Spatial changes in species composition and structure of sublittoral soft-bottom polychaete assemblages along a transect parallel to the shoreline off the Barcelonès and Maresme regions on the Spanish Catalan coast (NW Mediterranean sea) are described. The transect (20 m mean depth) covers 30 km through an area affected by both water sewage discharges and river outflow. These effluents generate a spatial gradient of both silt and organic enrichment, according to the prevailing southerly currents in the zone. South of the effluents, eutrophized sediments supported a macrofaunal assemblage which was typical of polluted bottoms, characterized by the massive presence of Capitella capitata and Malacoceros fuliginosus, high abundance and biomass, low values of species richness, and a simplified trophic structure dominated by surface and subsurface-deposit feeders. North of the effluents, both the mud and organic matter content in sediment decreased drastically and the polychaete assemblages described were progressively less affected with distance. Thus, a clear change in species composition, as well as low values of abundance and biomass, increases in species richness, and a more complex trophic structure on the polychaete assemblages was observed north rather than south of the effluents.

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