The effect of food limitation on vertical migration in Daphnia longispina

Diel vertical migration of a natural population of Daphnia longispina was investigated experimentally in enclosures twice in 1985. The amount of available food was manipulated and vertical distributions were recorded at times of low (night) and high (day) risk of predation from fish. In the first experiment, the large D. longispina displayed diel migrations. When food was depleted experimentally, however, the large animals were near the surface during both day and night. In the second experiment, large D. longispina first began to migrate after being provided with ingestion-rate-saturating concentrations of food. Vertical migrations in D. longispina seemed to reflect a trade off between two conflicting interests: avoiding predation and risking starvation. This trade off was balanced by the overall amount of available food, restricting migrations of the large animals when food was scarce.