Laboratory streams, a 50-liter respirometer chamber, and a Gilson differential respirometer were used to study factors influencing rates of periphyton respiration in lotic environments. Rates of respiration, expressed on an ash-free dry weight basis, determined for sections of intact communities in air-saturated water, varied between 0.8 mg 02/g hr at 7.20C and 2.3 mg 02/g hr at 18.60C for a community grown at 38 cm/sec current velocity, and between 0.4 mg 02/g hr at 7.2'C and 1.7 mg 02/g hr at 18.5'C for a community grown in a current of 9 cm/sec. These values were in good agreement with rates obtained mano- metrically in Warburg experiments and with values reported in the literature for various aquatic plants. The periphyton that developed in the swifter current exhibited a greater metabolic response to shaking than material grown in a slow current velocity. The fast-current community usually had a higher respiratory rate than the slow-current community. Rates of respiration in both communities decreased rapidly when the dissolved oxygen concentration was reduced below air saturation, indicating that the sessile lotic communities were particularly susceptible to oxygen exhaustion in the vicinity of the respiring cells. The enhancement of respiration by increasing 02 tension above air saturation was more pronounced at 50C than at 13TC, and concentration of oxygen above air saturation had a greater effect on respiration when the flasks were not shaken.
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