The routine output of a mathematical model for Cladophora growth provides a useful integration of the biological, chemical and physical factors involved, but affords little information regarding the individual importance of key mechanisms. This manuscript describes model calculations of the seasonal and spatial variation in the processes regulating the growth of Cladophora. The impact of light, temperature, internal phosphorus levels, and self-shading are described on a seasonal basis. Seasonal cycles of Cladophora growth and the distribution of the alga with respect to depth are examined in terms of a critical energy balance between light-enhanced respiration and gross photosynthesis.