Long-term monitoring of marine nematode assemblages in the Morlaix estuary (France) following the Amoco Cadiz oil spill

Estuarine,

Coastal

and Shelf

Science

(1987) 24,657-670

Long-term Monitoring of Marine Nematode Assemblages in the Morlaix Estuary (France) Following the Amoco Cudiz Oil Spill

N. E. Gourbault Mustkn 75231

national d’tiistoire naturelle, Paris Cedex 05, France

Received

14 April

Keywords:

1986

and in revised

Laboratoire form

des Vers, 61 rue de Buffon,

9 September

1986

nematomorpha,estuaries;monitoring; pollution; channels

In March 1978the coastof Brittany washeavily pollutedwith oil from the Amoco Cadiz. Marine nematodeassemblages from the Morlaix estuarywere regularly monitored at three sitesfrom October 1978to November 1984.Differences amongthe assemblages and indicationsof the effectsof oil contaminationwere detectedby diversity, correspondenceanalysisand fits to empirical models. Clearestevidenceof the effectswere seenat the shallowestupstreamsite, and were detectablefour yearsafter the spill. It wasdifficult to demonstrateunequivocaleffectsat the deepersitesagainstthe backgroundof naturalvariation. It was concludedthat by 1984the faunahadrecoveredat all sitesto a situationsimilarto that pertaining in October 1978. Introduction

On 16 March 1978 the supertanker Amoco C&z went aground off the north Brittany coast causing severe local oil pollution. Many reports on the effects of the catastrophe on marine life have already appeared(seeAmoco Cadiz, 1981for arecent report). The impact on the meiofauna has been described by Boucher (1980a, 1981, 1983, 1985), RenaudMornant and Gourbault (1980, 1982), Renaud-Mornant et al. (1981) and Bodin and Boucher (1983). Becauseof the importance of marine nematodes in sedimentsand their potential use as indicators of pollution (Ferris & Ferris, 1979; Platt & Warwick, 1980; Larnbshead, 1986) it was decided to study them in more detail. This paper reports on temporal fluctuations of nematode assemblagesin the heavily contaminated estuary of the River Morlaix between 1978 and 1984. Material

and methods

Three stations (A, B and C) along a transect in the Morlaix estuary were selected(Figure 1, corresponding to stations 1, 4 and 6 of Gourbault, 1981). Their location was between the upper and lower parts of the Bay of Morlaix studied by Boucher (1985) and Boucher et al. (1984). The mean low water depth at stations A, B and C was 0.1, 15 and 12m, respectively. Each station was sampled in October 1978 and approximately each four 657 0272-7714/87[050657+

14 $03.00/O

0 1987 AcademicPress Inc. (London) Limited

658

N. E. Gourbault

Pierng

Figure 1. Map of the Morlaix C distance = 4.4 km.

Noire

Bay and estuary

showing

the locations

of stations

A-C.

A-

months between October 1979 and November 1984, the complete data set consists of 15 single samples from each station (Table 3). Sediment was collected with a SmithMcIntyre grab and preserved in 5% buffered seawaterformalin. The sediment wassubsequently washed through 250~pmand 40-pm sieves;nematodesin the mud which collected on the latter sieve were extracted using the Ludox method (modified after Heip etal., 1974; cf. McIntyre & Warwick, 1984). The first 200 nematodes picked at random from each sample were identified to species:it was determined that the ‘ collectors curve ’ had levelled off with this number of identified individuals (Gourbault, 1984). The raw data may be obtained by writing to the author. The method of Lambshead et al. (1983) was followed in comparing diversity and speciesabundance distribution results using the Pareto model according to Gourbault and Lecordier (1985). Results Physical

and chemical parameters

The general physical parameters of these marine stations were reported in Gourbault (1981). Oil concentrations in the surface sedimentswere more than 1000ppm after July

