Is buoyancy regulation in cyanobacteria an adaptation to exploit separation of light and nutrients?

Fogg and Walsby's (1971) hypothesis that buoyancy regulation in cyanobacter ia might be an adaptation to exploit the separation of light and nutrients has since become a paradigm. The evidence of its veracity is examined withi n 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 filam entous genera such as Anabaena and colony-forming genera such as Microcysti s are capable of changing their buoyancy within a diurnal cycle. However, e vidence for population migration to exploit separation of light and nutrien t availability is tenuous, with most field observations of the vertical dis tribution of phytoplankton populations showing no evidence of vertical migr ation to sufficient depth to reach nutrients in stratified systems. Instead, changes in the vertical distribution of phytoplankton suggest a re sponse either to the dynamics of the surface mixed layer or to lateral adve ction. In natural systems, algal buoyancy appears to be dependent much more on light than on nutrients, this being consistent with the carbohydrate ba llast mechanism. Physical mechanisms can provide sufficient replenishment o f epilimnetic nutrients to explain the observed net growth rates of phytopl ankton populations in situ.