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Plexiglas plates were placed in laboratory streams and in a spring and removed at 2-to 4-day intervals to determine the increase in standing crop of periphyton.The instantaneous growth rate, calculated from the growth curve of periphyton on plates in the streams, was 0.51 g mea day-l as compared to an estimate of 0.60 g m-' day-' obtained by oxygen measurements for growth of periphyton on the stream bottom. The average growth rate of periphyton on plates in the streams over a single exposure period was 0.31 g m-' day-l. Under the conditions of this experiment, the instantaneous growth rate was a better estimate of productivity than the average growth rate.

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