Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convection is unspecified, convection due to the effects of thermal expansion and buoyancy can be assumed. Convection may also take place in soft solids or mixtures where particles can flow. (wikipedia)
Instead of being evenly distributed in the culture medium, clouds or bands run through the culture. Euglena gracilis is ideal for illustrating this pattern formation (Lebendkulturen on bioconvection)