MODELING THE SIGNIFICANCE OF ZOOPLANKTON GRAZING FOR EXPORT PRODUCTION

We present a theoretical model and different scenarios for the impact of herbivorous zooplankton grazing on planktonic production and vertical flux of particulate organic matter. Sedimentation of phytoplankton and fecal matter from the mixed layer (i.e. export production P(e)) is given by P(e) = [(1 - alpha) G/(w(s) + G)]P1, where alpha is a ''pelagic assimilation efficiency'' of the mixed layer, w(s) is phytoplankton sinking rate, G is grazing pressure of the herbivore stock, and P1 is the integrated phytoplankton production in the mixed zone. The parameter a is influenced by herbivorous assimilation and decomposition of fecal matter in the upper water column. In the above expression the influence of grazing acts like a Michaelis-Menten limitation term [G/(w(s) + G)], where sinking rate is the half-saturation parameter and pelagic assimilation efficiency represents the maximal influence from grazing. The model seems predictive as it gives reasonable explanations to P(e) : P(t) relationships for both freshwater and marine systems, and therefore improves our understanding of several empirical models presented in the literature linking export production to primary production.