Modelling functional groups of phytoplankton in three lakes of different trophic state

A recently developed approach to divide phytoplankton into functional groups was implemented in the biogeochemical lake model BELAMO and tested by comparing model results with data from three lakes of different trophic state. The groups are distinguished by different values of specific growth rate (kgro,ALG), half-saturation light intensity of primary production (KI,ALG), edibility (kgro,ZOO), sedimentation velocity (vsedvsed), and half-saturation concentration with respect to phosphate uptake (KP,ALG). After calibration, the occurrence pattern of the functional groups in the three lakes can qualitatively be represented by the model. However, some quantitative and systematic differences between model results and data remain. Furthermore, the division of total phytoplankton into functional groups makes these groups very sensitive to changes in kinetic model parameters. There is a very high sensitivity of each group not only to the parameters characterizing this group, but due to very strong interactions, also to the parameters determining the dynamics of the other groups. In contrast, the sensitivity of total phytoplankton to parameter changes remains at a similar level as in the aggregated model. The resulting high sensitivity of the composition of the phytoplankton community on model parameters indicates a poor predictability of the occurrence of functional groups in these lakes. Strong phosphorus limitation during the summer could be the cause for the better predictability of total phytoplankton concentrations in both, the aggregated and the functional group phytoplankton models.