Parameterising competing zooplankton for survival in plankton functional type models

32 33 Marine plankton ecosystems are an important component of biogeochemical cycling in the oceans. 34 Operational plankton functional type (PFT) models, that group plankton according to their 35 biogeochemical properties, are currently being developed to resolve biogenic gas exchange between 36 the ocean and atmosphere, and to model the lowest trophic levels in fisheries models. A 37 fundamental problem with these models is that PFTs often go extinct in computer simulations, 38 effectively removing the biogeochemical processes from the models. Cropp and Norbury (Journal 39 of Plankton Research, 31: 939-963) demonstrated that parameter combinations that allowed all 40 PFTs to stay extant for all time in stable, homogeneous environments were rare in a PFT model 41 with two competing phytoplankton and one zooplankton (NP1P2Z). In this paper, we examine the 42 dynamical properties of a generic predator-predator-prey PFT model, and apply the analysis 43 techniques developed by Cropp and Norbury to a simple example PFT model with one 44 phytoplankton and two zooplankton (NPZ1Z2) in order to explore its properties and parameter 45 space. We find that the properties of predator-predator-prey PFT systems are fundamentally 46 different from those of predator-prey-prey PFT systems. The likelihood of parameter combinations 47 for which all PFTs stay extant for all time in predator-prey-prey PFT systems depends critically on 48 the process formulations used, while the properties of co-existing zooplankton (as defined by their 49 parameter values) are quite different to those of co-existing phytoplankton. 50 51 52