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A three-year ecological survey in benoit and wrac'h abers following the Amoco Cadiz oil spill
.¥etherlands Journal of Sea Research 16:483-490 (1982)
ATHREE-YEAR ECOLOGICAL SURVEY IN BENOIT AND WRAC'H ABERS FOLLOWING THE AMOCO CADIZ OIL SPILL by M. GLI~MAREC and E. HUSSENOT Laboratoire d'OcganographieBiologique, Institut d'Etudes Marines, Facultg des Scienceset Techniques, 29283 Brest, France
CONTENTS I. II. III. IV. V. VI.
Introduction . . . . . . . . . . . . . . . . . . . . Methods . . . . . . . . . . . . . . . . . . . . . . Analysisof benthic faunal successionsin the areas sampled General discussion . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . ....... . . . . . . . . . . . .
. . . . . . . . . .
483 484 485 488 489 489
I. INTRODUCTION The two Benoit and Wrac'h Abers (coastal inlets) are located very near the place where the Amoco Cadiz, a super-tanker loaded with some 250,000 t of light crude oil, grounded in March 1978 (Fig. 1). This area is the most affected by the oil spill, and numerous studies have been concerned with these abers, in particular the analysis of ecological successions within the infaunal communities of the area has provided much information on medium and long-term changes attributable to oil pollution. Non-seasonal but progressive temporal fluctations have been documented (GL~MAREC& HVSSENOT, 1981) and it is now possible to give a comprehensive account of these processes 3 years after the oil-spill. After the complete destruction of the initial communities, recolonisation resulted first in the transitory development of a special fauna, characterising an excess of organic material on the sea-floor, betore proceeding through successive intermediate phases to the development of stable populations characteristic of the unpolluted communities of the area. The approach taken in this brief review of benthic changes in the area is to recognize taxonomic groups, composed of species of equal sensitivity to decreasing levels of organic pollution. The successive appearance of these various groups, their relative importance, and their disappearance, are the key features of this dynamic approach. This type of analysis indicates that biological perturbations can persist within the ecosystem well beyond the time when most physical and chemical factors have apparently returned to normal.
484
M. GLI~MAREC& E. HUSSENOT
Acknowledgements.--We express our thanks to the editing committee and to our friend Dr T o m Pearson; by their help this paper appears in an acceptable form. This study was supported by funds provided through the N O A A / C N E X O A M O C O CADIZ Commission. II. METHODS The macroinfauna of sedimentary areas was studied at a series of stations along the sub.tidal channels of the two abers (Fig. 1); 5 samples were collected at each station with a Smith-MacIntyre grab (0.1 m -2) 3 times a year (winter, spring and summer). More than 15 stations were sampled in each aber, representing the range of different communities present. Their spatial arrangement from downstream to upstream is described by GLI~MAREC& HUSSENOT (1981) after GENTIL • CABIOCH (1979), and follows a gradient of decreasing hydrodynamic energy and increasing hydrocarbon enrichment in the sediments of the abers (MARCHAND & CAPRAIS, 1981). In the Benoit Aber, the distribution of these factors is somewhat different because dune sands are well developed in that area (Fig. 1).
Fig. 1. Location of the Aber Wrac'h, the Aber Benoit and the wreck of the Amoco Cadiz (*) on the north coast of Brittany and the regions with the subtidal sediments dune sands (DU), fine muddy sands (FV), finesands (SF), coarse sands (SG), heterogeneous muddy sand (SHV), and sandy muds (VS).