Nematode assemblages in the Morlaix

Estuary

659

1978 (e.g. Station C), decreased to W-300 ppm by the beginning of 1979; oil was not detected a year later. However, Dauvin (1984) reported another peak of 500ppm in January 1981. The first three years of contamination are described in detail by Beslier (1981). Table 1 gives the detailed granulometric data for each sample. Stations A and B consisted of muddy sand with the percentage of sediment passing through the 63-urn sieve (i.e. silt) averaging 34 + 9.3 and 39.9 + 7.9, respectively. Station C, which was closerto the open sea,consisted of fine sand: the silt percentage was 17.8f 5.9. Species composition

A total of 9000 specimens representing 191 species were identified. The 100 most abundant specieswere the sameasthose previously found at this site (Gourbault, 1981). All the others had previously been found in the North Sea (Heip et al., 1983) with the exception of a small number which are new to science and for which descriptions are in preparation. Table 2 gives the amalgamated total abundance, percentage and frequency of occurrence at each station of the 15 most common species.The muddy sand stations (A and B) had fewer species (28 f9 and 30+ 8, respectively) than the line sand station (41 f 8). Sabatieria pulchra and Terschellingia communis characterized station A, T. longicaudata and Spirinia parasitifera were present in equal numbers at all three stations and Metalinhomoeus biformis, S. celtica, Terschellingia sp. nov. and Neotonchus meeki were found in largest numbers at Station C. Only T. longicauduta occurred in all samplesfrom the three stations. Some were present in all samplesof two stations (A. torosum and M. turgofrons at stations B and C) or a single station (S. pulchra and T. communis at station A; Comesa sp. nov. at station B; M. biformis at station C). Of the remaining species,83 had abundances over 0.1%. Frequencies were lessthan 500/b except for Viscosia viscosa (87% at station C), Paracomesoma dubium (80% at B and C), Daptonema trabeculosum (80% at B), Cyartonema zosterae (68% at B) and Ptycholuimellus ponticus (60% at A and C). The other 108 speciesall occurred in four samplesor lessat each station. Assemblage

structure

The data were subjected to three different kinds of treatment to determine whether any significant trends could be detected. The methods used were correspondence analysis, numerical diversity analysis, and fits to empirical statistical models. (1) Correspondence analysis. Correspondence (reciprocal averaging) analysis (Benzecri et al., 1973; Hill, 1974) was performed on the 72 specieswhich contained 95% of the available information (sensuMargalef, 1958). The rare specieswere excluded to avoid distorting the ordination. The ordination of the 45 samplesis shown in Figure 2, together with the position of 25 of the characteristic species.The first two principal axes explained only 270/b of the variance, and the first four only 44%, suggesting that no overriding factors were operating. The negative part of axis I was determined by four samples and two species. The stations were: A, Autumn 1980 (relative contribution 0.58) and Spring 1983 (0.49); B, Autumn 1982 (0.50); and C, Autumn 1981 (0.40). The specieswere Sabatieria pulchra (0.79) and Terschellingia communis (0.34). On the positive part of axis I were two of the 15 Station C samplesand three species;these were Summer 1981 (O-40) and Autumn 1978 (0.36) and Metalinhomoeus biformis (0*53), M. filf z ormis (0.40) and Neotonchus meeki

