Evaluation de la qualité de l'eau des lacs : Proposition d'une méthode appliquée au lac de Joux (Canton de Vaud)

The aim of this report is to propose a method for assessing the compliance of lakes in the Canton of Vaud, Switzerland, with the regulatory water quality standards defined by the Swiss Ordinance on the Protection of Waters (OEaux), using existing monitoring data. The criteria evaluated focus on the trophic state and oxygen concentration.

The method is applied to monitoring data from Lake Joux (Vaud) to evaluate its potential for broader application to other lakes. It relies on indicators of phosphorus availability (phosphorus loads and concentrations in the lake), trophic state (transparency and chlorophyll concentrations), and oxygenation status (hypolimnetic oxygen demand and minimum oxygen concentrations in the hypolimnion). Proposed thresholds allow for comparison of the observed conditions with the OEaux criteria.

Application of this method to Lake Joux indicates an improvement in trophic state, likely due to phosphorus reduction measures implemented in the lake’s catchment area since the 1960s. However, oxygen levels in the deep waters remain insufficient and non-compliant with regulatory criteria (often below 4 mg/L), primarily due to persistently elevated phosphorus loads.

This assessment demonstrates that it is possible to evaluate a lake’s water quality and identify the causes of any deficiencies using a limited set of indicators, many of which are routinely measured in existing monitoring programs.