Light utilization efficiency and quantum yield of phytoplankton in a thermally stratified sea1

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Abstract
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−1) and the second at the depth of about 1.5% light level (ca. 2,500 cal m−2 h−1) 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−1 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.