Relationships among substrate , flow , and benthic microalgal pigment density in the

Measurements of photosynthetic pigments have been used to estimate the biomass of benthic microalgae on soft and hard substrates in a small river in the Virginia Piedmont. The irregular distribution of periphyton was partially related to substrate distribution and is explained as a result of spatial variations in the current regime in the river. High flow due to unpredictable and heavy rainfall is shown to be the main factor controlling algal biomass. Deterministic predictions of algal crop are therefore not possible in rivers of this type. The factors that control the growth and biomass of periphyton (using the term to include all microalgae living on or close to any submerged surface) in a river include temperature, light intensity, current and scour, substrate, grazing, and nutrients (Hynes 1970). Little is known about seasonal changes in periphyton biomass (as opposed to species composition) because few rivers have been sampled quantitatively for a sufficient time. With a few exceptions (e.g. Douglas 1958), direct estimation of the numbers of one or more species has proved too slow for use in extensive surveys, and the use of artificial substrates to facilitate sampling is open to criticism (Tippett 1970). Photosynthetic pigments can be determined more quickly and, despite variability in chlorophyll:organic biomass ratios, can be used as rough measures of plant biomass. We used estimates of pigment densities in the Mechums, a small river in the Virginia Piedmont, to investigate the hypothesis that periphyton biomass is ultimately controlled more by river flow than by any other factor. In our hypothesis, 1 The work on which this publication is based was supported in part by funds provided by the u.S. Department of the Interior as authorized under the Water Resources Research Act of 1964, as amended, and in part by National Science Foundation grant GB 32914. 2 Present address: Scottish Marine Biological Association, Dunstaffnage Marine Research Laboratory, P. O. Box 3, Oban, Argyll, Scotland. variations in flow, acting through changes in the rates of scour, washout, and sedimentation, determine the spatial pattern of, and the time course of changes in, the biomass of periphyton. Spatial variations in river flow are reflected in substrate distribution. Thus periphyton density should relate to substrate as well as flow. In the Mechums as in many rivers, short periods of greatly increased flow result from heavy rainstorms. We suppose that periphyton biomass is very sensitive to these floods. If periphyton otherwise grow steadily, then biomass ought to be proportional to time elapsed since the last flood. If our hypothesis is correct, and if the rainstorms that produce high river flows are random events, then deterministic predictions of algal productivity will not be possible for rivers like the Mechums. We thank S. Shoup for some of the velocity data, the Virginia State Water Control Board for the Rivanna North Fork discharge data, and NOAA for the Charlottesville-Albemarle Airport rainfall data.