CONTRIBUTION OF MATHEMATICAL-MODELING TO LAKE ECOSYSTEM UNDERSTANDING- LAKE BOURGET (SAVOY, FRANCE)

This paper demonstrates how mathematical modeling can contribute to im prove understanding of lake behavior. Since the 60's Lake Bourget, one of the largest in France, had been suffering from eutrophication whic h was checked in 1980 by the diversion of the main sewers entering the lake. A research program was implemented between 1987 and 1990, inclu ding an on-site sampling campaign conducted concurrently with thermal and biogeochemical modeling of lake behavior. The model helped provide a better understanding of the ecosystem, displaying some processes hi therto misunderstood: (1) Winter overturn does not reach the bottom of the water column when the weather is mild. This leads to a incomplete reoxygenation of the hypolimnion and to redox conditions inducing the release of orthophosphate from the sediment, (2) Grazing by herbivoro us zooplankton is getting more important in the control of spring alga l growth as eutrophication of the lake regresses, (3) Settling of part iculate phosphorus seems a complex and very important process in Lake Bourget, showing high sedimentation rates for particulate mineral phos phorus.