ECOLOGICAL
SURVEY FOLLOWING
OIL SPILL
485
III. ANALYSIS OF BENTHIC FAUNAL SUCCESSIONS IN THE AREAS SAMPLED In the first stage following on oil spill event (called primary phase by MARCHAND, 1981) the toxicity of the hydrocarbons induces heavy mortalities in the existing fauna. New, mainly opportunist species settle during a second phase (GLI~MAREC & HUSSENOT, 1981; LE MOAL & Q UILLIEN-MONOT, 1981). These new species are similar to those which elsewhere, in sewage areas for instance, are characteristic of an excess of organic matter. After a number of investigations of these problems (PEARSON & ROSENBER~, 1978; BELLAN et al., 1980) it appeared possible to regroup these species according to the extent of their sensitivity to pollution (GLI~MAREC & HILY, 1981): Group I: sensitive species, largely dominant in normal conditions. They differ according to each kind of community. Group II: tolerant species, always in small densities, which do not fluctuate significantly in time. Group III: sensitive species which at first disappear and then reappear over a greater ecological range than in normal conditions. Group IV: opportunist species, essentially cirratulid and capitellid polychaetes. Group V: extreme opportunist species, which form monospecific, high-density populations in the places of m a x i m u m pollution. These different groups can coexist, and along the gradient of organic pollution 6 steps can be defined (Fig. 2a): Step 1: groups I, II and III are in smaller densities than in normal conditions, but no qualitative change appears. Step 2: the ecosystem is unbalanced and group III is dominant. Steps 4 and 6 are defined by proliferation of groups IV and V, respectively. Steps 3 and 5 are ecotonal, group II is locally found alone, because competition between groups is lessened. This diagram (adapted from GLI~MAREC & HILY, 1981) is static, however, and we will describe here the progression in time of the different groups (Fig. 2b) and the successive regression of these special faunas (Fig. 2c). Three months after the disturbance (t3) pollution was broadly stabilized at the beginning of the secondary phase, according to MARCHAND'S (1981) terminology. At ts (8 months after the grounding) all the communities were largely destroyed (Fig. 3). At Wrac'h Aber only the down-stream coarse sands had not been affected and the community showed some seasonal variation (step 0). In the dune sands (DUI) of Benoit Aber, the amount of hydrocarbons following the oilspill had always been less than 50/zg- g-1 and the community showed lower densities (step 1) with a transitory appearance of step 2.
486
M. G L I ~ M A R E C
& E. H U S S E N O T
For all other sediments in the two abers, where the amount of hydrocarbons had always been more than 1,000 fig.g-1 (MARCHAND & CAPRAIS, 1981), communities were very impoverished. After t8 group 6
5
4
3
2
1
0
a
6
4-2
4
l
r
b
6-2 2-6 2-4
2
1
0
Fig. 2. a. Relative occurrence of the faunal groups I to V along a gradient of organic enrichment (after GLgMAREC & HILY, 1981), defining the successive steps 0 to 6. b. Situation after settlement of the opportunist fauna (Groups IV and V) and the stabilization of pollution (group II left out). c. Situation after the decline of the opportunist fauna and development of the tolerant group III (group II left out!.
IV was predominant over groups I and III (step 4). In upstream muddy areas the communities were more damaged and group V took over, reaching high densities (step 6). Transitory steps 6 2 and 4-2 illustrate the appearance of group III, still dominated by group V or IV, respectively. Under winter climatic conditions frequent storms stir and oxygenate the sediments. After the first winter (t12) MARCHAND~.~ CAPRAISreported a noticeable decrease in the hydrocarbon content of the dune sands in the Benoit Aber, where the values were less than 100/zg' g 1 while in the other sediments the values were generally higher than 1,000/ag g - x and even figures over 10,000 /zg.g t could be found in muds and heterogeneous muddy sands. Only during the second winter (t2t), however, could decontamination be shown by biological data (in the whole Wrac'h Aber), as illustrated by the decline of opportunist groups V and IV (Fig. 2c) and the transient dominance and eventual replacement of
ECOLOGICAL
SURVEY
FOLLOWING
OIL
487
SPII,I,
group I I I , with later steps the persistance of which proves that the ecosystem is still unbalanced. T h e beginning of the decline of the opportunist groups appeared to occur at about the same time at the different places within W r a c ' h i6
6
6-2
6-2
6-2
44-2~
il~~ 2-, i~-" ,12
16
~-'
14-2
6-2
2-6
2
2
2
4
!4
2
12-4 i
14-2
11
1
1
1
4-2
4-2
J
iJ4
:3i
!6
2
J3
~4
J
il
3000i
I0
I0
8
13
17
iO
Zl
/0
25
2
I" //'
i
/0
1 ....--
1
4000~
0
i2 ,
I0
[0
2;
.
