Distribution and air-sea fluxes of CO2 in coral reefs in the Greater Bay Area, China

From May to June 2021, the sea surface partial pressure of CO₂ (pCO₂) and air-sea CO₂ fluxes around six coral reefs in the Great Bay Area (contain Pearl River Estuary (PRE) and Daya Bay) were investigated respectively. The distribution of seawater pCO₂ around different coral reefs were quite different. The seawater pCO₂ of the coral reef region of Daya Bay is relatively high, ranging from 268 to 496 µatm. In contrast, the pCO₂ levels in the coral reef area of the PRE are significantly lower, with measurements ranging from 84 to 374 µatm. Further analysis showed that the distribution of seawater pCO₂ around the coral reefs of the PRE were mainly affected by biological metabolism and seawater mixing, while those in Daya Bay were mainly controlled by temperature and seawater mixing. During the survey, the three coral reefs in the PRE were affected by algal blooms showing strong sinks of atmospheric CO₂ ranging from −12.3 to −19.2 mmol CO₂ m⁻² d⁻¹. However, the difference between the seawater pCO₂ and atmospheric pCO₂ in the three coral reefs of Daya Bay were all very small, and they were weak sources or sinks of atmospheric CO₂, respectively.