660

N. E. Gourbault

TABLE 1. Granulometric Date of sampling

analysis

of sediment

samples

Y. <63lrn

So

Sample

Md

Ql

43

A2 A3 A4 A5 A6 A7 A8 A9 A10 All Al2 Al3 Al4 Al5

180 84 105 56 54 70 240 83 87 175 68 83 132 75

55 40 47 40 40 41 76 48 56 57 47 45 73 52

270 205 170 115 200 135 375 160 145 330 147 175 215 150

22 35 28 43 48 36 20 37 31 28 45 39 20 48

2.2 2.2 1.9 1.7 2.2 1.8 2.2 1.8 1.6 2.4 1.7 1.9 1.7 1.7

B2 B3 B4 B5 B6 B7 B8 B9 BlO Bll B12 B13 B14 B15

62 54 60 54 49 64 64 115 84 70 78 170 80 68

40 40 40 40 40 43 48 55 45 46 60 64 53 51

120 98 115 88 100 130 110 400 255 170 109 550 240 150

39 46 43 45 50 38 49 29 37 45 29 24 37 47

1.7 1.5 2.2 1.4 1.5 1.7 1.5 2.7 2.3 1.9 1.3 2.9 2.1 1.7

c2 c3 c4 c5 C6 c7 C8 c9 Cl0 Cl1 Cl2 Cl3 Cl4 Cl5

119 290 170 330 150 320 510 410 660 410 165 410 650 197

47 86 70 96 54 92 130 86 145 125 60 148 130 61

440 1050 980 1050 1050 1050 1000 1000 1000 1000 275 1000 1900 600

26 15 17 14 23 18 14 14 11 13 28 12 13 26

3.1 3.4 3.7 3.3 4.4 3.3 2.7 3.4 2.6 2.8 2.1 2.6 3.8 3.1

Station A 16.10.1979 28.05.1980 26.08.1980 01.12.1980 01.04.1981 28.08.1981 17.11.1981 15.04.1982 19.08.1982 24.11.1982 26.04.1983 30.08.1983 30.11.1983 08.11.1984

Station B 16.10.1979 28.05.1980 26.08.1980 01.12.1980 01.04.1981 28.08.1981 17.11.1981 15.04.1982 19.08.1982 24.11.1982 26.04.1983 30.08.1983 30.11.1983 08.11.1984

Station C 16.10.1979 28.05.1980 26.08.1980 01.12.1980 01.04.1981 28.08.1981 17.11.1981 15.04.1982 19.08.1982 24.11.1982 26.04.1983 30.d8.1983 30.11.1983 08.11.1984

Abbreviations: Md, median diameter in pm; Ql, grain size at first quartile in pm; 43, grain size at third quartile in pm; y0 < 63 pm, percentage of sediment passing through 63-pm sieve (i.e. silt comer@; So, Sorting coefficient = Ql/Q3 (Trask 1932). No data available for samples Al, B 1 and Cl.

(0.33), respectively. Those sampleswith a negative co-ordinate on axis I had a significantly smaller median grain size and higher silt content than those with a positive coordinate (r-test at P= 0.05). It appears,therefore, that stations were located along the first axis according to sediment grain size.

Nematode

assemblages

in the Morlaix

2. Amalgamatedabundance overall total abundance of 76%

TABLE

661

Estuary

dataat eachstation for the 15 speciescomprising an

Station A RgT Sabarieria pulchra Aponema torosum Terschellingia longicaudata Metalinhomoeus biformis Molgolaimus turgofrons Terscheilingia communis Sabatieria celtica Spirinia parasitifera Meralinhomoeus filiformis Comesa n. sp. Neotonchus meeki Daptonema svalbardense Metachromadora vivipara Terschellingia n. sp.

1 2 3 4 5 6 7 8 9 10 11 12 13

Odontophora Dominance

15

wieseri

14

Station B

Station C

Nb

70

F

Nb

y0

F

Nb

O$

F

1100 113 201 85 34 246 34 145 105 29 5 109 103 8 ~ 55

54 16 31 14 7 61 8 38 27 18 4 87 83 7

100 80 100 73 67 100 47 87 73 73 20 87 87 40

698 372 219 167 290 116

34 51 34 27 61 29

93 100 100 73 100 87

111

28

94 89 89 34 7 11 10

24 23 56 25 5 9 8

256 237 221 371 152 42 254 149 193 41 95

12 33 35 59 32 10 64 38 50 26 71

73 100 100 100 100 93 93 93 87 87 93

80 87 73 100 73 20 53 40

10 10 102

8 8 85

27 33 73

\ “,l 79.0

93,

24

“,” 77.7

73,

72-l

Abbreviations: RgT, overall rank; Nb, total number; %, percentageof each species at eachstation (e.g. of the total numbers of Sabatieriapulchra found during the study, the percentages at stations A, B and C were 54, 34 and 12, respectively); F, percentage frequency of occurrence at eachstation.