/
2-4
.<..... /"
i
2g
33
8
13
17
21
25
29
33
Fig. 3. Mean numbers of individuals per m -2 of faunal groups I to V (Fig. 2) present and steps reached (0 to 6) during 33 months of observation in Wrac'h Aber (left hand) and Benoit Aber (right hand). Aber. In Benoit Aber, this decline occurred more quickly where sediments were well oxygenated, e.g. after t 8 for dune sands (DUz), after t13 for heterogeneous m u d d y sands (SHV), after tit for fine sands (SF), dune sands (DU3) and sandy muds (VS). T w o years after the oil spill (t25) the dune sands (DU1) were in a healthy condition (step 0); all the other communities were u n b a l a n c e d (steps 1, 2, 2-4) and in the m u d d y areas d e c o n t a m i n a t i o n was just beginning (step 6 2). Thereafter dynamic processes proceeded at a slower pace. Following an attempt to remove pockets full of m u d and oil during April 1980 by mechanical dredging near the heterogeneous m u d d y sands, the c o m m u n i t y in this
488
M. G L I ~ M A R E C
& E. H U S S E N O T
area showed a temporary increase in perturbation to step 6 at t29. Hydrodynamic effects were weaker in Wrac'h Aber than in Benoit Aber, thus the rate of chemical decontamination was not simultaneous in the two abers. The decline of the opportunist fauna was slower in Wrac'h Aber, and it can be seen that at the same time (t25 for example) the same communities are more perturbed in Wrac'h Aber than in Benoit Aber. By t3a this difference disappears (Table I). TABLE
I
Comparison of the faunal steps reached in areas with differentsediments (Fig. 1) in the two Abers 95 and 33 months after the grounding of the Amoco Cadiz. Aber
Slep reached at: ~25
Benoit Wrae'h
~33
DU 2
SF
VS
DG 2
SF
['5'
1 4-2
2-4 4-2
6-2 4
2 2
24 24
6 2 2 6
IV. GENERAL DISCUSSION Independently of annual cyclic fluctuations, the temporal modification of benthic subtidal communities along the spatial gradients found in the two abers, showed non-cyclic changes quite similar to those described by LE MOAL (1981) for the intertidal communities in the same areas. As a first step, the original populations almost disappear within the first 3 months (CHASS~, 1978; CABIOCn et al.~ 1980). Group II did not fluctuate significantly and only the disappearance ofthc other groups explains the dominance of this group (steps 3 and 5) in muddy areas o f W r a c ' h Aber. From t 3 to t13 , at the beginning of the second phase, the opportunist fauna settles. Group IV is abundant everywhere from ts onwards in Wrac'h Aber, but its importance decreased in the communities of the Benoit Aber after ts, t13 o r t17 according to the hydrodynamic conditions. Group V only appeared in muddy areas of the two abers and 2 years after the oil spill this group was poorly represented. For all sediments where the content of hydrocarbons was still more than 1,000/~g'g-1 one year after the oil spill, the greatest development of the opportunist fauna was found between t a and t20. Group III reappeared at ha and its development was very important everywhere during the second winter. Simultaneously, group I reappeared, but after 2 years its recovery was obscured by the rapid development of the competing group III. Thus after the development and
ECOLOGICAL SURVEY FOLLOWING OIL SPII.I.
489
decline of the o p p o r t u n i s t fauna, a third period c a n be defined c h a r a c terized b y the d o m i n a n c e of g r o u p I I I , essentially after t20. T h e s e dyn a m i c processes o c c u r quickly a n d one y e a r after p e r t u r b a t i o n some c o m m u n i t i e s are in " g o o d h e a l t h " , whilst others are still u n b a l a n c e d . It a p p e a r s therefore t h a t 3 or 4 years will be a reasonable estimate for the time lag necessary to allow a general r e t u r n of the c o m m u n i t i e s to n o r m a l conditions or to a new equilibrium, This a p p r o a c h of assessing c h a n g e in c o m m u n i t i e s seems m o r e efficient t h a n a study o f one p a r t i c u l a r p o p u l a t i o n , a n d even if the m e a s u r e m e n t of abiotic p a r a m e t e r s seems of m o r e i m m e d i a t e relevance: to the c h a r a c t e r i s a t i o n of a polluted e n v i r o n m e n t , the collection