The research was directed at developing and establishing methods for measuring rates of planktonic production resulting from heterotrophic microorganisms. Since production of organic matter increases with increasing levels of eutrophication, heterotrophy (the use of organic matter) should become increasingly more important to the ecological processes in aquatic systems. A method was developed which includes the heterotrophic processes in the measurement of total planktonic production. The basic approach was to examine sulfur metabolism, rather than carbon, as the production yardstick. (Author Modified Abstract)