How critical is the critical N:P ratio?

The value of cellular N:P that corresponds to colimitation by N and P, the critical (R-crit) 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 constant KQP), and transport kinetics in affecting the utility of R-crit are considered. For a low PCo to endow an organism with a high R-crit over a wide range of growth rates, the P-quota curve must be more hyperbolic ( low) rather than linear ( high). PCo and KQP also 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 affecting KgP than changing the kinetics of internal P usage. Thus, the value of R-crit is 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.