of such ecological d a t a reveals p e r t u r b a t i o n in ecosystems w h i c h are impossible to detect by physical analysis, and are essential to an u n d e r s t a n d i n g of the long t e r m and residual effects of h y d r o c a r b o n c o n t a m i n a t i o n . V. S U M M A R Y T h e succession in time of different ecological groups along a g r a d i e n t of organic e n r i c h m e n t was studied for 3 years after the A m o c o C a d i z oil spill in 2 a d j a c e n t tidal inlets or abers. F r o m the results, time p a t t e r n s are established, t h r o u g h which the ecosystem evolves towards a new e q u i l i b r i u m or b a c k to a f o r m e r one. I n a first stage b o t h pollution sensitive a n d tolerant species were wiped out as a result of the high toxicity o f the kind of oil involved. O n c e the level o f c o n t a m i n a t i o n was s o m e w h a t stabilized, an opportunistic f a u n a d e v e l o p e d t h a t e v e n t u a l l y was o v e r t a k e n b y a c o m m u n i t y of pollution tolerant species. T h e latter exhibited an excessive d e v e l o p m e n t before a new e q u i l i b r i u m was reached. Differences in the rate of this t e m p o r a l succession were tound b e t w e e n the abers, d e p e n d e n t on the intensity of h y d r o d y n a m i c processes. N e a r l y 3 years after the oil spill, most c o m m u n i t i e s had still not reached equilibrium. VI. R E F E R E N C E S
BELLAN, G., D. BELLAN-SANTINIt~ J. PICARD, 1980. Mise en evidence de modele dcobiologique dans des zones soumises 5. perturbations par matieres organiques. Acta Oecologica, Oecol. Applic. 3 (3) :383 390. CABIOCH,g., J. C. DAUVIN,J. MORA-BERMUDEZ& C. RODRIGUEZ-BAB10,1980. Effets de la maree noire de l'"Amoco Cadiz" sur le benthos sub-littoral du nord de la Bretagne. Helgol~inder wiss. Mecresuntcrs. 33 (1 4): 192--208. CnAss~, C., 1978. Impact ecologique dans la zone eetiEre concernee par la mardc noirc de l'"Amoco Cadiz". Mar. Pollut. Bull., NS. 11:298 301. GENT1L,F. & L. GABIOCH,1979. Premieres donnees sur le benthos de l'Aber Wrac'h (Nord-Bretagne) et sur l'impact des hydrocarbures de l'"Amoco Cadiz".--lRech. Oceanogr., 4 (1): 35.
490
M. GLI';MAREC & E. HUSSENOT
GLt~MAREC, M. & C. HILY, 1981. Perturbations apportdes ~ la macrofaune benthique
de la baie de Concarneau par les effluents urbains et portuaires.--Acta Oecologica, Oecol. Applic. 3: 139-150. GL~AREC, M. & E. HVSSENOT, 1981. Definition d'une succession ecologique en milieu meuble anormalement enrichi cn matiere organique/: la suite de la catastrophe dc 1' "Amoco Cadiz". In: "Amoco Cadiz", consequences d'une pollution accidentellc par les hydrocarbures. C.N.E.X.O., Paris: 499-512. LE MOAL, Y., 1981. Ecologie dynamique des plages touchees par la maree noire de l'"Amoco Cadiz", Universit6 de Bretagne Occidentale: 1 131 (thesis). LE MOAL, Y. & M. QUILL1EN-MONoT,1981. Etude des populations de la macrotaune et de leurs juveniles sur les plages des Abers BenMt et Wrac'h. In: "Amoco Cadiz, consequences d'une pollution accidentelle par les hydrocarbures". C.N.E.X.O., Paris: 311-326. MARCHANDM., 198 l. Bilan du colloque sur les consequences d'une pollution accidentelle par les hydrocarbures. Rapport Scient. & Tech. C.N.E.X.O. 44:1 -86. MARCHAND~ M. ~z M. R. CAPRAIS, 1981. Suivi de la pollution de l'"Amoco Cadiz" dans l'eau de met et les sediments marins. In: "Amoco-Cadiz, consequences d'une pollution accidentelle par les hydrocarbures". C.N.E.X.O., Paris: 23 54. PEARSON,m. n . • R. ROSENBERG~1978. Macrobenthic succession in relation to organic enrichment and pollution of the marine environment. Oceanogr. mar. Biol. Ann. Rev. 16:229 311.
ATHREE-YEAR ECOLOGICAL SURVEY IN BENOIT AND WRAC'H ABERS FOLLOWING THE AMOCO CADIZ OIL SPILL by M. GLI~MAREC and E. HUSSENOT Laboratoire d'OcganographieBiologique, Institut d'Etudes Marines, Facultg des Scienceset Techniques, 29283 Brest, France
CONTENTS I. II. III. IV. V. VI.
Introduction . . . . . . . . . . . . . . . . . . . . Methods . . . . . . . . . . . . . . . . . . . . . . Analysisof benthic faunal successionsin the areas sampled General discussion . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . ....... . . . . . . . . . . . .