Only the negative part of axis II was well determined by four samples and two species. They were C, Summer 1983 (0.47) and B, Autumn 1983 (0.43), Autumn 1979 (0.32) and Spring 1980 (0.30), and Aponema torosum (0.50) and Molgolaimus turgofrons (0.43), respectively. Axis II showsa clear separation between the silty stations A and B. The latter muddy station corresponds to the flocculation zone: mid-estuary trough where fine particles accumulate trapping hydrocarbons; organic carbon values were higher here than in any other part of Morlaix Bay (Beslier, 1981). Biodegradation is a major component of the low oil degradation and may be responsible for the eutrophication of the habitat. Looking at someof the individual samplesand species,the remote position of sampleA6 with respect to axis II is associatedwith the presence of large numbers of Diploscapter coronata (23% of the total fauna), a freshwater speciesnormally considered to be an indicator of organic pollution (Zullini, 1982). Sample C8 wasdistinguished from the other station C samplesby axis I. Despite a large grain size and low silt content, the samplewas dominated by Sabatieria pulchra (68%) a species with an established preference for muddy sediments(Heip et al., 1985). Possibly this occurrence is associatedwith the buildup of numbers of this speciesat upstream sitesat that time (Figure 3). Aponema torosum and Microlaimus turgofrons are very abundant at station B: they also constitute 45% of the fauna in sample C13, which may explain its position in relation to axis II. As seven of the 15 samplesat each station were taken in late Autumn, it is interesting to consider just these data in an attempt to reduce any effect of seasonality. The temporal

N. E. Gourbault

662

t.D.cor

I

q6

AX.11

St.Ao st.s. st.c* M.vir

sp.

0 l.Al

-1 silty

sp.

-

0’0

.r.ion

;

0 9

1, -9

A4

.

*lo

N.spn V.“iS

. DA m

. N.mes

3

Mxom

.7 M.iur . 5 A.4

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Eutrophication -1 13 .

t Figure 2. Ordination of the 45 samples and 24 of the species from Stations A-C in the space of the first two principal axes of the correspondence analysis based on 72 species. The names of the species mainly responsible for sample locations, mentioned in the text, can be deduced from the abbreviations given in the figure. The Autumn samples from each station are joined together sequentially by solid lines with arrows.

variation in the position of these samples as indicated by the first two axes of the correspondence analysis is shown in Figure 2. At stations A and C, there appears to be a tendency for a more negative location along axis II from 1978 to 1983 with a return towards the 1978-9 position in 1984. At each of the most negative positions (A14, B 14, and C14) the samples contained a large proportion of the associated species A. torosum and M. turgofrons (18, 49 and 29%, respectively). Hitherto, these two species have only been recorded in the North Sea near Helgoland (Lorenzen, 1972, 1973) and in the Oresund (Jensen, 1978). Recently they were found to be the dominant species in the marine section

Nematode assemblages in the Morlaix

Estuary

663

.-

.A *B

I

*

* _.

A--y---

n

70

79

l

80

._-

0

. . I

81

I

a2

I

a3

a4

sample Figure 3. Absolute and Station C (0),

abundances (Nb) of Sabazieriapulch with only the Autumn data joined

at station by lines.

A ( l ), Station

B (*)

of the Rance estuary (Gourbault & Renaud-Mornant, 1986) and A. torosum was found to be present in the Tamar estuary (Warwick & Gee, 1984). (2) Diversity analysis. Various diversity indices were calculated (Table 3), all of which show the samegeneral pattern. Diversity waslower for the nearshore (upstream) stations. Mean H’ values for stations A, B and C were 3.2 f 0.9,3.4 f 0.8 and 4.2 f 0.7, respectively: i.e., no significant difference between A and B, but significant between B and C, and A and C (P= 0.05). Within each station, the diversity was highest during the summer months. Only the Autumn samplesat station A showed reduced diversity after 1978 with a subsequent increasein the last two samplestaken; but a stochastic explanation may pre-empt a biological one. Figure 4 showsthe Lorenz (equitability) curves for the Autumn samples,again selected to reduce variation due to seasonality. To a large extent the curves reflect the data in Table 3. Only station A showed a clear trend, with a decreasein equitability from 1978 to 1980 followed by a recovery to the 1978 situation by 1984. The curves were all closetogether at each of the other two sites, with the exception of 1982at station B and 1981at station C. Increased dominance is often associatedwith pollution (Shaw et al., 1983). The number of speciescomprising 80% of the total fauna (Table 3) was generally low at around five at station A in the period 1979-81 with somesignsof an increasethereafter. However, any effect of the oil may have taken sometime to influence the nematode assemblagebecause dominance was still relatively low in Autumn 1978. (3) Empirical statistical models. These assemblagesdo not conform to any of the most frequently used models, such as the log-normal or log-series models (Gourbault & Lecordier, 1985). A similar general conclusion was reached previously by Shaw et al. (1983) and Lambshead and Platt (1985), although Warwick and Gee (1984) fitted log-

