Competition of phytoplankton under fluctuating light

Light is an essential resource for phytoplankton and fluctuates on a wide r ange of timescales. To understand how light fluctuations affect phytoplankt on community structure and diversity, we have studied a set of simple model s using a combination of analytical and numerical techniques. Light fluctua tions can affect community structure when species exhibit the gleaner-oppor tunist trade-off between competitive ability and ability to reach carrying capacity quickly. Fast fluctuations can switch the competitive dominant fro m a gleaner to an opportunist; slow fluctuations can cause this switch and also lead to stable coexistence. Coexistence is easiest between species tha t are highly differentiated along the gleaner-opportunist trade-off. Our re sults remain qualitatively unchanged when more realistic light fluctuations such as daily and seasonal changes in irradiance and the presence of a spa tial gradient in light are considered. Seasonal change in day length may be one of the factors driving the seasonal succession of phytoplankton, from opportunist species dominant under shorter day lengths (spring and autumn) to gleaner species dominant under longer day length (summer). These results illustrate how resource fluctuations can have an important role in structu ring ecological communities.