Intertidal biogenic reefs built by the polychaete Boccardia proboscidea in sewage-impacted areas of Argentina, SW Atlantic

The rocky intertidal zone around the city of Mar del Plata (SW Atlantic, 38 degrees S-57 degrees W) is characterized by dense mussel beds of Brachidontes rodriguezii. This intertidal community develops on natural and artificial hard substrates, including abrasion platforms in sewage-impacted areas. A monitoring program, designed to assess the effect of sewage discharge on intertidal macrobenthic communities, has been conducted since 1997. During the spring season of 2008, a new spionid polychaete (Boccardia proboscidea) was found near the sewage outfall, forming large biogenic reefs. This is the first report of biogenic reefs being built by a non reef-forming spionid polychaete in areas organically impacted by sewage discharges. The aim of this work was to evaluate the spatial-temporal dynamics (% cover and density of B. proboscidea) of these reefs. These biogenic reefs covered almost the entire impacted site, reaching a density of 650,000 ind center dot m-2. This phenomenon is unique in that there is no other record available worldwide of any other biogenic polychaete reefs that could be sewage-induced. The presence and stability of these biogenic reefs is discussed in relation to increased organic contamination as a structuring factor.