Thirty-two two-station diel dissolved oxygen surveys were made in the experimental streams of the U.S. EPA Monticello Ecological Research Station. The data were analyzed by a numerical Dissolved Oxygen Routing Model (DORM) to determine total community respiratory and photosynthetic rates. The artificial channels consisted of pools and riffles and had pH of 8.0, 6.3 and 5.4. Community photosynthetic production and respiration ranged from highs of 14.8 and 10.7gm-2 day-1 to lows of 1.5 and 2.6gm-2 day-1 between June and September 1979. P/R ratios varied from 2.1 in May to 0.6 in August. Seasonal trends were affected by pH and/or age of the channel. The channels also exhibited large diel variations in photosynthetic efficiency, where dependence of photosynthetic production upon light was nonlinear and had a strong hysteresis. This hysteresis cannot be explained by physical processes and is attributed to several biological processes.