Preliminary observations on pollution of the sea bed and disturbance of sub-littoral communities in northern Brittany by oil from the Amoco Cadiz

Volume 9 / N u m b e r 11/November 1978

Preliminary Observations on Pollution of the Sea Bed and Disturbance of Sub-Littoral Communities in Northern Brittany by Oil from the A M O C O CADIZ LOUIS CABIOCH, JEAN-CLAUDE DAUVIN and FRANCK GENTIL Station Biologique de R oscoff, 29211, R oscoff, France

When, on 17 March 1978, the oil slickg released from the wreck of the A M O C O CADIZ began to spread along the coast of north-west Brittany, they rapidly reached their full extent on the beaches, covering an area from Pointe Saint Mathieu to Br6hat. These initial events have been the subject of preliminary reports dealing mainly with surface phenomena and the arrival of the oil on shore (O'Sullivan, 1978; Hess, 1978) and describing the initial effect on the intertidal fauna. This will form the subject matter of the majority of observations. But one of the peculiarities of the A M O C O CADIZ oil spill, compared to that of the TORREY CANYON, has been the rapid penetration of a considerable amount of oil into the sea bed. Consequently, we present the first observations on this phenomenon, using our previous knowledge of the area (Boillot, 1964; Cabioch, 1968).

The Sea Bed Morphology, Currents and Sediments Morphology Two very dissimilar morphological entities are recognized in the submarine relief of northern Brittany:

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the inshore slope and the offshore flats. The inshore slope is the continuation at depth of the complex structure of the littoral; here, the rocky outcrops of the Breton paleozoic pediment alternate with depressions filled with sediment. The off-shore flats continue on from this with, generally, a very distinct limit (Figs 1, 2, line G) which in the Roscoff area occurs at a depth of 65 to 70 m. They owe their low relief to the presence at depth of a tabular stratum of Eocene limestone overlying the ancient pediment and hiding its irregularities. The surface of the limestone is covered by a layer of Quaternary blocks, stones and gravels.

Currents and distribution of sediments The presence of strong tidal currents makes its mark on the whole area. Almost everywhere they exceed 2.5 knots up to 16 km from the coast. Their speed decreases gradually farther north and they are further weakened in places in bays (Bay of Morlaix, Bay of Lannion). The clearly visible succession of sediments on the sea bed corresponds to this hydrodynamic gradient; strong currents produce a pebbly bottom (Fig. 2, C, D),

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progressively enriched with gravels and sand as the speed of the currents decreases (Fig. 2, E). Finally in the North-West we find a layer of coarse zoogenous sands (Fig. 2, F). Despite the intensity of the currents, on the inshore slope the broken relief permits the deposition of coarse sediments in the depressions between rocky outcrops, The local weakening of the current in bays results in the presence of fine sands and silt. Figure 3 (B, F, G, H) illustrates this type of succession on entering the Bay of Morlaix. This example also shows the complex mosaic formed of areas of sediment and rock.

Benthic Communities Two important factors govern the distribution of benthic communities in the area under consideration; light absorbtion, which imposes vertical zonation on the communities and the nature of the sediment which is linked to the hydrodynamic gradients, The reduction in light intensity with depth leads, on hard substrates, to the disappearance of Laminarian populations at about 25 m and to the progressive appearance of a deep-water fauna (Axinella dissimi/is community). This limit which is common to many different species separates, in terms of communities, the infralittoral and circalittoral zones. The upper part of the infralittoral includes the lower part of the intertidal zone and the base of the circalittoral extends out over the whole continental shelf, 3O4

Where the sea bed is sheltered, light penetration permits the proliferation in shallow water of the free arbuscular Corallinaceans (Lithothamnium) which form beds of maerl (Fig. 3, D, E). However, the compositon of communities on a soft bottom is generally decided primarily by the nature of the sediment. Their distribution therefore follows hydrodynamic and sedimentary gradients (Figs 2, 3). In conclusion, the sublittoral benthic ecosystems upon which hydrocarbon contamination has fallen show hydrodynamic and sedimentary zonation characterised by a preponderance of substrates and communities subject to strong tidal currents, which are therefore well oxygenated and almost devoid of fine sediments. Fine sediments are confined to shallow waters in bays and estuaries. But there is also a vertical zonation which separates a circalittoral zone made up of continuous communities spread over wide areas or arranged in units close to each other and a scattered infralittoral zone in the bays forming a patchwork of small units, some of which (the Abra alba community in fine sand, maerl) are only to be found again at considerable distance from the polluted area.

