Trophic status evolution of Lake Nantua as revealed by biological records in sediment

Stratigraphic analyses of fossil pigment and diatom frustules in a 0.5 m sediment core were used to reconstruct the trophic history of Lake Nantua (France), with a particular interest about the appearance of the cyanobacterium Oscillatoria rubescens, the appearance of this species 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 Pb-210 and Cs-137 dating. The first step of the colonization by O. rubescens as well as in changes in diatom assemblages, eutrophic species replacing oligo-mesotrophic ones, indicate that eutrophic status of Lake Nantua took place around 1880.