The Cladophora growth model (CGM) was used to estimate the importance of light, temperature, phosphorus, and self-shading on the spatial and temporal variability of Cladophora growth rates and biomass accrual in eastern Lake Erie during 2002. The CGM predicted that Cladophora growth was highly sensitive to spatial and temporal variations in soluble phosphorous concentration (SRP). Specifically the CGM predicted that: 1) Site-to-site differences in SRP concentration resulted in a 2×difference in depth-integrated biomass; 2) maximum growth rates were strongly influenced by SRP concentrations during periods of rapid biomass accrual (mid-June to mid-July); 3) inter-annual differences in SRP concentration during the spring period (∼ 1 μg/L) could result in up to a 3.5×difference in depth integrated biomass; 4) Spatial variations in water clarity could result in a 2×difference in depth-integrated biomass between sites, with variations betweens sites occurring primarily between 2–6 m depth; 5) the midsummer sloughing phenomenon likely resulted from self-shading by the algal canopy; and 6) the seasonal growth pattern of Cladophora was strongly regulated by temperature.