. . . . . . . . . .
483 484 485 488 489 489
I. INTRODUCTION The two Benoit and Wrac'h Abers (coastal inlets) are located very near the place where the Amoco Cadiz, a super-tanker loaded with some 250,000 t of light crude oil, grounded in March 1978 (Fig. 1). This area is the most affected by the oil spill, and numerous studies have been concerned with these abers, in particular the analysis of ecological successions within the infaunal communities of the area has provided much information on medium and long-term changes attributable to oil pollution. Non-seasonal but progressive temporal fluctations have been documented (GL~MAREC& HVSSENOT, 1981) and it is now possible to give a comprehensive account of these processes 3 years after the oil-spill. After the complete destruction of the initial communities, recolonisation resulted first in the transitory development of a special fauna, characterising an excess of organic material on the sea-floor, betore proceeding through successive intermediate phases to the development of stable populations characteristic of the unpolluted communities of the area. The approach taken in this brief review of benthic changes in the area is to recognize taxonomic groups, composed of species of equal sensitivity to decreasing levels of organic pollution. The successive appearance of these various groups, their relative importance, and their disappearance, are the key features of this dynamic approach. This type of analysis indicates that biological perturbations can persist within the ecosystem well beyond the time when most physical and chemical factors have apparently returned to normal.
484
M. GLI~MAREC& E. HUSSENOT
Acknowledgements.--We express our thanks to the editing committee and to our friend Dr T o m Pearson; by their help this paper appears in an acceptable form. This study was supported by funds provided through the N O A A / C N E X O A M O C O CADIZ Commission. II. METHODS The macroinfauna of sedimentary areas was studied at a series of stations along the sub.tidal channels of the two abers (Fig. 1); 5 samples were collected at each station with a Smith-MacIntyre grab (0.1 m -2) 3 times a year (winter, spring and summer). More than 15 stations were sampled in each aber, representing the range of different communities present. Their spatial arrangement from downstream to upstream is described by GLI~MAREC& HUSSENOT (1981) after GENTIL • CABIOCH (1979), and follows a gradient of decreasing hydrodynamic energy and increasing hydrocarbon enrichment in the sediments of the abers (MARCHAND & CAPRAIS, 1981). In the Benoit Aber, the distribution of these factors is somewhat different because dune sands are well developed in that area (Fig. 1).
Fig. 1. Location of the Aber Wrac'h, the Aber Benoit and the wreck of the Amoco Cadiz (*) on the north coast of Brittany and the regions with the subtidal sediments dune sands (DU), fine muddy sands (FV), finesands (SF), coarse sands (SG), heterogeneous muddy sand (SHV), and sandy muds (VS).
ECOLOGICAL
SURVEY FOLLOWING
OIL SPILL
485
III. ANALYSIS OF BENTHIC FAUNAL SUCCESSIONS IN THE AREAS SAMPLED In the first stage following on oil spill event (called primary phase by MARCHAND, 1981) the toxicity of the hydrocarbons induces heavy mortalities in the existing fauna. New, mainly opportunist species settle during a second phase (GLI~MAREC & HUSSENOT, 1981; LE MOAL & Q UILLIEN-MONOT, 1981). These new species are similar to those which elsewhere, in sewage areas for instance, are characteristic of an excess of organic matter. After a number of investigations of these problems (PEARSON & ROSENBER~, 1978; BELLAN et al., 1980) it appeared possible to regroup these species according to the extent of their sensitivity to pollution (GLI~MAREC & HILY, 1981): Group I: sensitive species, largely dominant in normal conditions. They differ according to each kind of community. Group II: tolerant species, always in small densities, which do not fluctuate significantly in time. Group III: sensitive species which at first disappear and then reappear over a greater ecological range than in normal conditions. Group IV: opportunist species, essentially cirratulid and capitellid polychaetes. Group V: extreme opportunist species, which form monospecific, high-density populations in the places of m a x i m u m pollution. These different groups can coexist, and along the gradient of organic pollution 6 steps can be defined (Fig. 2a): Step 1: groups I, II and III are in smaller densities than in normal conditions, but no qualitative change appears. Step 2: the ecosystem is unbalanced and group III is dominant. Steps 4 and 6 are defined by proliferation of groups IV and V, respectively. Steps 3 and 5 are ecotonal, group II is locally found alone, because competition between groups is lessened. This diagram (adapted from GLI~MAREC & HILY, 1981) is static, however, and we will describe here the progression in time of the different groups (Fig. 2b) and the successive regression of these special faunas (Fig. 2c). Three months after the disturbance (t3) pollution was broadly stabilized at the beginning of the secondary phase, according to MARCHAND'S (1981) terminology. At ts (8 months after the grounding) all the communities were largely destroyed (Fig. 3). At Wrac'h Aber only the down-stream coarse sands had not been affected and the community showed some seasonal variation (step 0). In the dune sands (DUI) of Benoit Aber, the amount of hydrocarbons following the oilspill had always been less than 50/zg- g-1 and the community showed lower densities (step 1) with a transitory appearance of step 2.
