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

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.

Effects of drill cuttings discharge on meiofauna communities of a shelf break site in the southwest Atlantic

“…This result is probably an opportunistic response from some epibenthic copepods, able to tolerate barium, and colonize drill cutting deposits with low TPH. Moore et al (1987) also observed a similar response near a drilling platform in North Sea, where the abundance of copepods was either unaffected or significantly increased, suggesting that living copepod species flourished in more oxygenated conditions where bacteria levels are enhanced and predation pressure reduced. Similarly, experiments with copepods and nematode assemblages in microcosms dosed with different toxicity levels of oil-based drill cuttings showed a markedly increase of copepods in all low-toxicity treatments (Leaver et al 1987).…”

“…Due to their ubiquitous distribution in nature, high abundance and diversity, intimate association with sediments, fast reproduction, and rapid life histories, meiofauna are widely regarded as ideal organisms to study the potential ecological effects of natural and anthropogenic impacts (reviewed by Coull and Chandler 1992). Despite this, information on the effects of cutting discharge on meiofauna community structure at lower taxonomic levels (e.g., genus composition) is, to our knowledge, limited to one study along the banks of an estuary in the North Sea, where the fluid used was mineral oil (Moore et al 1987) and another at the slope of SW Atlantic, drilled with paraffin-based NAF type III (Netto et al 2009). Also, most previous studies have so far reported on the impact of oilbased fluids (Olsgard and Gray 1995;Moore et al 1987) which toxicity is known to be much higher than water-based fluids (Neff et al 2000).…”

“…Despite this, information on the effects of cutting discharge on meiofauna community structure at lower taxonomic levels (e.g., genus composition) is, to our knowledge, limited to one study along the banks of an estuary in the North Sea, where the fluid used was mineral oil (Moore et al 1987) and another at the slope of SW Atlantic, drilled with paraffin-based NAF type III (Netto et al 2009). Also, most previous studies have so far reported on the impact of oilbased fluids (Olsgard and Gray 1995;Moore et al 1987) which toxicity is known to be much higher than water-based fluids (Neff et al 2000). Particularly the effects of cuttings discharge embedded in NAF-type III, one of the lightest (with lower amount of aromatic chains) among the syntheticbased fluids, is largely unknown.…”

“…The discharge of cuttings may affect the marine benthic fauna by two processes-physical and chemical (e.g., Moore et al 1987;Leaver et al 1987;Netto et al 2009). Effects with a physical nature are mainly those conditioned by the cuttings' properties, which vary regarding their shape and size, as well as the final volume deposited.…”

In situ experiments on recolonization of intertidal mudflat fauna to sediment contaminated with different concentrations of oil

“…Macrofaunal colonization did not seem to be affected by oil concentrations up to 600 ppm, and increasing oil concentrations above 1 000 ppm led to reduced densities of most species. Some field and experimental studies indicate that oil concentrations of about 2 000 ppm in sediments is 182 the level where detrimental effects begin (DICKS & HARTLEY 1982;DECKER & FLEEGER 1984;MOORE & al. 1987).…”