Carbon budget of coral reef systems: an overview of observations in fringing reefs, barrier reefs and atolls in the Indo-Pacific regions

The seawater CO₂ system and carbon budget were examined in coral reefs of wide variety with respect to topographic types and oceanographic settings in the Indo-Pacific oceans. A system-level net organic-to-inorganic carbon production ratio (R_OI) is a master parameter for controlling the carbon cycle in coral reef systems, including their sink/source behavior for atmospheric CO₂. A reef system with R_OI less than approximately 0.6 has a potential for releasing CO₂. The production ratio, however, is not easy to estimate on a particular reef. Instead, observations planned to detect the offshore—lagoon difference in partial pressure of CO₂ (pCO₂) and a graphic approach based on a total alkalinity—dissolved inorganic carbon diagram can reveal system-level performance of the carbon cycle in coral reefs. Surface pCO₂ values in the lagoons of atolls and barrier reefs were consistently higher than those in their offshore waters, showing differences between 6 and 46 µatm, together with a depletion in total alkalinity up to 100 µmol kg⁻¹, indicating predominant carbonate production relative to net organic carbon production. Reef topography, especially residence time of lagoon water, has a secondary effect on the magnitude of the offshore—lagoon pCO₂ difference. Terrestrial influence was recognized in costal reefs, including the GBR lagoon and a fringing reef of the Ryukyu Islands. High carbon input appears to enhance CO₂ efflux to the atmosphere because of their high dissolved C:P ratios. Coral reefs, in general, act as an alkalinity sink and a potentially CO₂-releasing site due to carbonate precipitation and land-derived carbon.