A multi-class, phytoplankton simulation model was developed and calibrated to an extensive set of field data acquired on Saginaw Bay, Lake Huron, during 1974. Phytoplankton biomass was partitioned into five functional groups. Process level analyses were conducted with the calibratred model to determine the relative importance of various factors affecting phytoplankton and nutrient dynamics. Results indicated that temperature and light were relatively more growth rate limiting than nutrients on an annual average basis. Phytoplankton production was found to be extremely sensitive to variations in the light extinction coefficient in the water column, and relatively insensitive to variations in incident solar radiation.