Assessment of Natural Cleaning and Biological Colonization On Oiled Rocky Shores: In Situ Experiments

ABSTRACT When exposed rocky shores are affected by oil spills, the advised cleanup option, in most cases, is the “do nothing” This assumes that natural processes should rapidly clean up such shores and that remedial actions should have great detrimental effects in regard of ecological recovery. Few studies however deal with quantitative rates of natural cleanup on rocky shores. Therefore, CEDRE (Centre de Documentation de Recherche et d'Experimentations sur les Pollutions Accidentelles des Eaux) has conducted field experiments to determine the rate of such processes. Granite plates have been polluted with Arabian Light crude oil and Bunker C and set on exposed and sheltered sites. On an exposed site, the influence of tidal elevation has also been studied. The plates were situated in diverse biological communities; the recolonization of polluted and non polluted plates has been recorded during a 13-month survey. The results show that the Arabian Light crude oil was rapidly washed away by the tide despite low wave energy even on the most exposed site. The persistence of Bunker C was much longer and seemed to be mainly a function of fauna and flora settlement. At the beginning of the survey, all the plates at the exposed site were colonized by barnacles in equal densities whatever the nature of the oil. A second recruitment wave of barnacle colonized preferably the “crude oil plates” which in fact were clean at that time. Along the tidal gradient, non polluted lower plates were colonized by Porphyra and Fucus while the polluted ones were essentially colonized by green algae.