Buoyancy of Phaeocystis globosa Scherffel (Prymnesiophyceae) colonies was investigated by measuring the
vertical distribution of colonies in quiescent water where convection had been removed. Over 60% of the colonies
exhibited negative buoyancy regardless of light condition or growth phase. Positively and neutrally buoyant colonies lost
their buoyancy in the dark, but regained buoyancy upon return to the light. Colonies with closer cell packing; i.e., more
cells per unit colonial surface area, had greater capability to remain buoyant. Our results suggest that colony
buoyancy was not uniform within a P. globosa population, and that biological regulation of colony buoyancy
required light energy.