The need for a better management of estuaries requires an environmental characterization following a methodology that allows the comparison of distinct estuarine systems and the assessment of their evolution. The peculiar characteristics of estuaries, in particular their intrinsic variability, make this characterization difficult and there is no commonly accepted general methodology available. The approach followed in freshwater ecosystems is generally based on the concept of environmental indicators, but such a concept has not been developed for estuaries. Indeed, a different approach is needed here, due to the spatial heterogeneity and the different time scales associated with the processes that control water quality.

This paper presents a proposal for a methodology that starts with observed values and uses a procedure to integrate these values in time and space in order to calculate significant values, upon which normalized indicators are built which take into account criteria based either on legal, scientific or heuristic concentration limits. A normalization is carried out in two steps: (1) application of a mathematical operator to the significant values, (2) transformation using the concept of penalty curves. This methodology may be complemented with the definition of quality classes, particularly appealing and useful, as tools to communicate with decision makers and the public in general.

Water quality data pertaining to the Tejo estuary are used to test the methodology.