Marine Pollution Bulletin
Notes on the Condition in September 1978 of some Intertidal Sands Polluted by A M O C O CADIZ Oil N. A. H O L M E
Marine Biological Association of the United Kingdom, Plymouth
At the end of the summer a conspicuous feature of some beaches which had been heavily polluted by AMOCO CADIZ oil six months previously was the strong development of grey or black layers close to the surface in sands which would not normally be discoloured at this depth,
Visits were made to the following beaches in north Brittany on 28-30 September, 1978: two sites on the W. side of the Presqu'ile de Ste Marguerite (8 km E. of the wreck) and sites at St Michel-en-Gr~ve and Ile Grande (ca. 90 km E.) which had also been very heavily oiled. At all these sites the rocks around high water were still polluted with oil, and on the Ile ~ Canton, o f f Ile Grande, heavy pollution of rocks extended well into the intertidal zone. A p a r t f r o m a few blobs of oil (of uncertain origin) at one site at Ste Marguerite the strand lines were remarkably clear of lumps of oil, there being no evidence at this time of submerged oil being washed ashore. Signs of slight pollution were seen here and there in the f o r m of an iridescent film or light brown scum at the water's edge. Continuing pollution of the sediments was revealed when holes were dug in the sand. The most severe instances were seen at three places: in the upper and midtide regions on the sheltered shore between Ile Grande and Ile ~ Canton; in a zone near high water m a r k along the beach at St Michel-en-Gr~ve (pits and trenches had been dug here during the clean up operations), and in a small west-facing cove on the Presqu'ile de Ste Marguerite. In the worst examples there was a distinct slimy film on the s u r f a c e - possibly formed of a mixture of oil, sediment and b a c t e r i a - b e l o w which the sand was very black f r o m the surface down, with brown oil visible in the sediment typically at 10-15 cm depth. Where there was no slimy layer on the surface, as at St Michel-enGr~ve, the surface sand was normal in colour, turning to black witbin 1 cm. At the latter site the blackened sand continued as far down as was investigated, to a b o u t 50 cm, there was a brown colouration at about 15 cm, indicating an oil-rich layer, and as the hole filled with water its surface soon became partially covered with liquid brown oil. In other areas, such as on the lower parts of the beach at St Michel-en-Gr~ve and on the greater part of the beach at Ste Marguerite, the presence of lower concentrations of oil was indicated by a slight brown scum or by iridescence on
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the surface of water filling holes dug in the sand. At Ste Marguerite grey or black layers often occurred at a depth of only I or 2 cm, and in places were even exposed at the surface, presumably resulting from very recent removal of sand by waves. The presence of grey or black sulphide layers is indicative of reducing conditions probably caused by bacterial and other oxidative processes. These had been acting on the oil, but were now largely arrested until such time as disturbance of the sediment a n d / o r effective flushing could renew the oxygen supply. W a r m and relatively stable conditions during the latter part of the summer would have favoured the development of such anaerobic sands. Grey and black layers were sought but not observed in mid May in some of these habitats (M. F. Spooner, personal communication). The black layers here described were quite distinct f r o m those attributable to high concentrations of rotting algal material which often occur near rocks. It is thought that the m a n y less-polluted sands, and those which were readily permeated by sea water, had already lost much of their oil content by this time, but where pollution had been heavy or where the sediment was less permeable because of its high silt or clay content biodegradation was occurring very slowly, so that it might take m a n y months, or even years, for the sediment to become clean. It is likely that on the more waveexposed beaches polluted sands will become dispersed through normal seasonal shifts in sediments during the winter, when beaches are often at a lower level than in summer. Where no such movements occur, as seems likely to be true for the sheltered areas between Iie Canton and Ile Grande, pollution may persist for a long time. Arenicola and certain other polychaete worms inhabited all but the most heavily polluted stretches of sand. Investigations were not sufficiently detailed to assess the survival of bivalve molluscs and small crustaceans. None were seen; they do not normally inhabit sediments in which an intense black or grey layer occurs close to the surface. Recolonisation by meiofauna and other infauna of these particularly polluted beaches may be hindered as much by the reducing conditions as by any continuance of toxicity of the remaining oil.