A population of buoyant colonies of Microcystis sp. dominated the phytoplankton of the small Vinkeveen Lake, in the Netherlands. The Microcystis colonies were circulated within the surface mixed layer whose depth (usually 2 to 4 m) usually exceeded the euphotic depth (usually less than 1.5 m) in this wind-exposed lake. Colonies held in bottles at the lake surface responded to the high irradiance by losing their buoyancy, while those kept at greater depths remained buoyant. For colonies circulating within the surface layer the degree of buoyancy loss that occurred during the day depended on the mean irradiance to which they were exposed, which could be determined from the average surface irradiance, the mixed depth and the vertical extinction coefficient. Buoyancy loss was also correlated, but less strongly, with the proportion of the daytime spent by colonies within the euphotic zone. Surface scums were usually prevented from forming by wind mixing.