Summary• Analyses were made to determine which changes in a Lake Zürich population of Planktothrix rubescens were dependent on light-and temperature-dependent growth rates, and when growth was limited by the mixing depth.• Changes in vertical distribution of the cyanobacterium, determined weekly from August 1998 to September 1999, were related to growth increments calculated at 1-h time and 1-m depth intervals from values of irradiance, attenuance, temperature and biomass in the lake, using algorithms based on growth rates in culture.• Population biovolume varied annually from 1.2 to 120 cm 3 m −2 . During summer, modelled growth in the metalimnion matched the 50-fold population increase. Modelled growth exceeded the observed increase when Planktothrix was mixed into the nutrient-depleted epilimnion, suggesting nutrient limitation. The measured increase ceased when the mixed depth exceeded the critical depth for growth in autumn (Sverdrup's principle). Light limitation explained the gradual decrease of the population in winter. The steep decline in spring had other causes.• Population changes were largely determined by interactions of light and depth distribution; decreases in nutrient loading have had little impact on Planktothrix growth in Lake Zürich.

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The vertical distribution of the cyanobacterium Planktothrir (Oscillazoria) rubescens in Lake Zürich 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 cm −3 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 Zürich 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