Pre-existing seawater drives CO2 sources in reef-building coral habitats-a case study in Dongshan coral nature reserve, China.

In reef-building coral habitats, the balance of community organic and inorganic carbon metabolisms can either create an oceanic sink or source of atmospheric CO2. However, researchers often overlook how pre-existing seawater-oceanic water that may be out of equilibrium with atmospheric CO2 before entering the reef system-affects reef system carbon pattern assessments. This study shows that the Dongshan coral habitat acts as an atmospheric CO2 source during our observations in the summers of 2022 and 2024. Through quantitative analysis of CO2 source drivers, we found that this primarily results from pre-existing seawater already releasing CO2, potentially amplified by external biological processes before reaching the reef system. In contrast, the balance of reef community metabolism has less impact on this dynamic and functions primarily as a carbon sink. Additionally, we found that major environmental changes-such as increased seawater turbidity or decreased chlorophyll levels-could transform this balance from a carbon sink into a source, leading to increased CO2 evasion. Our study highlights that pre-existing seawater plays a crucial role in shaping the CO2 source characteristics of Dongshan coral habitats, largely masking whether the reef system itself functions as a sink or source of CO2.