Growth rates of phytoplankton under fluctuating light

1. The effect of light fluctuations on the growth rates of four species of freshwater phytoplankton was investigated. Experimental light regimes inclu ded constant irradiance and fluctuations of a step function form, with equa l proportion of high (maximum of 240 mu mol photons m(-2) s(-1)) and low li ght (minimum of 5 mu mol photons m(-2) s(-1)) (or dark) in a period. Fluctu ations of 1, 8 and 24-h periods were imposed over several average irradianc es (25, 50, 100 and 120 mu mol photons m(-2) s(-1)). 2. Growth rate responses to fluctuations were species-specific and depended on both the average irradiance and the period of fluctuations. Fluctuation s at low average irradiances slightly increased growth rate of the diatom N itzschia sp. and depressed growth of the cyanobacterium Phormidium luridum and the green alga Sphaerocystis schroeteri compared to a constant irradian ce. 3. Fluctuations at higher average irradiance did not have a significant eff ect on the growth rates of Nitzschia sp. and Sphaerocystis schroeteri (fluc tuations around saturating irradiances) and slightly increased the growth r ates of the cyanobacteria Anabaena flosaquae and Phormidium luridum (when i rradiance fluctuated between limiting and inhibiting levels). 4. In general, the effect of fluctuations tended to be greater when irradia nce fluctuated between limiting and saturating or inhibiting levels of a sp ecies growth-irradiance curve compared to fluctuations within a single regi on of the curve. 5. The growth rates of species under fluctuating Light could not always be predicted from their growth-irradiance curves obtained under constant irrad iance. When fluctuations occur between limiting and saturating or inhibitin g irradiances for the alga and when the period of fluctuations is long (gre ater than 8 h), steady-state growth-irradiance curves may be insufficient t o predict growth rates adequately. Consequently, additional data on physiol ogical acclimation, such as changes in photosynthetic parameters, may be re quired for predictions under non-constant light supply in comparison to con stant conditions.