Plankton functional type modelling: running before we can walk?

Biogeochemical cycling in marine systems is intimately linked to the activity of specific plankton functional types (PFTs) such as diatoms, coccolithophores and nitrogen fixers, thereby providing a focus for contemporary modelling studies. Incorporating extra complexity beyond simple nutrient-phytoplankton-zooplankton-detritus (NPZD) models is, however, fraught with difficulties: poorly understood ecology; lack of data; aggregating diversity within functional groups into meaningful state variables and constants; sensitivity of output to the parameterizations in question and their physical and chemical environment. Although regional models addressing the seasonal succession of plankton types have achieved some degree of success, predicted distributions of PFTs in global biogeochemical models have thus far been less than convincing. While the continued articulation of detail in ecosystem models is surely the way forward, I argue that this can only be so with due care and attention to the formulations employed and a healthy dose of scepticism regarding model outcomes. Future directions should emphasize building up complexity gradually, objective assessment of the resulting parameterizations, and variety in approach such as the use of empirical alternatives to the fully dynamic representation of PFTs in models.