An Incubation Method for Estimating the Carbon Content of Phytoplankton in Natural Samples

In this method, the phytoplankton standing stock as carbon was calculated from carbon assimilation rates, measured at 24 and 48 hr, during incubation of seawater samples on shipboard. The method requires exponential growth at constant specific growth rate of the phytoplankton in the sample during incubation and relies on the fact that the rate of synthesis of cell substance is proportional to the quantity of cell substance present. Growth was so extensive that nutrients had to be added to the samples before the 48-hr incubations to maintain exponential growth. Light intensity during incubation affected results, and matching the incubation intensity to that in situ seemed best. Carbon : chlorophyll cc ratios found in phytoplankton off La Jolla, California, averaged about 90 in nutrient-deplctcd surface layers and about 30 in deeper, nutrient-rich water. Photosynthetic rates per unit chlorophyll a, calculated from 24-hr incubations, were high due to chlorophyll a synthesis during incubation.