated small atolls/reefs in the Xisha Islands, northern-central South China Sea. Wide, deep channels connect the Yongle Atoll lagoon with the open ocean. The sea surface partial pressure of CO 2 ( p CO 2 ) and sea–air CO 2 flux was measured at different geomorphological locations on the atoll. Atmospheric p CO 2 generally showed small diurnal variations, while seawater p CO 2 showed large diurnal variations. Lingyang Reef lagoon, an atoll within Yongle Atoll, was used to check the spatial difference in seawater p CO 2 and air–sea CO 2 flux. The diurnal variation in seawater p CO 2 was greater in the lagoon (258–748 μatm) than outside the lagoon area (354–476 μatm). Differences were also observed in the air–sea CO 2 fluxes depending on geomorphological zone. The mean seawater p CO 2 at Yongle Atoll is in the order of Lingyang Reef lagoon > inner reef slope of the Yongle Atoll > Yongle Atoll channel slope ≈ outer reef slope of the Yongle Atoll. The degree mixing between the lagoon and open sea through channels may have a key role in the horizontal distribution of seawater p CO 2 in the Yongle Atoll. Additional analyses indicated that the daily variations in surface seawater p CO 2 are significantly influenced by biological metabolic processes. The mean air–sea CO 2 flux in the Yongle Atoll was ~ 3.2 mmol CO 2 m −2 d −1 , indicating that the Yongle Atoll acts as a net source of CO 2 to the atmosphere.