Weiying Zhao

Charles-Alban Deledalle

Loïc Denis

Henri Maître

Jean-Marie Nicolas

Florence Tupin

RABASAR: A FAST RATIO BASED MULTI-TEMPORAL SAR DESPECKLING

Keywords: Multi-temporal SAR series, ratio image, super-image, SAR, despeckling

In this paper, a generic method is proposed to reduce speckle in multi-temporal stacks of SAR images. The method is based on the computation of a "super-image", with a large number of looks, by temporal averaging. Then, ratio images are formed by dividing each image of the multi-temporal stack by the "super-image". In the absence of changes of the radiometry, the temporal fluctuations of the intensity at a given spatial location are due to the speckle phenomenon. In areas affected by temporal changes, fluctuations cannot be ascribed to speckle only but also to radiometric changes. The overall effect of the division by the "super-image" is the spatial stationarity improvement: ratio images are much more homogeneous than the original images. Therefore, filtering these ratio images with a speckle-reduction method is more effective, in terms of speckle suppression, than filtering the original multitemporal stack. After denoising of the ratio image, the despeckled multi-temporal stack is obtained by multiplication with the "super-image". Results are presented and analyzed both on synthetic and real SAR data and show the interest of the proposed approach.

INTRODUCTION

Synthetic aperture radar (SAR) imaging is a widely used remote sensing acquisition system due to its all time acquisition capability. However, the inherent speckle attached to any coherent imaging system affects the analysis and interpretation of SAR images. Therefore, a preliminary speckle reduction step is often proposed for a successful exploitation of SAR images. Plenty of denoising methods dedicated to the restoration of a single SAR image have been proposed in the past decades (see [START_REF] Argenti | A tutorial on speckle reduction in synthetic aperture radar images[END_REF][START_REF] Deledalle | Exploiting patch similarity for SAR image processing: the nonlocal paradigm[END_REF] for a review). With the launch of recent radar satellites (Cosmo-SkyMed, TerraSAR-X, ALOS-2, Sentinel-1, etc.), more and more SAR images, with short Thanks to China Scholarship Council for funding and to the CNES (French Space Agency) for funding (project DAJ/AR/IB-2016-10117102).

revisit time or high resolution, are now available. Jointly processing several images of the same area can potentially provide better denoising results than the restoration of a single image. This is the path followed by several multi-temporal denoising methods such as 2SPPB [START_REF] Su | Twostep multitemporal nonlocal means for synthetic aperture radar images[END_REF] or MSAR-BM3D [START_REF] Chierchia | Multitemporal SAR image despeckling based on blockmatching and collaborative filtering[END_REF]. A drawback of such approaches is the increased computational complexity with longer time series. In this paper, we take a different approach by forming a summary of the multitemporal series through a "super-image" [START_REF] Quin | MIMOSA: An automatic change detection method for SAR time series[END_REF], and using this "super-image" to denoise the SAR image at a given date.

Here, we propose a ratio-based multitemporal denoising method which fully exploits the significant information of the multi-temporal stack through the "super-image". We consider multi-temporal images acquired on the same orbit, with similar incidence angles, and that have been accurately registered. The paper is organized as follows. In section 2, the general framework of the method called RABASAR (RAtio-BAsed multitemporal SAR despeckling) is described. Different methods to compute a "super-image" are discussed in section 3. Section 4 introduces the denoising of the ratio image. Section 5 presents experimental results of the method. Finally, some conclusions are drawn in section 6.

PRINCIPLE OF THE METHOD

Having huge multi-temporal stacks allows to produce an image with reduced noise thanks to temporal averaging (also called temporal multi-looking). In this paper we propose to exploit this "super-image" to build a ratio-based denoising framework (Fig. 1). The proposed method is divided in 3 main steps.

First (step 1), a "super-image" is computed from the multi-temporal stack (serie of registered and calibrated SAR data). The average of the intensity images is the simplest way to compute a multi-look image and corresponds to the maximum likelihood estimation of the reflectivity if the temporal samples are i.i.d. (temporal fluctuations due to fullydeveloped speckle). In practice, different points have to be taken into account. First, since the data are temporally correlated and potentially changing in time, the noise reduction is spatially variable. Second, for changing areas, the mean value has no physical meaning. Therefore, it can be interesting to introduce binary weights to average only similar data as done in [START_REF] Su | Twostep multitemporal nonlocal means for synthetic aperture radar images[END_REF]. These points will be discussed in section 3. This "super-image" is noted by ûSI in the following.

In the second step, starting from this "super-image", the ratio R t between a noisy image y t and the "super-image" should contain only the residual speckle noise and the temporal variations between the two images.

R t = y t ûSI (1)
Then, this ratio is filtered to eliminate the noise fluctuations and keep only relevant information about the changes. In stable areas (where no change occurs other than speckle-related fluctuations), this ratio image is expected to be gamma distributed with mean 1 (i.e., independent of the radiometry at each pixel of the SAR image). The division in equation ( 1) therefore strongly increases the spatial stationarity (i.e., homogeneity) of the image. Any speckle filtering method can then be used for this step, as discussed in section 4.

In the last step, the original filtered image is recovered by simple multiplication of the denoised ratio and the "superimage". Experimental results on synthetic and real data are discussed in section 5.

"SUPER-IMAGE" OF A TEMPORAL STACK

The computation of a "super-image" from a multi-temporal stack can be done in different ways. Indeed, different means (arithmetic mean, geometric mean, Hölder mean) can be used, providing different kinds of information [START_REF] Quin | MIMOSA: An automatic change detection method for SAR time series[END_REF]. Besides, the changes between the dates can possibly be taken into account.

In this paper we propose to use the arithmetic mean of intensity images, with the option of using binary weights to discard the intensity at some dates when a change occured.

