A model of river benthic algal photosynthesis in response to rapid changes in light1

A linear relationship has been found between light and community productivity in a river, but the correlation is reduced when radiation varies rapidly due to the passage of cumulus clouds. To investigate the relationship during periods of patchy cloudiness we developed a method of calculating short term variations in oxygen exchange based on oxygen measurements taken at very frequent intervals. The method has a high degree of resolution when used with data recorded at l-min intervals and the results indicate that large fluctuations in oxygen exchange on time scales of about 5 min can be discerned. Cross-spectral analysis of the productivity data and measured radiation indicated that a linear lead model for algal photosynthesis in rivers is appropriate. This model has the property that the change in net oxygen productivity per unit change in light is enhanced during periods of rapidly varying radiation relative to periods of slowly varying light. The relationship between primary productivity and solar radiation in aquatic communities has been studied by a variety of methods. Most of the work in standing water communities has indicated that photosynthesis becomes light saturated or inhibited at moderate intensities, but these studies have required containment of the populations and separate productivity estimates at each light intensity. Several investigators have presented data from river communities in which net productivity calculated from free-water measurements of oxygen remained directly proportional to radiation (Duffer and Dorris 1966; Kelly et al. 1974; Hornberger et al. 1976). The light-productivity plots presented by Kelly et al. (1974) and Hornberger et al. ( 1976) exhibited considerable scatter about a generally linear trend when radiation varied rapidly because of patchy cloudiness. The deviation from the linear relationship between light and productivity during periods of rapidly varying radiation prescnted by Hornberger et al. (1976) sugl This work was supported in part by National Science Foundation grant GB 32914 and in part by U.S. Army Engineers Waterways Experiment Station contract DACW-39-74-0030. Recorders were provided by National Oceanic and Atmospheric Administration grant N22-9-72 ( N ) made available by M. Garstang. B. J. Cosby provided assistance in fieldwork and data processing. LIMNOLOGY AND OCEANOGRAPHY 226 gcsts either that net productivity is not being resolved on a sufficiently fine time scale (and therefore the linear relationship cannot be discerned) or that the direct proportionality model relating light to productivity fails to hold when incident light is not fairly constant. This prompted us to examine the light-productivity relationship using data collected at very frequent intervals to see if the direct proportionality could be recovered or if some other model might be appropriate. Productivity determination Data available for analysis consist of digital records oE oxygen concentration, tempera ture, and incident radiation recorded at 15-s intervals on the Mechums River, Virginia. To examine short term variations, we devised an efficient procedure for performing productivity calculations using