This paper describes the elaboration of an observation system based on a participative multicriteria valuation method in order to take into account the sustainability problem related to the Thau lagoon in the Mediterranean area of France. These methodologies enable the elaboration, selection and prioritisation of sustainable development indicators according to expectations and uses to be complied with. It is a question of associating the indicator `supply' based on research works with the `demand' issued by managers. Using this approach enables the two to be combined following indicator elaboration logics: * a normative approach allowing a list of reference indicators to be defined by researchers * a participative approach in association with territory managers for selecting an indicator panel in this list. In this paper, the emphasis is put on the methodological aspects and the significance of this approach with respect to participation-based methods.