Pont-de-Salars reservoir is of the canyon type, horizontally long and dendritic, sheltering the water surface from the wind and is a warm monomictic lake.

We observed great fluctuations in interannual and annual phytoplankton biomass and abundance from 1993 to 1996. Moreover, a fortuitous biomanipulation (total removal of fish), involving whole-lake emptying, was carried out in 1995.

These inter- and intra-annual changes in phytoplankton biomass and abundance were accompanied by compositional changes in phytoplankton groups and species, as shown by descriptive methods, cluster analysis, and CCA(canonical correspondence analysis). It was difficult to identify a mean seasonality, a typical year for the typical seasonal succession, and a characteristic species in the season.

Mallomonas acaroides and Cryptomonas were pioneer taxa for recolonization after the biomanipulation.

These inter- and intra-annual fluctuations in phytoplankton biomass and abundance resulted from the combination of abiotic factors : hydrological effects (e.g. seasonality of NO3-N, Si in relation to inflow and outflow respectively), loadings of the inflow (e.g. NH4-N, NO3-N and TP : ˇ°bottom-upˇ±) and biotic factors (biomanipulation).

Fish removal biomanipulation in 1995 resulted in a general improvement in the quality of water although the reservoir was deep, large and stratified: a drastic summer decrease in phytoplankton biomass and abundance, a spectacular early summer increase in water transparency, an increase in hypolimnetic dissolved oxygen, and perhaps an increase in the biomass of large grazers, after the biomanipulation.
The Viaur river exerts a major effect on watershed inflows. The seasonality of the alkalinity, conductivity and nitrate nitrogen in the reservoir seemed to follow that of the Viaur inflow. Moreover this reservoir has never shown nutrient deficiencies probably due to high nutrient loadings of its inflow and surrounding land use patterns (i.e. agriculture, cattle farming).