Statistics of SAR images

Under Goodman's hypothesis, the fully developed intensity speckle is following a Gamma distribution:

G[u, L](y) = L uΓ(L) Ly u L-1 e -Ly u (2)
where L is the number of looks, y is the intensity value, u is the mean intensity related to the reflectivity of the scene. The intensity data can be written with a multiplicative noise model

Arithmetic mean of a temporal stack

Given a series of M well registered SAR images indexed by time t, we now consider the intensity variables along the temporal domain. The arithmetic mean is defined by:

ûAM (s) = 1 M M t=1 y t (s) (3)
where s is the position in one image. If y t (s) is distributed according to Goodman's fully developed speckle model, with an underlying constant radiometry u t (s), then the arithmetic mean corresponds to the maximum likelihood estimator of u t (s). Because of the temporal coherence of SAR images in interferometric configuration, the resulting number of looks (ENL, Equivalent Number of Looks) may be less than the theoretical value, which is L × M under complete incoherence.

Binary weighted arithmetic mean

Changes may occur during the acquisition of the time series. In this case, the arithmetic mean is not reliable since it mixes different sample populations. To improve the computation of the reflectivity of the scene, we may focus on a specific image and select only the stable samples. To select these samples without being too much affected by speckle noise, a patchbased similarity criterion (using a generalized likelihood ratio test) proposed in [START_REF] Su | Twostep multitemporal nonlocal means for synthetic aperture radar images[END_REF] is used. This sample selection step, although improving the reflectivity estimation is time consuming since a patch-based comparison is done for each pixel in the multi-temporal stack.

Denoising the "super-image"

Because of varying coherence or unstable areas, or when using the binary weights, the "super-image" has a spatially varying ENL. This ENL is locally estimated [START_REF] Tison | A new statistical model for Markovian classification of urban areas in high-resolution SAR images[END_REF] using a sliding window and a global ENL for the "super-image" is deduced by taking the maximum value. A global value is used to speed up the denoising process that is possibly applied on the "super-image".

Therefore, 4 "super-images" can be computed: arithmetic mean image (AM), binary weighted arithmetic mean image (BWAM) and their denoised versions.

RATIO IMAGE DENOISING

If ûSI is close to the noise free image u t and intensity fluctuations follow the fully-developed speckle model, the ratio image R t is distributed as a standard gamma distribution. When changes occur, the super-image ûSI does not completely normalizes the radiometry and speckle fluctuations have an average value that differs from 1. Nonetheless, the spatial homogeneity is largely improved in the ratio image compared to the original image.

Non-local methods offer a powerful approach to estimate the noise-free values Rt , leading to smooth despeckled results while preserving edges and textures. For a given position s, similar positions i are searched within a window W s centered at s. The similarities are computed based on patch comparisons. Then, the noise free value Rt (s) is estimated by the weighted average:

Rt (s) = i∈Ws ω(s, i)R t (i) (4)
where ω(s, i) is the weight of the i-th pixel in the estimation at pixel s, and weights are normalized ( i∈Ws ω(s, i) = 1). This processing could benefit from any adaptive denoising method. In this paper, MuLoG-BM3D [START_REF] Deledalle | Mulog, or How to apply Gaussian denoisers to multi-channel SAR speckle reduction?[END_REF] is chosen to process the ratio image (Fig. 2

DENOISING RESULTS ANALYSIS

To evaluate the performances of the proposed method, different experiments have been conducted on simulated images and actual Sentinel-1 images. Two different multi-temporal denoising methods are also compared with 2SPPB [START_REF] Su | Twostep multitemporal nonlocal means for synthetic aperture radar images[END_REF] and MSAR-BM3D [START_REF] Chierchia | Multitemporal SAR image despeckling based on blockmatching and collaborative filtering[END_REF]

Using different "super-images"

As explained in the previous analysis, the "super-image" can be calculated in different ways. Two different "super-images" are used to evaluate the ratio-based denoising performances (Fig. 3). All of them provide good filtering results, as can be observed by close inspection of the filtered ratio images.

When using the arithmetic mean to calculate the ratio, small areas with low values could be smoothed which leads to the apparition of new points in the denoised results (Fig. 3(c) red rectangular). This phenomenon is obvious for the changes in the building areas. Using binary weighted arithmetic mean reduces the artifacts in these areas (Fig. 3(e)). In addition, using binary weights for temporal averaging may lead to noisy areas in some changing part of the image.

Comparison with MSAR-BM3D and 2SPPB

To numerically evaluate the performances of these four different denoising methods, we produced simulated time series with changes from Sentinel-1 images (Fig. 4). Different kinds of changes are introduced in the synthetic images: disappearing buildings, appearing buildings, forest areas and farmland areas. The changed values are extracted from the real Sentinel-1 SAR time series. We present results for the two different denoised "super-images" in this section. Since MSAR-BM3D released code requires a number of time series equal to a power of 2, we only tested such cases, so as to keep the original performances of this method. Ratio-based denoising results always provide better PSNR than others. When using the denoised arithmetic mean image, RABASAR can provide better PSNR values than when using denoised binary weighted arithmetic mean image. With larger time series, RABASAR seems to obtain better denoising results. 2SPPB method results in less good PSNR and obtains better SSIM values when more than 16 images are used. However, 2SPPB acquired results have obvious bias in some farmland areas. MSAR-BM3D has been developed to keep the original behavior of bright points, so it gives smaller PSNR and SSIM.

CONCLUSION

This paper proposed a general framework for the reduction of speckle fluctuations in multi-temporal SAR images. It uses a "super-image" to exploit temporal information during the restoration of each SAR image. Standard single-image speckle reduction can be applied within this framework. The success of this simple approach comes from the improved spatial stationarity in the ratio images. The statistical hypotheses of the speckle reduction methods are more easily fulfilled with the ratio image than with the original images. Based on the processing of simulated time-series and actual Sentinel-1 stacks, a quantitative comparison with MSAR-BM3D and 2SPPB methods showed the potential of RABASAR to better preserve structures in multi-temporal SAR images while efficiently removing speckle. The "superimage" can be easily updated when new data become available so as to process new images on-line.

