TRADE‐OFFS IN DAPHNIA HABITAT SELECTION

Diel vertical migration (DVM) is a dynamic behavioral pattern found extensively in the world's oceans and lakes, yet the role of food and temperature distribution on DVM is still unclear. While DVM has been mostly studied in systems with surface food maxima, deep‐water food maxima are quite common in lakes and oceans. In such ecosystems, optimal conditions of temperature and food are uncoupled. In a Swiss high‐mountain lake (Oberer Arosasee) with a deep‐water food maximum, we found that Daphnia galeata adults and juveniles exhibit DVM behavior almost throughout the year and migrated upwards, out of food‐rich environments, at night. In a large indoor mesocosm experiment, we were able to show that Daphnia respond to fish‐mediated cues by migrating into deeper water layers. In the presence of fish and using natural vertical food and temperature distributions in the mesocosms, we could reproduce the vertical distribution of adult Daphnia observed in the field. The indoor experiments show that food and temperature modulate the actual depth at which the animals stop migrating, whereas fish and ultraviolet radiation likely determine the timing of migration (its synchronization with dawn and dusk). Overall, our results show that the nighttime movement into the surface waters contributes to the fitness of Daphnia , given the costs and benefits associated with the trade‐off between food and temperature.