Submarine Groundwater Discharge Releases CO₂ to a Coral Reef

Submarine groundwater discharge (SGD) flows into coral reefs. In volcanically active areas, the incoming groundwater is typically CO₂-rich which can alter the carbon balance and views on how coral reefs function at prevailing high CO₂. We quantified dynamic hydrothermal SGD and CO₂ fluxes to a Philippine coral reef over a spring-neap tidal cycle. SGD rates, with mean of 35 cm d^–1 and 5–95% range of 0–147.8 cm d^–1. The groundwater-CO₂ fluxes (266 mmol m² d^–1; range: 0–1111 mmol m² d^–1) were up to ∼300-fold larger than evasion of CO₂ to the atmosphere. The reef seawater pCO₂ (493 μatm; range: 421–680 μatm) remained above atmospheric values and spanned the upper end of the range of atmospheric levels (400–500 μatm) expected for the next century. Because of the hydrothermal SGD, the reef has prevailing above-atmospheric CO₂ and is a source to the atmosphere and nearby waters.