Spatial structure of a subtidal macrobenthic community in the Bay of Veys (western Bay of Seine, English Channel)

www.elsevier.com/locate/jembe

Journal of Experimental Marine Biology and Ecology

307 (2004) 217–235

Spatial structure of a subtidal macrobenthic

community in the Bay of Veys

(western Bay of Seine, English Channel)

Jean-Claude Dauvina,*, Eric Thiebautb, Jose Luis Gomez Gesteirac,Konstantinos Ghertsosa, Franck Gentild,

Michel Roperte, Bernard Sylvandf

aStation Marine de Wimereux, Universite des Sciences et Technologies de Lille, CNRS UMR 8013 ELICO,

28 Avenue Foch, B.P. 80, 62930 Wimereux, FrancebMuseum National d’Histoire Naturelle, Departement Milieux et peuplements Aquatiques,

UMR 5178 BOME (MNHN, CNRS, UPMC), 61 rue Buffon, 75005 Paris, FrancecGrupo de Fısica Oceanografica y de Costas, Facultade de Ciencias, Universidade de Vigo,

32004 Ourense, SpaindStation Biologique de Roscoff, Universite P.&M. Curie, CNRS UMR 7127, B.P. 74,

29682 Roscoff Cedex, FranceeLaboratoire conchylicole de Basse-Normandie, Ifremer, B.P. 32, 14520 Port-en-Bessin, France

fLaboratoire CNRS UMR 6143 M2C, Station Marine, rue du Dr Charcot, 14530 Luc-sur-mer, France

Received 5 March 2003; received in revised form 5 February 2004; accepted 9 February 2004

Abstract

The spatial distribution of the muddy fine sand community from the Bay of Veys (western

English Channel) were investigated during spring and autumn 1997. A grid of 55 and 54 sites

was sampled in March and October, respectively, using two replicates per site of a Hamon grab

(0.25 m2) for macrofauna collection and an additional one for sediment analysis. A total of 172

species were sampled with a dominance of polychaetes, followed by crustaceans and bivalves.

The species richness and abundance show low temporal changes despite higher values in October

than in March. In March, the mean abundance was 165 ind. 0.5 m� 2; in October, the mean

abundance was 212 ind. 0.5 m� 2. Four assemblages from the Abra alba–Pectinaria koreni

community were identified corresponding to a bathymetric and sedimentary gradient from muddy

fine sands with high levels of fine particles in shallow water to fine sands in deeper water. The

0022-0981/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jembe.2004.02.005

* Corresponding author.

E-mail address: [email protected] (J.-C. Dauvin).

J.-C. Dauvin et al. / J. Exp. Mar. Biol. Ecol. 307 (2004) 217–235218

discussion focuses on factors prevailing on the spatial structure of sandy communities in the

English Channel.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Abra alba–Pectinaria koreni community; Spatial patterns; Sediment–animal relationship; English

Channel

1. Introduction

As composition of benthic communities is highly variable in space at a hierarchy of

different scales, analysis of spatial pattern of species is an essential basis for understanding

scales at which organisms interact with one another or with their environment (Underwood

and Chapman, 1996). Once the spatial distribution is described the next question that

arises concerns the identification of the biotic and abiotic processes that govern this

observed distribution. Spatial distribution of benthic communities and processes involved

in its control have been detailed in a depth review by Constable (1999). At large scale (>10

km), physical environmental factors including general circulation, tidal currents or

sediment grain size determined broad patterns of benthic organisms distributions (Warwick

and Uncles, 1980; Barry and Dayton, 1991). At smaller scale, Thrush (1991) suggested

that different biotic and abiotic factors (e.g. life cycle characteristics, predation, compe-

tition, sediment properties), or interactions between them can regulate organisms distri-

bution. As an example of interaction between biotic and abiotic factors, the sediment grain

size, which renders an environment more or less favourable to certain species, is primarily

governed by water circulation but may also be altered by the inhabiting organisms

themselves, especially sediment feeders. Finally, small-scale and short-term perturbations

generated a mosaic of habitats at different successional stages that can induce habitat

heterogeneity and small-scale variations of benthic communities (Morrisey et al., 1992a).

In the English Channel, intense tidal currents control the large-scale distribution of

superficial sediments and macrobenthic communities (Gentil and Cabioch, 1997). The

sandy communities are located along the English and French coasts, mainly in bays and

estuaries, where currents are weaker; conversely the gravel and pebble communities are

distributed in open sea and near the capes in areas with strong tidal currents. Thus, along

the French coasts of the eastern English Channel, three isolated sandy communities were

reported: in the eastern part of the Bay of Seine off the Seine estuary covering an area of

f 400 km2, in the western part of the Bay of Seine (i.e. the Bay of Veys) occupying an

area of f 80 km2 and along a narrow coastal band in the Picarde Bay, from Dieppe to

Dunkerque, on a surface of f 800 km2 (Gentil and Cabioch, 1997; Thiebaut et al., 1997;

Desroy et al., 2003).

