Ongoing climate change is one of the most pressing environmental concerns, particularly also for ecosystem management and nature conservation. As coldwater ecosystems, mountain lakes are exceptionally vulnerable to climatic warming to the extent that they are considered “early warning systems”, showing the evidence of climate impacts earlier than other ecosystem and lake types. However, considerably less attention has been given to understanding the impacts of recent climatic warming on Swiss mountain lakes compared with other similarly sensitive and vulnerable geo- and ecosystems (e.g. glaciers, alpine treeline, alpine meadows). This project will use an innovative, two-pronged palaeoecological approach to document how Swiss mountain lakes have responded to pronounced climatic warming experienced during recent decades, particularly in respect to summer temperatures. Palaeoecological reconstructions with an exceptionally high temporal resolution will be developed from the remains of organisms preserved in the sediments of 10 Swiss mountain lakes. The records will cover the past ca. 100-150 years with a particular focus on the past 30 years that are characterized by a pronounced rise in summer temperatures in the Swiss Alps. Analyses will focus on state-of-the-art and established palaeolimnological and palaeoenvironmental indicators such as chironomids and other aquatic insect remains, diatoms, cladocerans, pollen and non-pollen palynomorphs, but also on novel, recently developed approaches such as stable carbon isotope analysis of chitinous invertebrate remains. This work will be supported by geochronological (¹³⁷Cs, ²¹⁰Pb) and basic geochemical (e.g. organic matter content, bulk sediment delta C-13/delta N-15, XRF) analyses. For the examined indicators these records will reveal recent changes in species composition and functional groups (e.g. planktonic versus benthic forms), changing abundances of taxa typical for variations in nutrient availability, lakewater pH or deepwater oxygenation, and also show whether there is evidence for variations in carbon cycling in lacustrine foodwebs with increasing temperatures, based on carbon isotope analyses of sensitive invertebrate groups. Surface sediments will be collected from a further 15 lakes that have been sampled with similar methods 30 years ago and will be examined for diatom, chironomid and cladoceran assemblages. These analyses will reveal changes in assemblage composition between two time intervals bracketing the most recent summer temperature rise in the Alps, and will allow an assessment of whether assemblage changes in these indicator groups in the high-resolution records are representative for a larger number of lakes and lake types. Interpretations will be supported by indirect evidence on changes in other global change drivers that may have affected the study lakes, provided by the analysed palaeoecological proxies (e.g. in respect to land use (pollen), fish predation pressure (cladocerans) or pH (diatoms)), and by a systematic collection of documentary, instrumental and remote sensing data for the study sites. Final analyses of the project results will combine the two lines of evidence (downcore records, surface sediment data) and will be used to critically evaluate, and potentially revise, climate responses expected for these sensitive ecosystems based on presently available ecological and ecosystem knowledge. The project will provide a broad-scale, regional assessment of the effects of ongoing climatic warming on Swiss mountain lakes and one of the most ambitious and extensive palaoeecological assessments of recent climate change response of aquatic ecosystems to date. The project will therefore be relevant, and potentially influential, for a range of different research and stake holder communities not restricted to palaeoecologists, palaeolimnologists, and Quaternary geologists but also including the broader community of ecologists, geoscientists and ecosystem managers studying and assessing ongoing global change impacts on sensitive eco- and geosystems in the modern environment.