OPTICAL-PROPERTIES OF WATER AND SPECTRAL LIGHT-ABSORPTION BY LIVING AND NONLIVING PARTICLES AND BY YELLOW SUBSTANCES IN CORAL-REEF WATERS OF FRENCH-POLYNESIA

Spectral optical properties in different kinds of water were studied a t several islands in French Polynesia. Optical differences between the waters of a high island (Moorea, Society archipelago), an atoll (Taka poto, Tuamotu archipelago) and the ocean above the outer reef slope we re first assessed in terms of irradiance and diffuse attenuation coeff icients. These differences were analysed with respect to the effect of phytoplankton, non-living particles and yellow substances, with these 2 latter components being responsible for most of the variations obse rved. Absorption coefficients for phytoplankton and non-living particl es, as well as absorption by yellow substances, were also measured in the waters of the high island of Tahiti (Society archipelago) using th e 'glass-fiber filter' technique and spectrophotometric measurements, respectively. These data showed large spatial variability of absorptio n, the main absorbing compound varying from one location to another de pending on whether a station was mainly influenced by emergent land or by the ocean. Specific absorption coefficients for phytoplankton show ed marked spatial and temporal variations among the stations. Spatial variations were related to concentrations of chlorophyll a and covaryi ng pigments, taxonomic differences and total suspended particles. Temp oral variations were probably caused by differences in the incident UV radiation between summer and winter. These results show that optical properties of water can be used to describe the medium in aquatic envi ronmental studies.