Comparison of measured growth rates with those calculated from rates of photosynthesis in Planktothrix spp. isolated from Blelham Tarn, English Lake District

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Author for correspondence: A. E. Walsby Tel. +44 (0)117 9287490 Email: a.e.walsby@bristol.ac.uk

Summary

  • • Differences in photosynthetic production and conversion to biomass of the red-coloured cyanobacterium Planktothrix rubescens and the green-coloured Planktothrix agardhii , were investigated in relation to their growth in Blelham Tarn, UK, using clonal isolates from the lake.
  • • Growth rates (µ) were measured in cultures under 12 h : 12 h light : dark cycles at 15 irradiances ( E ) in temperatures (Θ) of 10–25°C. Photosynthetic rates ( P ) were measured under the same conditions.
  • • For P. rubescens , µ reached a maximum of 0.33 d −1 at 25°C in photon irradiances > 40 µmol m −2  s −1 and exceeded µ for P. agardhii over the range of temperatures in Blelham Tarn (< 21°C), although not at temperatures > 25°C. In P. rubescens , the dif ference (Δµ) between the growth rate of cell carbon (µ C ), calculated from P , and µ was only 3% at 10°C but increased with temperature to 30% at 25°C; in P. agardhii , Δµ values were higher at low temperatures and lower at the higher temperatures.
  • • Using algorithms describing the irradiance- and temperature-dependent growth rates and measured values of E and Θ at different depths in Blelham Tarn, it was demonstrated that P. rubescens would outgrow P. agardhii , though the latter might grow better in warmer and shallower lakes. We discuss the problems of modelling phytoplankton growth from measurements of in situ photosynthesis.

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