While processes governing the general distribution of benthic communities within the

English Channel is well known, spatial structure of benthic populations as well as

ecological mechanisms that drive within each community remain poorly documented.

Previous studies performed in the eastern Bay of Seine and along the coasts of the

Picarde Bay suggested that the relative importance of factors explaining the distribution

of macrofaunal assemblages (e.g. sediment grain size, freshwater inputs and biotic

J.-C. Dauvin et al. / J. Exp. Mar. Biol. Ecol. 307 (2004) 217–235 219

interactions) may be variable and change between both areas (Thiebaut et al., 1997;

Desroy et al., 2003). Although sediment grain size partly controls the distribution of

dominant species and of faunistic assemblages along Picarde Bay, its role in the eastern

Bay of Seine is less significant. While each factor could be relevant for one area, their

general influence can be only determined by comparing patterns over different scales

among communities.

Different methods have been recently proposed to assess spatial patterns of marine

benthic communities. While spatial autocorrelation has been used to describe small-scale

spatial patterns of distribution (e.g. Hewitt et al., 1997, 1998, 2002), Morrisey et al.

(1992a) suggested that nested ANOVAs coupled with a hierarchical sampling design is

more powerful to cover a large spectrum of spatial scales. However, these two univariate

methods focused only on peculiar species or global descriptors of the communities (e.g.

total abundance, species richness). Multivariate analyses (e.g. PCA, MDS) have been

commonly used in benthic ecology since several decades but they have rarely incorporated

explicitly spatial information into the analysis (e.g. Warwick and Clarke, 1991; Thiebaut et

al., 1997). When incorporated explicitly into the analysis, spatial information has been

generally taken into account as a polynomial regression of geographical coordinates of

sampling sites (Borcard et al., 1992; Ysebaert and Herman, 2002) or as a geographical

distance matrix (Legendre and Troussellier, 1988). Otherwise, Ghertsos et al. (2001) used

on macrofauna data from the eastern Bay of Seine a method developed by Thioulouse et

al. (1995) in which spatial information is integrated as a neighbourhood graph.

The objectives of the present study are: (1) to describe the spatial structure of the

subtidal sandy community of the Bay of Veys at two dates (March and October) at a local

and a global scales using the method applied by Ghertsos et al. (2001); (2) to identify the

relationship between this structure and the sediment properties; and (3) to assess the

variability of environmental conditions implied in the structure of fine sand communities

in the eastern English Channel. While all these questions are specific to such communities,

they can be generalised to the broader question concerning the diversity of ecological

processes involved in the spatial patterns of benthic organisms within a community.

2. Materials and method

2.1. Sampling site

The Bay of Veys is located in the western part of the Bay of Seine within the eastern

English Channel (Fig. 1). The sandy subtidal zone occupies an area of f 80 km2. The

depth is less than 20 m at low tide, with a tidal range about 8 m during spring tides.

Instantaneous tidal currents are parallel to the Cotentin coasts with a velocity reaching 3

knots (Ropert and Dauvin, 2000). From the coast to open sea, two main subtidal

communities are described in this area: the fine to medium sand Ophelia borealis

community, offshore at depths of between 10 and 20 m, and the muddy fine sand Abra

alba–Pectinaria koreni community located at depth between 0 and 10 m (Gentil and

Cabioch, 1997). Freshwater inputs are low and do not exceed 50 m3 s� 1 (Ropert and

Dauvin, 2000).

Fig. 1. Location of the sampling sites in the Bay of Veys (western Bay of Seine) . sites sampled in March and


2.2. Sampling

The macrofaunal distribution in the Bay of Veys was described by two surveys

conducted in 1997. The first was in March before the recruitment of dominant species,

and the second in October after the main period of recruitment. As shown in Fig. 1, a grid

of 55 sites was sampled in March, and 47 of these ‘winter’ sites were sampled again in

October. The 8 remaining sites were not sampled due to bad weather conditions.

Conversely, 7 additional sites were sampled opposite the Cotentin coast giving a total

of 54 sites sampled in October. Most of the sites were located in the muddy fine sand A.

alba–P. koreni community but some offshore sites were located in the fine sand O.

borealis community (Gentil and Cabioch, 1997). All benthic sampling was carried out

using a Hamon grab sampling an area of about 0.25 m2 to a sediment depth of f 10 cm

October 1997; n sites sampled only in March 1997; + sites sampled only in October 1997.


(Eleftheriou and Holme, 1984). At each site, three grab samples were collected, two for the

biological analysis and one for the sediment analysis. Distance between stations was about

1 nautical mile while replicates at each station were located within a 50-m radius.

