TRADE-OFFS IN DIEL VERTICAL MIGRATION BY ZOOPLANKTON - THE COSTS OF PREDATOR AVOIDANCE

Diel vertical migration (DVM) of zooplankton is a behavioral antipredator defense that is shaped by the trade-off between higher predation risk in surface waters and reduced growth in deeper waters. We conducted two laboratory experiments to quantify the costs connected with DVM. In the first experiment, Daphnia magna were kept individually in thermally stratified flow-through tubes and exposed to seven different concentrations of fish-exuded kairomones. Above a threshold concentration, the strength of migration increased with increases in the concentration of fish exudates. Enhancement of migration resulted in a lower mean ambient temperature experienced by the animals and marked decreases in individual growth and reproduction rates. In order to separate costs due to low hypolimnetic temperatures in a stratified system from costs due to reduced food concentrations in deeper waters, we conducted an experiment with a 2 x 2 factorial design (fish presence vs. absence and high vs. low food conditions). Differences in mean ambient temperature between Daphnia that performed DVM and nonmigrating animals were found to have a much stronger impact on life history parameters than food effects. A reevaluation of field data on DVM in Daphnia further supports the view that vertical temperature gradients are more important than food gradients.