DIFFERENTIAL SENSITIVITY OF MACROZOOBENTHIC SPECIES TO DISCHARGES OF OIL-CONTAMINATED DRILL CUTTINGS IN THE NORTH-SEA

To assess the effects on macrozoobenthos of discharged drill cuttings with adhering drilling mud, monitoring studies have been performed aro und several drilling locations in the Dutch sector of the North Sea si nce 1985. Altogether 11 surveys were carried out at well sites where o il-based drilling muds (OBM) had been used, and 4 at locations where w ater-based muds (WBM) had been used. The resulting data-matrices, comp rising the numerical densities of species at increasing distances from the discharge sites, were inspected for consistent patterns in the sp atial abundance of individual species around the sites. Of the species that were abundant by nature at most locations, 15 showed reduced abu ndance near OBM discharge sites on five or more occasions. These speci es are considered useful indicators of OBM stress in the Dutch sector of the North Sea. None of them showed reduced abundance at WBM locatio ns. Only one species, the opportunist polychaete Capitella capitata, c onsistently occurred in increased numbers near OBM locations. It did n ot show this response at WBM locations. Oil on drill cuttings is appar ently a major stress factor affecting the macrofauna around drilling s ites. The species showing the strongest response to OBM cutting discha rges were the amphipod Harpinia antennaria, the echinoid Echinocardium cordatum and the bivalve Montacuta ferruginosa. Their numbers were fr equently found to be reduced up to a few km from OBM locations. In vie w of the commensal relationship between M. ferruginosa and E. cordatum , their relative sensitivity is discussed in detail. The host species E. cordatum (particularly adult specimens) is probably the really sens itive species of the two.