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Marine Ecology Progress Series

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MEPS 122:277-306 (1995)  -  doi:10.3354/meps122277

A comprehensive analysis of the effects of offshore oil and gas exploration and production on the benthic communities of the Norwegian continental shelf

Olsgard F, Gray JS

Multivariate statistical analyses of data on environmental variables and benthic fauna from 14 oil and gasfields obtained from 24 surveys collected between 1985 and 1993 are presented. At all fields oil-based drilling mud was used. The purpose of this study was to investigate contamination gradients, assess effects on benthic fauna both spatially and temporally and to evaluate measures such as diversity indices, indicator species and multivariate analysis techniques in assessment of pollution. Results from analyses of baseline surveys of environmental variables and fauna were characterised by a lack of distinct gradients in station placement, having a typical shot-gun pattern in PCA-, DCA- and MDS-ordination analyses. Likewise there was no consistency in which environmental variables correlated with the fauna. Contamination was assessed using all the physical and chemical data in classification and PCA-ordination analyses. Clear patterns were found using 4 categories, conveniently termed initial, moderate, severe and gross. The categories were usually apparent as rings radiating from the platform. Initial contamination of the outermost areas at most fields was shown as elevated levels of barium and total hydrocarbons (THC) and sometimes also by elevated levels of zinc, copper, cadmium and lead. Three fields were studied in particular and showed contaminated areas of over 100 km2 (Valhall), over 15 km2 (Gyda) and over 10 km2 (Veslefrikk). After a period of 6 to 9 yr contamination had spread, so that nearly all of the outermost stations 2 to 6 km away from the platforms showed evidence of contamination. Thus, the existing sampling design is no longer suitable for assessment of the area contaminated. Effects on the fauna showed, as with contamination, 4 categories. Analyses linking fauna and environmental variables indicated that the effects were mainly related to THC, barium and strontium, but also to metals like zinc, copper, cadmium and lead, which are all discharged in drill-cuttings. Effects on the fauna closely followed the patterns of contamination with only a few stations at each field that were contaminated not showing effects. Thus the areas showing effects were only slightly less than the areas contaminated. Subsequent to cessation of discharges biodegradation of oil and reduced concentrations of THC were observed. Yet there was an extension of areas where the fauna was affected several years after cessation of drill-cutting discharges.This may indicate that barite and related compounds associated with the discharges also have an environmental impact. However, preliminary results from fields using only water-based mud clearly indicate a reduction in environmental contamination and biological impact, compared to effects reported here, for oil-based drill-cuttings. Diversity indices applied to the data did not show the extent of effects and such indices alone should not be used to interpret changes. The consistent patterns that the multivariate techniques were able to detect showed that these methods were far superior. Analyses of the initial effects on the fauna showed that there were no consistent patterns in changes in species composition over fields or time, and thus the search for 'universal' sensitive indicator species does not seem to be rewarding. Yet under gross effects of pollution there were consistent patterns with the same species dominating.Finally, the initial effects of pollution included severe reductions in organisms that are key components of the benthic communities and also food for bottom-living fish, and are thus ecologically important. The new fauna which establishes in the contaminated sediments close to platforms, often with high abundance, will probably be less valuable as a food source for fish populations since it is of small size and lives sub-surface.

Macrobenthos · Multivariate analyses · Norway · Offshore · Oil drilling

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