Abstract

Fogg and Walsby’s (1971) hypothesis that buoyancy regulation in cyanobacteria might be an adaptation to exploit the separation of light and nutrients has since become a paradigm. The evidence of its veracity is examined within observations of algal abundance and chlorophyll distributions in several Australian freshwater systems and is also reviewed from the literature. It is clear from both laboratory experiments and field measurements that filamentous genera such as Anabaena and colony-forming genera such as Microcystis are capable of changing their buoyancy within a diurnal cycle. However, evidence for population migration to exploit separation of light and nutrient availability is tenuous, with most field observations of the vertical distribution of phytoplankton populations showing no evidence of vertical migration to sufficient depth to reach nutrients in stratified systems.

Instead, changes in the vertical distribution of phytoplankton suggest a response either to the dynamics of the surface mixed layer or to lateral advection. In natural systems, algal buoyancy appears to be dependent much more on light than on nutrients, this being consistent with the carbohydrate ballast mechanism. Physical mechanisms can provide sufficient replenishment of epilimnetic nutrients to explain the observed net growth rates of phytoplankton populations in situ.