Mathematical model Food ingestion Zooplankton as animals Phytoplankton

This paper presents the numerical simulations of the influence of food ingestion by a herbivorous copepod on phytoplankton and zooplankton biomasses (PZB) in the sea. The numerical studies were carried out using the phytoplankton-zooplankton-nutrient-detritus PhyZooNuDe biological upperlayermodel. This takes account both of fully developed primary production and regeneration mechanisms and of daily migration of zooplankton. In this model the zooplankton is treated not as a ‘biomass’ but as organisms having definite patterns of growth, reproduction and mortality. Assuming also that {Zoop} is composed of i cohorts of copepods with weights Wi and numbers Zi ,t hen{Zoop} = WiZi .T he PhyZooNuDe model consists of three coupled, partial second-order differential equations of the diffusion type forphytoplankton, zooplankton and nutr ients, and one ordinary first-order differential equation for the benthic detritus pool, together with initial and boundary conditions. The calculations were made during 90 days (April, May and June) for the study area P1 (Gdansk Deep) in an area 0 ≤ z ≤ 20 m with a vertical space step of 0.1 m and a time step of 300 s. The simulation given here demonstrated the importance of food ingestion by zooplankton in that it can alter the nature of the interactions of plants and herbivores. The analysis of these numerical studies indicate that the maximal ingestion rate and the half-saturation constant for grazing strongly affect the magnitude of the spring bloom and the cyanobacterial bloom, and also the total zooplankton biomass.