Definition of a synthetic index for classifying the quality of water bodies is a key aspect in integrated planning and management of water resources systems. In previous works (Salis et al., 2005; Sechi and Sulis, 2005), based on EU and Italian guidelines for water quality in surface reservoirs, we proposed water system optimization modelling that considered a quality index based on the adoption of a small number of parameters. This paper analyses the feasibility of an index that considers

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... ty parameters estimated both on the basis of the overall trophic state of the water body as on concentration density values of the most toxic species of algae. This index makes it possible to describe the conditions limiting water use due to excessive nutrient enrichment in the water body and to the health hazard linked to toxic algae blooms. Application to the Flumendosa-Campidano water system highlighted the usefulness of this approach for obtaining a synthetic quality index in complex water resource systems.