A bifurcation analysis of a simple phytoplankton and zooplankton model

The paper presents a detailed study of a four component population dynamical model consisting of the biomasses of carbon of phytoplankton x1x1, nitrogen of phytoplankton x2x2, carbon of zooplankton x3x3 and total free nutrient x4x4. In this model, both the influence of the internal (carbon) and external (nitrogen) nutrient on the growth of phytoplankton is included. This is usually termed a variable yield model in contrast to the constant yield model analyzed by J. Monod [Recherches sur la croissance des cultures bacteriennes, Herman, Paris, 1942.  [21]]. The existence and stability of various steady states are discussed analytically and numerically using a computer simulation (AUTO) designed by Doedel [E.J. Doedel, AUTO: A program for the automatic bifurcation analysis of autonomous systems, Congr. Numer. 30 (1981) 265–284; E. Doedel, X. Wang, T. Fairgrieve, AUTO: Software for continuation and bifurcation problems in ordinary differential equations, in: Applied Mathematics Reports, California Institute of Technology, 1994]. Various bifurcation diagrams so obtained support the analytical results and identified complex behavior which cannot be obtained analytically. The purpose of this paper is as a preliminary step to determine the change in the richness of behavior as complexity, such as spatial compartments, is added, which in limiting form would be a continuous spatial dependence (as a partial differential equation model).