Impact of two off-shore gas platforms on the surrounding benthic communities (Western Adriatic Sea, Italy)

The soft-bottom macrobenthic communities living around two 4-leg platforms, (Barbara NW, pB, and Calipso, pC) located at different depth in the Western Adriatic sea, were investigated for 3 years after their installation, to detect the eventual impacts due to the drilling operations. The sampling sites were arranged along two ideal transects intersecting at the structure, the first one oriented Northeast-Southeast and the second one orthogonal to the first. The sampling stations were spaced at increasing distance from the centre up to 120 m for pB and up to 250 m for pC. Six field cruises were performed from 2000 to 2002 and from 2003 to 2005 at pB and pC respectively and at open-sea control sites. In the first survey the benthic community at pB site was very poor in terms of individuals and taxa. However, from 30 m far from the rig onwards all the biological indices (species abundance, mean richness and Shannon diversity) increased reaching similar values to those recorded at the reference sites and this situation was recorded during all the subsequent sampling periods. Also the benthic community living around pC was composed by a small number of individuals and species during the first survey, without any particular difference with the reference sites. This scenario still persisted along the transects during the second cruise, while species abundance and mean richness increased at the reference sites. From the second year, all the biological indices along the transects became similar to those recorded at the controls. Differently, a gradual change in the benthic assemblage occurred since the second monitoring year at pB and pC sites, due to mussel mound development with associated hard bottom species, which increased the natural community complexity.