Interrelations of photosynthetic behaviour and buoyancy regulation in a natural population of a blue-green alga

The photosynthetic activity of Anabaena cirdnalis and associated changes in buoyancy were determined from prepared suspensions exposed in the natural light field of Crose Mere. The observations are related to variations in subsurface irradiance and temperature. Parallel experiments, aimed at trapping algal colonies undertaking controlled vertical movements within the lake system, are also described. Buoyancy loss and downward migration are clearly associated with specific photosynthetic rates: rates as low as 1.8 mg O₂ (mg chlorophyll a) h^?1 are shown to be sufficient to effect buoyancy loss, while movements in the lake tend towards a depth where rates of 5–7 mg O₂ (mg chlorophyll a)^?1 h^?1 are possible. These rates are significantly less than those possible at light saturation. The effect of increasing temperature is to depress the population in the light-gradient. The significance of this response is discussed in relation to the growth of natural populations of blue-green algae.