Composition, abundance and stratification of macrobenthos in the marine area impacted by tar aggregates derived from the Haven oil spill (Ligurian Sea, Italy)

The impact of tar aggregates deriving from the Haven oil spill that occurre d in 1991 on the soft-bottom macrobenthos off Arenzano (Ligurian Sea, NW Me diterranean; Italy) was investigated in this study. Core samples were colle cted to evaluate the composition, abundance and stratification of macrobent hos in three contaminated and three control sites. The distribution of tar aggregates (often settled by hydroids, bryozoans and serpulids) in the impa cted area did not show distinct patterns related to sediment depth. The mac robenthos was numerically dominated by polychaetes, followed by crustaceans (amphipods, isopods and tanaids) and sipunculids, whereas bivalve molluscs were less represented. Ophiuroids and nemerteans were only occasionally fo und, Stratification was the main factor explaining the distribution of bent hic invertebrates, highest abundances being observed in the upper 5 cm of s ediment, while densities of individuals declined sharply in deeper sediment layers, No significant differences were observed in the abundance of both the whole benthic assemblage and of the main benthic taxonomic groups betwe en contaminated and control sites. The higher abundance of sipunculids at t he contaminated sites, instead, was attributed to the availability of empty tubes of polychaetes in which they take refuge, The polychaetes/amphipods ratios did not show strong differences between impacted and control sites, Multivariate analysis (MDS) showed a wide scattering among deep bottom samp les and a comparatively high similarity among intermediate and superficial layers, regardless of the contamination level. In conclusion, the results r eported here suggest that tar aggregates do not determine appreciable detri mental effects on the soft-bottom macrobenthos, which appeared to have reco vered towards natural pristine conditions. (C) 2000 Elsevier Science Ltd. A ll rights reserved.