ABSTRACT: Air-sea CO₂ exchanges and the partial pressure of CO₂ were measured in surface water overlying 2 coral reefs: Moorea (French Polynesia, austral winter, August 1992), where coral diversity and surface cover are low, and Yonge Reef (Great Barrier Reef, austral summer, December 1993), where coral diversity and cover are comparatively higher. A procedure is proposed to estimate the potential CO₂ exchange with the atmosphere by taking into account both the saturation level of oceanic seawater and the equilibration process occurring after water leaves the reef. It is shown that both sites were net sources of CO₂ to the atmosphere as a result of the effect of calcification on the dynamics of the inorganic carbon system. The potential global CO₂ evasion from the ocean to the atmosphere is about 4 times higher at Yonge Reef than at Moorea. It is also demonstrated that, at both sites, the major exchange of CO₂ from sea to air occurs as seawater returns to chemical equilibrium after it has crossed and left the reef. The dynamics of inorganic carbon were studied using the so-called homogeneous buffer factor [beta = dln(pCO₂)/dln(DIC)] (where pCO₂ is the CO₂ partial pressure in surface water and DIC is dissolved inorganic carbon), which gave estimates that approximately 80% of the change in inorganic carbon was related to photosynthesis and respiration. This approach showed that the calcification rate was proportional to the net organic production during the day and to the respiration rate at night.

KEY WORDS: Coral reefs · Inorganic carbon · Air-sea CO₂ exchanges

Published in MEPS Vol. 145 (1996) on December 31
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1996