At 32 stations covering a wide variety of phytoplankton biomass, results have been obtained for in situ production, daily irradiation, photosynthetically available radiation (PAR), and spectral composition of submarine radiant energy. PAR is compared to the photosynthetically stored radiant energy (PSR), and a dimensionless quantity, ϵ, describing the efficiency of the energy storage, is ... [Show full abstract] calculated. ϵ varies from less than 0.01% to about 0.1% between the surface and the base of the euphotic layer in oligotrophic waters (Sargasso Sea). The corresponding values for productive waters (Mauritanian upwelling area) range between 0.1 and 0.8%. On average, for the whole productive column, about 3% (±1.5%) of the total incident energy (including infra-red) is stored by photosynthesis per unit area of ocean per unit of chlorophyll (1 g Chl-a m−2), despite the variability of biomass (0.003 to 0.3 g Chl-a m−2).The photosynthetically usable radiant energy (PUR), i.e. the part of PAR that can be absorbed by algae, is evaluated. Comparison of PSR and PUR allows the estimation of the quantum yield φ and its variation with depth. The exact calculation of PUR requires the introduction of an effective absorption coefficient, aeff, for the living phytoplankton in its actual environment. The coefficient aeff must take into account both the algal absorption and the spectral composition of the remnant light. Low values obtained for PUR and the consequent excessive values for φ emphasize the necessity for chromatic adaptation, especially for an increase in accessory pigments absorbing green-yellow light.