In Lake Maarsseveen (The Netherlands), the daphnids migrate vertically at dawn and dusk and the mean population depth at noon is much lower than at midnight. This diel vertical migration is considered a behavioural stategy to avoid predation by visually hunting predators, such as juvenile perch (Perca fluviatilis). Since light intensity declines with depth, moving downward into deeper water layers upon increases in light intensity at dawn, reduces predation risk. The disadvantage of this migration is diminished reproduction, either directly because migration requires energy, or indirectly because migrating Daphnia stay at greater depth during the daytime, where they experience a lower temperature and less food. It would thus be advantageous if a daphnid could estimate advantages and disadvantages of given migrations and if they could decide when to migrate. It is assumed that when both juvenile fish and food are abundantly present, the conditions will allow an early and extensive migration.

Light-induced swimming was considered the behavioural mechanism underlying diel vertical migration. The objective of the research was to study the mechanism of this light-induced swimming in order to investigate to what extent migration behaviour in the field can be predicted. In laboratory experiments, individuals of the hybrid Daphnia galeata ( hyalina from Lake Maarsseveen were observed while responding to different types of light change. The influence of food availability and predator associated chemicals (kairomones) on behavioural response variables was quantified. In order to test whether the plasticity of light-induced swimming could lead to seasonal variation in diel vertical migration amplitude in Lake Maarsseveen, response variables were used to study displacements in the case of a light change at dawn.

Both food and kairomones had a modifying, additive, but independent effect on displacement velocity during a swimming reaction. In a field situation this would result in a larger migration amplitude when food is abundantly present and juvenile fish density is high. Clonal variability of this environmentally induced plasticity in photo-behaviour was also studied. Two clones, one from the epilimnion and one from the hypolimnion, showed a particularly significant difference in displacement velocity elicited by the light stimulus. Phenotypic plasticity and clonal variability in this light-induced swimming are considered to be at the base of the seasonal variation in migration amplitude in Lake Maarsseveen.