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The increase in atmospheric carbon dioxide is leading to an increase in dissolved CO2 in the oceans, leading to another increase in hydrogen ions and therefore a relative acidifi cation, although the pH still remains slightly alkaline. In addition, this will also lead to fewer available carbonate ions. This concentration contributes to the transformation of calcium ions from a solid state (calcium carbonate) to a liquid state. Thus, the calcifi cation rate will similarly decrease in all carbonate skeletal organisms, including corals. The risk of such a drop in calcium carbonate saturation is that dissolution factors, combined with mechanical destruction and bioerosion, will reverse coral reef construction and start fragmentation of the structure. It is generally thought that the concentration of atmospheric carbon dioxide, currently of 386 ppm, should not exceed 450-500 ppm to avoid this situation. However, according to IPCC scenarios, such values will be reached in less than a century.

The islands of French Polynesia are scattered throughout a considerable oceanic area located on the eastern boundary of the Indo-Pacific Province. This area stretches from 134°28' W (Temoe Island) to 154°40' W longitude (Stilly Island), and from 7°50' S (Motu one Island) to 27°36' S latitude (Rapa Island). Out of the 118 islands constituting French Polynesia, 35 are high volcanic islands and 83 are low-relief islands or atolls. Altogether, the territory of French Polynesia represents an area of 4000 km2 of dry land, 12,000 km2 of lagoonal water and a huge Exclusive Economic Zone (EEZ) covering 5,500,000 km2 of oceanic water (Gabrie and Salvat, 1985).