Cyanophyte blooms: The role of cell buoyancy1

Abstract

A one‐dimensional model of growth diffusion and scaling arguments show that bloom formation in epilimnetic cyanophytes is attributable to the passive mechanism of positive buoyancy of the cells, which enhances the average exposure of the population to light. The effects of interacting turbulent diffusion, photic depth, mixed‐layer depth, and diurnal mixed‐layer excursions on population growth rate and biomass production differ radically for positively and negatively buoyant cells. Over the typical range of sinking and rising velocities for lake phytoplankton and the characteristic velocities of turbulent mixing, the proportion of the maximum achievable growth rate attained is always greater for a positively buoyant species until significant self‐shading occurs. The magnitude of the difference is determined by the ratio of photic depth (zp) to mixed‐layer depth (z,); the advantages of positive buoyancy decrease as zp approaches zm. Results furthermore suggest that a positively buoyant population is able to track diurnal mixed‐layer excursions and thus to experience a more favorable average daily zp : zm ratio than is possible for a negatively buoyant population.