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Adrianna A. Kowalewska, Nina Krebs, Ralph Tollrian, Linda C. Weiss

• Aquatic acidification is a major consequence of fossil fuel combustion. In marine ecosystems it was shown, that increasing pCO\(_2\) levels significantly affect behavioural and sensory capacities in a diversity of species. This can result in altered predator and prey interactions and thereby change community structures. Just recently also CO\(_2\) dependent acidification of freshwater habitats has been shown. Also here, increased levels of pCO\(_2\) change organisms’ behaviour and sensory capacities. For example, the freshwater crustacean \(\it Daphnia’s\) ability to detect predators and accurately develop morphological defences was significantly reduced, rendering \(\it Daphnia\) more susceptible to predation. It was speculated that this may have cascading effects on freshwater food webs. However, for a comprehensive understanding of how increased levels of CO\(_2\) affect trophic interactions, it is also important to study how CO\(_2\) affects predators. We tested this using the dipeteran phantom midge larva \(\textit {Chaoborus obscuripes}\), which is a world-wide abundant inhabitant of freshwater impoundments. We monitored activity parameters, predation parameters, and predation rate. \(\it Chaoborus\) larvae are affected by increased levels of pCO\(_2\) as we observed an increase in undirected movements and at the same time, reduced sensory abilities to detect prey items. This is likely to affect the larvae’s energy budgets. \(\it Chaoborus\) is a central component of many freshwater food-webs. Therefore, CO\(_2\) effects on predator and prey levels will likely have consequences for community structures.