Fig. 1 :

 1 Fig. 1: Simple multitemporal SAR image denoising framework.

y

  = uv, with expectation E[y] = u and variance V ar[y] = u 2 /L, where v follows a Gamma distribution G[1, L]. With the increase of the number of looks L, the variance u 2 /L decreases.

Fig. 2 :

 2 (b)). The resulting image is then ût = ûSI Rt . Sentinel-1 image over Paris area. (a)ratio image, (b)MuLoG-BM3D denoised result. Appearing (dark areas in (b)) and disappearing buildings are located in the middle of the image.

Fig. 3 :

 3 methods. All the Sentinel-1 images are decorrelated by under-sampling by a factor 2, at the cost of spatial resolution reduction and are accurately registered using a sub-pixel image registration method. (a)noisy Sentinel-1 data, (b)arithmetic mean image, (c)RABASAR denoised results with AM, (d)ratio between noisy data and result in (c), (e)RABASAR denoised results with BWAM, (f)ratio between noisy data and result in (e).

Fig. 4 :

 4 Fig. 4: PSNR and SSIM with different changed images. 64 synthetic Sentinel-1 images with changes are used.


Jean-Claude Dauvin
email: jean-claude.dauvin@unicaen.fr

Ali Bakalem

Alexandrine Baffreau

Claire Delecrin

Gérard Bellan

Claudio Lardicci

Elena Balestri

Rafael Sarda

Samir Grimes

Erard Bellan

Rafael Sard

The well sorted fine sand community from the western Mediterranean Sea: A resistant and resilient marine habitat under diverse human pressures

Keywords: Mediterranean Sea, Diversity, Benthic structure, Benthic indices, Biological Traits, Analysis Human pressures

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The Biocenosis of Well Sorted Fine Sands (WSFS) (SFBC, Sables Fins Bien Calibr es in French, P er es and [START_REF] Picard | Nouveau manuel de bionomie benthique de la mer M editerran ee[END_REF]) is a Mediterranean community very well delimited by bathymetry (2e25 m) and sedimentology (very low proportion of fine particles < 63 mm; Organic Matter < 2%; and proportion of fine sand >90%) occurring in zones with a relatively strong hydrodynamic regime. Its typology is well recognized at the scale of the western basin of the Mediterranean Sea and has been studied at numerous locations mainly along the French coast [START_REF] Mass E | Contribution a l' etude de la macrofaune des peuplements de sable fins infralittoraux des côtes de Provence, I. Baie de Bandol[END_REF](Mass e, , 1971a(Mass e, ,b,c, 1972[START_REF] Mass E | Trente ans d'am enagements dans la baie du Prado (golfe de Marseille) comment r eagit la macrofaune des sables fins?[END_REF][START_REF] Mass E | Cons equences a long terme de travaux d'am enagements littoraux sur la macrofaune des sables fins de deux stations de la baie du Prado (M editerran ee nord-occidentale, golfe de Marseille[END_REF][START_REF] Mass E | Long-term changes in sand-bottom macrofauna along the coast of Provence (northwest Mediterranean Sea)[END_REF][START_REF] Guille | Bionomie benthique du plateau continental de la côte catalane française. II-Les communaut es de la macrofaune[END_REF][START_REF] Guille | Bionomie benthique du plateau continental de la côte catalane française IV-Densit es, biomasses et variations saisonni eres de la macrofaune[END_REF]Gr emare et al., 1998a[START_REF] Gr Emare | The effects of a strong winter storm on physical and biological variables at a shelf site in the Mediterranean[END_REF]Labrune et al., 2007aLabrune et al., , b, 2008;;[START_REF] Sard A | Shallow-water Polychaete Assemblages in the Northwestern Mediterranean Sea and its Possible Use in the Evaluation of Good Environmental State[END_REF], Spanish [START_REF] De-La-Ossa-Carretero | Effect of Sewage Discharge in Spisula subtruncata (da Costa 1778) Populations[END_REF], 2010;Sard a et al., 1999[START_REF] Sard A | Changes in the dynamics of shallow sandy-bottom assemblages due to sand extraction in the Catalan Western Mediterranean Sea[END_REF][START_REF] Sard A | Changes in the dynamics of shallow soft-bottom due to man-made disturbance processes in the Catalan Western Mediterranean Sea[END_REF][START_REF] S Anchez-Moyana | Dreding impact on the benthic community of an unaltered inlet in southern Spain[END_REF], and Italian [START_REF] Fresi | Benthic community and sediment types: a structural analysis[END_REF][START_REF] Somaschini | A Mediterranean fine-sand polychaete community and the effect of the tube-dwelling Owenia fusiformis delle Chiaje on the community structure[END_REF][START_REF] Somaschini | Characterization and cartography of some Mediterranean soft-bottom benthic communities (Ligurian Sea, Italy)[END_REF][START_REF] Diviacco | Classification of soft-bottom communities off the Apulian coast (Mediterranean Sea)[END_REF][START_REF] De Biasi | Monitoring of marine macrobenthic communities at a dumping site: are cause/effect relationships clear?[END_REF][START_REF] Simonini | The effects of sand extraction on the macrobenthos of a relict sands area (Northern Adriatic Sea): results 12 months post-extraction[END_REF], but also along the Algerian coast [START_REF] Bakalem | Contribution a l' etude des peuplements benthiques de la baie d'Alger[END_REF][START_REF] Bakalem | Les peuplements des sables fins de la côte Alg erienne. Th ese de Doctorat d'Etat, option Oc eanographie Biologique[END_REF][START_REF] Bakalem | Benthic indices and ecological quality of shallow Algeria fine sand community[END_REF][START_REF] Grimes | Les peuplements macrobenthiques des substrats meubles algériens: organisation et structure[END_REF][START_REF] Grimes | Ecological Quality Status of the soft-bottom communities on the Algerian coast: general patterns and diagnosis[END_REF]. Based on polychaete assemblages near the French-Spanish border, Labrune et al. (2007a) proposed a new terminology for the WSFS: Littoral Fine Sands; however, this proposal has not yet been adopted. Therefore, we conserve here the classic WSFS terminology as currently used in the European EUNIS classification ('Mediterranean communities of well sorted fine sands': http://eunis.eea. europa.eu/habitats/2197). Studies on this benthic habitat concern mainly the description of community structures, i.e. the species living in the WSFS community and their abundances, as well as the estimation of species or taxonomic richness in terms of diversity index, particularly the Shannon H 0 diversity index, and, in some cases, biomass and more rarely production [START_REF] Mass E | Quantitative investigations of sand-bottom macrofauna along the Mediterranean north-west coast[END_REF]. Some authors have also assessed ecological status using benthic indicators [START_REF] Tataranni | Performance of some biotic indices in the real variable world: a case study at different scales in North-Western Mediterranean Sea[END_REF][START_REF] Sard A | Shallow-water Polychaete Assemblages in the Northwestern Mediterranean Sea and its Possible Use in the Evaluation of Good Environmental State[END_REF]. This oligotrophic community is well recognized and characterized by a relatively low number of dominant species associated with a high diversity at the scale of the western basin of the Mediterranean Sea. Moreover, over the last few decades, WSFS communities located in shallow waters have been subject to a multitude of human pressures: outfalls of treated and untreated urban waste, new constructions of dykes, dredging and recharging of beaches with sand collected in the WSFS at shallow depths (<10 m), as well as embankment of terrestrial territories for urban development, industries, harbours, airport extensions, etc. Fish trawling continues to have an impact, in spite of its prohibition at a distance of less than three nautical miles from the coastline due to the importance of this habitat for flatfishes which depend on the WSFS community for feeding. In addition, the WSFS is also changing in response to the global increase in sea temperatures leading to the warming of shallow waters in the Mediterranean Sea, which is mainly observed in summer [START_REF] Nykjaer | Mediterranean Sea surface warming 1985-2006[END_REF][START_REF] Shaltout | Recent sea surface temperature trends and future scenarios for the Mediterranean Sea[END_REF]. Since the WSFS is located above the thermocline, it can be directly damaged by a greater increase of Sea Surface Temperature and the overall rise in temperature of the water masses of the Mediterranean Sea, probably in relation to global climate change.

