A MODEL OF THE YEARLY CYCLE OF NUTRIENTS AND PLANKTON IN THE BALTIC SEA

A model of plankton dynamics in parts of the Baltic Sea is constructed which is as simple as possible but able to simulate the yearly cycle of nutrients and plankton in a horizontally averaged two layer system. State variables are limiting nutrients, phytoplankton, zooplankton and detritus expressed by concentrations relative to a certain density of limiting nutrients. The physical control is reduced to the formation and destruction of a thermocline in spring and fall. The dynamical ingredients are nutrient limited growth, grazing, loss and recycling. Owing to problems of observational undersampling of marine ecosystems and to the integrated nature of the presented model, no attempt has been made to calibrate the model against data of a single field campaign. The model parameters are adjusted in such a manner that timing of the spring bloom and of the zooplankton response as well as the relative magnitude of the peak values and the summer concentrations are consistent with the accumulated knowledge from many observations in the Arkona and the Bornholm basin of the Baltic Sea. Nutrient limited growth and food limited grazing are accounted for by quadratic modified versions of the Michaelis-Menten and the Ivlev approach. These expressions reduce consumption for low concentrations and do not require threshold assumptions. The presented model is stable and robust for a relatively wide range of parameter values. The effects of eutrophication can be described in a reasonable manner. The model is developed as a first step towards a coupled model of circulation and plankton dynamics in the Baltic Sea.