Facteurs physiques du milieu dans les lagons des Tuamotu

Physical environmental factors in the coral-reef lagoons of the Tuamotu ln coral-reef ecosystems, as in other aquatic ecosystems. physical characteristics are key forcing variables Various rescarch programs have been carried out betweell 1994 and 2002 in severallagoons, mostl)' in the Tuamotu atolls (French Polynesia), TIleir cornmon theme was the study of the relationships between water column characteristics and planktonic processes Although "nonnal" atoll lagoons are rather similar, our comparative approach could detect an hydraulic gradient, mirrored by a trophic gradient. These trends are confirmed by some few "abnormal" atoll lagoons. At the scale of a whole lagoon, water renewal rate (from the ocean into the lagoon) and lagoon level arc controlled by S'vell height and reef-rim morphometrv Permanent passes. allowing tidal exchanges, have a negligible role, The trophic state of the water column, which is weil correlated with water residence time, can be monitored through simple physical descriptors (turbidiry by Secchi disk, water colour by Forel scale). Significant saIinity (and ( 0) variations are found only in extreme cases of nearly land-locked lagoons At a smaller spatial scale, inside a given \agoon, physical properties such as light absorption. vertical temperature profile, and internai circulation are mainly controlled by local wind. The role of physical factors is examplified by the relationships berween water residence time and selected biological processes (planktonic bloorns and pearl-oyster growth). a Centre IRD de Tahiti, B.P. 529. Papeete (Tahiti Polynesie francaise) b Service de la Perliculrure. BP 9047 Motu Dta (Tahiti. Polynesie francaise) c. LODyC UPMC 4, pl Jussieu. F-75252 Paris cedex 05 d: Centre IRD de Noumea. BP 5 A. Noumea (Nouvelle Caledonie)