HOW CRITICAL IS THE CRITICAL N:P RATIO?1

Abstract

The value of cellular N:P that corresponds to co‐limitation by N and P, the critical (Rcrit) or optimum ratio, has been used to infer the competitive advantage of phytoplankton growing in P‐impoverished systems. Using a revised quota model, with a normalized quota function and capable of simulating surge transport, the interactions between the minimum P‐quota (PCo), the shape of the P‐quota–cell growth relationship (affected by constantKQP), and transport kinetics in affecting the utility ofRcritare considered. For a lowPCoto endow an organism with a highRcritover a wide range of growth rates, the P‐quota curve must be more hyperbolic (KQPlow) rather than linear (KQPhigh).PCoandKQPalso affect the half saturation constant for growth,KgP ; this and the capacity to transport nutrients at rates above those required to sustain steady‐state growth endows a competitive advantage. However, the kinetics of transport into the organism have a greater potential for affectingKgPthan changing the kinetics of internal P usage. Thus, the value ofRcritis not a critical factor affecting competition except in extreme oligotrophic conditions. For competition between species, nutrient transport, accumulation, and resource utilization are all important. However, the efficiency of internal resource utilization is of lesser importance, and certainly not of greater importance, than resource acquisition. Multinutrient models intended to describe competition need to recognize these interactions; the traditional quota model is poorly equipped to do so.