On phytoplankton growth rates and particulate C: N: P ratios at low light1

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Abstract
In a series of continuous culture experiments involving N‐limited growth of the chrysophyte Pavlova lutheri (Mono) I found that large deviations in cellular C: N: P ratios from the Redfield ratio of 106 : 16 : 1 occurred only as a function of low relative growth rate and not of low light intensity. By contrast, light‐limited phytoplankton in continuous culture can attain the Redfield chemical proportions at all combinations of light intensity and dilution rate. Because the absolute magnitude of the maximum growth rate is variable with respect to light intensity and other abiotic factors, the chemical composition of particulate matter can be used as a diagnostic tool to estimate the degree of nutrient deficiency of both laboratory‐grown and natural phytoplankton populations at all light intensities. There is an abundance of data from recent oceanic studies to substantiate the possibility that the chemical composition of oceanic phytoplankton typically is in the Redfield proportions. These results support the hypothesis that oceanic phytoplankters are growing at relative growth rates close to unity.