Extensive laboratory studies were conducted to evaluate on a seasonal basis the respiratory response of selected aquatic plants to changes in oxygen concentration, temperature, and exogenous nutrient additions. Sampling sites included (1) a nutrient rich stream section which received the effluent from a secondary sewage treatment plant and (2) an unenriched section supported solely by watershed drainage. Test species for the study, which was run in continuous flow and batch type respirometers, included the aquatic macrophytes Potamogeton orispus and Elodea canadensis. Consumption rates were found to be dependent on temperature, state of plant maturity, the oxygen concentration of the test water, and the level of enrichment at the growth site. Consumption rates varied from 0.2 to 2.8 mg 02/gm dry weight/hr for dissolved oxygen concentrations of 0 to 18 mg/1. A methodology is presented to estimate primary productivity in aquatic ecosystems from observations of the diurnal variation of dissolved oxygen incorporating a variable community respiration rate. (LWR)