[1] Temperate deep freshwater lakes are important resources of drinking water and fishing, and regional key recreation areas. Their deep water often hosts highly specialised fauna surviving since glacial times. Theoretical and observational studies suggest a vulnerability of these hydro-ecosystems to reduced mixing and ventilation within the ongoing climatic change. Here we use a numerical thermal lake model, verified over the 20th century, to quantify the transient thermal behaviour of two European lakes in response to the observed 20th-century and predicted 21th-century climate changes. In contrast to Lac d'Annecy (France) which, after adaptation, maintains its modern mixing behaviour, Ammersee (Germany) is expected to undergo a dramatic and persistent lack of mixing starting from $2020, when European air temperatures should be $1°C warmer. The resulting lack of oxygenation will irreversibly destroy the deepwater fauna prevailing since 15 kyrs. [2] The impacts of future climate change on freshwater systems have been investigated using three approaches: (i) synoptic-scale vulnerability assessments based on modern climate/biodiversity relationships [Magnuson et al., 1997;Moore et al., 1997;Stenseth et al., 2002]; (ii) lake-specific observations showing recent changes in ice-cover duration [Magnuson et al., 2000], instrumental lake vertical temperature profiles, mixing behaviour, and eventually ecological consequences Verburg et al., 2003;Quayle et al., 2002;Carvalho and Kirika, 2003;Dabrowski et al., 2004;King et al., 1999;Livingstone, 2003]; (iii) lake-specific steady-state numerical modelling for idealized climatic changes [Blenckner et al., 2002;Peeters et al., 2002]. However, in order to face future potential impact as expected from these studies, regional administrations require quantitative and temporal forecasting for each lake, which can only be provided by dynamical lake-specific modelling .[3] Here we explore the transient thermal behaviour of two European lakes to recent and future climatic changes. Lac d'Annecy (France), and Ammersee (Germany) are both deep pre-alpine lakes, well documented by instrumental meteorological and lake observations. The benthic faunal assemblage preserved in the deep lake sediments indicates that the bottom waters have been regularly ventilated throughout at least the last 15 000 years [von Grafenstein et al., 1999]. These specific lakes were chosen originally because they are representative of the two main types of mid-latitude deep lakes. In deep and fresh lakes, the springsummer surface heating induces the formation of a warm and light water layer (epilimnion), which ''floats'' over the colder and denser hypolimnion. The winter turn-over occurs when the density of the epilimnion is close enough to the density of the hypolimnion to allow wind-driven full mixing and oxygenation of the bottom water [Famer and Carmack, 1982]. Due to the large seasonal cycle of local temperatures and the high water-discharge rates, we consider only the thermodynamic control of temperature on...