According to Ellingsen (2001) who suggested that a sampling area of 0.5 m2 was

sufficient to obtain a meaningful measure of local biodiversity, we assume that two grab

replicates provide a reliable estimate of the abundance of the dominant species. Total

abundance of individuals is greatly affected by the onset of recruitment which occurs in

the study area around late May to September, being largely dependent upon temperature

(Dauvin et al., 1993). In choosing these particular sampling dates, October and March, a

generalised image of the macrobenthic communities could be attained assessing the

survival of the individuals over the winter period and after the spring–summer recruitment

period. To minimise the effects of larval supply on the benthic community structure, the

sieving of samples was carried out with a mesh size of 2 mm which is sufficient to collect

most of the macrobenthic species in the English Channel after the early post-settlement

mortality (Thiebaut et al., 1997).

Fauna was preserved in 10% buffered formaldehyde prior to sorting, identifying to

species level and counting in the laboratory. Species number and abundance were

expressed in number of species and number of individuals per 0.5 m2, respectively.

Particle size distribution of sediment was analysed by dry sieving the sediment through

a stack of Wentworth grade sieves according to the technique of Buchanan (1984). The

sediment was characterised by the percentage of fine particles < 63 Am, the percentage of

particles >500 Am, and the percentage of particles >2 mm.

2.3. Data analysis

Relationships between the abundance of main macrobenthic species versus sediment

parameters were tested using Spearman’s rank correlation coefficients (Scherrer, 1984). As

the same data set was used in several tests, the Bonferroni correction was applied.

Temporal variations in abundances of the top ranked species were assessed using a sign

test for the 47 stations sampled at both dates (Scherrer, 1984).

For both surveys (March and October), the spatial structure of the community was

analysed using a method of multivariate analysis of spatial patterns described by

Thioulouse et al. (1995) used in identifying benthic assemblages in the eastern part of

the Bay of Seine by Ghertsos et al. (2001). The global/local method of Thioulouse et al.

(1995) is based on incorporating spatial information into multivariate statistical analysis.

Correspondance analysis (CA) or principal coordinates analysis (PCA) sites are weighted

with respect to geographical proximity. These are created in our case by construction of a

neighbourhood graph (Ghertsos et al., 2001).

This method functions like any PCA or CA with the added advantage that similarities/

differences between sites nearer to one another are given more importance to those of sites

further apart from one another. The result is that one may decompose a general analysis

and extract two different images. One termed the local analysis where major differences

between sites are highlighted (especially sites with strong localised abundances of less

common species). This part of the analysis relies on maximisation of the variance between

neighbouring sites relying on the Geary (1954) index.


The other termed the global analysis where one works across the whole sampling grid

searching for larger-scale trends and similarities. Ghertsos et al.’s study showed at their

scale that global analysis was a good tool for the identification of assemblages within a

marine community and local analysis served well to identify sporadic or localised patches

of certain species. This other part of the analysis relies on maximisation of the covariance

between neighbouring sites relying on the Moran (1948) index of autocorrelation. Further

details including mathematical formulation may be found in Meot et al. (1993),

Thioulouse et al. (1995), Gaertner (1997) and Ghertsos et al. (2001).

In summary, global analysis was used to identify main assemblages within the

community and local analysis to identify major heterogeneities occurring at smaller scales.

In a first step, a ‘strictly linked’ neighbourhood graph was applied to the data. The term

strict is applied as only sites closer than a maximum of 6 nautical miles where linked to

one another. This graph is created from the map of the sampling sites in order to designate

neighbours. We used a connection network, know as Delaunay triangulation using Systat

version 9. First a Voronoi tesselation was made on the sampling sites based on a matrix of

Y–Y coordinates. Sites sharing at least one side of their respective Voronoi polygons were

linked. This is the basis of Delaunay triangulation. Finally, sites were thus linked to one

another on the basis of their geographical proximity. Sites linked together in this way were

designated as neighbours (Ghertsos et al., 2001).

In a second step, a total analysis was performed. It differs from a classical CA in that

the values per sampling site are weighted according to their corresponding number of

neighbours deduced from the neighbourhood graph: more importance in the analysis is

assigned to sites with a larger number of neighbours. Next, two different analyses are

carried out, the first called ‘global analysis’ and the second ‘local analysis’ to decompose

the total variance. The global analysis searches a linear combination of initial variables that

maximises the covariance between neighbouring sites while local analysis searches a

linear combination of initial variables that maximises the variance between neighbouring

sites. The representation of species in local and global factorial space may be used to

identify those playing important roles in the structuring of the sites at both spatial scales.

Analyses were carried out with species-site matrices where species occurring in less

than 5% of the sites at each campaign were removed. The only adaptation to the original

method is that in order to gain a generalised idea of seasonal assemblage distribution, a

hierarchical classification was carried out on the global analysis site scores of each

respective date (March five axes, October three axes). The classification used a Euclidian

distance metric with a flexible linkage (Fromentin et al., 1997). Finally, a generalised map

was conceived giving an overall image of both seasons.

