Vertical distribution of Daphnia in a trade-off between food and temperature
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 found (1) that Daphnia actively chose the best vertical habitat, which implies that they can assess their fitness in the different layers. (2) Their distribution was “dynamic”, i.e., individuals frequently migrated between different habitats and allocated the time they spent there according to the fitness value. (3) 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.