Additions of deep-ocean water reduced the rate of carbon fixation by phytoplankton both in natural populations and in single species chemostat-grown cultures. Laboratory experiments indicated that much of the inhibition observed was due to high nutrient concentrations, although for one of two diatom species there was a component of inhibition not explained by the nutrient additions alone. Inhibition was transitory and was followed by stimulation after periods of more than a few hours. Also, the presence of micromolar concentrations of phosphate caused the time course of nitrate uptake from deep-ocean water to differ from the time course observed in response to the addition of nitrate alone.
