Stratigraphic analyses of fossil pigment and diatom frustules in a 0.5m sediment core were used to reconstruct the trophy history of Lake Nantua (France), with a particular interest about the appearance of the cyanobacterium Oscillatoria rubescens, the appearance of this psecies being usually considered as a sign of deleterious conditions due to enrichment with nutrients originating from sewage or fertilizer. The chronology of sediment was based on 210Pb and 137Cs dating. The first step of colonization by O. rubescens as well as changes in diatom assemblages, eutrophic species replacing oligo-mesotrophic ones, indicates that the eutrophic status of Lake Nantua took place around 1880.