Anabaena circinalis blooms are a frequent occurrence in freshwaters throughout Australia, spanning tropical to cool temperate regions. These paralytic shellfish toxin producing blooms pose serious management and health issues, with blooms sometimes extending for over 1000 km and with stock deaths associated with blooms. In order to understand and therefore better manage A. circinalis blooms, we are investigating factors regulating the bloom dynamics, and in particular the role of akinetes in blooms. Akinetes have been presumed to have a resistant resting stage inherent in the bloom dynamics due to their capacity to survive environmental conditions which are deleterious to vegetative cells and because of their ability to form germling cells. We are using cultured strains isolated from blooms representing the diversity of biogeographic regions in Australia. We have found that akinete formation is not a ubiquitous strain characteristic, and that the effect of potential environmental triggers such as nutrients, light and temperature on akinete production do not demonstrate that akinete differentiation is promoted by environmental extremes as a response to stress. However, field data of bloom populations show a strong relationship between bloom density and proportion of akinetes and proportion of trichomes containing akinetes. This matches observations of akinete differentiation in late logarithmic phase in culture and introduces the possibility of stimulation due to biological factors such as increasing cell density and interaction. The relative significance on akinete production of genetically determined strain / population characteristics, the effect of environmental factors, and biological interactions and the implications for A. circinalis bloom dynamics will be discussed.