Changes of phytoplankton populations in Rostherne Mere in 1996 were examined by means of simulation mathematical models. Simple models, solely based on Monod or Michaelis Menten equations, failed to give a reasonable simulation of the phytoplankton succession. A more complex model Rostherne (version 1.1a) calibrated on an extensive set of XRMA and conventional data, however, proved to be useful both for prediction of the outcome of the spring and summer competition and for the estimation of values of certain non-measured variables. It also helped to identify the limiting factors for different times of the year. Alteration of the simulated magnitude of the spring diatom bloom had a major influence on summer cyanobacterial maxima, demonstrating fine regulation of the biogeochemical balance within the modelled system.