There has been a notable increase in the number of benthic indices due to the implementation of the European Water Framework Directive (WFD) and the Marine Strategy Framework (for example, see [START_REF] Borja | A marine biotic index to the establish ecology quality of soft-bottom benthos within European estuarine coastal environments[END_REF][START_REF] Borja | Marine management -towards an integrated implementation of the European marine Strategy framework and the water framework directives[END_REF][START_REF] Blanchet | Use of biotic indices in semi-enclosed coastal ecosystems and transitional waters habitats -implications for the implementation of the European Water Framework Directive[END_REF][START_REF] Rice | Indicators for sea-floor integrity under the European marine Strategy framework directive[END_REF][START_REF] Dauvin | Polychaete/Amphipod ratios: an approach to validating simple benthic indicators[END_REF]. Nevertheless, most of these benthic indices are based on the assumption that the response of macrobenthic species is linked to the increase of organic matter (OM) in the sediment, in view of 1) the increase in the abundance of tolerant and opportunistic species with increased OM and 2) on the contrary, the disappearance of sensitive species. According to this paradigm, the species are classified into five Ecological Groups (EG) (EG-I, species very sensitive to organic enrichment and disturbance, usually present only under unpolluted conditions; EG-II, species indifferent to enrichment or disturbance; G-III, species tolerant of excess organic matter enrichment, that may occur under normal conditions, but their populations are stimulated by organic enrichment; EG-IV, second-order opportunistic species and EG-V, first-order opportunistic species, able to resist high disturbance) and their relative abundances serve to calculate benthic indices [START_REF] Borja | Marine management -towards an integrated implementation of the European marine Strategy framework and the water framework directives[END_REF]. Subsequently, these indices, such as AMBI and BOPA (as well as its associated modifications), were tested to detect other human pressures unconnected with the OM content (i.e. long-term changes of benthic communities under natural stressors) in areas affected by harbour construction, zones of granulate extraction, metal contamination, etc. [START_REF] Borja | Response of single benthic metrics and multi-metric methods to anthropogenic pressure gradients, in five distinct European coastal and transitional ecosystems[END_REF][START_REF] Borja | Is there a possibility of ranking benthic quality assessment indices to select the most responsive to different human pressures?[END_REF][START_REF] Tataranni | Performance of some biotic indices in the real variable world: a case study at different scales in North-Western Mediterranean Sea[END_REF][START_REF] Dauvin | Polychaete/Amphipod ratios: an approach to validating simple benthic indicators[END_REF]. Along the same lines i.e. the classification of macrobenthic species in terms of ecological groups corresponding to the response to pollution is mostly related to organic matter input from urban waste water outfalls, [START_REF] Bellan | Pollution indices[END_REF] proposed an environment quality index and a Pollution-Degradation index based on the percentage of six groups of sentinel, indicative or characteristic species. However, the generalization of these OM benthic indicators to other human pressures has hindered the search for specialized indicators based on the impact of pressures acting alone, i.e. a single type of pressure leading to a particular impact.