3. Results

3.1. Sediments

Fig. 2 shows the distribution of sediment in the Bay of Veys in March and October

1997. According to a northeast/southwest gradient and the depth, four main types of

superficial sediment were observed from fine sand in offshore areas to muddy sand in

Fig. 2. Superficial sediments in the Bay of Veys in March and October 1997. Fine sand, very fine sand, muddy

sand and sandy mud according to the classification of Larsonneur et al. (1982).


inshore areas. Nevertheless, a small patch of very fine sand was located in the central part

of the muddy sand area just in front of the estuary zone. Sandy mud was present at a single

site along the Cotentin coast only in October.

3.2. Faunal composition

A total of 122 and 144 species were identified in March and October, respectively, with

a total number of 172 species (Table 1). The polychaetes accounted for 78 species, the

crustaceans 38, the bivalves 32, the echinoderms 7, the gastropods 6, and all other groups

11. Twenty-nine species were sampled in March only and 51 exclusively in October, 93

species were common to both dates. In March and October, 51 species were sampled in

one or two sites exclusively. The total number of species at each site varied from 3 to 31 in

March (mean 17.5F S.D. 6.9), and from 7 to 57 in October (mean 25.0F S.D. 9.5).

Table 1

Number of species, and mean abundance (ind. 0.5 m� 2) of the main zoological taxa in March (grid of 55 sites)

and October (grid of 54 sites) 1997

Polychaetes Bivalves Gastropods Echinoderms Crustaceans Others Total

Species richness March 59 25 4 7 19 8 122

October 62 29 4 5 33 11 144

Abundance March 70.7 17.2 2.0 71.7 1.8 2.0 165.4

October 120.6 20.9 10.3 53.2 2.9 4.2 212.1

3.3. Abundance

In March, abundance varied between 9 and 585 ind. 0.5 m� 2 (mean 165 ind. 0.5

m� 2F S.D. 159), and in October from 18 to 829 ind. 0.5 m� 2 (mean 212 ind. 0.5

m� 2F S.D. 170) (Fig. 3A, B). Total abundance distribution exhibited seasonal changes

with a significant increase of abundance between March and October (U-test, Wilcoxon–

Mann–Whitney; p = 0.045).

Polychaetes and echinoderms were numerically dominant in March, with each taxon

accounting for 43% of the total collected individuals. Bivalves represented about 10% of

the individuals, followed by gastropods, crustaceans and others groups forming each more

than 1% of the individuals (Table 1). In October, polychaetes dominated numerically with

57% of all individuals, according to an important increase in abundance. The echinoderms

formed 25% of the individuals having a lower abundances than in March. Molluscs

(bivalves and gastropods) showed an increase between March and October forming 15%

of the individuals, crustaceans and other groups remained low (i1% of the individuals).

3.4. Distribution of the dominant species

For both surveys, in March and October, the top 10 ranked species and their mean

abundances are given in Table 2. Eight species were common for both dates. Thyone fusus

and Nephtys hombergii were reported among the dominant species only in March,

conversely Crepidula fornicata and Lanice conchilega in October.

Sediment grain size distribution played a major role in explaining the species

distribution (Table 2). While some species like L. conchilega, Echinocardium cordatum


Fig. 3. Total abundances (ind. 0.5 m� 2) in March (A) and October (B) 1997.

Table 2

The first 10 major species arranged in a decreasing order of mean density (ind. 0.5 m� 2) and their relationships

with the sediment parameters in March and October 1997

Rank Species Number

of stations

Density Relationships with the sediment

parameters

(% occurrence)Range Mean

(F S.D.)

% Silt-clay

( < 63 Am)

% Coarse

sand-gravel

(>500 Am)

% Gravel

(>2 mm)

