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Studies covering long-term climatic variability have shown that the similar climate change can lead to completely opposite consequences on lake ecological status, depending on its location and morphological characteristics. Our knowledge of climate influences on lake ecology is then considerably hampered by the complexity of the pathways under which climate can act on these ecosystems. Indeed, such a singularity of lake responses to climate results from the fact that the different climate components can affect the lake directly, through modifications of the water column physical, chemical and biological properties, but also indirectly through their impacts on the watershed characteristics and subsequent inputs to the lake (Leavitt et al, 2009). Depending on the lake context, one pathway or the other shall predominate, leading to a lake-specific response to climate change.

We use, for that purpose, a set of three peri-alpine lakes (Annecy, Bourget, Geneva) exhibiting rather similar morphologies and submitted to the same climatic variability. These lakes have undergone, over the last century, similar local forcings (changes in nutrient inputs and fisheries management practices) but with varying intensities.

The first step of the program has been dedicated to the paleolimnological reconstruction of the lakes ecological and geochemical processes over the last 150 years. To reach a large picture of ecological processes, we implemented the classical paleolimnological approach by several new proxies developed within the program (molecular methods for microbial diversity, stable isotope analysis for food web structure and geochemical indicators of anoxia). We could then achieve a holistic and ecosystem-wide view on how these three lakes changed over the last century.

A second, on-going step consists in hierarchizing the drivers of the ecological changes observed over time. For instance, detected changes in food web structure resulted from the interactions between local and global forcings in all cases. However, using adequate models, we could show that climate impacts on food webs increased with the intensity of local stressors, and especially eutrophication. We could also detect the appearance of lake bottom anoxia, a major symptom of ecosystem degradation, from the 1950’s and its persistance since then, in spite of undertaken remediation measures. The contribution of climate change to the persistance of lake bottom anoxia was shown to be uneven between the three lakes.