Initial effects of a moderate-sized oil spill on benthic assemblage structure of a subtropical rocky shore

The environmental impacts of very large oil spills are well documented across a range of settings. However, there is a dearth of information about the immediate effects, and post-spill trajectories, of small to moderate (<1000 t) oil spills on intertidal biota. The published studies are from very different environments, and are contradictory in terms of the severity of initial impacts. This study reports on the effects of a 270 t spill of bunker fuel oil on 11 March 2009, approximately 13 km east of Cape Moreton, eastern Australia. We examined the initial effects of this moderate sized spill on the rocky shore biota of Cape Moreton, and quantified the trajectory of oil removal and change in assemblage structure over the next 5 months. Compared to adjacent reference sites, the initial effects were very marked, especially on the upper shore. Oiling was heavier and more persistent on the upper shore than the mid-shore, and biological effects were more pronounced higher in the intertidal. At both levels, however, there was little evidence of recovery up to 5 months after oiling, and visible oil residues were still apparent. The effect size was larger than previously reported for spills of this magnitude, comparable to that of larger spills, although over a smaller stretch of coastline.