Optical properties of water and spectral light absorption by living and non-living particles and by yellow substances in coral reef waters of French Polynesia

Abstract

Spectral optical properties in different kinds of water were studied at several islands in French Polynesia. Optical differences between the waters of a high island (Moorea, Society archipelago), an atoll (Takapoto, Tuamotu archipelago) and the ocean above the outer reef slope were first assessed in terms of irradiance and diffuse attenuation coefficients. 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 observed. Absorption coefficients for phytoplankton and non-living particles, as well as absorption by yellow substances, were also measured in the waters of the high island of Tahiti (Society archipelago) using the 'glass-fiber filter' technique and spectrophotometr~c measurements, respectively. These data showed large spatial variability of absorption, the main absorbing compound varying from one location to another depending on whether a station was mainly influenced by emergent land or by the ocean. Specific absorption coefficients for phytoplankton showed marked spatial and temporal vanations among the stations. Spatial variations were related to concentrations of chlorophyll a and covarying pigments, taxonomic differences and total suspended particles. Temporal 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 environmental studies.