In a thermally stratified water column with a deep-water chlorophyll maximum (DCM), Daphnia face a trade-off between optimum temperature and food availability. The high temperatures in the surface layer (epilimnion) accelerate development, whereas the high food availability in the deep layer (hypolimnion) increases fecundity. I investigated the influence of this trade-off on the fitness and on the vertical distribution of Daphnia. I tested (1) whether Daphnia actively choose the best vertical habitat, implying that they can assess their fitness in the different layers, and (2) whether their distribution is “dynamic”, i.e., individuals frequently migrate between different habitats and allocate the time they spent there according to the fitness value.

I measured the vertical distribution of a Daphnia population in 11 m high indoor meso¬cosms, in which I established a hypolimnetic DCM in different temperature gradients. The zooplankton samples obtained from this experiment were counted and sized with the bench-top version of an optical plankton counter, which gave very accurate measure¬ments of the total number and the size composition of Daphnia in each depth. In parallel to the vertical distribution, I measured the fitness that Daphnia can achieve in the different layers of the mesocosms. I transferred water from the surface and from the hypolimnion into flow-through systems, in which I assessed the juvenile growth rate of Daphnia as a proxy for fitness.

Both the relative fitness and the proportion of Daphnia in the hypolimnion increased with the hypolimnetic temperature (lower costs). However, ontogenetic stages differed in their vertical habitat choice. Large Daphnia avoided light, resulting in diel vertical migration. This was most likely a “residual” response of Daphnia to the potential threat of being preyed upon by visually hunting fish. Small Daphnia that are less conspicuous did not show such a response and mostly dwelled near the surface, because new-born Daphnia still dispose of egg-yolk reserves, which makes feeding in the cold hypolimnion less important.

Most Daphnia selected the vertical habitat where they achieved the highest fitness. However, a certain proportion of the population was always found in the less profitable habitat. Using a system of vertical perspex tubes (with a DCM and various temperature gradients), I demonstrated that individual egg-bearing females asynchronously move up and down the water column in intervals of about 5 hours, leading to a dynamic distribution of the population. These frequent migrations might be a mechanism to take advantage of both, high temperature in the epilimnion and high food availability in the hypolimnion. The time they spent feeding in the colder hypolimnion increased with the hypolimnetic tem¬perature. Neonates with egg-yolk reserves, which did not experience the trade-off, con¬tinuously stayed in the warm epilimnion, regardless of the temperature gradient.

In summary, Daphnia apply several complementary behavioural strategies to optimise their fitness in the vertical trade-off between food and temperature. Additionally, other vertical gradients (e.g., of oxygen or predation) should similarly influence the fitness and hence the vertical distribution of Daphnia in the field.