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

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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.