PHYSICOCHEMICAL AND BIOLOGICAL FEATURES OF A DRILLING SITE IN THE NORTH-SEA, 1 YEAR AFTER DISCHARGES OF OIL-CONTAMINATED DRILL CUTTINGS

In the framework of the Paris Convention for the prevention of marine pollution, the Dutch government initiated a research programme on environmental effects of discharges of contaminated drill cuttings from platforms in the North Sea. The programme has run since 1985 and includes field surveys at drilling sites in different parts of the Dutch sector. One of the locations investigated is the former drilling site F18.9, situated on the Oyster grounds. The (abandoned) location was visited in 1988, 1 y after the exploratory drilling ended. This paper gives a brief overview of the 1988 study at this discharge site, which was investigated again in 1990 as part of the Bremerhaven Workshop. Chemical analyses of the sediment over a transect in the residual current direction revealed a clear gradient in contamination levels, with oil concentrations ranging from 250 to 450 mg kg-1 dry sediment within 250 m and gradually decreasing to < 20 mg kg-1 beyond 750 m. Barium concentrations showed a similar pattern. Subtle biological effects, indicated by the absence of a very sensitive species (Montacuta ferruginosa), were found as far as 2 km from the discharge site. Abundance patterns of 2 other sensitive species (Harpinia antennaria and Callianassa subterranea) suggested environmental stress to occur up to 1 or 2 km. Within 750 m from the discharge site an increasing number of species, including the dominant Amphiura filiformis, appeared to decrease in abundance, successively resulting in a reduced total macrofauna abundance and species richness. Typical opportunistic species were observed more frequently only within a radius of 250 m. An alternative measure of diversity ('relative macrofauna abundance') was introduced and its merits are discussed. It is suggested that a combination should be used of community-based parameters (e.g. species richness, relative abundance) and species-based parameters (e.g. presence of opportunistic species, absence of susceptible species) to describe the impact of pollution. In this way 7 accumulating effects were distinguished. At contamination levels > 100 mg oil kg-1 dry sediment all defined effects occurred, whereas below 20 mg kg-1 the number of effects rapidly decreased.