The in water penetration of the solar light (namely the Photosynthesis Active Radiation, PAR) is described by theattenuation coefficient Kd. Kd depends upon (i) the solar angle, (ii) the state of the sky (clear or cloudy), and (iii) the watercomposition. Knowing the above parameters as well as the inherent optical properties of the atmosphere and of the water, aradiative transfer code can be run to derive Kd at different wavelengths.Ocean color satellite missions provide the water leaving radiances after atmospheric correction. For ocean case 1water, L, allows to derive the chlorophyll concentration and the associated yellow substances. It is then possible to deriveKd. Ifl ocean case 2 waters, chlorophyll concentrations are also proposed but Kd also depends upon the sediments and uponthe yellow substance absorption. At present time, no operational algorithm is proposed to derive the water composition inocean coastal waters.