An ecosystem-based approach to assess the status of the Mediterranean coralligenous habitat.

International audience ; Coralligenous outcrops are a Mediterranean sciaphilic habitat whose three-dimensionally layered communities develop on a basal biogenic concretion of calcareous red algae, along with a rich assemblage of sciaphilic sessile animals. The complexity of the evaluation of the ecological status of coralligenous is on a par with the complexity of this habitat. Biotic indices developed to implement the EU Water Framework Directive reflect the ecological status of the environment using species whose function, population or status depend on the quality of the water column. However, the state or achievement of good quality in a water body and the apparent health of some biological indicators are not always indicative of good structure and functioning of the whole ecosystem, even if they are key-species or ecosystem engineers. The new indices to be developed according to the Marine Strategy Framework Directive (MSFD) of the EU require an ecosystem approach, which takes into account the functioning of the ecosystem. Here, on the basis of a simplified conceptual model of the ecosystem, we propose an Ecosystem-Based Quality Index (EBQI) focused on the structure and functioning of coralligenous outcrops. The coralligenous EBQI is based upon (1) a set of representative functional compartments, (2) the weighting of these compartments and (3) the assessment of their quality by comparison with an assumed baseline. The implementation of the EBQI is non-destructive, relatively robust, according to the selection of the compartments and to their weighting, and is associated with confidence indices (both at the level of each compartment, and for the overall score), thus indicating possible weaknesses and biases in the data and therefore the need for further field data acquisition.