Internal dynamics of NPZD type ecosystem models

The marine ecosystem is an essential part of the Earthʹs carbon cycle. Therefore, to create model-based scenarios of the future of the Earthʹs climate, it is inevitable to consider the relevant processes of this system within numerical simulations. In the last two decades, a wide range of submodels of varying complexity have been established representing the marine ecosystem. Their specific behaviour and characteristic responses to, e.g., external forcings, however, are only merely understood. In this study, we present a mathematical method to assess the fundamental behaviour of such models. Three marine ecosystem models of NPZD type, varying in their formulation of phytoplankton loss and zooplankton loss, are analyzed. Following the qualitative theory of dynamical systems, model specific behaviours as well as similarities of the individual models are revealed. For each model, three equilibria are detected and their stability properties are analyzed. Parameters that significantly affect the model solutions are pointed out. Parameter constellations and initial values that cause characteristic model behaviours are determined and discussed. The results obtained provide the basic, theoretical knowledge which is essentially needed to use models in an effective and appropriate way.