486
M. G L I ~ M A R E C
& E. H U S S E N O T
For all other sediments in the two abers, where the amount of hydrocarbons had always been more than 1,000 fig.g-1 (MARCHAND & CAPRAIS, 1981), communities were very impoverished. After t8 group 6
5
4
3
2
1
0
a
6
4-2
4
l
r
b
6-2 2-6 2-4
2
1
0
Fig. 2. a. Relative occurrence of the faunal groups I to V along a gradient of organic enrichment (after GLgMAREC & HILY, 1981), defining the successive steps 0 to 6. b. Situation after settlement of the opportunist fauna (Groups IV and V) and the stabilization of pollution (group II left out). c. Situation after the decline of the opportunist fauna and development of the tolerant group III (group II left out!.
IV was predominant over groups I and III (step 4). In upstream muddy areas the communities were more damaged and group V took over, reaching high densities (step 6). Transitory steps 6 2 and 4-2 illustrate the appearance of group III, still dominated by group V or IV, respectively. Under winter climatic conditions frequent storms stir and oxygenate the sediments. After the first winter (t12) MARCHAND~.~ CAPRAISreported a noticeable decrease in the hydrocarbon content of the dune sands in the Benoit Aber, where the values were less than 100/zg' g 1 while in the other sediments the values were generally higher than 1,000/ag g - x and even figures over 10,000 /zg.g t could be found in muds and heterogeneous muddy sands. Only during the second winter (t2t), however, could decontamination be shown by biological data (in the whole Wrac'h Aber), as illustrated by the decline of opportunist groups V and IV (Fig. 2c) and the transient dominance and eventual replacement of
ECOLOGICAL
SURVEY
FOLLOWING
OIL
487
SPII,I,
group I I I , with later steps the persistance of which proves that the ecosystem is still unbalanced. T h e beginning of the decline of the opportunist groups appeared to occur at about the same time at the different places within W r a c ' h i6
6
6-2
6-2
6-2
44-2~
il~~ 2-, i~-" ,12
16
~-'
14-2
6-2
2-6
2
2
2
4
!4
2
12-4 i
14-2
11
1
1
1
4-2
4-2
J
iJ4
:3i
!6
2
J3
~4
J
il
3000i
I0
I0
8
13
17
iO
Zl
/0
25
2
I" //'
i
/0
1 ....--
1
4000~
0
i2 ,
I0
[0
2;
.
/
2-4
.<..... /"
i
2g
33
8
13
17
21
25
29
33
Fig. 3. Mean numbers of individuals per m -2 of faunal groups I to V (Fig. 2) present and steps reached (0 to 6) during 33 months of observation in Wrac'h Aber (left hand) and Benoit Aber (right hand). Aber. In Benoit Aber, this decline occurred more quickly where sediments were well oxygenated, e.g. after t 8 for dune sands (DUz), after t13 for heterogeneous m u d d y sands (SHV), after tit for fine sands (SF), dune sands (DU3) and sandy muds (VS). T w o years after the oil spill (t25) the dune sands (DU1) were in a healthy condition (step 0); all the other communities were u n b a l a n c e d (steps 1, 2, 2-4) and in the m u d d y areas d e c o n t a m i n a t i o n was just beginning (step 6 2). Thereafter dynamic processes proceeded at a slower pace. Following an attempt to remove pockets full of m u d and oil during April 1980 by mechanical dredging near the heterogeneous m u d d y sands, the c o m m u n i t y in this
488
M. G L I ~ M A R E C
& E. H U S S E N O T
area showed a temporary increase in perturbation to step 6 at t29. Hydrodynamic effects were weaker in Wrac'h Aber than in Benoit Aber, thus the rate of chemical decontamination was not simultaneous in the two abers. The decline of the opportunist fauna was slower in Wrac'h Aber, and it can be seen that at the same time (t25 for example) the same communities are more perturbed in Wrac'h Aber than in Benoit Aber. By t3a this difference disappears (Table I). TABLE
I
Comparison of the faunal steps reached in areas with differentsediments (Fig. 1) in the two Abers 95 and 33 months after the grounding of the Amoco Cadiz. Aber
Slep reached at: ~25
Benoit Wrae'h
~33
DU 2
SF
VS
DG 2
SF
['5'
1 4-2
2-4 4-2
6-2 4
2 2
24 24
6 2 2 6