As an alternative to analysing relative taxon composition or using integrative indices [START_REF] Bremner | Assessing functional diversity in marine benthic ecosystems: a comparison of approaches[END_REF][START_REF] Bremner | Methods for describing ecological functioning of marine benthic assemblages using biological traits analysis (BTA)[END_REF], Biological Traits Analysis (BTA) takes into account the biological characteristics of the benthic species (life-history, morphological, behavioural aspects) to study the functioning of benthic communities [START_REF] Verissimo | Assessment of the subtidal macrobenthic community functioning of a temperate estuary following environmental restoration[END_REF]. In this way, BTA offers a motivating approach that is complementary to benthic indicators for detecting changes in the functioning of benthic communities in response to human pressures.

This study is focused on some selected sites in the northern, south-western and eastern parts of the Western Basin of the Mediterranean Sea (WBMS) along the Algerian, French, Italian and Spanish coasts. The main aim of the paper is to compare the structure, ecological status and diversity of the macrofauna and to examine the effects of recent human pressures on the ecological status of this shallow macrobenthic community established on Well Sorted Fine Fands (WSFS). In the results section, firstly, we describe the assemblage classification and species composition of the community as well as the quality status of the analysed sites, and secondly, we perform a biological trait analysis to assess its functionality. Then, we discuss the quality status and functionality against human pressures in the regional area of analysis. Finally, we present some general concluding remarks to understand the status of the WSFS community.

Materials and methods

Study sites and available data

The selected data comes from studies carried out in four areas in the western Basin of the Mediterranean Sea, from sites located on the Spanish, French and Italian coasts in the north and on the Algerian coast in the south (Fig. 1).

For the French coasts, two main kinds of data are selected (Table 1). Firstly, the historic data collected in the 1970s in the area around Marseilles by Mass e (1970, 1971a,b,c, 1972, 1996, 1998, 2000) in five small bays (Bandol, Faraman, Fos, Prado and Verdon) can be considered as representing the 'original communities' of the WSFS community before the increase of human activities in shallow waters along the French coast [START_REF] Mass E | Trente ans d'am enagements dans la baie du Prado (golfe de Marseille) comment r eagit la macrofaune des sables fins?[END_REF][START_REF] Mass E | Cons equences a long terme de travaux d'am enagements littoraux sur la macrofaune des sables fins de deux stations de la baie du Prado (M editerran ee nord-occidentale, golfe de Marseille[END_REF][START_REF] Mass E | Long-term changes in sand-bottom macrofauna along the coast of Provence (northwest Mediterranean Sea)[END_REF]. In the case of the Bay of Banyuls-sur-Mer, near the Spanish border, station 43 sampled in 1994 by Gr emare et al. (1998a) is considered as an outlying site of the Marseilles area. Secondly, a recent survey of shallow stations along 55 km of the coast of the Camargue (from Lat. 4 13,474 0 E to 4 54,473 0 E) provides information on the state of the community under fishing pressure in the western part of the zone, in spite of being subject to regulatory prohibitions [START_REF] Bellan | Le probl eme de la d etermination de la nature des habitats et de leur etat ecologique dans un programme de cartographie[END_REF] and the effects of contamination in the east coming from the Rhône River. This survey was undertaken in the framework of Natura 2000 [START_REF] Labadie | Inventaires biologiques et analyse ecologique des habitats marins patrimoniaux du lot Natura 2000 (FR930 1592) Camargue[END_REF].

The data for Spain come from the Bay of Blanes (NW Mediterranean). Samples were taken at two stations located at a depth of 15 m (stations A and B;Sard a et al., 1999[START_REF] Sard A | Changes in the dynamics of shallow sandy-bottom assemblages due to sand extraction in the Catalan Western Mediterranean Sea[END_REF]. The first station (A), located near the town and around 300 m off Blanes harbour, was sampled for five years (from 1992 to 1996). The second station (B) was only sampled for two years (1992 and 1993), and was located near the mouth of the Tordera River (Table 1). The organisms retained by a 0.5 mm mesh sieve were counted and classified to the lowest possible taxonomic level for polychaetes, bivalves and echinoderms. The rest of the taxa were classified only according to major groups.

For the Algerian coast (Southwestern Mediterranean Sea), the data come from the PhD work of [START_REF] Bakalem | Les peuplements des sables fins de la côte Alg erienne. Th ese de Doctorat d'Etat, option Oc eanographie Biologique[END_REF] and [START_REF] Grimes | Les peuplements macrobenthiques des substrats meubles algériens: organisation et structure[END_REF] (see also [START_REF] Bakalem | Benthic indices and ecological quality of shallow Algeria fine sand community[END_REF][START_REF] Grimes | Ecological Quality Status of the soft-bottom communities on the Algerian coast: general patterns and diagnosis[END_REF] for historic data collected between 1979 and 1990 in the bays of Algiers, Arzew, Bejaia, Bou Ismail, Jijel and[START_REF] De-La-Ossa-Carretero | Effect of Sewage Discharge in Spisula subtruncata (da Costa 1778) Populations[END_REF]in 1997 and[START_REF] Nykjaer | Mediterranean Sea surface warming 1985-2006[END_REF] for the Djendjen area (Table 1).

For Italy (eastern part of the North-Western Mediterranean Sea), the data cover two maritime areas on the coast of Tuscany (Pisa and Grosseto) which are about 120 km apart. The area near Pisa is located on a stretch of the coast that is more intensively affected by urbanization and industrialization, although there are no major urban or industrial centres. This area is characterized by a prevailing northward longshore drift associated with the estuary of the Arno River, giving rise to freshwater, organic and nutrient inputs which can influence the coastal macrobenthic communities. Site A is located to the north, while site C is father south and site B is near the river mouth; the sites are separated by distances of thousands of metres. The Grosseto coastal area is located father south, and is characterized by weak anthropogenic pressure compared to other western Italian sandy coasts. Only a few small tourist centres and marinas are situated along this part of the coast, while there is also an extensive terrestrial natural park. All the samples from the Pisa and Grosseto coastal areas were collected in September 2005 and in May 2006 (Table 1; [START_REF] Tataranni | Performance of some biotic indices in the real variable world: a case study at different scales in North-Western Mediterranean Sea[END_REF].