664

N. E. Gourbault

TABLE 3. Diversity Date of sampling

data for each sample Sample

Nb

D

H’

J’

SR

a

Al A2 A3 A4 A5 A6 A7 A8 A9 A10 All Al2 A13 A14 Al5

41 18 26 26 13 25 44 18 21 45 16 19 34 37 37

17 4 6 6 2 5 16 8 4 16 5 4 11 10 14

4.6 2.6 3.2 3.0 1.8 3.0 4.5 2.3 2.8 4.1 2.2 2.4 3.9 3.7 4.4

0.85 0.63 0.69 064 0.48 0.64 0.83 0.56 0.64 0.75 0.56 0.57 0.77 0.72 0.84

7.55 3.21 4.72 7.72 2.26 4.53 8.12 3.21 3.78 8.30 2.83 3.40 6.23 6.80 6.80

15.62 4.79 7.97 7.97 3.11 7.54 17.44 4.80 5.92 18.07 4.09 1.16 11.76 13.35 13.35

Bl B2 B3 B4 B5 B6 B7 B8 B9 BlO Bll B12 B13 B14 B15

26 23 18 39 25 22 33 28 36 41 22 38 46 25 27

7 5 4 14 6 4 11 6 8 14 4 17 19 6 6

3.3 3.0 2.8 4.4 3.3 2.4 4.1 3.1 3.4 4.2 2.3 4.7 4.6 3.2 3.2

0.70 0.66 0.67 0.83 0.71 0.54 0.81 0.64 0.66 0.78 0.52 0.90 0.84 0.68 0.67

4.72 4.15 3.21 7.17 4.53 3.96 6.04 5.10 6.61 7.55 3.96 6.98 8.49 4.53 4.91

7.97 6.71 4.80 14.46 7.54 6.31 11.25 8.86 12.81 15.62 6.31 13.90 18.71 7.54 8.41

Cl C2 c3 c4 c5 C6 C7 C8 c9 Cl0 Cl1 Cl2 Cl3 Cl4 Cl5

47 48 37 53 43 39 32 21 37 32 42 47 41 50 43

18 21 11 22 14 8 12 4 11 11 15 17 26 18 14

4.6 4.7 4.1 4.9 4.6 4.1 3.5 2.1 4.1 4.2 4.4 4.6 3.9 4.7 3.8

0.83 0.84 0.79 0.85 0.84 0.79 0.69 o-49 0.79 0.84 0.81 0.83 0.73 0.83 0.70

8.68 8.87 6.79 9.81 7.93 7.17 5.85 3.78 6.80 5.85 7.74 8.68 7.74 9.25 7.93

19.36 20.03 13.35 23.55 16.82 14.47 10.75 5.92 13.35 10.76 16.21 13.35 16.86 2140 16.82

Stat&m A 23.10.1978 16.10.1979 28.05.1980 26.08.1980 01.12.1980 01.04.1981 28.08.1981 17.11.1981 15.04.1982 19.08.1982 24.11.1982 26.04.1983 30.08.1983 30.11.1983 08.11.1984

Station B 23.10.1978 16.10.1979 28.05.1980 26.08.1980 01.12.1980 01.04.1981 28.08.1981 17.11.1981 15.04.1982 19.08.1982 24.11.1982 26.04.1983 30.08.1983 30.11.1983 08.11.1984

Station C 23.10.1978 16.10.1979 28.05.1980 26.08.1980 01.12.1980 01.04.1981 28.08.1981 17.11.1981 15.04.1982 19.08.1982 24.11.1982 26.04.1983 30.08.1983 30.11.1983 08.11.1984

