In many areas nutrient loadings to aquatic ecosystems have increased considerably as a result of population growth, industrial development and urbanization. This has resulted in enhanced growth of phytoplankton, shifts in composition of the plankton community, and changes in the structure of ecosystems, which are often considered to be objectionable. To help understanding these processes and to predict future conditions, a mathematical model, BLOOM, has been developed and applied since 1977. It simulates the biomass and composition of phytoplankton and macro algae in relation to the amount of nutrients, the under water light climate and grazing. It can be applied as a relatively simple screening tool, but also as part of advanced integrated modelling systems including additional hydrodynamic, suspended matter and habitat components. The model has been extensively validated, which means that its credibility was demonstrated systematically for certain types of applications. It has been applied as a supporting management tool to a very large number of aquatic systems worldwide: lakes, channel systems, estuaries, lagoons and coastal seas, using generic coefficients (one set for fresh water, one set for marine simulations) as much as possible. The principles of the model, its validation and a number of representative applications are described in this thesis.