Light is a major resource for phytoplankton and has a complex pattern of spatial and temporal variability, yet little is known how temporal variability in light supply affects populations and communities of phytoplankton. This study investigates the effects of fluctuating light on phytoplankton, with an emphasis on species competition and coexistence, through experiments with two- and multispecies communities, individual species and modeling.

The temporal distribution of light supply had a significant effect on absolute and relative abundances of species, dominance patterns and diversity in both two- and multispecies communities. Diatoms were the dominant group under low fluctuating light, while cyanobacteria and green algae had higher abundances at higher average irradiance and constant light supply. Under high average irradiance, fluctuations increased species diversity compared to constant light.

Experiments with individual species demonstrated that growth rate responses of phytoplankton to light fluctuations were species-specific and depended on the average irradiance, period and amplitude of fluctuations. Fluctuations had a greater effect if they occurred between limiting and saturating or inhibiting light levels, i.e., over a nonlinear region of the growth-irradiance curve of a species. Growth rates under fluctuating light could be adequately predicted from the steady state growth irradiance curves, except for long fluctuation periods (T > 8 h) or when fluctuations occurred over a nonlinear part of the curve. Species responses in monocultures agreed with their responses in communities.

In a long-term competition experiment with two cyanobacteria (Anabaena flos-aquae and Phormidium luridum), fluctuating light, including light:dark daily cycle, promoted persistence of both competitors; under constant light Anabaena was competitively excluded.

The effects of light fluctuations on phytoplankton competition and coexistence were investigated further through modeling. As analytical and simulation results suggest, fluctuations over a wide range of temporal scales (from hourly to seasonal) may slow competitive exclusion, reverse the competitive outcome reached under constant light or allow stable coexistence.

This study demonstrates that temporal variability in light supply may have a significant effect on the structure and dynamics of phytoplankton communities by differentially affecting growth rates of species and mediating competitive interactions.