Overall, the database is made up of 306 samples, with a great disparity of information as regards number of replicates as well as seasonal sampling between the different sites, i.e. ranging from only one sample for the Bay of Banyuls to 56 sampling dates at station A in the Bay of Blanes. The database contains more than 800 taxon names, but after updating with WORMS (http://www. marinespecies.org; accessed on 15 December 2015), removing doubtful species and pooling the disparities due to different persons carrying out identification of the species to the genus level, the number of taxa is reduced to 521.

The abundances are normalized to the number of individuals per m 2 .

Data analysis

Assemblages of species are determined with multivariate analyses using the PRIMER 6 software [START_REF] Clarke | PRIMER V6: User Manual/Tutorial[END_REF]. Hierarchical cluster analysis (HCA) with group average and multidimensional scaling (MDS) are based on Sørensen's coefficient for Presence/Absence data (concerning species richness) or the Bray Curtis coefficient with log 10 x transformation (concerning abundances). First of all, clustering is carried out on the total matrix, i.e. 306 samples and 521 taxa. Four samples (one from Camargue, and three from Djendjen) are excluded because they contain too few taxa (between four and seven), and can be statistically separated in the first analyses, especially in the cluster. In a second step, clustering (302 samples and 520 taxa, one taxa being present only in one of the four excluded samples) reveals the influence of sampling between the various locations, i.e. the separation of samples according to sites. Finally in a third step, the data are averaged per site and per year in the case of the Spanish and Italian stations used for temporal monitoring, amounting to a total of 28 situations.

In a last (step), only the ten most abundant taxa of each situation are selected and a new matrix is constructed including a total of 108 taxa for the 28 situations (considering the densities per m 2 of the 108 taxa). HCA and MDS are performed in order to describe and characterize these 28 situations.

Benthic indices

In this study, we select three categories of indices (see Appendix A for the thresholds of the different indices, taken from [START_REF] Dauvin | Response of the different indices to the diverse human pressures[END_REF][START_REF] Dauvin | Polychaete/Amphipod ratios: an approach to validating simple benthic indicators[END_REF].

The five indices used here are based on species classification in ecological groups. The AZTI Marine Biotic Index (AMBI), as developed by [START_REF] Borja | A marine biotic index to the establish ecology quality of soft-bottom benthos within European estuarine coastal environments[END_REF], is used to analyse the proportions of five ecological groups in terms of the gradient of organic matter enrichment, and is calculated according to the guidelines of [START_REF] Borja | Guidelines for the use of AMBI (AZTI's Marine Biotic Index) in the assessment of the benthic ecological quality[END_REF]. The Benthic Opportunistic Annelida Amphipods index (BO2A) and the Benthic Polychaete Opportunistic Families Amphipods index (BPOFA) are also calculated; these indices are based on the principle of taxonomic sufficiency and use the frequencies of two well-known zoological groups as indicator taxa: the opportunist annelids and the sensitive amphipods [START_REF] Dauvin | Polychaete/Amphipod ratios: an approach to validating simple benthic indicators[END_REF]. The indices of environmental quality (IQ) and Pollution-Degradation (PD) used here are based on the relative percentages of six groups of sentinel, indicative or characteristic species [START_REF] Bellan | Pollution indices[END_REF]Appendix B). These groups of species show the sensitivity or resistance of species to pollution. The term sentinel species was established by [START_REF] Bellan | Pollution et peuplements benthiques sur substrat meuble dans la r egion de Marseille[END_REF][START_REF] Bellan | Pollution indices[END_REF] for a species that, by virtue of its presence or relative abundance, 'warns' an observer about the possible impact of pressures in the nearby environment (see also [START_REF] Bellan | Benthic ecosystem changes associated with wastewater treatment at Marseilles: implications for the Mediterranean protection and recovering[END_REF].

One index based on trophic groups, the Infaunal Trophic Index (ITI) proposed by [START_REF] Mearns | Forecasting effects of sewage solids on marine benthic communities[END_REF], is built on the assumption that the ecological quality of the community increases with the dominance of suspension feeders and decreases with the dominance of subsurface deposit feeders.

The index based on diversity is denoted by H' (the Shannon Index with log 2 ; [START_REF] Shannon | A mathematical theory of communication[END_REF]. High values of H' indicate a rich community where no dominant species are present, whereas low values are typical of poor communities or communities dominated by a few species.

We also make use of the five ECOlogical Quality Status (EcoQS) classes recommended by the WFD: high for unpolluted sites, good for slightly polluted sites, moderate for moderately polluted sites, poor for heavily polluted sites, and bad for extremely polluted or azoic sites.

Biological traits of life

A total of ten Biological Traits (BT) were selected covering different aspects of the life history, morphology and behaviour of each taxon: position on the substratum, habit, feeding mode, adult mobility, bioturbation, size, life span, developmental mechanisms, substratum affinity and ecological groups (Table 2). The choice of BTs is based on seven studies [START_REF] Garcia | Approche fonctionnelle des communaut es benthiques du bassin oriental de la Manche et du sud de la Mer du Nord[END_REF][START_REF] Paganelli | Functional structure of marine benthic assemblages using biological traits analysis (BTA): a study along the Emilia-Romagna coastline (Italy, North-West Adriatic Sea)[END_REF][START_REF] De Juan | A trawl disturbance indicator to quantify large scale fishing impact of ecosystems[END_REF][START_REF] Verissimo | Assessment of the subtidal macrobenthic community functioning of a temperate estuary following environmental restoration[END_REF][START_REF] Bolam | Macrofaunal production and biological traits: spatial relationships along the UK continental shelf[END_REF][START_REF] Rigolet | Benthic control freaks: effects of the tubiculous amphipod Haploops nirae on the specific diversity and functional structure of benthic communities[END_REF][START_REF] Rivi Ere | Evaluation de la sensibilit e des habitats benthiques de M editerran ee -Version 1. Service du patrimoine naturel[END_REF], in accordance with their supposed importance in community functioning and their potential ability to detect the main impacts of human activities on the WSFS habitats. Each trait is divided into modalities (42 over the 10 studied traits) (Table 2). The significance of the selected BTs in terms of benthic functioning is given in Garcia (2010), [START_REF] Verissimo | Assessment of the subtidal macrobenthic community functioning of a temperate estuary following environmental restoration[END_REF] and [START_REF] Bolam | Macrofaunal production and biological traits: spatial relationships along the UK continental shelf[END_REF].

