SEASONAL,SUCCESSION OF ALGAE IN RIVERS. I. EXAMPLES FROM THE AVON,A LARGE SLOW-FLOWING RIVER1

The epipelic, epilithic and epiphytic algae in a large slow-flowing river (the Avon) situated in southern England usually exhibited a pronounced bloom during early spring probably in response, to increasing daylength. Although floods caused a reduction in the standing crop of most communities both through scouring action and possibly long-term alteration of microhabitat, they also seemed essential for the renovation of sediment and, rock substrates after periods of rapid algal growth. Because of the attachment characteristics of the predominant epiphytic species, the community attached to Cladophora remained relatively unaffected by flooding. Water velocity may have been important in determining the species composition of some communities, particularly the epiphytic associations. Nutrients probably seldom limited development, but during part of the summer competition with planktonic diatoms for silicon appeared to limit attached algae. During the summer, large scale detachment (which greatly reduced algal density) apparently correlates with a high photosynthetic rate. Algal, numbers in the plankton occasionally decreased in response to a loss of buoyancy of the predominant species.