Das Projekt "Chemische Kontrolle von Weidern durch Futterorganismen" wird vom Umweltbundesamt gefördert und von Universität Zürich, Institut für Pflanzenbiologie, Limnologische Station durchgeführt. It is now generally acccepted that higher plants have developed chemical defence mechanisms against grazer and pathogenic micro-organisms. The existence of molecules with similar features that may regulate aquatic ecosystems have frequently been proposed and many experiments clearly show the presence of such molecules but up to now in no single case has the structure of such a compound been determined. In the current research programme we will capitalize on our experience on ultratrace analysis of odour and allelopathic compounds. Our interest focuses on the chemical interactions between grazers and food organisms in the aquatic environment and the chemical analysis of compounds responsible for these interactions. Planktothrix rubescens is the major cyanobacterium in Lake Zuerich and exhibits high resistance against grazers. Experiments with an axenic strain have shown two different strategies of the grazers to avoid the toxicity of this cyanobacterium. Either ingestion of Planktothrix filaments is rejected (food selection takes place by perception of Planktothrix specific compounds that are different from the microcystins) or the grazers have developed resistence against the toxins. The chemical characterization of the responsible compounds is under investigation.