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Aquatic Sciences

, Volume 63, Issue 3, pp 326–349 | Cite as

The effects of diel changes in photosynthetic coefficients and depth of Planktothrix rubescens on the daily integral of photosynthesis in Lake Zürich

  • A. E. Walsby
  • Z. Dubinsky
  • J. C. Kromkamp
  • C. Lehmann
  • F. Schanz

Abstract:

In late summer and autumn, before the vertical circulation reaches the thermocline, the phytoplankton population of Lake Zürich is dominated by the red-coloured filamentous cyanobacterium Planktothrix rubescens, which stratifies in the metalimnion at depths close to the photosynthetic compensation point. The filament volume concentration reached a maximum of 12 cm3 m-3; the depth of the maximum varied from 10.5 to 12.5 m. Changes in the depth distribution were attributed to a combination of (1) seiche movements, which raised or lowered the thermocline by up to 2 m over 36 h, and (2) flotation by the buoyant filaments relative to the isotherms, by up 0.4 m d-1. These changes caused a 2-fold change in insolation at the Planktothrix peak. Estimates were made of the daily integral of photosynthetic O2-production, ΣΣ(NP), by the population of P. rubescens over a period of four cloudless days. The estimates were calculated from measurements of surface irradiance (at 5-min intervals), vertical light attenuation, temperature, filament volume concentration and the photosynthesis/irradiance (P/I) curves of filaments concentrated from the metalimnion. Despite the similar, high insolation on each of the four days, the calculated values of ΣΣ(NP) varied from 9 to 53 mmol m-2 d-1, owing to the changing depth distribution of the filaments. Measurements of P/I curves of lakewater samples incubated at a depth of 11 m showed changes in the photosynthetic coefficients during the day. These also generated large changes in calculated values of ΣΣ(NP). The computer spreadsheet used to calculate ΣΣ(NP) was modified to incorporate time-based changes in the photosynthetic coefficients and vertical distribution of the organism. These refinements provide a more accurate description of photosynthesis by the deep-living P. rubescens, which adjusts its position by buoyancy regulation to exploit the light field in the metalimnion, where it outcompetes other phytoplankton.

Key words: Lake Zürich, stratification, Planktothrix rubescens, diel photosynthesis changes. 

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Copyright information

© Birkhäuser Verlag, 2001

Authors and Affiliations

  • A. E. Walsby
    • 1
  • Z. Dubinsky
    • 2
  • J. C. Kromkamp
    • 3
  • C. Lehmann
    • 4
  • F. Schanz
    • 4
  1. 1.School of Biological Sciences, University of Bristol, Bristol BS8 1UG, United KingdomGB
  2. 2.Faculty of Life Sciences, Bar-Ilan University, Ramat-Gan 52100, IsraelIL
  3. 3.NIOO-Centre for Estuarine and Coastal Ecology, PO Box 140, NL-4400 AC Yerseke, The NetherlandsNL
  4. 4.Institut für Pflanzenbiologie der Universitat Zürich, Limnologische Station, Seestrasse 187, CH-8802, Kilchberg, SwitzerlandCH

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