Hydrocarbon Pollution-Penetration a t Depth The first two weeks of the oil spill were marked by a series of gales from the south- and north-west. The oil slicks accumulating along the coast were doing so in

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Fig. 3 Example of the distribution of communities in a bay: biosedimentary m a p of the Bay of Morlaix (After Cabioch, 1968). A: rocky bottoms. B - C : Amphioxus-Venusfasciata coarse sediment community which is relatively independent of vertical zonation (C: epifauna facies, with Sabellaria spinulosa). D - E : rnaerl community (D: coarse facies with Lithothamnium corallioides var. coralloiodes, E: mixed sediment facies, with L. corallioides vat. minima). F: .4bra prismatica fine sand dune community. G to I: Abra alba fine sediment community: (G: sandy facies with Hyalinoecia bilineata, H: muddy facies, with Melinna palmata, I: mixed sediment facies, with Pista cristata). 1-2-3: stations where population dynamics are being studied.

extremely turbulent conditions on a particularly varied coastline. It is known that a dispersion of oil in water in the form of fine droplets is thus liable to be produced, followed by adsorption onto particles of sediment in suspension (MacAuliffe, 1977; Huang & Elliott, 1977) which are abundant near coasts during bad weather. This results, on the one hand, in the stabilization of oil-inwater emulsion and, on the other, in the deposition of the oil-sediment particles on the sea bed when turbulence decreases. The possible effect of dispersants used to treat oil slicks at sea should also be born in mind, in this respect, On 21 March, the oil came ashore at Roscoff and on the eastern shore of the Bay of Morlaix between Saint Samson and Primel (Fig. 3). Its entry into the sediment was observed by one of us (J.C. Dauvin) on 3 April during a benthic sampling programme in the fine sand of Pierre Noire (Fig. 3, G) at a depth of 18 m, near the rocky coast which had been exposed to the oil for two

weeks. Shortly afterwards (13 and 18 April) it was also confirmed that the sublittoral channels of the Morlaix and Penz6 rivers had been equally heavily polluted, even though they had not been affected by large surface slicks. Taken together, these observations suggest that a process of dispersion and deposition occurred at depth, accompanied by transport along the bottom or in the water column with preferential deposition in areas where fine particles tend to accumulate as a result of the dynamic zonation of the whole region, that is to say in more or less silty fine sands (Fig. 3, G, H) or in mixed deposits composed of both coarse and fine sediments (Fig. 3, E, I). Meanwhile, scientists from the Centre Oc6anologique de Brettagne working on board the SUROITfrom 7-10 April had also observed contamination at various points in the Bays of Morlaix and Lannion down to a depth of 70 m (Trezen Vraz, Fig. 2) (M. Marchand, personal communication). We encountered the same conditions 3O5

Marine Pollution Bulletin

in the Bay of Lannion on 27 April with oil especially heavily concentrated in the fine Abra alba sand at 22 m and even present in coarse sands, More precise knowledge of the behaviour of oil on the sea bed was to be obtained by the use of underwater television. In May, we were able to observe the presence of dark irregular patches on the surface of the fine sand at Pierre Noire which, when sampled by divers, proved to be an oil contaminated silt. It is reasonable to assume that this oily silt, easily resuspended by turbulence, would be transported and deposited elsewhere. In other places the sand appeared normal on the surface despite contamination below. Thirdly, the sediments in places also contain little brown masses of 'chocolate mousse' (water-in-oil emulsion). In conclusion, these observations showed us the presence of oil in different types of sediment in shallow bays and suggested a tendency towards its dynamic accumulation in areas of fine sediment. We know that these sediments occupy only very limited areas and that their cleansing is liable to be slow, since oxygenation occurs only in a shallow superficial layer. It is probably here that the most long lasting disturbance resulting from the oil slicks will occur. It is even more worrying that the sublittoral channels of the Morlaix and Penz6 rivers, both heavily polluted, cross vast oyster beds and promise to form a reservoir of pollution, a source of chronic oyster contamination. It is different for the coarse sediments of the coastal slope and offshore pebble-gravel substrates. Here, hydrodynamics will discourage the persistence of particulate oil whilst facilitating its breakdown by good oxygenation, except perhaps in thick deposits, which may be polluted in their deeper layers,

