Effective variables in ecosystem models with an application to phytoplankton succession

To account for changes in the distribution of species grouped together in an ecosystem compartment we introduce averaged physiological parameters and derive equations for their dynamics. These parameters may describe shifts from larger to smaller species or between species with high and low demand of some nutrient, for example. Specifically, in a model for the phytoplankton of Lake Constance, these effective variables describe the shift between diatomic and non-diatomic as well as between rapidly and slowly growing algae. With reasonable values for the remaining biological and physical parameters and a simple model for zooplankton grazing, the dynamics of the effective variables then reproduces the succession of algal species in Lake Constance, as observed and described by Sommer (1987, Progr. Phycol. Res., 5: 123–178).