EXPERIMENTAL FIELD-STUDY OF THE EFFECTS OF CRUDE-OIL, DRILL CUTTINGS AND NATURAL BIODEPOSITS ON MICROPHYTOZOOBENTHIC AND MACROZOOBENTHIC COMMUNITIES IN A MEDITERRANEAN AREA

Effects of an experimental pollution by neogenous and fossil organic m atter on microphyto- and macrozoobenthic communities were studied in s itu over a 1 yr period (July 1989 to May 1990) in a shallow microtidal bay (Gulf of Fos, south coast of France). Three experimental enclosur es 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 i n the third enclosure contained diesel oil-based cuttings (CUT). Pollu tion by contaminants did not prevent microphytobenthos from colonizing sediments. Population changes over time were quite similar in all enc losures, except in CUT, where a four times higher chlorophyll a conten t appeared to be related to a decreased number of grazers and conseque ntly lower grazing rates of animals. Toxicity to the fauna was immedia te in the case of BAL and occurred within 3 mo at CUT. Opportunistic s pecies settled in all contaminated sediments; this occurred quite rapi dly in BD and BAL which recovered within 3 mo to levels comparable wit h control sediments. In CUT, natural populations had not recovered aft er 1 yr, whereas a quasi-monospecific population of Capitella capitata was still present. During the first 3 mo, the oxygen demand of the se diment was higher in oil-contaminated sediments than in controls. On t he whole, the changes in fluxes and organism assemblages in our weakly tidal area appear to be consistent with other findings in macrotidal seas.