WATER‐BLOOMS

Summary 1. Peculiarities in the ecology of planktonic blue‐green algae are reviewed in relation to recent advances in understanding their physiological characteristics. 2. Dense water‐blooms are always the result of buoyant migration of existing populations to the lake surface under calm weather conditions. The size of the population is the direct result of photoautotrophic growth, and is dependent upon light and the availability of inorganic nutrients; it is apparently enhanced by moderately high water temperatures, high pH, low oxygen tensions and possibly, the presence of organic solutes. The relative effectiveness of these factors is untested. 3. Buoyancy is imparted by gas vacuoles whose principal function is to regulate the position of the alga in the water column. Control is effected by two mechanisms: (i) ‘dilution’ of newly produced vacuoles during active cell division; (ii) changes in cell turgor‐pressure acting on the gas‐vacuole structure. Gas‐vacuole production is greatest at low light intensities and the alga becomes more buoyant; at higher light intensities, increased turgor‐pressure collapses the weaker vacuoles causing the alga to lose buoyancy. 4. Potentially, algae are able to poise themselves at an optimum point in the light gradient, usually towards the bottom of the euphotic zone, where the algae are likely to encounter the conditions most favouring their growth. 5. Different species of blue‐green algae differ in the typical sizes of their colonies and, hence, in their rates of controlled movement. These differences are interpreted as hydrodynamic adaptations to the variations in turbulent water movements to which the algae are subject. 6. Populations of single‐filamentous Oscillatoria agardhii and O. rubescens come to occupy the stable metalimnia of stratified lakes, provided that they are located within the euphotic zone. 7. The large stream‐lined colonial forms occur mainly in polymictic lakes and in the unstable epilimnia of stratified lakes where light penetration is restricted to the superficial layers. These algae are adapted to sink or float rapidly to the optimum depth when turbulence subsides. Because of their potentially high rates of movement, it is the large colonial forms that commonly form blooms. 8. Bloom formation can occur when most of the algae possess excess buoyancy. Excess buoyancy is acquired when the photosynthetic rate is insufficient to develop the necessary turgor‐pressure to cause collapse of the vacuoles. Photosynthesis may be sufficiently impaired under four circumstances: (i) during turbulent circulation of the population over a depth that significantly exceeds the euphotic depth; (ii) in the absence of light (e.g. at night): (iii) at limiting concentrations of carbon dioxide: and (iv) when the algal population is senescent. 9. Because bloom‐formation depends upon the coincidence of persistent algal overbuoyancy with calm weather, its occurrence is incidental, and serves no vital function in the biology of blue‐green algae. 10. Some possible causes for the occurrence of blue‐green algal blooms in a relatively restricted range of water bodies are discussed. Large bloom‐forming populations are probably restricted to moderately rich, mildly alkaline, thermally unstable lakes in all regions, except those which are permanently cold. Extremes of poverty or richness of nutrients, short water‐retention times and low pH seem to be factors which select against planktonic blue‐green algae.