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

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 phy-toplankton 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 phyto-plankton 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 phytoplank-ters are growing at relative growth rates close to unity. It is well documented that the chemical composition of phytoplankton species in culture varies tremendously as a function of the degree of nitrogen or phosphorus limitation and approaches the Redfield proportions of C106N16P, when steady state dilu-' Contribution 6083 from the Woods Hole Oceanographic Institution. tion rates (=growth rates) are close to the washout rate D, (=b the maximum growth rate) (Goldman et al. 1979). Because the chemical composition of particulate matter in surface waters of the ocean frequently is close to the Redfield proportions, it has been suggested that growth rates of resident phy-toplankton populations are close to the maximum possible and are not controlled by the availability of nitrogen or phosphorus (Goldman et al. 1979). Clearly, this argument is valid only for steady state conditions. Such conditions are met in the laboratory with continuous culture, and in nature they are characteristic of pelagic surface waters where phytoplankton growth, her-bivore grazing, and nutrient regeneration are in reasonable balance on a temporal basis (Goldman 1984a, b). An additional proviso is that b is not a constant, but is variable with respect to abiotic factors such as light intensity and temperature. Changes in the chemical composition of phytoplankton would be expected to occur as a function of the relative growth rate SL. : I;(ZJ and not of the absolute rate I;(ZJ when, for example, light intensity I, is variable …