FISH AND NUTRIENTS INTERPLAY DETERMINES ALGAL BIOMASS - A MINIMAL MODEL

An analysis is presented of a simple zooplankton-phytoplankton interaction model, accounting for the effects of nutrients and planktivorous fish. The overall behaviour is shown to be analogous to that of the cusp catastrophe model with nutrients and fish as control variables. Predictions that emerge are: gradual change in fish density can lead to discontinuous changes in zooplankton and phytoplankton density under eutrophic conditions. planktivorous fish tends to damp zooplankton-phytoplankton oscillations. enrichment in the absence of fish enhances zooplankton density, but has no effect on algal density, whereas in the presence of a high fish stock the opposite applies. These and other predicted patterns are demonstrated to be present indeed in much field data and experimental results. It is concluded that several, often counter intuitive and complex, patterns in plankton dynamics can be understood simply from the basic features of the considered three trophic level interaction.