BIOEROSION CARBONATE CYCLE - APPROACH OF QUANTIFICATION OF ITS PROCESSES IN FRENCH-POLYNESIA

The contribution to bioerosion of the major boring and grazing species on coral reefs was estimated on French Polynesia by three methods: (1 ) experimental models using dead coral samples exposed for 5 years. Bi oerosion was mainly due to microboring organisms during the first two months, and to macroborers after 6 months of experiment. Grazing was t he dominant process since one year and accounted for 66% of the total bioerosion which reached 2.6 kg CaCO3 m2.y-1 after two years of exposu re; (2) bioerosion rates of grazers. The quantity of carbonate eroded from the reef framework by echnids and scarid fishes was determined th rough the study of gut contents production. On Moorea barrier reef fla t, bioerosion was estimated to 4.5 kg CaCO3 m-2.y-1 for echnids and to 1.7 kg CaCO3 m-2.y-1 for scarids; (3) influence of reef communities u pon carbonate budget. On the reef studied calcification processes domi nated over dissolution processes (2 kg CaCO3 m-2.y-1). In laboratory e xperiments, the living corals depleted the carbonate content of seawat er (3.7 g CaCO3 m-2.y-1) which is in turn highly restored by the activ ity of boring organisms and grazers. These results demonstrated the gr eat importance of bioeroders and grazers in determining the calcium ca rbonate budget of coral reefs.