March 1997

1 Echinocardium cordatum 51 (92.7) 0–335 55.1 (82.4) � 0.0840 � 0.2502 � 0.3114

2 Pectinaria koreni 35 (63.6) 0–329 33.9 (67.6) 0.6032*** 0.3617 0.2919

3 Acrocnida brachiata 38 (69.1) 0–86 11.4 (18.2) 0.5850*** 0.3527 0.3752

4 Abra alba 38 (69.1) 0–58 9.5 (15.1) 0.5919*** 0.2154 0.1639

5 Euclymene oerstedii 27 (49.1) 0–283 6.7 (38.0) 0.5860*** 0.2572 0.1479

6 Scoloplos armiger 35 (63.6) 0–42 4.9 (8.2) � 0.2458 � 0.2504 � 0.3223

7 Owenia fusiformis 25 (45.5) 0–88 4.5 (14.9) 0.4511** 0.0730 0.0145

8 Thyone fusus 3 (5.5) 0–145 3.8 (21.2) 0.2044 0.3736 0.3632

9 Nephtys hombergii 47 (85.5) 0–19 3.4 (3.7) � 0.1506 � 0.3133 � 0.3450

10 Marphysa belli 35 (63.6) 0–21 3.1 (4.5) 0.7637*** 0.2493 0.2656

October 1997

1 Lanice conchilega 52 (96.3) 0–251 53.8 (62.9) 0.0518 0.2139 0.2084

2 Echinocardium cordatum 48 (89.0) 0–235 39.8 (69.3) � 0.1699 0.0951 0.0238

3 Acrocnida brachiata 43 (79.6) 0–57 11.6 (13.9) 0.4410* 0.5209** 0.5827***

4 Pectinaria koreni 41 (75.9) 0–124 10.8 (23.1) 0.5091** 0.2947 0.3337

5 Abra alba 37 (68.5) 0–95 9.2 (19.9) 0.4956** 0.1960 0.1801

6 Scoloplos armiger 41 (75.9) 0–90 8.5 (16.9) � 0.3395 � 0.0218 � 0.1160

7 Owenia fusiformis 37 (68.5) 0–89 8.4 (22.0) 0.5650*** 0.1300 0.2148

8 Crepidula fornicata 6 (11.1) 0–399 7.5 (162.2) 0.1206 � 0.0348 0.0119

9 Marphysa belli 40 (74.1) 0–29 6.1 (7.7) 0.2337 0.3673 0.4653**

10 Euclymene oerstedii 32 (59.3) 0–29 5.2 (8.5) 0.4227* 0.4167* 0.4249*

Species for which densities increased from March to October are indicated in bold while species for which

densities decreased from March to October are underlined (sign test; p< 0.05). Relationships between

macrobenthic species density and sediment parameters were calculated using the Spearman’s rank correlation

coefficient. The significance level was determined after the Bonferroni correction.

*p< 0.05.

**p< 0.01.

***p< 0.001.


and Scolopos armiger seemed indifferent to the sediment type, A. alba, Owenia fusiformis

and P. koreni were primarily present in muddy sand with high percentages of silt-clay.

Finally, other species as Acrocnida brachiata and Euclymene oerstedii were sampled

preferentially in heterogeneous sediment with high percentages of silt-clay and coarse

sand-gravel.

Most dominant species like A. alba, A. brachiata and E. oerstedii showed very low

changes of mean abundance between both seasons (Fig. 4; Table 2). Three species (i.e.

Marphysa bellii, Scoloplos armiger and L. conchilega) exhibited a significant increase of

abundances from one season to another while only one species, P. koreni, showed a

significant decrease in abundance between both campaigns (Figs. 5 and 6; Table 2).

� 2


3.5. Distribution of the assemblages

Graphs displaying the eigenvalue inertia distributions resulting from global and local

analyses after implementation of the method of Thioulouse et al. (1995) permitted to select

the numbers of axes to be retained. Eigen values inertias decrease axis by axis; the point at

which a significant decrease is found is often used as the criterion for axis retention

Fig. 4. Abundance (ind. 0.5 m ) of A. brachiata population in March and October 1997.

Fig. 5. Abundance (ind. 0.5 m� 2) of L. conchilega population in March and October 1997.

Fig. 6. Abundance (ind. 0.5 m� 2) of P. koreni population in March and October 1997.


(Ghertsos et al., 2001). Thus, in March, five axes were retained for the global analysis and

three for the local analysis. In October, three axes were retained for global analysis and

four for local analysis. These selected axes were then used in the hierarchical classification

Fig. 7. Maps of factorial scores for sampling sites in March 1997, for three axes of each analysis, global and local,

after using the method of Thioulouse et al. (1995).


in order to have the generalised seasonal assemblages. However, in order to homogenise

the observations and for reasons of simplicity, the first three factorial scores were

represented for both dates in Figs. 7 and 8. The scores for each site were projected as

symbols onto maps of the bay with white squares depicting positive values and black

circles as negative values with sizes being proportional to value as in Ghertsos et al.

(2001).

In March, global analysis illustrated three partitions (Fig. 7). The first axis displayed

oppositions between western sites (positive) and eastern sites (negative). The second axis

showed oppositions between coastal (positive) and seaward (negative) sites. The third axis

illustrated the opposition of the central part (negative) and the outside part (positive) of the

bay. Local analysis illustrates the dominance of some species in a limited number of sites:

axis 1, sites 122 and 131 with dominance of the holothurian T. fusus and the polychaete

Pista cristata compared to the rest of the sites; axis 2, identified the site 11 influenced by

the dominance of the cnidarian Sagartia troglodytes, and axis 3, displayed the opposition

of site 91 with the amphipod Bathyporeia gracilis and the polychaete Magelona mirabilis

to the rest of the sites.

In October, global analysis illustrated again three partitions (Fig. 8). The first axis

showed oppositions between coastal (positive) and seaward (negative) sites. The second

axis displayed oppositions between eastern (positive) and western sites (negative). The

third axis illustrated the opposition of the central part (negative) and the western and

Fig. 8. Maps of factorial scores for sampling sites in October 1997, for three axes of each analysis, global and

local, after using the method of Thioulouse et al. (1995).


eastern (positive) of the bay. Local analysis illustrated the importance of some species in

a limited number of sites: axis 1, site 11 with the gastropod C. fornicata; axis 2, showed

the opposition of site 131 with the holothurian T. fusus and the polychaeta P. cristata to

the rest of the sites; axis 3, displayed the opposition of sites 33, 101 and 115 with the

polychaete Nephtys cirrosa and the gastropod Nassarius reticulatus to the rest of the

sites.