Abbreviations: Nb, number of species; D, minimum number of species comprising 80% of the total number (= 200) of individuals; H’, Shannon-Wiener index (log 2); J’, evenness; SR, species richness; a, Fisher, Corbet and William’s index.

normals to estuarine nematodes and copepod assemblages.However, the nematodes, which are capable of extensive resource partitioning, fitted the Pareto model very well. This model, more usually encountered in the field of economics, is essentially an inverted J

Nematode assemblages in the Morlaix

665

Estuary

,$&...“

10 0 2

10

100

I 2

I 10

100

R Figure 4. Lorenz equitability the years 1978-81 plotted R = relative species rank.

plots for the Autumn assemblages at each station, with separately from 1982-84. Ab=curnulative abundance,

curve with the slope of the regression line (Figure 5) being a measure(s) of a geometrical progression corresponding to a diversity index (Table 4). The correlation coefficient (r) between observed and theoretical data for specimen clusters was generally good, with the sole exception of sampleC5 (Table 5). The Autumn data are shown in Figure 5 and there is a clear difference between station A, with a large degreeof variability ins, and the other two sites.The exception to the generally low degree of variation at stations B and C is the C8 sample, which was dominated by Sabatieria pulchra.

Discussion The analysis of population structure in relation to pollution is often bedevilled by large natural fluctuations. In addition, relatively few studies have dealt with long-term natural fluctuations in nematode assemblages(Warwick & Buchanan, 1970, 1971; Juario, 1975; Boucher 1980b; de Bovee, 1981), probably due to a perceived difficulty in identifying the

666

N. E. Gowbadt

Nematode assemblages in the Morlaix

TABLE 4. Correlation coefficient(r) (the Autumn samples are identified Station Al* A2* A3 A4 A5* A6 A7 A8* A9 A10 All* Al2 Al3 A14* A15*

A

I

s

0.956 0.974 0.975 0.995 0.982 0.987 0.981 0.971 0.986 0.980 0.987 0.987 0.989 0.993 0.979

1.031 0.576 0.683 0.739 0.534 0.667 1.024 0.639 0.626 0.983 0.557 0.648 0.870 0.833 0.875

Estuary

667

and slope (5) of the Pareto by an asterisk, see text)

StationB Bl* B2* B3 B4 B5* B6 B7 B8* B9 BlO Bll* B12 B13 B14* B15*

r 0.983 0.985 0.972 0.971 0.975 0.989 0.975 0.992 0.976 0.994 0.978 0.903 0.983 0.981 0.983

s 0.731 0.650 0.597 O-983 0.687 0.629 0.899 0.731 0.749 0.955 0.659 1.178 1.042 0.721 0.712

distribution

StationC cl* c2* c3 c4 c5* C6 c7 a* c9 Cl0 c11* Cl2 Cl3 c14* c15*

of each sample

r

s

0.990 0.984 0.965 0.986 0.879 0.993 0.987 0.979 0.986 0.934 0.991 0.972 0.983 0.974 0.976

1.053 1.094 0.864 1.129 0.950 0.775 0.917 0.626 0.916 0.912 0.987 1.034 0.883 1.039 0,890

