INCORPORATING THE MICROBIAL LOOP IN A SIMPLE PLANKTON MODEL

The microbial loop, which recycles nutrients in the upper layers of th e ocean, is an integral part of plankton dynamics. The usual method fo r modelling the complex patterns involved has been to consider the 'Z' in N/P/Z (nutrient/phytoplankton/zooplankton) models as containing al l possible grazers on P and, implicitly, relegate the carnivorous meta zoans to the loss term on Z. I propose the opposite approach-to define Z explicitly as the metazoans responsible for export fluxes-and to si mulate the effects of the microbial loop implicitly in terms of grazin g and excretion rates. The reasons for taking this alternative route a re (i) the importance of copepods in the carbon/nitrogen flux from the euphotic zone to deeper water compared with (ii) the predominantly in ternal role of the microzooplankton in recycling nutrients; and (iii) the problems of sampling the microbial component, compared with sampli ng larger metazoans. Finally, there is the need to keep plankton model s as simple as possible for later use in coupled physical/biological s ystems.