Hierarchical classification distinguished two main groups of sites which can be each

subdivided in two subgroups for a total of four assemblages in the Bay (excluding the

three sites N1, N2 and N3 at the north which were only sampled in March) (Fig. 9).

The first assemblage corresponded to nine sites distributed in three patches located in

the shallow water on muddy sand, with low percentages of fine particles (Fig. 2) and the

presence of gravels and pebbles. In March, the total abundance was low ( < 70 ind. 0.5

m� 2); T. fusus, P. cristata and A. brachiata dominated in abundance (Table 3). In October,

the total abundance of this group was doubled of March (i140 ind. 0.5 m� 2), due to the

sampling of patches of C. fornicata and L. conchilega; A. brachiata remained the third

Fig. 9. Generalised map of the four Bay of Veys assemblages occurring over both dates (in March and October

1997).

Table 3

The 10 first top species arranged in a decreasing order of mean abundance (N ind. 0.5 m� 2) in March and October 1997 for each of the four assemblages identified in the Bay of Veys

Group 1 Group 2

March October March October

Species N Species N Species N Species N

Thyone fusus 23.1 Crepidula fornicata 44.7 – – Owenia fusiformis 54.3

Pista cristata 10.0 Lanice conchilega 19.9 – – Abra alba 27.9

Acrocnida brachiata 4.9 Acrocnida brachiata 6.2 – – Lanice conchilega 24.1

Nucula hanleyi 4.2 Scoloplos armiger 5.6 – – Nucula turgida 16.7

Sagartia troglodytes 4.0 Marphysa belli 3.6 – – Acrocnida brachiata 15.3

Echinocardium cordatum 2.0 Chaetozone setosa 3.2 – – Pectinaria koreni 11.6

Nephtys hombergii 1.9 Sigalion mathildae 2.8 – – Nephtys hombergii 9.0

Notomastus latericeus 1.6 Notomastus latericeus 2.7 – Sigalion mathildae 7.1

Abra alba 1.3 Magelona mirabilis 2.3 – – Euclymene oerstedii 6.3

Pectinaria koreni 1.3 Sagartia troglodytes 2.1 – – Notomastus latericeus 5.4

Total 68.3 Total 137.8 – – Total 215.1

% of the 10 top species 79.5 % of the 10 top species 67.8 – – % of the 10 top species 82.6

Group 3 Group 4

March October March October

Species N Species N Species N Species N

Pectinaria koreni 62.8 Lanice conchilega 75.6 Echinocardium cordatum 110.5 Echinocardium cordatum 91.2

Echinocardium cordatum 30.8 Echinocardium cordatum 34.9 Pectinaria koreni 15.7 Lanice conchilega 54.5

Acrocnida brachiata 19.6 Pectinaria koreni 18.3 Scoloplos armiger 7.5 Scoloplos armiger 20.8

Abra alba 18.1 Acrocnida brachiata 14.3 Magelona filiformis 6.1 Acrocnida brachiata 9.2

Euclymene oerstedii 13.6 Abra alba 9.5 Acrocnida brachiata 5.2 Lumbrineris gracilis 6.1

Owenia fusiformis 9.5 Marphysa belli 8.6 Chaetozone setosa 3.8 Marphysa belli 5.3

Marphysa belli 5.6 Euclymene oerstedii 7.4 Nephtys hombergii 3.4 Pectinaria koreni 4.7

Scoloplos armiger 4.9 Scoloplos armiger 4.3 Abra alba 3.4 Euclymene oerstedii 4.4

Nephtys hombergii 3.8 Sigalion mathildae 4.3 Nassarius reticulatus 2.0 Abra alba 4.3

Notomastus latericeus 2.1 Nephtys hombergii 3.2 Nucula hanleyi 1.9 Nucula hanleyi 3.1

Total 195.7 Total 217.6 Total 178.9 Total 247.5

% of the 10 top species 90.1 % of the 10 top species 82.4 % of the 10 top species 89.2 % of the 10 top species 82.3

Total: total abundance N. ind. 0.5 m� 2, and cumulative frequency (%) of the 10 top species.

J.-C.Dauvin

etal./J.

Exp.Mar.Biol.Ecol.307(2004)217–235

230


species (Table 3). At both dates, this assemblage was dominated by suspension feeders as

bivalves Spisula spp., Venerupis spp., and echinoderms as T. fusus and A. brachiata.