According to [START_REF] Verissimo | Assessment of the subtidal macrobenthic community functioning of a temperate estuary following environmental restoration[END_REF], three different types of data matrices are required: 1) 'taxa by station' (taxa abundance for each sampling station); 2) 'taxa by traits' (biological traits for each taxon) and (3) 'traits by station' (biological traits at each sampling station). Information for assigning taxa to functional traits is obtained from different sources including the PhD thesis of [START_REF] Garcia | Approche fonctionnelle des communaut es benthiques du bassin oriental de la Manche et du sud de la Mer du Nord[END_REF], the WORMS site (http://www.marinespecies.org; accessed on 15 December 2015), the UK Marlin site (http://www.marlin.ac.uk/ biotic/; accessed on 15 December 2015), scientific journals and the scientific expertise of the authors. When reliable information is missing, data are considered from the phylogenetic nearest neighbour taxa. The resulting 'traits by station' data matrix is then subjected to multivariate analysis.

The Biological Traits Analysis is performed according to the [START_REF] Rigolet | Benthic control freaks: effects of the tubiculous amphipod Haploops nirae on the specific diversity and functional structure of benthic communities[END_REF] approach. The fuzzy coded 'species by trait' matrix is computed using a Fuzzy coded multiple analysis (FCA).

The FCA output (coordinates of taxa on the first axes) is used to plot a dendrogram applying Ward's linkage method based on Euclidean distances [START_REF] Jr | Hierarchical grouping to optimize an objective function[END_REF]. Clusters of species exhibiting similar traits are then defined by selecting a given partitioning level. Finally, a biological profile is created for each cluster, showing for each trait the proportion of modalities exhibited by the cluster [START_REF] Usseglio-Polatera | Biological and ecological traits of benthic freshwater macroinvertebrates: relationships and definition of group with similar traits[END_REF].

The « species x trait » matrix is then multiplied by the abundance of the species at each station to obtain a « station x trait » matrix. This new matrix is ordinated by an FCA. A cluster analysis is then performed on the results by applying Ward's linkage method using Euclidean distances. This analysis groups stations with similar biological trait patterns. A PERMANOVA is used to test the statistical validity of the groups of stations obtained by cluster analysis.

Results

The WSFS community: assemblage classification and species composition

Four HCA and four MDS are performed taking into account the Fig. 3. MDS ordination plot of BrayeCurtis similarities based on abundance data [with log (x þ 1) transformation] using the 108 top ten species and the 28 situations matrix; solid lines show groups identified at 33% similarity levels in the cluster analysis (Fig. 2).

density at these sites coupled with low percentages of fine particles.

Table 3 summarizes the results of the four cluster analyses. For each analysis, the Banyuls and Blanes sites always contain the same assemblages, while Marseilles forms a single group which includes Skikda when using the P/A analyses and the 108 ten top species. Similarly, Italy forms a single group which includes Algiers when based on the P/A analysis and the 108 ten top species. Algiers is separated with the abundance analyses, and can be regrouped with other assemblages by using the P/A analyses. Camargue is always clustered with the Algeria situations (all seven or only four). The two Djendjen situations are grouped together with Arzew, and Djendjen97 forms a cluster alone with Arzew (analyses on the 108 ten top species). The Djendjen sites correspond to semi-enclosed areas with sedimentation of fine particles, while the shallow Bay of Arzew receives a regular input of fine particles coming from the main Algerian 'oueds'. In both areas, the fauna is enriched by muddy species.

The enrichment of the Djendjen sites and the Bay of Arzew is clearly seen in Table 5 were the main species composition for the sites is given. This particular group does not contain the most important species that we can observe for the other formed groups where the bivalve Spisula subtruncata and the polychaete Owenia fusiformis are always present in the obtained groups being dominant and characterizing this WSFS community. Other small polychaetes, amphipods and bivalves become the accompanying species.

Banyuls is the only site characterized by a high density of the Polychaete Ditrupa arietina in 1994, as well as two other dominant species: the polychaete Owenia fusiformis and the bivalve Spisula subtruncata.

The Spanish site is characterized by the Owenia fusiformis - 7) Blanes ( 7) Blanes (7) Group III A Marseilles (5) Marseilles ( 5) Marseilles ( 5) Marseilles (5) þ Skikda (Algeria) Group III B Italy ( 6) Italy ( 6) Italy ( 6)

Italy (6) þ Alger (Algeria) Group III C Alger (Algeria) Alger (Algeria) e e Group IV Algeria (7) þ Camargue (France) Algeria (4) þ Camargue (France) Algeria (8) þ Camargue (France) Algeria (4) þ Camargue (France) Group V e Djendjen (2) þ Arzew (Algeria) e Djendjen97 þ Arzew (Algeria) Table 4
Values of indices measuring the Quality status (based on 28 situations and 108 top 10 species) according to the European Water Framework Directive. High in Blue, Good in green, Moderate in yellow, Poor in orange and Bad in black. For ITI, only three statuses are assessed represented by three colours: Normal in blue, Perturbed in yellow and Degraded in red. In the fisrt column of the table, the situations are ordered following the groups recognized in the Fig. 2.

