Carbon fluxes in coral reefs. II. Eulerian study of inorganic carbon dynamics and measurement of air-sea CO2 exchanges

Abstract

Air-sea CO, exchanges and the part~al pressure of CO, were measured in surface water overlying 2 coral reefs: Moorea (French Polynesia, austl-a1 lvlnter, August 1992). where coral di\,ers~ty and surface cover are low, and Yonge Reef (Great Barr~er Reef, a~istral summer, December 1993), where coral diversity and cover are comparatively h ~g h e r A procedure is proposed to estimate the potential CO2 exchange with the atmosphere by t a k ~n g ~n t o account both the saturation level of oceanlc 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 seacvater 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 [P = dln(pC02)/dln(D1C)] (where pCO, 1s the CO2 partial pressure In surface ivdter and DIC 1s d~ssolved 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 calc~fication rate was proportional to the net organic production during the day and to the respiration rate at night.