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 th e cyanobacterium Oscillatoria rubescens, the appearance of this specie s being usually considered as a sign of deleterious conditions due to enrichment with nutrients originating from sewage or fertilizer. The c hronology 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 dia tom assemblages, eutrophic species replacing oligo-mesotrophic ones, i ndicate that eutrophic status of Lake Nantua took place around 1880.