In Lake Zürich, populations of the cyanobacterium Planktothrix rubescens develop in the metalimnion during the summer and become gradually entrained in the deepening surface mixed layer during the autumn. It had previously been demonstrated that the daily integrals of photosynthetic production accounted for the growth observed in the metalimnion and greatly exceeded the smaller increase during the autumn. We have now determined the relationship between growth rate (μ) and irradiance (I) in cultures of P. rubescens strain Pla 9316 maintained at 20 °C on a 12[ratio ]12 h light[ratio ]dark cycle: the highest net growth rate averaged over the 24 h (μh) was 0.123 d−1, the dark rate (μD) was −0.020 d−1, the gross rate (ϕm = μh−μD) was 0.144 d−1, the affinity coefficient (α) was 0.0273 (d μmol m−2 s−1)−1 and the compensation point (IC) was 1.76 μmol m−2 s−1. Using the corresponding coefficients calculated for the light period (μLh = 0.267 d−1, ϕLm = 0.287 d−1 and αL = 0.0547 (d μmol m−2 s−1)−1), instantaneous growth rates could be calculated from the irradiance. Comparison with growth rates at 10 °C indicated a Q10 of 1.48. These coefficients were used in a modification of the Smith equation to calculate potential growth rates of Planktothrix from the irradiance and temperature at each time and depth in Lake Zürich. Data on irradiance, vertical light attenuation and temperature were used to calculate the daily integrals of biomass increase over a period of 136 d. These growth integrals gave a closer correspondence to the observed population increase than the photosynthetic integrals calculated previously from measurements made with lakewater samples dominated by Planktothrix. Photosynthetic measurements made with the Planktothrix culture indicated a maximum rate of carbon increase (0.467 d−1 that exceeds the maximum growth rate, which suggests that other factors limit growth over long periods.

It has been suggested that the populations of planktonic cyanobacteria that occupy the metalimnion of stratified lakes during the summer months may be aestivating between the main periods of growth during entrainment in the epilimnion in spring and summer. We determined the vertical distribution of the biomass and daily integral of photosynthesis of the population of Planktothrix (Oscillatoria) rubescens in Lake Zürich for 136 d from July to November 1995. The population showed an 80-fold increase during the stratified period but it only doubled over the subsequent period of entrainment. During the first eight days, part of the increase was attributed to recruitment of filaments floating up from greater depths but all of the subsequent production could be accounted for by photoautotrophic growth. On sunny days the biomass-specific photosynthesis of this population reached some of the highest values over the whole period despite its depth (>13 m). On very cloudy days, however, primary productivity was very low and on 4 days, when the mean depth of the population exceeded 15 m, there was no net production. Over the whole period of the study, the accumulated photosynthetic production exceeded the increase in biomass of the population by a factor of 9·5. Although much of this production occurred during the period of entrainment only a small proportion was translated into growth of the population. It is concluded that the growth that takes place in the period of stratification in the metalimnion is essential to subsequent production.

Secchi depths recorded in Lake Zürich are determined mainly by concentrations of autochtonous material such asplankton and other material related to biomass production. The surface contrast transmittance factor has a negligibleeffect on a Secchi depth of up to 7 m and is of minor importance up to 13 m. Alterations in transparency values after1970 as a result of the reduced nutrient input were most pronounced in the autumn-winter period (October 28 to February19) as well as in the clear water phase (April 29 to July 29). After 1970, almost all buildings in the Lake Zürich drainagearea were connected to a sewage treatment plant with a three stage purification. In the autumn-winter period, the Secchidepth is related to the development of Oscillatoria rubescens filaments and is not correlated with changes in trophicstate. The restoration measures led to a statistically significant increase in the Secchi depth during the clear water phase,and additionally to a prolongation of the period with the highest tranparency values (May 20 to July 4). The Secchitechnique is simple but seems to be valuable for recognizing long-term changes in the optical properties of a water body.