The Physiological State With Respect to Nitrogen of Phytoplankton From Low‐nutrient Subtropical Water as Measured by the Effect of Ammonium Ion on Dark Carbon Dioxide Fixation1

The ammonium ion enhancement method has been used to estimate the state of nitrogen nutrition in natural populations of marine organisms, with the advantage that the native population can be assessed at time zero. The ammonium ion does not stimulate dark CO 2 fixation by natural populations of phytoplankton in the low nitrogen subtropical waters of the Florida Strait, indicating that the cells are not physiologically nitrogen deficient; however nitrogen may still be limiting population size. When enhancement was not noted at time zero, incubation of the water in a carboy resulted in depletion of the available nitrogen and a physiological state of nitrogen deficiency subsequently became measurable by this method. The eventual positive results from the test therefore confirm the negative results at time zero.