Grazer control of benthic algal biomass: The role of food quality on different levels of spatial organization

在食物网中，自下而上(即资源)和自上而下(即由下一个营养水平取食)因素都控制着中间营养水平上的生物量，如底栖藻类生物膜(围生生物)。这两种机制都被证明是显著的，然而，在不同的自然环境中，自上而下控制的强度有很大的差异。决定自上而下控制力度的因素迄今知之甚少。该项目的中心假设是，自上而下控制附生植物的强度由藻类的食物质量决定，而藻类的食物质量又受基本资源(营养物质、能量)的分配调节。在围生等空间显性群落中，食物质量表现出显著的空间异质性。这一点，连同食草动物的相应行为适应，在控制附生生物生物量方面可能具有特别重要的意义，尽管迄今为止很少考虑这些因素。在这里，我们将在高度受控的实验室实验和与野外相关的中观实验中，在不同的规模和复杂程度上测试这一假说。这包括均匀的局部斑块大小，具有空间异质性的多斑块水平和观察者的选择，以及在中观实验中考虑食草动物的生长和迁徙行为时的高度复杂性。在这些水平上，将操纵资源分配(限制养分P；光能)，并将通过(I)藻类和放牧动物之间的物质流动和(Ii)放牧与无放牧对照相关的放牧控制来量化附生植物的放牧控制。总体而言，该项目将揭示自上而下对附生植物的压力强度是如何调节的机制，并将有助于理解天然地表水富营养化的控制。

In food webs, both bottom-up (i.e., resources) and top-down (i.e., feeding by next trophic level) factors control the biomass on intermediate trophic levels such as benthic algal biofilms (periphyton). Both mechanisms were demonstrated to be significant with, however, strong variations in the strength of top-down control in different natural settings. Factors that determine the strength of top-down control are as yet poorly understood. The central hypothesis of this project is that the strength of top-down control of periphyton is determined by the food quality of the algae, which is in turn regulated by allocation of essential resources (nutrients, energy). In the case of spatial explicit communities such as periphyton, the food quality shows remarkable spatial heterogeneity. This, together with the corresponding behavioural adaptations of the grazers, is probably of particular relevance in the control of periphyton biomass, although these factors have so far rarely been considered. Here we will test this hypothesis on different scales and complexity levels in both highly controlled laboratory experiments and field-related mesocosm experiments. This includes the homogenous local patch size, the multi-patch level with spatial heterogeneity and the choice for gazers as well as a high level of complexity under consideration of growth and migration behaviour of grazers in mesocosm experiments. On these levels, resource allocation (limiting nutrient P; light energy) will be manipulated and grazing control of periphyton will be quantified via (i) flux of matter between algae and grazers and (ii) reduction of periphyton biomass by grazers in relation to grazer-free control. Overall, this project will reveal mechanisms how the strength of top-down pressure on periphyton is regulated and will contribute to understand the control of eutrophication of natural surface waters.