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 vertic al 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 ''pela gic assimilation efficiency'' of the mixed layer, w(s) is phytoplankto n sinking rate, G is grazing pressure of the herbivore stock, and P1 i s the integrated phytoplankton production in the mixed zone. The param eter a is influenced by herbivorous assimilation and decomposition of fecal matter in the upper water column. In the above expression the in fluence of grazing acts like a Michaelis-Menten limitation term [G/(w( s) + G)], where sinking rate is the half-saturation parameter and pela gic assimilation efficiency represents the maximal influence from graz ing. 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 pres ented in the literature linking export production to primary productio n.