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Free keywords:
Daphnia; environmental gradients; fitness optimisation; predation; vertical distribution
Abstract:
1. The vertical distribution of zooplankton results from active habitat choice aiming to optimise
fitness gain in a system of trade-offs.
2. Using large, controlled indoor mesocosms (Plo¨n Plankton Towers), we monitored the
behavioural response of Daphnia pulicaria to vertical gradients of temperature, food, oxygen and
light, in the presence and absence of fish predation.
3. In the absence of fish, Daphnia distributed as predicted by an ideal ‘free distribution with costs’.
If the food was distributed homogeneously, they stayed in the warm epilimnion, while they
balanced their time dwelling in epi- and hypolimnion if the food was concentrated in a deep-water
maximum.
4. However, oxygen depletion in the hypolimnion, representing an additional cost, prevented
Daphnia from completely exploiting the hypolimnetic food maximum. Consequently, the
proportion dwelling in the hypolimnion was larger if oxygen was not limiting.
5. Fish predation had an overwhelming effect, driving Daphnia into the hypolimnion under all
experimental conditions. If permitted by oxygen availability, Daphnia used the whole hypolimnion,
but oxygen depletion reduced their possible habitat to the upper hypolimnion with oxygen
concentrations above c. 0.7 mg L)1. As fish were less tolerant of low oxygen, the layer below the
thermocline formed a predation refuge for Daphnia