The second assemblage corresponded to seven shallow sites located along the Cotentin

coasts, sampled only in October, on muddy sand with a high percentage of fine particles

(Fig. 2). This is an assemblage dominated by O. fusiformis, and other species such as A.

alba, L. conchilega, Nucula turgida, A. brachiata and P. koreni. The total abundance were

in the same order of magnitude of the two other assemblages (i215 ind. 0.5 m� 2).

The third assemblages corresponded to 25 sites located in the central part of the Bay

of Veys on very fine sand with a medium percentage of fine particles (Fig. 2). In terms

of abundance, the dominant species were P. koreni, E. cordatum, A. brachiata, A. alba,

at both seasons and L. conchilega in October. Total abundance remained stable at both

dates i200 ind. 0.5 m� 2.

The last of the four assemblages was formed by offshore and eastern sites on fine sand

with low percentages of fine particles (Fig. 2). E. cordatum dominated at both seasons,

while P. koreni was abundant in March and L. conchilega in October. Sandy species like S.

armiger and Nucula hanleyi were among the 10 top species at both seasons. Total

abundance showed a low increase between March and October (180 against 250 ind. 0.5

m� 2, respectively).

Only 27 species, of 172 sampled, were among the top species in the four assemblages.

The total abundance was the lowest in the first assemblage while the abundance remained

around 200 ind. 0.5 m� 2 in the three other assemblages at both seasons.

4. Discussion

The understanding of spatial distribution have become important in defining the

sampling strategies in benthic ecology. Morrisey et al. (1992a) suggested that a stratified

sampling strategy could be used to identify the variance at the different scales of

observation within variance analysis. Nevertheless, these methods take into account only

global variables characterizing a community (e.g. species number, species abundance,

etc.). The global/local method of Thioulouse et al. (1995) permitted the separation of

assemblages in a systematic way. Firstly in observing local analysis results, small-scale

localised patches of certain species were easily distinguished. Strong localised patches can

sometimes influence and therefore hinder larger-scale detection of assemblages. This case

was identified by Ghertsos et al. (2001) as occurring in previous assemblage identifica-

tions from the eastern part of the Bay of Seine. Similarly, global analysis showed its

advantages in detection of larger-scale groups with little to no localised influences.

Moreover, in this case, structures were linked to certain abiotic parameters. It would be

interesting to make a neighbourhood graph based on these abiotic parameters, such as

sediment type, as opposed to simple geographical distance, in order to asses the local and

global components of these parameters and their influence on assemblage/species

distribution.

The advantages of using the global/local method were firstly demonstrated in our

results in the clear identification of four main assemblages according to abiotic parameters

such as sediment texture and depth (Fig. 9). Another advantage of this method lies in the


fact that the data set as a whole may be used since less common or ‘rare’ species do not

affect the final results. Previous and preliminary analysis using simple PCAs and then CAs

showed all patterns and patches confounded together and so very little information was

extracted (Ghertsos, 1998). Univariate methods would not be capable of capturing the full

complexities of the revelaed images (Morrisey et al., 1992a). The use of more complex

methods would have surely added a new angle to our analysis but the method used for the

purpose of this study was not only simple to apply but also suited the answering of our

initial study aims very well. Since there is a constant development of new methods with

the advent of expanding computer technology, one must be careful not to get carried away

with new methods but also look to answer the initial objectives in as simple and clear a

way as possible. On a practical note, the way in which resulting data may be plotted on the

maps also aids this greatly.

Local analysis illustrated the dominance of some species in a limited number of sites

(four in March, Fig. 7, and five in October, Fig. 8, for a total of seven different sites) at

both dates in the Bay of Veys. The echinoderm T. fusus and the polychaete P. cristata were

patchy and dominant in the first assemblage. Other patches (e.g. the gastropod C.

fornicata, the cnidarians S. troglodytes) occured only in October also in the first

assemblage. These species are practically absent in the other sites. This probably high-

lights the importance of the local heterogeneity of the sediment (e.g. patches of gravels and

pebbles) on the distribution of such species at a small spatial scale. Other more largely

distributed species, such as the gastropod N. reticulatus and the polychaete N. cirrosa,

dominated in October in three sites, showing low species richness and low abundance, of

the offshore and eastern assemblage (Group 4) on fine sand with low percentages of fines

particles (Fig. 3, axis 3 of the local analysis). In the eastern part of the Bay of Seine,

Ghertsos et al. (2001) showed that there was a high spatial heterogeneity on a small spatial

scale at the mouth of the estuary, probably as a result of the more extreme nature of this

highly unstable environment.

In summary, local analysis emphasized sites, with patches of a limited number of

species, which are in general drowned by traditional multivariate analyses multivariate

(e.g. PCA, CA AFC or MDS) and difficult to join in an assemblage. This is important to

notice that strong spatial variability of the specific richness and abundances of the benthic

macrofauna at a small scale seems a general rule in the studies on the distribution of the

soft-bottom macrofauna of the continental shelf of the Northeastern Atlantic (see

Ellingsen, 2001, 2002).