Effects of Pollution on Benthic Communities We have seen that the impregnation of sediments by oil constitutes the longest lasting result of oil spill, Sedentary species inhabiting these substrates provide the most reliable evidence of the effects of pollution. Our precise knowledge of the previous state of these communities is a valuable guide here, the more so since one of us (J.C.D.) has been making a quantitative and dynamic study for the past year of the communities of the Bay of Morlaix. Three stations were chosen; one in the coarse Amphioxus sand (Primel) (Fig. 3, station 1), another in the Hyalinoecia bilineata facies in the fine

Abra alba sand (Pierre Noire) (station 2) and a third in the silted facies with the same population (Channel of the Morlaix River) (station 3). The object of the study was to determine the density and biomass of the communities with a view to estimating the productivity of these substrates and elucidating the evolution of their diversity. The main effort has been concentrated on the fine sand community of Pierre Noire (sampled at least once a month). Indeed it shows a very dense and rich community with more than 100 species. Particular attention has been paid to the biology of the dominant species, Amphipods of the genus Ampelisca, very abundant here for several years, and an important constituent of the diet of fish. Ampelisca was represented initially by 6 species which accounted for 90% of the numbers and 40°7o of the biomass under summer conditions. Population dynamics of the 4 principal species of the genus have been followed (identification of the various cohorts, growth in size and variation in number in each cohort as a function of time). A summer phase of recruitment and growth, with maximum abundance in September, was followed by heavy mortality in November and December, then by a winter stabilization (c 8000 individuals per m 2 for the four species together) which was reached in January and continued into March. The penetration at depth by hydrocarbon pollution from the AMOCO CADIZ was translated into massive mortality of the Amphipod population (Table 1). On 21 March, surface oil reached Pierre Noire. On 3 April, heavy pollution of the sediment was observed and the Amphipod population was reduced to a single species (A. sarst) at about 350 individuals per m 2. The density of this species fell to 15 individuals per m 2 on 27 April and to just a few individuals at the end of May. Parallel to this, the total number of Amphipod species fell from 24 on 1 March to 10 on 3 April then to 7 on 27 April. During this period, the mollusc and polychaete populations have not been visibly affected in the adult state. A considerablerecruitment ofAbraalbahas been observed. In May, the fine sand of the Bay of Lannion was strewn with large numbers of Echinocardium cordatum tests down to about 20 m. This mortality was first noted on 7 April by scientists on board the SURTOfT(P. Chardy, personal communication). In contrast, the communities on the coarse Amphioxus sands do not appear to have suffered during this first phase of pollution. Similarly, samples taken in April/ May from the flat pebbly bottom offshore did not show

TABLE 1

Density of Ampeliscidae and n u m b e r of species of A m p h i p o d s per m 2, from January to July 1978, in the A bra alba fine sands of Pierre Noire (station 2, Fig. 3) (data from J.C. Dauvin).

Ampelisca sp. * A mpelisca sarsi A mpelisca tenuicornis A mpelisca brevicornis A mpelisca spinipes A mpelisca typica

N u m b e r of A m p h i p o d species

18/1/78

1/2/78

1/3/78

3/4/78

27/4/78

29/5/78

29/6/78

17/7/78

5531 2424 245 41 5 1

5226 2724 178 24 0 0

4998 2157 175 30 2 2

0 367 2 0 0 0

0 15 0 0 0 0

0 4 0 0 0 0

0 2 0 0 0 0

0 1 0 0 0 0

20

24

23

10

7

6

9

7

• New species, in process of description.

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Volume 9/Number 11/November 1978

any obvious community modification. However, sponges collected from 75 m, 5 km NNW of the Ile de Batz gave off a strong smell of oil. Levels of hydrocarbons in their tissues are currently being determined at the Museum National d'Histoire Naturelle.

