The daily integral of photosynthesis by Planktothrix rubescens during summer stratification and autumnal mixing in Lake Zürich

Abstract

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 Zu$ rich 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 9n5. 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.

The daily integral of growth by Planktothrix rubescens calculated from growth rate in culture and irradiance in Lake Zürich

“…4). On this sunny day, when the photon insolation was at the highest value in the period studied (50 mol m −# , equivalent to 58 mol m −# in Micheletti et al, 1998), the irradiance was sufficient to support growth at depths down to 19.7 m at midday (between 11 : 00 and 13 : 00 hours).…”

“…3. The plot of growth rate (µ) vs irradiance (I ) (given in the 12-h light period of each diel cycle) was of similar form to the photosynthesis\irradiance (P\I ) curve for Planktothrix (Micheletti et al, 1998), showing light limitation at low irradiance and light saturation at high irradiance. In the first set of measurements the maximum irradiance investigated was 68.7 µmol m −# s −" , which was expected to produce inhibition of growth in this organism, associated as it is with deep, shaded habitats.…”

“…8 we compare the changes in biomass of the Planktothrix population measured by sampling the lake population (Micheletti et al, 1998) with the biomass increase calculated for 7-d periods with ' grow-int.xls '. The calculated increase due to growth should usually equal or exceed the observed increase, which might not achieve the light-limited growth rate owing to nutrient limitation or might be subject to additional losses through predation, parasitism and lysis.…”

“…In a previous paper we compared the biomass increase and photosynthesis of a population of Planktothrix rubescens in the metalimnion of Lake Zu$ rich (Micheletti et al, 1998) over a period of 136 d (10 July-22 Nov 1995). The increase in the population was determined by integration of the filament volume concentration at different depths.…”

“…Calculations of the daily integrals of growth and photosynthesis of the P. rubescens population on each day from 10 July (Day 190) to 22 November (Day 325) of 1995 in Lake Zu$ rich were made using data collected by Micheletti et al (1998) from the ratio of 2.116 µmol J −" obtained with the LI-COR sensor used previously by Micheletti et al (1998) ; the photon irradiance values at the lake used here are therefore 0.8705 of those in Micheletti et al (1998).…”