The "one-out, all-out" principle entails the risk of imposing unnecessary restoration costs: a study case in two Mediterranean coastal lakes

The Water Framework Directive uses the "one-out, all-out" principle in the assessment, i.e. the worst status of the elements used in the assessment determines the final status of the water body. In this study, we assessed the ecological status of two coastal lakes located in Italy. Indices for all biological quality elements used in transitional waters from the Italian legislation and other European Countries were employed and compared. From our analyses, the two lakes need restoration, despite they harbor lush seagrass beds, articulated macrobenthic communities and rich fish fauna. The "one-out, all-out" principle tends to inflate Type I errors, i.e. to conclude that a water body is below "good" status, even if the water body is really of "good" status. This entails the risk of imposing restoration costs where they are not strictly needed. Results from this study strongly support the need to alternative approaches to the "one-out, all-out" principle