Photosynthesis in the Ocean as a Function of Light Intensity1

Photosynthesis was measured in 14 species of marine plankton algae in relation to light intensites within the entire range to which natural phytoplankton populations may be exposed. A mean curve was constructed showing relative photosynthesis in marine phytoplankton as related to light intensity. From this curve and surface radiation data, relative values for photosynthesis were determined throughout the day at the surface and at depths in the ocean at which the surface intensity is reduced by given amounts. Integration of these values over the entire day gave values for daily relative photosynthesis at each depth considered, and integration of the daily values with depth resulted in a single figure for daily relative photosynthesis beneath a unit of sea surface. An empirically derived relationship was discovered between daily surface radiation and daily relative photosynthesis within the water column. From this relationship the effect of light intensity upon photosynthesis throughout the year at a temperate latitude was demonstrated. A preliminary method is described for estimating absolute daily photosynthesis beneath a square meter of sea surface from measurement of (1) photosynthesis of a sample of the population at light saturation, (2) the extinction coefficient of light in the water, and (3) the daily surface radiation. From an estimate of the ratio of photosynthesis to respiration at light saturation, the daily ratio P:R for the entire population is calculated. It is shown that phytoplankton populations cannot photosynthesize sufficiently to compensate for their own respiration during periods of low solar radiation, as may occur in the winter months at temperate or northern latitudes.