Mass occurrence of the cyanobacteria Microcystis aeruginosa in the hypereutrophic Villerest reservoir (Roanne, France) : usefulness of the biyearly examination of N/P (nitrogen phosphorous) and P/C (protein/carbohydrate) couplings

The biyearly (i.e. 1990-1991) distribution of the biochemical composition of particulate matter together with that of phytoplankton composition, chlorophyll-a, primary production coupled with environmental conditions were studied in the hypereutrophic Villerest reservoir (located near the city of Roannne, France). The results showed that the P/C (protein/carbohydrate) ratio appears to be a relatively sensitive index integrating all metabolic functions of the cells. It responded well to variations of light level, temperature, nutrient supplies, hydrodynamic conditions and algal species composition. In addition, the coupling of P/C and N/P ratios along the three studied stations A, B and C allowed a discrimination of three types of ecosystems. Station A is a distinct riverine zone characterized by flow and mixing. It is followed by station B which is a transitional zone. Station C located near the dam is the most lake-like. This would imply that Microcystis cells are station-specific. The chemical composition of water and mainly the variations of N/P ratios are the deterministic factors of the mass occurrence of Microcystis aeruginosa. The summer growth phases of this species observed both sampling years started as soon as N/P levels decreased below 5. The proliferation of Microcystis did not seem to be regulated by biological factors. Any manipulation to lower N-inputs will translate into low N/P ratios allowing Microcystis to become dominant. A first step in the restoration of the Villerest reservoir passes through a dephosphoryzation of tributaries. In addition, in station C in order to limit the diffusive fluxes of phosphorus from the sediment, an appropriate management of the water gates of the dam should be attempted.