Lake Geneva long-term changes of phytoplankton biomass and environmental parameters revealed a silicate influence between1980-1989, a transition phase between 1989-1995/99, a mild phosphorus influence between 1995/99-2007 and a period of strong phosphorus influence between 2007-2012. Moreover, shifts in the phytoplankton community from species characteristic of eutrophic waters towards those characteristic of meso-oligotrophic waters were reported. Yet, total phytoplankton biomass remained un-changed. Additionally, micronutrients had not a critical role in Lake Geneva, possibly due to low phytoplankton biological requirements or efficient recycling. Furthermore, species with high light requirement were dominant in the upper-lake (aggregated around 10m) and were thus more likely seasonally nutrient limited. In the lower-lake, species adapted to low light intensities dominated (up to 25m) and benefited from a greater nutrient supply; they were thus more likely seasonally transiently light and nutrient limited. The outcomes of this thesis are expected to benefit Lake Geneva monitoring program.