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

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 (P) were measured in cultures under 12 h : 12 h light : dark cycles at 15 irradiances (E) in temperatures (Theta) of 10-25degreesC. Photosynthetic rates (P) were measured under the same conditions. For P. rubescens, p reached a maximum of 0.33 d(-1) at 25degreesC in photon irradiances >40 mumol m(-2) s(-1) and exceeded mu for P. agardhii over the range of temperatures in Blelham Tarn (<21°C), although not at temperatures >25degreesC. In P. rubescens, the difference (Deltamu) between the growth rate of cell carbon (mu(C)), calculated from P, and mu was only 3% at 10degreesC but increased with temperature to 30% at 25degreesC; in P. agardhii, Deltamu 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 Theta 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.