Lampert Winfried
Department of Physiological Ecology, Max Planck Institute for Limnology, Postfach 165, 24302 Plön, Germany.
BMC Biol. 2005 Apr 6;3:10. doi: 10.1186/1741-7007-3-10.
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.
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.
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.
在具有深水藻类最大值的湖泊中，食草性浮游动物面临着一种权衡，即在表层高温但食物供应不足，与深层低温但食物充足之间做出选择。有人提出，面临这种权衡的浮游动物（水蚤）会根据“有成本的理想自由分布（IFD）”进行垂直分布。已设计了一项实验来测试垂直分布的密度（竞争）依赖性，因为这是IFD理论的一个基本假设。
实验在大型室内中宇宙（浮游生物塔）中进行，温度梯度为10摄氏度，在温跃层下方建立了深水藻类最大值。正如预期的那样，当水蚤密度较低时，它们聚集在两个不同栖息地的界面处。当密度增加时，分布不对称地向藻类最大值方向扩展，直到80%的种群在高密度下栖息在凉爽、食物丰富的水层中。小个体在水柱中的位置比大个体更高，这与不平等竞争者的模型相符。
水蚤的分布模拟了有成本的IFD模型的预测。这一概念对于分析在塑造栖息地适宜性的一大系列环境条件下的浮游动物分布很有用。导致昼夜垂直迁移的鱼类捕食可作为额外成本纳入其中。这很重要，因为湖泊中食草浮游动物的垂直位置会影响浮游植物的产量和物种组成，即生态系统功能。
