A multi-class, internal nutrient pool, phytoplankton simulation model was developed and applied to an extensive set of field data acquired in 1974 on Saginaw Bay, Lake Huron. Phytoplankton biomass was partitioned into five functional groups: diatoms, greens, heterocystous blue-greens, non-heterocystous blue-greens, and "others". The nutrients included in the model were phosphorus, nitrogen, and silicon. The model was applied to a single spatial segment encompassing the inner portion of Saginaw Bay.