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 organicto- inorganic carbon production ratio (ROI) 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 ROI 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 100μmol 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.
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