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（即下一营养层的摄食）因素控制着中营养层（如底栖藻类生物膜）的生物量。这两种机制都被证明是重要的，然而，在不同的自然环境中，自上而下的控制强度有很大的差异。决定自上而下控制强度的因素目前还不太清楚。这个项目的中心假设是，自上而下对周围生长的控制强度是由藻类的食物质量决定的，而藻类的食物质量又由必需资源（营养物质、能量）的分配来调节。在外围植物等空间外显群落中，食物质量表现出显著的空间异质性。这一点，连同相应的食草动物的行为适应，可能与控制周围植物生物量特别相关，尽管这些因素迄今为止很少被考虑。在这里，我们将在高度控制的实验室实验和现场相关的中观实验中，在不同的尺度和复杂性水平上检验这一假设。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对食草动物的选择，以及在中观实验中考虑食草动物生长和迁移行为的高度复杂性。在这些水平上，资源分配（限制营养物P；光能）将被操纵，并且通过(i)藻类与食草动物之间的物质通量和（ii）相对于无食草动物的控制，食草动物减少了周围植物的生物量来量化对周围植物的放牧控制。总的来说，本项目将揭示自上而下对周围植物的压力强度是如何调节的机制，并将有助于理解自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.