This Observatory involves 3 French alpine lakes (Léman, Annecy and Bourget), corresponding to different situations with regard to the water quality and ecosystems status. The main goal of the Observatory is to understand and to model the ecological status changes (structure, biodiversity, fluxes) of the lacustrine systems submitted to changes (in terms of nature and intensity) of the external drivers (anthropization, climate). Limnological surveys are organized since 1957 (Léman), 1980 (Annecy) and 1990 (Bourget) and concerned several environmental variables: physico-chemistry (water, meteo), Phyto-, Zoo-, Bacterio- and Virioplankton, fishes (stocks and dynamics). Collections of biological elements are also organized (Fish scales; phytoplankton; algae) and the data are gathered and organized in a database. The initial question of ALO was about phosphorus loads increase and eutrophication risk. But since 1960-1970 and the improvement of wastewater treatment, the main scientific questions are about the consequences of re–oligotrophication and about the consequences of global change (temperature, new pollutants, ..) on the lakes ecology. Three main results will illustrate the contribution of ALO these questions: - the links between the organization of planktonic communities and lacustrine ecosystem functioning, in response to local or global environmental changes, including a “retro observation” approach using paleolimnological data. - The responses of communities and populations to habitat disturbances by developing a functional bioindication linked to risk assessment (including ecotoxicology approach) and by contributing to the management and bioconservation of fish stocks. - The use of models describing the mechanisms and regulation process of suspensions, nutrients (dissolved, particulates) and contaminants of the rural catchment flowing into lakes.