Conclusions These first fragmentary notes just presented can only lead to tentative conclusions. However, it can be assumed that the final development of the disruption produced by hydrocarbons in sublittoral benthic communities is going to depend largely o n t w o general conditions: o n the one hand on the natural capacity of the various types of habitat present to speed up or hinder their own cleansing; on the other on the proximity of more or less large reserves for recolonisation. The peculiar benthic structure of northern Brittany, cha,racterised by the presence of coarse, pebbly substrates continually exposed to strong tidal currents, has resulted in the harmful consequences of pollution being felt primarily by and

even being concentrated in communities in fine sediment, isolated from each other in bays and estuaries. Without any doubt, we have here a long term problem which, in any event, is a very unhappy blow to a maritime region whose exceptional faunistic and floristic diversity is the result of, among other things, the variety of its ecological conditions.

Boillot, G. (1964). G~ologie de la Manche occidentale. Ann. Inst.

Oc~anogr.,42, 1-220.

Cabioch, L. (1968). Contribution/t la connaissance des peuplements benthiques de la Manche occidentale. Cab. Biol. mar., 9(5), 489-720. Hess, W. D. (ed). (1978). The Amoco Cadiz 0il spill, a preliminary scientific report. U.S. Dept. of Commerce&U.S. Environ.Agency. Huang, C. P. & Elliott, H. A. (1977). The stability of emulsified crude oil as affected by suspended particles. In Fate and Effects o f PetroleumHydrocarbons in Marine Organisms and Ecosystems. D.A. Wolfe(ed.). pp. 413--420.Pergamon Press. MacAuliffe, C. D. (1977). Dispersal and alteration of oil discharged on a water surface, ln Fate and Effects o f Petroleum Hydrocarbons in Marine Organisms and Ecosystems. D.A. Wolfe (ed.). pp. 19-35.

PergamonPress. Bull., 9(1), 123-128.

O'Sullivan, A. J. (1978). The Amoco Cadiz oil spill. Mar. Pollut.

Birds Oiled during the A M O C O CADIZ Incident--an Interim Report P. HOPE JONES*, J.-Y. MONNAT+, C. J. CADBURY* and T. J. STOWE* *Royal Society fo r the Protection o f Birds, The Lodge, Sandy, Bedfordshire ÷Institut d'Etudes Marines and Soci~t~ pour l'Etude etla Protection de la Nature en Bretagne, Brest

Over 4500 oiled birds were collected from beaches in Northwest France and the Channel Islands following the oil spillage from the A M O C O C A D I Z in March 1978. Auks were the most abundant casualties: 1391 puffins Fratercula arctica, 978 razorbills Alca torda and 731 guillemots Uria aalge, but there were also 126 divers Gavia spp. A total of 33 bird species were recorded oiled. A corpse drift experiment suggested that after 30 March at least 3450 seabirds died off north Finist~re alone; the total mortality in the first fortnight of the incident was probably considerably larger, The super-tanker A M O C O C A D I Z was wrecked on rocks near Portsall, on the northwest coast of Brittany, on 16 March 1978, and she subsequently spilled her cargo of 220 000 tons of light Iranian and Arabian crude oils. The incident occurred almost on the eleventh anniversary of the grounding of the infamous T O R R E Y C A N Y O N . Oil from that incident enveloped the seabird reserve of Les Sept Isles and apparently severely reduced the auk population (Monnat, 1969). In recent years these auks had begun to show a slight increase but the latest incident is likely to have reduced the numbers

again. This report examines the situation concerning oiled seafowl; a detailed collation will be made at a later date. In the first fortnight of the incident there were predominantly strong westerly winds, and later southwesterly winds. The sequence of events following the wreck has been described by O'Sullivan (1978) for the period up to 31 March. After that date, a change in wind direction to the northeast caused extensive secondary contamination of beaches. Oil also drifted around the western tip of the Brittany peninsula, and came ashore, under the influence of north- and southwesterly winds, on the coast as far as Pointe du Raz, about 66 km south of the wreck. Emergency arrangements With the previous experience of oil pollution in Brittany from the T O R R E Y C A N Y O N in 1967, and the wrecking of two other tankers, O L Y M P I C B R A V E R Y and BOHLEN, both in 1976, an operations centre was quickly established at Brest by the Soci~t~ pour l'Etude e t l a Protection de la Nature en Bretagne (SEPNB). Members of this society, together with university 307