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The vertical distribution of the cyanobacterium Planktothrix (Oscillatoria) rubescens in Lake Zurich was investigated from March 1993 to June 1995 by collecting filaments on filters and measuring them by epifluorescence microscopy and computer image analysis. The initial population, which began to stratify in April, decreased by up to 99% by June. During the summer, the population peaked at depths of 8—15 m; it reached a maximum areal filament-volume concentration of -60 cm3 m~2 of lake surface in early September and was then entrained in the deepening surface layer. It became mixed progressively deeper, to the lake bottom in the cold winter of 1993-94, but less completely in the milder winter of 1994-95. Most of the filaments remained viable during the winter. At the end of the mild winter of 1994-5, 70% of filaments in the water column retained buoyancy, but after the cold winter of 1996-7 only 22% were buoyant. Few remained buoyant below 80 m, where the hydrostatic pressure caused gas vesicle collapse. The proportion that remain buoyant decreases with the depth and duration of winter mixing, and increases with the critical collapse pressure (pc) of the gas vesicles, which provide buoyancy. Strains of P.rubescens isolated from Lake Zurich differed in mean pc of their gas vesicles, from 0.9 to 1.1 MPa, the highest values in freshwater cyanobacteria. Allowing for a turgor pressure of 0.2 MPa, these strains would remain buoyant at depths down to 70 and 90 m, respectively. Natural selection for gas vesicles of high pc will operate by increasing the proportion of filaments that remain buoyant in the upper parts of the water column after circulation to various depths during the winter because only buoyant filaments will form the inoculum for the following season.

The critical pressures of gas vesicles in Planktorhrix rubescens in relation tothe depth of winter mixing in Lake Zürich, Switzerland