Community structure changes with pollution or stress. In the Water Framework Directive, high ecological status through biological parameters is defined as a slight or minor deviation from the reference community, while the good status is defined as a small deviation. To assess the importance of this deviation, and then to measure the degradation of ecological status along a river, an index based on the concept of ecological distance between species was created and called EDI (Ecological Distance Index). It was tested on diatom data from a pilot watershed (the Garonne river basin, South-West France). The results show a good correlation between the EDI and the IPS (Indice de Pollusensibilité Spécifique - Specific Pollusensitivity Index-, a diatom-based biotic index) ecological ratios, which means first that the EDI is a valuable indicator of ecological status, and that it can account for ecoregional specificities. This index can be applied to any communities (macro-invertebrates, fish, ), since i) typical reference communities are found for each river type and ii) species are characterised then ranked by pollution sensitivity values.