Growth, gas vacuolation and buoyancy in a natural population of a planktonic blue-green alga

(1)The growth and development of a natural population of Anabaena circinalisis described in relation to physical and chemical conditions within a small, shallow eutrophic lake. (2)Germination of resting spores took place when the lake was destratified, in response to improving conditions of light and temperature. (3)The growth of the population was monitored for chlorophyll and nutrient content, and parallel determinations of heterocyst ratio, gas vacuole volume, turgor and buoyancy were made. (4)Growth of the population was probably limited by phosphorus deficiency. (5)Cessation of growth was marked by increases in gas vacuole volume and buoyancy. (6)Gas vacuole volume is determined primarily by the rate of increase of the alga. It is modified by turgor changes, associated with photosynthetic rates. (7)Surface blooms are the result of the redistribution of buoyant algae when turbulence currents are weak. (8)Buoyancy is increased in algae lodged at the surface.