Bloom formation and stratification by a planktonic blue-green alga in an experimental water column

The growth and buoyancy of the planktonic blue-green alga Anabaena flos-aquae Bréb. was studied under different nutrient conditions in small batch cultures and in a thermally stratified water column. When conditions permitted the development of high algal concentrations, the diffusion of CO2 into the culture limited the photosynthetically driven buoyancy-regulating response and the filaments became buoyant. Under similar conditions in the water column CO2 again became limiting and the alga floated up to form a waterbloom at the surface, even though the incident light intensity there was 10 klx (200 μE s-1 m-2). When growth of the alga in culture was limited by reducing the concentration of K2HPO4 the alga sank, even when the light intensity was only 1·7 klx. The alga no longer formed a surface water bloom in the column when the K2HPO4 concentration was reduced to 10 μm, but instead it stratified at a depth of about 1 m, where the light intensity was about 1·2 klx. The behaviour of the alga in the column is reminiscent of that shown by certain planktonic blue-green algae in natural lakes.