Selective transmission of light in the ocean waters and its relation to phytoplankton photosynthesis

Occurrence of the depth differences in pigment composition and photosynthetic properties of marine phytoplankton were examined in relation to the spectral changes of light with depth. Phytoplankton were taken from various depths in the northwestern North Pacific, and their absorption spectra were determined with intact cells and in 90% acetone extract. The photosynthetic activities of phytoplankton were concurrently measured under blue, green, red and white light. The difference in absorption spectra for the surface and deeper samples was considerably small, indicating that the prevailing green or blue light in the deeper layers may have little significance for depth-variations of the pigment composition in marine phytoplankton. The depth differentiation in the shape of the light-photosynthesis curve was marked in a well stratified water column but no active response of deeper phytoplankton to green light could be confirmed. The photosynthetic efficiencies of phytoplankton for blue and green light were approximately 105–115 % and 80–90 % of white light, respectively, irrespective of sampling depth.Contribution No. 261 from Shimoda Marine Biological Station