Experimental field study of the effects of crude oil, drill cuttings and natural biodeposits on microphyto- and macrozoobenthic communities in a Mediterranean area

Effects of an experimental pollution by neogenous and fossil organic matter on microphyto- and macrozoobenthic communities were studied in situ over a 1 yr period (July 1989 to May 1990) in a shallow microtidal bay (Gulf of Fos, south coast of France). Three experimental enclosures of 1 m² non-defaunated sediments were covered with 1 cm of polluted defaunated sediments. The sediment in one enclosure contained natural biodeposits with a high organic matter content (BD), that in a second enclosure contained Arabian light crude oil (BAL), and the sediment in the third enclosure contained diesel oil-based cuttings (CUT). Pollution by contaminants did not prevent microphytobenthos from colouizing sediments. Population changes over time were quite similar in all enclosures, except in CUT, where a four times higher chlorophyll a content appeared to be related to a decreased number of grazers and consequently lower grazing rates of animals. Toxicity to the fauna was immediate in the case of BAL and occurred within 3 mo at CUT. Opportunistic species settled in all contaminated sediments; this occurred quite rapidly in BD and BAL which recovered within 3 mo to levels comparable with control sediments. In CUT, natural populations had not recovered after 1 yr, whereas a quasi-monospecific population of Capitella capitata was still present. During the first 3 mo, the oxygen demand of the sediment was higher in oil-contaminated sediments than in controls. On the whole, the changes in fluxes and organism assemblages in our weakly tidal area appear to be consistent with other findings in macrotidal seas.