Uncertainty analysis along the ecological quality status of water bodies: The response of the Posidonia oceanica multivariate index (POMI) in three Mediterranean regions

► Uncertainty estimation is a central element in WFD-compliant classification methods. ► Uncertainty associated to POMI index changes depending on the region of application. ► Uncertainty associated to POMI depends on the quality status of water bodies. ► Monitoring programmes design must respond to the specific conditions of water bodies.