Long-term effects of the water-accomodated fraction (WAF) of diesel oil on rocky shore populations maintained in experimental mesocosms

The long-term effects of continuous doses (average hydrocarbon concentration =129.4 μg L-1 and 30.1 μg L-1) of the water-accommodated fraction (WAF) of diesel oil on 15 rocky littoral populations were determined at three tidal levels in experimental mesocosms over two years. At each tidal level, most species exhibited similar abundance changes in both oil-contaminated and control (average background hydrocarbon concentration = 5.6 μg L-1) mesocosms. Significant changes in species abundances attributable to oil (WAF) were demonstrated for only two of ten seaweeds and three of five invertebrates. Compared with the other mesocosms, significantly greater reductions in upper-level cover were recorded in the basin receiving the highest oil dosage for the seaweeds Phymatolithon lenormandii and Fucus evanescents together with lower recruitment of the barnacle Semibalanus balanoides. The mussel Mytilus edulis was storngly affected by the oil treatments and essentially disappeared from both oil-contaminated mesocosms. Numbers of the starfish Asterias rubens also fell to zero at the lowest tidal level in the basin receiving the highest oil dosage. There were no demonstrable differences in the abundance patterns of the gastropod Littorina littorea, the crab Carcinus maenus, and a total of eight brown (Ascophyllum nodosum, Fucus serratus, F. vesiculosus, Laminaria digitata), red (Chondrus crispus), and green (Cladophora rupestris, Enteromorpha spp., Ulva lactuca) seaweeds in the oil-contaminated compared with the control mesocosms. (orig.)