GROWTH RATE DEPENDENT OPTIMUM RATIOS IN SELENASTRUM MINUTUM (CHLOROPHYTA): IMPLICATIONS FOR COMPETITION, COEXISTENCE AND STABILITY IN PHYTOPLANKTON COMMUNITIES 12

ABSTRACT Steady‐State growth equations predict that the optimum C:P ratio (R) of Selenastrum minutum (Naeq.) Collins should change by a factor of 20 over the growth range of this organism. Chemostat cultures were established at fixed inflow C:P ratios chosen such that a transition from carbon to phosphorus limitation should occur solely as a result of increasing the steady‐state growth rate. Measurements of residual dissolved inorganic carbon (DIC), cellular C:P, the kinetics of photosynthesis with respect to [DIC] and the response of culture biomass lo DIC or K2HPO4 additions were obtained. These results show that optimum ratios are growth rate dependent and that this dependency can be predicted based on steady‐stale algal growth equations. A theoretical analysis was undertaken evaluating the range of growth rate dependent changes in the optimum ratio which could be expected for different nutrient pairs. Further analysis showed that, under certain conditions, the growth rate dependence of the optimum ratio may alter the breadth of zones of stable coexistence between species and allow for either the formation or complete elimination of such zones.