The Benthic Algae of Some Streams in Southern England: I. Biomass of the Epilithon in Some Small Streams

Abstract

The seasonal changes in the biomass of algae growing on the gravel and flints in a small hard-water stream were compared with those in 2 soft-water streams in southern Britain. The gravel occurred between large stands of submerged macrophytes and occupied an area varying between 70% of the river bed in late winter and 20% in late summer. Algal biomass was estimated in terms of chlorophyll a after corrections for degradation products. Phaeopigment concentrations rarely exceeded 30% of the chlorophyll a and were usually less than 20%. Algal cover on gravel in the hard-water stream varied from less than 20 mg m-2 chlorophyll a in winter to a maximum of 200-300 mg m-2 in April 1970. Populations in summer were smaller than in April, normally varying between 40-100 mg m-2 chlorophyll a. The decline, following the spring maximum, did not appear to be associated with nutrient depletion. The April maximum was dominated by a variety of pennate diatoms. During the summer a lime-encrusted community occurred. Scattered growths of Vaucheria and Cladophora overlay these lime encrustations in the later summer. In the soft-water streams, densities were lower and there was no clear spring maximum. Densities varied from below 10 mg m-2 chlorphyll a in winter to levels which rarely exceeded 50 mg m-2 in summer.