CURRENT VELOCITY AND ITS EFFECT ON AQUATIC MACROPHYTES IN FLOWING WATERS

Current velocity significantly affected the biomass and shoot density of aquatic macrophytes in two slow-flowing rivers in western Canada. Studies of aquatic macrophyte communities at three sites on the Bow River, Alberta, Canada, between 1982 and 1985 showed that biomass decreased with increasing current velocity within the weed bed over the range 0.01-1 m/s; at current speeds in excess of 1 m/s, aquatic macrophytes were rare. Transplant experiments in which Potamogeton pectinatus was grown in pails containing three sediments differing in texture at three sites with different current velocities also demonstrated that biomass and shoot density were affected by both the direct effects of current velocity on plant shoots and its indirect effects on sediment nutrient concentrations. These results indicate that current velocity is an important factor regulating aquatic macrophyte biomass in flowing waters and suggest that even a relatively modest increase in current velocity within weed beds reduces the abundance of submerged aquatic plants.