pFreshwater ecosystems are under severe pressure by a multitude of anthropogenic stressors. A central assumption of the European environmental legislation is that degraded ecosystems return to their ”natural state” after relief from anthropogenic pressures. Unfortunately, we still lack sufficient understanding of the responses of ecosystems to changing conditions to predict whether restoration measures indeed result in the reversal of past changes in lake ecosystems. The proposed research training group (RTG) aims to improve such understanding using Lake Constance, one of the best-studied lakes in the world, as a model system. Lake Constance is perfectly suited for studying response patterns, including reversibility, as the lake has been affected by a multitude of environmental stressors (e.g. eutrophication, climate change, neobiota), and because eutrophication, the stressor of main concern during the 1960s to 1980s, has been removed successfully. The effects of these changes on the lake ecosystem have been documented in unique long-term data sets.p Combining classical and novel methods, i.e. population genomics and metagenomics approaches, metabolic profiling, compound-specific stable-isotope analyses, comparative multi-species experiments, paleo-limnological and resurrection ecology approaches, time-series analyses, and numerical simulation models, we will investigate the response of community structure, biological interactions and carbon and nutrient flows to changing conditions. The focus will be on the response patterns including resilience or reversibility of the ecosystem in the presence of confounding environmental changes (changing baselines).p The structured doctoral qualification programmme combines a mentoring program, specific lectures on research and applied questions in limnology, a seminar and visiting program among students, teaching of Master students in research projects, flexible one-week courses, transferable skills courses, visits by guest researchers, and annual retreats. The goals of this qualification program are to support doctoral researchers in conducting excellent research by providing access and understanding of a large suite of classical and modern ecological and evolutionary methods and research approaches, to foster collegial cooperation and interdisciplinary research, to enable teaching experiences and to improve the job perspectives of the doctoral researchers in and outside academia.