Light-dependent growth rate determines changes in the population of Planktothrix rubescens over the annual cycle in Lake Zürich, Switzerland.

• 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  cm3  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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