Aquatic organisms are typically exposed simultaneously to several organic compounds released from human activities like agriculture, industries, or simply from people living in cities. The ecological risk assessment of mixtures of such compounds has therefore to be addressed by scientists. The aims of this paper are (1) to describe the current mixture risk assessment procedures, (2) to apply such approach to a specific case study, Lake Geneva and the River Rhône in Switzerland, and (3) to discuss the outcomes of such an application. Two models, called concentration addition and independent action, are recognized to be robust enough to predict the mixture effect of substances on a given species. They are classically used also to assess the risk of mixtures for the ecosystem, but their use is often limited by the lack of available ecotoxicity data. Adopting a first level assessment, we describe the evolution of the mixture risk for several years of Lake Geneva, and for 2010 for the River Rhône. These first assessments allow identification of the most problematic substances demanding risk reduction measures. Furthermore, again for the two cases studies, we show that the risk levels associated with mixtures of compounds can rapidly exceed critical aquatic thresholds, and therefore, it is the sum of the substances that is problematic, which is more challenging in term of risk management. Further analysis of effects in compound mixtures as well as a better characterization of the overall ecological risk are necessary for the thousands substances co-occuring at very low concentrations.
Conflict of interest: The authors have declared no conflicts of interest for this article.