Physico-chemical and biological features of a drilling site in the North Sea, 1 year after discharges of oil-contaminated drill cuttings

Abstract

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-' dry sediment within 250 m and gradually decreasing to < 20 mg kg-' 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-' dry sediment all defined effects occurred, whereas below 20 mg kg-' the number of effects rapidly decreased.

Contaminants of Concern in Dutch Marine Harbor Sediments

“…These compounds are persistent in silty anaerobic marine sediments and can cause toxic effects in marine life (Daan et al 1992;Ten Hallers et al 1994;Brouwer et al 1989;Wolfe et al 1998). Bioaccumulation is a particular phenomenon for PCBs and mercury.…”

Introduction to the Bremerhaven Workshop on Biological Effects of Contaminants

“…The latest information on migration of the dab in the German Bight (Rijnsdorp et al 1992) is necessary for the correct interpretation of fish disease as a measure of the effects of pollution (see Vethaak 1992). Essential background information on the drilling site gradient is provided by Daan et al (1992), who describe the site and its contamination by drilling muds, and review their earlier studies which demonstrated a pollution gradient, relying on benthic ecology alone.…”

“…The site was located at 54O06'09"N 04"45'25"E and the transect was oriented in the direction of the residual current ( = 66"). R is the reference station 15 km NNW at 54'1 1'03"N 04O38'55"E contaminants which provided a clear gradient of biological impact on benthic community structure when studied earlier (see Daan et al 1992).…”

“…While historical data demonstrate stable contamination gradients associated with the Elbe/Weser plumes (Dethlefsen 1989) and the Dutch drilling platform (Daan et al 1992), it was essential to have synoptic chemical data for compounds Likely to be toxic to interpret the biological data from each gradient. The contaminant regime can change so rapidly that historical contaminant data may have little relevance.…”

“…1): a transect of 9 stations running northwest from an area where the Rivers Elbe and Weser flow into the North Sea out to the Dogger Bank, and a second transect down-current from a disused drilling site off the Dutch coast. The second gradient was due to a single type of input (drilling muds) which had created a steeper gradient (Daan et al 1992), less likely to be confounded by natural variables, or the complexity and number of inputs. The final format of the workshop was presented to the Statutory Meeting of ICES in The DRILUNG AREA.…”