The diversity and distribution of metazooplankton across the Mediterranean Sea was studied along a 3000 km long transect from the eastern to the western basins during the BOUM cruise in summer 2008. Metazooplankton were sampled using both a 120 µm mesh size bongo net and Niskin bottles at 17 stations. Here we report on the 5 stock, the composition and the structure of the metazooplankton community. The abundance was 4 to 8 times higher than in several previously published studies, whereas the

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... omass remained within the same order of magnitude. An eastward decrease in abundance was evident, although biomass was variable. Spatial (horizontal and vertical) distribution of metazooplankton abundance and biomass was strongly correlated 10 to chlorophyll-a concentration. In addition, a clear association was observed between the vertical distribution of nauplii and small copepods and the depth of the deep chlorophyll maximum. The role of environmental factors is also discussed. Cluster analysis allowed us to define a regionalization of the Mediterranean Sea based on the abundance and diversity of metazooplankton. We found a north-south distinction in the 15 western basin and a longitudinal homogeneity in the eastern basin. The Sicily Channel appeared as an intermediate region. The specific pattern of distribution of remarkable species was also described.