Differential sensitivity of macrozoobenthic species to discharges of oil-contaminated drill cuttings in the North Sea

Abstract

To assess the effects on macrozoobenthos of discharged drill cuttings with adhering drilling mud, monitoring studies have been performed around several drilling locations in the Dutch sector of the North Sea since 1985. Altogether 11 surveys were carried out at well sites where oil-based drilling muds (OBM) had been used, and 4 at locations where water-based muds (WBM) had been used. The resulting data-matrices, comprising the numerical densities of species at increasing distances from the discharge sites, were inspected for consistent patterns in the spatial abundance of individual species around the sites. Of the species that were abundant by nature at most locations, 15 showed reduced abundance near OBM discharge sites on five or more occasions. These species are considered useful indicators of OBM stress in the Dutch sector of the North Sea. None of them showed reduced abundance at WBM locations. Only one species, the opportunist polychaete Capitella capitata, consistently occurred in increased numbers near OBM locations. It did not show this response at WBM locations. Oil on drill cuttings is apparently a major stress factor affecting the macrofauna around drilling sites. The species showing the strongest response to OBM cutting discharges were the amphipod Harpinia antennaria, the echinoid Echinocardium cordatum and the bivalve Montacuta ferruginosa. Their numbers were frequently found to be reduced up to a few km from OBM locations. In view 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 sensitive species of the two.