In many areas nutrient loadings to aquatic ecosystems have increased considerably as a result of population growth, industrial development and urbanisation. 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.