In this paper we extend the P1P2ZN model, introduced by Cropp and Norbury [5], to investigate the effects of specialist (or discriminate) and generalist (or indiscriminate) grazing (as parameterised by p) on a prey-prey-predator model for plankton, in the presence of a limiting nutrient. We also examine the influence of facultative and obligate omnivory on the survival of Z as a generalist predator, as we vary the linear mortality parameter oz • This leads to bifurcation transition diagrams, which also include steady state stability branches for certain critical points. For specialist grazing (p = 0) the bifurcation transition diagram shows steady states, periodic and chaotic dynamics, with very small windows of periodic behaviour, as az varies, while for generalist grazing (p = 1), we only find periodic or steady state behaviours. The dynamics is interpretable in terms of facultative/obligate omnivory of Z. Results suggest that green ocean plankton code in global climate change modelling might run more stably with generalist grazing terms and careful control of grazer mortality.
