Incorporating the microbial loop in a simple plankton model

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Abstract
The microbial loop, which recycles nutrients in the upper layers of the ocean, is an integral part of plankton dynamics. The usual method for modelling the complex patterns involved has been to consider the ‘Z’ in N/P/Z (nutrient/phytoplankton/zooplankton) models as containing all possible grazers on P and, implicitly, relegate the carnivorous metazoans to the loss term on Z. I propose the opposite approach—to define Z explicitly as the metazoans responsible for export fluxes—and to simulate the effects of the microbial loop implicitly in terms of grazing and excretion rates. The reasons for taking this alternative route are (i) the importance of copepods in the carbon/nitrogen flux from the euphotic zone to deeper water compared with (ii) the predominantly internal role of the microzooplankton in recycling nutrients; and (iii) the problems of sampling the microbial component, compared with sampling larger metazoans. Finally, there is the need to keep plankton models as simple as possible for later use in coupled physical—biological systems.