Four assemblages of the A. alba–P. koreni community were identified along a

northeast/southwest gradient in the Bay of Veys by global analysis. Mortality and

recruitment cause continuously changes of abundance of benthic populations. Season-

ally, these changes will be most pronounced during the period of spring–summer

recruitment and then decrease in amplitude towards winter (Dauvin, 1992, Dauvin et al.,

1993; Armonies, 2000). The moderate abundance changes recorded between March and

October 1997 in the large (80 km2) sampling area did not affect the spatial structure of

assemblages neither the dominant species (Table 2). For dominant species such as L.

conchilega, spring–summer recruitment and exchanges of populations by carriage

between both intertidal and subtidal zones, explain the increase of the abundance

between March and October (Ropert and Dauvin, 2000). Conversely, some species such


as P. koreni showed a small spring–summer recruitment in 1997 (Ellien et al., 2000).

For this last benthic species, with planktonic larvae, the local Bay of Veys population

may survive only at the metapopulation level at the scale of the whole Bay of Seine with

exchanges of larvae between the eastern part of the Bay of Seine and the Bay of Veys

subpopulations (Ellien et al., 2000). Nevertheless, short-term variations which result

mainly from factors acting at small spatial scales (Morrisey et al., 1992b) should be

identified for such fine sand populations and communities (Fromentin et al., 1997).

Morrisey et al. (1992b) show that in spite of different results according to taxonomic

groups, there is a significant part of the variation at small temporal scales ranging

between the day and few months that could be due to processes on a small spatial scale.

For coastal North Sea sandy community, Armonies (2000) showed that, in many benthic

species, the spatial patterns within a 2-km2 area changed considerably within 2 weeks in

relation with migrations and re-suspension even during moderate weather. If such

variability exists in the Bay of Veys, for species such as L. conchilega, there is a weak

impact on the community structure at a global scale (80 km2) between the two surveys.

This suggest that the main factors structuring the benthic community should be

independent to the replacement between top dominant species. For example, the increase

of the Lanice population does not implicate any changes of the structure of the

assemblages and the presence of other species.

Distribution of macrobenthic assemblages may be explained by different environmental

factors. In the Bays of Veys, sediment especially the percentage of fine particles and depth

play major roles in explaining the species and assemblage distributions in this area. In the

eastern part of the Bay of Seine (Thiebaut et al., 1997; Ghertsos et al., 2001), sediment

seems to be a poor indicator; however, freshwater with a high input of nutrients and

particular organic matter from the Seine river, and the very high abundances of dominant

species (e.g. the polychaetes P. koreni and O. fusiformis, >100,000 ind. m� 2 just after the

spring recruitment) with high biotic interactions could be responsible for the spatio-

temporal structure of macrobenthic assemblages at the mouth of the Seine estuary. The

shallow soft-bottom sediments along the eastern English Channel and southern North Sea

(Desroy et al., 2003) were more uniform than in the Bays of Veys and eastern part of the

bay of Seine with single A. alba assemblages covering about 80% of the subtidal area.

Two other assemblages (e.g., an O. borealis medium to fine sand assemblage, and a

muddy heterogeneous assemblage) were also identified along these coasts. Relative

proximity to inshore waters, outflows from bays, harbours and the Scheldt estuary

structured the benthic community in this area, with increases in species diversity,

abundance and biomass just in front freshwater inputs which indicated an increased food

supply at these locations.

Consequently, several abiotic and biotic factors could interact to explain the pattern of

distribution of similar fine sand assemblages in the English Channel. Nevertheless, the

broad-scale distribution of benthic communities in the Bay of Veys as in other part of the

English Channel could be considered to depend on sediment structure as a response to

hydrodynamic factors at a meso-scale as suggested previously by several authors (see

Morrisey et al., 1992a). Biotic factors seemed dominate at a smaller scale (Thiebaut et al.,

1998). In laboratory experiments, Olivier et al. (1997) observed that mortality rates of P.

koreni were greatly affected by O. fusiformis adults. In the case of O. fusiformis, Dauvin


(1992) showed that the juvenile survivorship was mainly controlled by the combined

action of density-dependent mechanisms and sediment food. Organisms play also an

important role on the ambient sediment such as grain size distribution and on stabilisation

or destabilisation of the sediment water interface (Armonies, 2000). Nevertheless,

experimental or in situ approaches will be necessary to identify such relationship between

edaphic and biotic factors at a small scale (cm–m) in the coastal soft-bottom benthic

communities.

Acknowledgements

This study was realized under the National Program of the Coastal Environment (PNEC

‘Chantier Baie de Seine’ and ‘ART2 population dynamics’) supported by the CNRS-INSU

and IFREMER. The authors thank the crews of the N.O. ‘Cotes de la Manche’ for their

valuable assistance in fieldwork, Richard Warwick and three anonymous referees for their

useful comments on the first two versions of the typescript. [RW]

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Documents

Spatial structure of a subtidal macrobenthic community in the Bay of Veys (western Bay of Seine, English Channel)