Top-down and bottom-up control of benthic grazers as key species in stream biomanipulation

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The focal point of this project is the interaction between periphyton and stream insect grazers. The central question is whether insect grazers like mayfly larvae are keystone species in stream food webs and, thus, are able to be used in a biomanipulation approach. Experiments as to the interactions between grazers and periphyton under different environmental conditions are planned. To investigate the top-down impact on grazers, a pairwise field experiment with different predator regimes will be carried out. Furthermore, in outdoor mesocosms the optimal grazer density to control periphyton during the spring peak will be investigated. Finally, in laboratory mesocosm experiments the effects of pesticides on grazers will be quantified. Insecticides with direct effects can be categorised as top-down impact while herbicides have an influence on quantity and quality of periphyton in the biofilm community and, therefore, act indirectly via bottom-up effects on the grazers. The results of these interconnected experiments will reveal which factors are important to control grazers and which have adverse effects on their activity.