Diurnal changes in buoyancy and vertical distribution in populations of Microcystisin two shallow lakes

Changes in the buoyancy of Microcystis populations were followedover 24 h periods in two shallow well-mixed lakes, Lake Vinkeveen(area 0 6 km²) and Lake IJsselmeer (1190 km²), in the NetherlandsThe Microcystis colonies collected from the surface layers inboth lakes showed a buoyancy decrease during the day and anincrease at night The buoyant colonies, and especially the faster-movinglarge ones, became concentrated by flotation into the surfacemixed layers As a result the mean position of the cyanobacterialpopulation became located nearer the surface than that of othernon-buoyant phytoplankton, such as Scencdesmus. The cyanobacteriawould, therefore, have received a higher average irradianceThe Microcystis in these shallow lakes had weaker gas vesiclesthan those found previously in deeper lakes but it was demonstratedthat the loss of buoyancy, which occurred at high irradiances,resulted from an increase in carbohydrate ballast rather thanthrough turgoi-driven gas vesicle collapse