A numerical model, based on some simplifying assumptions, was used to evaluate spatial and seasonal patterns in primary production in Tuttle Creek Reservoir, Kansas, from June 1970 through May 1971. The assumptions of the model best fit turbid, homothermal lakes, such as those in Kansas and the Great Plains Province. The model describes the photosynthesis-depth profile on the basis of light attenuation with depth and allows integral photosynthesis to be obtained by a simple analytical calculation. The use of the model had its greatest advantage in allowing pelagic algal photosynthesis to be monitored in shipboard incubators. Various chemical, physical, and biological features were measured simultaneously with primary production to describe the diversity of the reservoir, to evaluate horizontal production patterns, and to validate the assumptions of the model.