Final Activity Report Summary - OBSERVER (Food web changes in response to increasing temperature and varying nutrient status: an experimental approach on freshwater mesocosms) Shallow lakes can rapidly switch from clear-water to turbid conditions with complete loss of macrophytes which are then replaced by phytoplankton or vice versa. Future climate warming might promote a state shift with its consequent severe food web alterations. To improve our understanding of the effects of a future temperature rise in shallow lakes we established an experimental flow-through mesocosm system allowing for investigations of the interactions between climate warming and eutrophication and their impacts on biological structure and ecosystem processes. The experimental set-up consisted of 24 tanks which were heated according to three temperature scenarios, one unheated and two heated according to the intergovernmental panel on climate change (IPCC) climate scenarios A2 and A2+50 % respectively, and two nutrient levels, enriched and non-enriched. The tanks were monitored continuously during the study period measuring key environmental variables such as pH, oxygen, nutrients, chlorophyll a, zooplankton and macroinvertebrates. Additionally, after two years of running, we sampled the main food web compartments including primary producers and consumers. We measured their elemental composition (carbon, nitrogen and phosphorus) and used the stable isotopes of carbon and nitrogen to describe their trophic relationships. Finally, we performed additional laboratory experiments designed to disentangle the effects of food quality and life history changes on the fractionation of stable isotopes and the internal nutrient allocation on invertebrates. The results obtained highlighted the importance of periphyton on the nutrient balance of the mesocosms, and the structuring role of macrophytes for the trophic structure of the studied food webs. Enrichment increased the total biomass of phytoplankton and periphyton and changed the elemental content of primary producers. In contrast, temperature was a much less significant factor, affecting only the elemental composition of macrophytes and periphyton, which was attributed to an effect of increasing respiration over production at higher temperatures. The amino-acid composition of food was found to be a key factor in the nitrogen metabolism and nitrogen isotopic fractionation. Life history differences were closely related to the elemental composition of the studied species, with faster growing species having higher phosphorus content. Slow growing species had a higher nutrient reserve pool and a lower nutrient turnover.