The spectral irradiance distribution at five stations on lakes and at sea was measured with a portable underwater spectral irradiance meter. Chlorophyll a concentration and the absorption coefficient of the water were concurrently measured. From measured spectral irradiance distributions, radiant energy absorbed per unit volume was computed. At these stations, the effect of upward irradiance on total quanta absorbed by the water was negligibly small for all layers. The relative contributions of phytoplankton, detritus, dissolved organic matter, and pure water to the total absorbed quanta were also computed by taking into consideration the spectral dependency of each component: the contribution of quanta absorbed by phytoplankton was about 3-10% in clear water and about 30-40% in the plankton-rich water.

Light utilization efficiency and quantum yield of phytoplankton in a thermally stratified temperate sea were evaluated. Underwater spectral irradiance was measured with a specially designed underwater irradiance meter and the specific absorption coefficient of phytoplankton was determined by a modified opal glass method. The light utilization efficiency of phytoplankton at each depth was derived from photosynthetically fixed energy divided by the energy penetrating into that depth, and quantum yield was estimated from photosynthetic rate divided by quanta absorbed by phytoplankton. Vertical profiles of in situ photosynthetic rates per unit volume of water showed two peaks, the first at the depth of about 30% light level (ca. 45,000 cal m-2 h-l) and the second at the depth of about 1.5% light level (ca. 2,500 cal m-2 h-r) with about the same rates. The light utilization efficiency was about 0.5% at the first peak and 5% at the second, The quantum yield was about 0.02 mol C Einst-' at the surface peak and 0.1 at the subsurface peak. The latter value was nearly the same as the maximum yield reported from culture experiments.

Measurements of underwater irradiance revealed that the vertical attenuance in upward irradiance for wavelengths above 520 nm decreased with increasing depth, while the attenuance in the remaining wavelength region and also the attenuance in the downward irradiance in the whole wavelength range kept almost constant values. In this paper, it is suggested that the decrease in the attenuance for the upvcard irradiance above 520 nm can be ascribed to the Raman scattering of water molecules excited by the intense blue-green light in the downward irradiance.The pure water Raman scattering function at a scattering angle of 90 ° is measured and the results are used for the theoretical computation of upward irradiance generated by Raman scattering. Then, the difference between observed upward irradiance and the upward irradiance obtained by extrapolation from that in the shallow layers is computed under the assumption of constant irradiance attenuance. Since this difference is expected to represent the upward irradiance generated by Raman scattering, its value is compared with the upward irradiance due to Raman scattering obtained by theoretical computation. The similarity between the two upward irradiances so evaluated supports the view that Raman scattering makes a significant contribution to upward irradiance in the longer wavelength region.

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