large array of speciesthat are encountered. Recently, the detection of differences among and between assemblagesof meiofaunal specieshas been receiving increased attention, with increasing use being made of graphical visualization of diversity in addition to numerical assessments(Boucher, 1980a; Gourbault, 1981, 1984; Heip & Decraemer, 1974; Lambshead et al., 1983; Lorenzen, 1974; Platt, 1984; Shaw et al., 1983; Tietjen, 1977). This investigation is a contribution to these attempts to detect differences in structural patterns on a long-term basis and is an extension of a preliminary study reported earlier (Gourbault, 1981). The first point to be made from the results is that there is a relatively clear overall distinction between the shallow upstream station (A) and the others, despite the closer similarity in sediment composition between stations A and B (both muddy sand). This is reflected in the correspondence analysis and the Pareto model. However, diversity (as measuredby the Shannon-Wiener index) separatesstation C from the others, presumably reflecting sediment composition. This relationship between diversity and silt content has been well documented and the results obtained in this work are comparable with others (cf. Warwick & Buchanan, 1970; Heip 81Decraemer, 1974). A secondpoint to be made is that it is very difficult to determine any unequivocal effects of the oil spill. The largest degree of variability appearsto be at station A, the shallowest station, which is more subject to natural variations of temperature and salinity. The greatest variation detected by the correspondence analysis occurred at this site, with a large alteration in the speciesassemblagesoccurring between 7-19 months after the oil impact. The other two sites showed little change during this period. However, it is difficult to make any further conclusions in the absenceof any information about how this kind of analysis behaves under ‘ normal ’ conditions or what the normal situation was at these sitesbefore the oil impact. One possibleindication of the effect of the oil is in the reduced diversity values at station A which occurred some 19 months after the spill, assuming the October 1978 and November 1984 values are normal. The trend would have been more cogent but for the August 1981result and if the results from station A had been significantly different overall from station B.

668

N. E. Gourbault

The Lorenz curves are more convincing. At station A there is a clear change in dominance away from the October 1978 position and back again, with little crossing over. The only two curves that stand out from the other two stations are those of 1982 at B and 198 1 at C, which both coincide with the period of maximum effect (1980-82) at station A. The general picture to emerge then is of assemblages of nematode species changing from and returning to a ‘ normal ’ structure in response to oil contamination. This was most clear at the shallow station. The assemblages did appear to recover close to their original structure (assuming this to be similar to that pertaining in October 1978). The assemblages therefore showed a certain degree of resilience (ability to return to a former state). This is not always the case, a progressive and permanent alteration of another Morlaix Bay nematode assemblage has been reported by Boucher (1983,1985); in contrast to estuarine nematodes assemblages, sublittoral marine assemblages are known to remain stable from month to month (Boucher, 1980b, 1981), as do most offshore nematode communities (Warwick&Buchanan, 1971). After a five-year survey, Boucher (1983) could not detect any return to an initial situation which could be interpreted as an end to the response of the assemblage to the disturbance: rather it changed to a new structure. A possible explanation for the difference between the situation in the estuary studied here and that found by Boucher is that the estuary may have been recolonized from less contaminated offshore sediments with a similar granulometric composition. Boucher’s site, in contrast, was an isolated area of fine sand among heterogeneous sediments, so that recruitment to replace the pollution-sensitive species was less likely to occur. Boucher (1982) showed experimentally that those species which disappeared from the natural sediment were the same as those eliminated from experimentally polluted sediment tanks. In Baie de Morlaix ofShore macrofauna assemblages, a short phase of selective mortality was observed followed by a four-year perturbation: absence of recruitment of some species, increase in opportunistic taxa, biostimulation, recolonization and return to normal hierarchic order. Long-term monitoring of the macrofauna of the Morlaix estuary area, however, has shown that pollution effects are more attenuated than in offshore situations (Dauvin, 1984). The effects of the oil pollution on the nematodes assemblages of the Morlaix estuary also appear to be similarly attenuated. This study has shown that durable alterations of nematode community structure can be detected underlining environmental change. Perhaps with further detailed analyses of community structure we may begin to approach with more confidence the assertion recently made by Platt (1984) that ‘ marine nematodes may reveal the “ health ” of the oceans-but only if we are able to distinguish one species from another ‘. Acknowledgements This study was supported by grants from CNEXO-COB (veille icologique des Cotes bretonnes). The author wishes to acknowledge the assistance provided by M. N. Helleouet for sorting the meiofauna samples collected by the Mysis Vessel, Roscoff, and C. Leroux and J. L. Douville for computing the data. The author is indebted to Dr G. Boucher and mainly Dr R. Rouch for helpful suggestions and to Dr H. Platt for manuscript revision and English rewriting of the text. References Amoco Cadiz (France),

1981 Fates and effects of the oil spill. Proceedings 19-22 November 1979 (CNEXO, ed.), Paris.

of the Znternational

Symposium

C.O.B.

Brest

Nematode

assemblages

in the Morlaix

Estuary

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