Light Adaptation by Marine Phytoplankton1

Abstract

Photosynthesis‐light intensity curves were obtained for natural phytoplankton populations in the Sargasso Sea from surface waters and from depths to which 10% and 1% of the surface light penetrated. In winter, when the water was isothermal and mixed to depths below the euphotic zone, phytoplankton from all three depths behaved like “sun” forms, becoming fully light saturated at 5000 foot candles. In summer, when the water and plankton were stratified, the surface plankton were similar to the winter plankton, those from the 1% light level behaved like “shade” plants, becoming light saturated below 1000 foot candles, those from the 10% light level were intermediate in their response to light. The effect of light adaptation on the calculation of primary production from chlorophyll and light is discussed.