Population and community responses of phytoplankton to fluctuating light

Light is a major resource in aquatic ecosystems and has a complex pattern o f spatio-temporal variability, yet the effects of dynamic light regimes on communities of phytoplankton are largely unexplored. I examined whether and how fluctuating light supply affects the structure and dynamics of phytopl ankton communities. The effect of light fluctuations was tested at two aver age irradiances: low, 25 mu mol quanta m(-2) s(-1) and high, 100 mu mol qua nta m(-2) s(-1) in 2- and 18-species communities of freshwater phytoplankto n. Species diversity, and abundances of individual species and higher taxa, depended significantly on both the absolute level and the degree of variab ility in light supply, while total density, total biomass, and species rich ness responded only to light level. In the two-species assemblage, fluctuat ions increased diversity at both low and high average irradiances and in th e multispecies community fluctuations increased diversity at high irradianc e but decreased diversity at low average irradiance. Species richness was h igher under low average irradiance and was not affected by the presence or absence of fluctuations. Diatom abundance was increased by fluctuations, es pecially at low average irradiance, where they became the dominant group, w hile cyanobacteria and green algae dominated low constant light and all hig h light treatments. Within each taxonomic group, however, there was no unif orm pattern in species responses to light fluctuations: both the magnitude and direction of response were species-specific. The temporal regime of lig ht supply had a significant effect on the growth rates of individual specie s grown in monocultures. Species responses to the regime of light supply in monocultures qualitatively agreed with their abundances in the community e xperiments. The results indicate that the temporal regime of light supply m ay influence structure of phytoplankton communities by differentially affec ting growth rates and mediating species competition.