Use, abuse, misconceptions and insights from quota models — the Droop cell quota model 40 years on

The Droop cell quota model is the most cited model of phytoplankton growth, even though many pay scant regard to the original description and to its limitations for the description of the interactions that define phenotypic plasticity. While the mechanistic basis of the concept and most ecosystem applications of quota models are C based, much experimental work is cell based, and most theoretical studies ignore the important differences between cell and C nutrient quotas. The future application of the quota approach would be enhanced by the adoption of a normalised quota ("Quota) description, employing a dimensionless constant (KQ) to define the response curve, rather than using the original fixed-curve form. Establishment of the range of these KQ values for different phytoplankton species would limit the number of free parameters in ecosystem variants of quota models while recognising the importance of curve shape for phenotypic variation. KQ for N is typically >3, while for P it is typically <0.2. In addition, appropriate control linkages are required to regulate nutrient transport to the quotas of limiting and non-limiting nutrients. Together, these would enable the establishment of a more coherent quota-based description of algal growth more fit for the development of plankton functional-type models.