The interactions of size, shape, and density of cyanobacteria result in a 5‐order of magnitude difference in flotation or sinking rates which, in turn, influence the extent of their dispersion in turbulent water masses. Active mixing through resource‐replete waters of high clarity favours fast‐growing, small‐celled species. Where photosynthetically active radiation is severely attenuated through the wind‐mixed layer, species may rely on turbulent entrainment but must be adapted toward efficient light harvesting (morphological attenuation, enhanced pigmentation). In both strongly segregated waters (light‐ and nutrient‐rich layers separated vertically) and waters experiencing high‐frequency fluctuations in vertical mixing and optical depth, emphasis is placed on the ability to make rapid, buoyancy‐adjusted vertical movements, favoured by large size. The cyanobacterial 1ife‐forms respectively typical of these contrasted limnological systems — unicellular coccoids (e.g.,Synechococcus), solitary filaments (e.g.,Oscillatoria) and colonial forms (e.g.,Microcystis) — illustrate the diversity of evolutionary adaptations to be discerned among the planktonic cyanobacteria and which contributes to their reputation as a prominent and successful group of organisms.