Distribution of Daphnia in a trade-off between food and temperature: individual habitat choice and time allocation

1. In a thermally stratified water column with a deep-water algal maximum, Daphnia face a trade-off between food (high fecundity) and temperature (fast development). Recent studies showed that Daphnia populations move up and down the entire water column to take advantage of both, but the proportion of time allocated by individuals to the epilimnion, metalimnion and hypolimnion with their specific food and temperature conditions is not yet known. 2. In a system of 1 m deep, vertical perspex tubes, I established three temperature gradients with 2, 5 and 10°C differences between the surface (epilimnion) and the bottom layer (hypolimnion). Algae were added to the hypolimnion to simulate a deep-water algal maximum. 3. The migration behaviour of individual neonate and egg-bearing Daphnia hyalina x galeata was monitored in order to measure the proportions of time the individuals allocated to the different vertical habitats and to assess the frequency of their shifts between epilimnion and hypolimnion. 4. Neonates stayed continuously at the surface, taking advantage of the higher temperature, possibly because feeding was less important for them because of egg yolk reserves. In contrast, egg-bearing females spent more time feeding in the hypolimnion when the temperature gradient was weak, but also migrated into the epilimnion to take advantage of the higher temperature. In the steepest temperature gradient, the egg-bearing females either shifted between epilimnion and hypolimnion, or dwelled constantly in the metalimnion with intermediate conditions