During summer stratification, Crooked and Little Crooked Lakes, Indiana, contained phytoplankton dominated by populations ofAphanizomenonj7o.vaquae that stratified in the metalimnion where photon irradiance was 6% of that at the surface. The gas vacuolate Aphanizomenon filaments from Little Crooked Lake lost buoyancy when exposed to irradiances > 100 pmol photons m-2 s-l. If 2 PM KH,PO, was added to the water samples, however, exposure to irradiances >200 pmol photons m -2 s-l was necessary to obtain buoyancy losses. Phosphate also affected the recovery of buoyancy by sinking filaments. Filaments that had lost buoyancy due to incubation in the light recovered it when incubated for 5 h in the dark. Addition of phosphate increased the rate of buoyancy recovery by 50%. The metalimnetic populations had stratified in a zone of relatively low irradiance and inorganic nutrient concentration. The physiological state of the populations was evaluated by determining the kinetic characteristics for phosphate, nitrate, and ammonia uptake and the cellular contents of surplus phosphate and chlorophyll a. On the basis of the premise that cells limited by a particular element will express a high uptake potential for that element, the metalimnetic populations showed characteristics of organisms not limited by the availability of light, phosphorus, or nitrogen.