Competition and coexistence of phytoplankton under fluctuating light: experiments with two cyanobacteria

The effects of light fluctuations on phytoplankton competition were examined in the experiments with 2 freshwater cyanobacteria, Anabaena flos-aquae and Phormidium luridum. Light regimes had the same average irradiance of 50 mumol photons m(-2) s(-1) and included constant light, high-low light fluctuations of 8 and 24 h periods and light:dark fluctuations of a 24 h period. A mechanistic model of light competition was used to predict competitive outcomes under these light regimes. The parameter values were obtained experimentally for constant light conditions. In the experiments, A. flos-aquae was rapidly excluded under constant light but persisted under fluctuating light. The model predicted well the dynamics and the outcome of competition under constant light but performed poorly under fluctuating light. The results indicate that light fluctuations may change the dynamics and outcome of competition and slow competitive exclusion and also that competition under fluctuating light cannot necessarily be predicted from the constant light monoculture dynamics.