SEASONAL DYNAMICS OF DAPHNIA AND ALGAE EXPLAINED AS A PERIODICALLY FORCED PREDATOR-PREY SYSTEM

The classical seasonal sequence of events in lake plankton is an early spring bloom of algae followed by a peak of zooplankton, typically of the genus Daphnia, grazing down algal biomass resulting in the spring clear-water phase around early June. Subsequently, Daphnia numbers us ually remain low and algal biomass increases until autumn when a secon d small Daphnia peak can cause another drop in algal biomass. In this paper we show that this entire scenario with the right timing of event s comes out naturally from a minimal model of the Daphnia-algae intera ction if we simply vary the parameters of the model sinusoidally over the year to mimic the effect of seasonal variation in temperature, lig ht and predation pressure from planktivorous fish. The model also repr oduces the observation that in lakes where fish biomass is low, Daphni a peaks can continue to occur regularly throughout the summer, while i n lakes with very high fish biomass Daphnia can be virtually absent th roughout the season.