The importance of the form of the quota curve and control of non-limiting nutrient transport in phytoplankton models

The description of phytoplankton growth and nutrient acquisition plays a pivotal role in affecting the performance of plankton ecosystem models. A comparison was made between the behaviour of multi-nutrient phytoplankton models with alternative descriptions of the relationships between the nutrient:C quota and growth rate (m) and for the control of the transport of the non-limiting nutrient. Using Droop kinetics (i.e. employing a hyperbolic curve form fixed by the ratio of maximum: minimum quotas), or conversely a linear quota–m relationship, gave models that could display behaviour in strong disagreement with that of the optimal model when operated in a dynamic scenario. The appropriate description of the control of the transport of the non-limiting nutrient is also important; in particular, a fixed algal N:P should not be assumed. Behaviour was most likely to be aberrant when P was limiting; behaviour under nutrient limitation when the N:P nutrient supply ratio accorded with Redfield expectations (16:1 mole) was least aberrant. Deviations of model output from expected behaviour would affect the simulation of the dynamics of phytoplankton growth, nutrient cycling, competition and have serious repercussions in models in which the stoichiometric quality of the phytoplankton prey affects zooplankton activity. It is important that the behaviour of phytoplankton behaviour accords with that of the groups they purport to represent. However, we currently lack the data upon which to parameterize such models.