Optimisation de la surveillance des masses d’eau DCE en Méditerranée. Utilisation de produits dérivés d’images satellites en complément du réseau de mesures in-situ

This study aims at showing the potential of « water colour » data derived from satellite imagery to optimise the monitoring of coastal waters within the WFD, with the French Mediterranean as a study area. The work is focused on one specific parameter, the surface chlorophyll-A (chl-a) concentration of which a monthly atlas for reference to “normal” values is provided, which consists of : (i) the monthly chl-a median computed from satellite data of 2003, 2004 and 2005 ; (ii) the integration in space of these values, represented by their average over the WFD water masses (except those of very small size) ; (iii) an indicator of the spatial variability of the chl-a through the spatial standard deviation within each water mass. Another important result is the characterisation of the ecological status of the WFD water masses by the 90th Percentile of the Chl-a concentration compared to a « good status threshold » as defined for the WFD for the Mediterranean. The study highlights the complementarity between in situ measured data, very local but precise and exhaustive, and the parameters accessible through satellite imagery that provide total coverage and allow to perform spatial analysis, and therefore to consider the notion of heterogeneity within water masses. This last aspect can have important implications in evaluating how the water masses were designed and in assessing the placement of the measuring points for a good representativity of the entire water mass