IV. GENERAL DISCUSSION Independently of annual cyclic fluctuations, the temporal modification of benthic subtidal communities along the spatial gradients found in the two abers, showed non-cyclic changes quite similar to those described by LE MOAL (1981) for the intertidal communities in the same areas. As a first step, the original populations almost disappear within the first 3 months (CHASS~, 1978; CABIOCn et al.~ 1980). Group II did not fluctuate significantly and only the disappearance ofthc other groups explains the dominance of this group (steps 3 and 5) in muddy areas o f W r a c ' h Aber. From t 3 to t13 , at the beginning of the second phase, the opportunist fauna settles. Group IV is abundant everywhere from ts onwards in Wrac'h Aber, but its importance decreased in the communities of the Benoit Aber after ts, t13 o r t17 according to the hydrodynamic conditions. Group V only appeared in muddy areas of the two abers and 2 years after the oil spill this group was poorly represented. For all sediments where the content of hydrocarbons was still more than 1,000/~g'g-1 one year after the oil spill, the greatest development of the opportunist fauna was found between t a and t20. Group III reappeared at ha and its development was very important everywhere during the second winter. Simultaneously, group I reappeared, but after 2 years its recovery was obscured by the rapid development of the competing group III. Thus after the development and
ECOLOGICAL SURVEY FOLLOWING OIL SPII.I.
489
decline of the o p p o r t u n i s t fauna, a third period c a n be defined c h a r a c terized b y the d o m i n a n c e of g r o u p I I I , essentially after t20. T h e s e dyn a m i c processes o c c u r quickly a n d one y e a r after p e r t u r b a t i o n some c o m m u n i t i e s are in " g o o d h e a l t h " , whilst others are still u n b a l a n c e d . It a p p e a r s therefore t h a t 3 or 4 years will be a reasonable estimate for the time lag necessary to allow a general r e t u r n of the c o m m u n i t i e s to n o r m a l conditions or to a new equilibrium, This a p p r o a c h of assessing c h a n g e in c o m m u n i t i e s seems m o r e efficient t h a n a study o f one p a r t i c u l a r p o p u l a t i o n , a n d even if the m e a s u r e m e n t of abiotic p a r a m e t e r s seems of m o r e i m m e d i a t e relevance: to the c h a r a c t e r i s a t i o n of a polluted e n v i r o n m e n t , the collection of such ecological d a t a reveals p e r t u r b a t i o n in ecosystems w h i c h are impossible to detect by physical analysis, and are essential to an u n d e r s t a n d i n g of the long t e r m and residual effects of h y d r o c a r b o n c o n t a m i n a t i o n . V. S U M M A R Y T h e succession in time of different ecological groups along a g r a d i e n t of organic e n r i c h m e n t was studied for 3 years after the A m o c o C a d i z oil spill in 2 a d j a c e n t tidal inlets or abers. F r o m the results, time p a t t e r n s are established, t h r o u g h which the ecosystem evolves towards a new e q u i l i b r i u m or b a c k to a f o r m e r one. I n a first stage b o t h pollution sensitive a n d tolerant species were wiped out as a result of the high toxicity o f the kind of oil involved. O n c e the level o f c o n t a m i n a t i o n was s o m e w h a t stabilized, an opportunistic f a u n a d e v e l o p e d t h a t e v e n t u a l l y was o v e r t a k e n b y a c o m m u n i t y of pollution tolerant species. T h e latter exhibited an excessive d e v e l o p m e n t before a new e q u i l i b r i u m was reached. Differences in the rate of this t e m p o r a l succession were tound b e t w e e n the abers, d e p e n d e n t on the intensity of h y d r o d y n a m i c processes. N e a r l y 3 years after the oil spill, most c o m m u n i t i e s had still not reached equilibrium. VI. R E F E R E N C E S
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