The impact of oily discharges on the meiobenthos of the North Sea

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Abstract
The impact of hydrocarbon discharges on the intertidal and subtidal meiobenthos of the North Sea is examined primarily by a consideration of two field investigations. The first study examines the effects of an oil refinery discharge on intertidal meiofauna in the Firth of Forth, while the second describes the impact of oil platform discharges on the surrounding meiobenthos. The impact of the refinery effluent is only clearly distinguishable upstream of the discharge, as downstream the effects are confused with those of a second petrochemical discharge. The meiofaunal community is only strongly affected on the upper shore and this appears to be chiefly the result of an organic enrichment effect causing a raising of the redox potential discontinuity (RPD) layer. All meiofaunal taxa examined are sharply reduced in density and species richness within 320 m of the discharge but at 600-900 m from the discharge meiofaunal densities are enhanced or depressed, relative to clean sediments, dependent upon the seasonal pattern of the RPD layer. Farther down the shore the impact is only felt at most by a slight reduction in species richness and subtle change in species abundance patterns on the middle shore for a distance of about 600 m. The meiofaunal responses to the petrochemical discharges seem similar to those described for the macrofauna in the same area, although a small meiofaunal population persists in the most polluted sediments in the absence of macrofauna. The discharge of drilling cuttings, contaminated with oil-based drilling mud, was found to strongly modify meiofaunal densities within 800 m of the Beryl A Platform. Nematode densities are strongly reduced in the vicinity of the platform and it is thought that the impact on this infaunal taxon may be due to slow degradation within the sediment of toxic fractions of the diesel base of the drilling mud. By contrast copepod densities were greatly enhanced in one survey and the difference in impact is considered to be due to the epibenthic habit of the species involved, enabling them to flourish in conditions of high food or low predation and competition or all three. The species involved seem typical members of meiofaunal communities of organically enriched sediments. Some improvement in meiofaunal densities throughout the period 1984-85 is thought to be possibly the result of a switch from diesel-based to lowtoxicity drilling muds. It is concluded from these and other studies that hydrocarbon discharges into the North Sea are unlikely to be causing extensive damage to meiofaunal communities.