A COMPUTER-MODEL OF BUOYANCY AND VERTICAL MIGRATION IN CYANOBACTERIA

We describe a computer model that simulates diurnal changes in buoyancy and vertical migration by planktonic cyanobacteria in lakes. The model is based on new measurements of the rates of buoyant density change by cyanobacteria in laboratory cultures. The overall rate of density change was found to be the result of two simultaneous processes: (i) a light-dependent increase in density, supported by photosynthesis; and (ii) a time-dependent decrease related to the previous light history. Using the experimentally determined rate constants of these processes, the computer model calculated how buoyant density changes of a colony, responding to the irradiance at a given depth and time of day, would affect its sinking or floating velocity. The model makes a number of interesting predictions on vertical migration, (i) Colony size affects the speed but not the depth range of vertical migrations, (ii) An increase in the proportion of cell volume in a colony has a similar effect to that of colony size, (iii) Surface blooms can be dynamic, with colonies floating up replacing colonies sinking out of the surface bloom, (iv) Phosphorus-limited colonies are likely to migrate to greater depths than light-limited colonies, enabling them to exploit the nutrient-rich depths. The vertical migration of a well-documented natural population is simulated. The model gives a good description of the buoyancy behaviour of the cyanobacterium in the lake. Other simulations explain why colony-forming cells cannot form a stratified layer, while single filaments can. The ecological implications of colony formation are discussed. © 1990 Oxford University Press.