While coastal ecosystems experience increasing pressures due to anthropogenic activities and climate change, measurement of functional diversity based on the biological traits analysis of species (BTA) is increasingly used as tool to assess ecosystem functioning and its responses to disturbance. Based on data from a long-term monitoring of the benthic macrofauna community of the eastern Bay of Seine (~ 50 stations from 1991 to 2016), an area exposed to numerous anthropogenic disturbances, our current study aims to investigate the link between species and functional diversity at local (α-diversity) and regional (ß-diversity) scales, as well as to assess how changes in the community structure and species composition may have altered the ecosystem functioning. A ‘traits-by-species’ matrix including seven traits (i.e. body size, life span, feeding type, development mode, mobility type, sediment reworking type and tolerance to disturbance) chosen according to their relevancy for three major ecosystem functions (i.e. secondary production, nutrient cycling, resilience) was built. To cover the different facets of α- diversity (i.e. richness, eveness and heterogeneity), a selection of species, taxonomic and functional diversity indices were calculated at each station. For each campaign, the links between these different metrics was determined from a Principal Component Analysis. The ß-diversity on a regional scale was analyzed by applying clustering and non-parametric Multidimensional scaling on both species and traits composition matrices (based on abundances of species and traits). The results showed that population fluctuations of a few very abundant species led to the major variations observed in the structure of the macrobenthic community in both taxonomic and functional aspects. A certain redundancy was found among species and functional diversity indices in terms of richness, evenness and heterogeneity. Likewise, at regional scale, similar patterns were reported on the spatial structure of the community along an inshore-offshore gradient described from species or traits composition. These results question the effort/profit of the FD for providing new information to the understanding of community structure and dynamics. To improve the measurements of functional diversity and facilitate the interpretation of our results, some suggestions are proposed (e.g. measure functional diversity on biomass rather than on density data; distinguish ‘effect traits’ and ‘response traits’). In parallel, a review of more than 75 publications using BTA on benthic communities from early 2000s to present was carried out and highlights large variations in the methods used in BTA in terms of the number of traits, the selected traits, or the variable used to describe the relative abundance of traits (density vs. biomass).