Growth-rate responses of Volvox aureus Ehrenb. (Chlorophyta,Volvocales) to variability in the physical environment

An analysis of the dynamic responses of a population of Volvox aureus to artificially-imposed alternations in mixing and stratification in a large limnetic enclosure (“Lund Tube” C, Blelham Tarn) is presented. The rates of population increase are shown to be regulated largely through the propagation of daughter colonies that are subsequently released into the medium. The efficiency of propagation is measured in terms of the numbers of developing daughter colonies in parent colonies at any time and the time required for their maturation prior to release. Efficiency of propagation increased when the vegetative population was unmixed and decreased when it was mixed through an optically-deeper water column. Self-regulation of vertical position in the depth profile also influences propagation and, hence, the rate of population increase.