Vertical distribution of zooplankton: density dependence and evidence for an ideal free distribution with costs.
BACKGROUND
In lakes with a deep-water algal maximum, herbivorous zooplankton are faced with a trade-off between high temperature but low food availability in the surface layers and low temperature but sufficient food in deep layers. It has been suggested that zooplankton (Daphnia) faced with this trade-off distribute vertically according to an "Ideal Free Distribution (IFD) with Costs". An experiment has been designed to test the density (competition) dependence of the vertical distribution as this is a basic assumption of IFD theory.


RESULTS
Experiments were performed in large, indoor mesocosms (Plankton Towers) with a temperature gradient of 10 degrees C and a deep-water algal maximum established below the thermocline. As expected, Daphnia aggregated at the interface between the two different habitats when their density was low. The distribution spread asymmetrically towards the algal maximum when the density increased until 80 % of the population dwelled in the cool, food-rich layers at high densities. Small individuals stayed higher in the water column than large ones, which conformed with the model for unequal competitors.


CONCLUSIONS
The Daphnia distribution mimics the predictions of an IFD with costs model. This concept is useful for the analysis of zooplankton distributions under a large suite of environmental conditions shaping habitat suitability. Fish predation causing diel vertical migrations can be incorporated as additional costs. This is important as the vertical location of grazing zooplankton in a lake affects phytoplankton production and species composition, i.e. ecosystem function.
Lampert
Winfried
W
Department of Physiological Ecology, Max Planck Institute for Limnology, Postfach 165, 24302 Plön, Germany. lampert@mpil-ploen.mpg.de
D000818:Animals; D003621:Daphnia / Q000254:growth & development / Q000502:physiology; D017753:Ecosystem; D005618:Fresh Water; D044444:Scenedesmus / Q000254:growth & development / Q000502:physiology; D013696:Temperature; D015048:Zooplankton / Q000254:growth & development / Q000502:physiology*