Comparison of the depth where Planktothrix rubescens stratifies and the depth where the daily insolation supports its neutral buoyancy

• Comparisons were made of the depths where the cyanobacterium Planktothrix rubescens stratified in lakes and the depths where the daily insolation supported neutral buoyancy of the organism. • The percentage of filaments floating and sinking were measured in cultures grown under light : dark cycles at different irradiances. Qn, the daily insolation for neutral buoyancy, was determined as that at which 50% of the filaments were floating, averaged over the 24-h period. • For P. rubescens 9316 from Lake Zürich, Qn was 0.28 mol m−2; during the summer period of stratification in Lake Zürich, the neutral buoyancy depth (zn) at which this insolation occurred varied between 5 and 14 m, depending on cloud cover and light attenuation. The weekly mean depth of the Planktothrix population maximum (zp) was usually within 1 m of the mean zn. For P. rubescens 9972 from Blelham Tarn, Qn was 0.51 mol m−2; during the stratified period in Blelham Tarn, zn varied between 2.3 and 6.2 m, also similar to zp; the shallower stratification depth is explained by the steeper light attenuation. • The depth at which Planktothrix stratifies in lakes is therefore explained by buoyancy regulation in relation to the irradiance. Metalimnetic stratification occurs only when zn exceeds the mixed depth, zm; when zm > zn, the organism becomes entrained in the surface mixed layer.