Spisula subtruncata assemblage, as well as D. arietina and the bivalve Lucinella divaricata, while the polychaetes Spio decoratus, Paradoneis armata and Mediomastus sp. are among the main species.

The Marseilles sites are characterized by very low density except at Prado, along with the presence of the phoronid Phoronis psammophila and various different dominant species recorded at the four other historic locations such as the polychaetes Owenia fusiformis, Spio decoratus, Scoletoma impatiens and Magelona mirabilis, the amphipod Ampelisca brevicornis, and the bivalves S. subtruncata, Chamelea gallina and Fabulina fabula.

The Italian sites are characterized by very high abundance of the dominant species S. subtruncata at the three Pisa sites and the bivalve Loripinus fragilis at the three other Grosseto sites. The tanaid Apseudopsis latreillii is also abundant and dominant in the six Italian situations; the small polychaetes Prionospio caspersi and Paradoneis armata are also abundant at these stations.

The Algiers site is characterized by the dominance of Scoletoma impatiens, S. subtruncata, Chamelea gallina, O. fusiformis and P. psammophila, while the seven other Algerian sites show very low density associated with different dominant species that are not among the 20 top species of the other locations (Appendix C), such as the opportunistic polychaete Heteromastus filiformis at Djendjen97, where Mediomastus sp. is also one of the main species; D. arietina is the dominant species at Bejaia; the bivalve Fabulina fabula and the amphipod Ampelisca brevicornis are the dominant species at Skikda. The polychaete Aponuphis bilineata is predominant at Arzew along with the opportunistic polychaete Chaetozone sp.

Finally, the Camargue site is characterized by very low density, as observed at most of the Algerian sites, and the dominant species are the polychaetes Lumbrineris latreilli and Glycera unicornis and the amphipod Urothoe pulchella.

Our results also show that each zone has a particular taxonomic richness and its specific dominant species (Table 5; Appendix C). Taking into account the mean total density of the macrobenthos per m 2 at the 28 situations, three categories of sites can be recognized: nine locations with very low densities <1000 ind.m 2 (including Fos, Camargue and all the Algerian locations except Algiers); eleven other locations with densities between 1000 and 5000 ind.m 2 (including Algiers, Blanes A in 1995 and 1996, Bandol, Faraman and Verdon, as well as Grasseto during three years of sampling, and Banyuls); eight locations with very high density > 5000 ind. m 2 , including three locations with >20,000 ind.m 2 (Blanes A in 1993, Pisa B and Pisa C) and the other locations are Blanes, Prado and Pisa A.

The WSFS community: quality status at selected sites

Table 4 gives the values of the seven indices along with the Faraman (Magelona mirabilis). Nevertheless, all these dominant species are characteristic of the WSFS and are not opportunistic; the high recorded abundances correspond to a very successful recruitment of few dominant species at these locations.

The WSFS community: Biological Traits Analysis (BTA)

When BTA is carried out for stations, the cluster analysis (FCA) separates five groups (Fig. 4; Table 5). Group D (two Djendjen situations) is discriminated on axis 1, being characterized by species with a long life-span and relatively low mobility (crawlers) as well as traits associated with opportunistic species (large non-specific deposit feeders; EG-IV with an affinity for mud and muddy sand). Axis 2 discriminates group E (Blanes and Prado), which is made up of a small number of species and a high proportion of EG-II species, as well as dominant species such as Owenia fusiformis and Spisula subtruncata. Groups B and C are opposed on axis 3 (not shown in Fig. 4), with group B being characterized by free-living species, whereas group C contains a higher proportion of very small sessile epifauna. Group A exhibits no specific set of traits, representing a mix of communities typical of other groups.

When BTA is done using species, the cluster analysis (not shown here) separates seven groups of species which correspond directly to its functional groups (FG) (Table 6). Axis 1 separates FG2, which is characterized by interstitial fauna (free-living species that move by walking or crawling, associated with weak bioturbation) having an affinity for coarse sand (Table 6). Axis 3 distinguishes FG7 from FG3 and FG6. FG7 is composed of large walking species with upward conveyor bioturbation. FG3 (all amphipods) and FG6 (15 crustaceans, including 12 amphipods) are characterized by very small species with high mobility (burrower/swimmer and walker/ swimmer), pooling together species of the main community (WSFS). On axis 4, FG6 is distinguished from group 3 by its bioturbation mode (diffusive mixing species). Axis 3 opposes FG4 and FG1. FG4 contains high proportions of opportunists associated with bioturbation (upward and downward conveyors) (EG-V), whereas FG1 is composed of long-lived sessile suspension feeders (i.e. molluscs). Both FG1 and FG4 are characterized by an affinity for muddy substratum. FG5 is not separated from the others on any of the axes because these species do not exhibit conventional set of traits (their traits are not correlated with each other according to a specific pattern) (see Tables 4,5,6).

Group A has no dominant FG (Fig. 5; Tables 5 and6). In addition, there is a large proportion of FG5. It seems that these stations contain a mixture of assemblages from other stations. Groups B and E are largely dominated by FG1 (over 80% of the abundance), which is one of the groups of the main community (WSFS). However, group E contains a higher proportion of FG4, indicating organic enrichment and the presence of FG2 (coarse sand species). Group C combines stations dominated by WSFS species (molluscs and the amphipods Urothoe spp., GF1 and FG6). Group D is dominated by FG4 (opportunistic group), suggesting that these stations have undergone organic enrichment. In addition, they also contain a high proportion of FG7 (carnivorous Polychaeta), which could be explained by the larger proportion of their prey item (FG4 species).

Discussion

The WSFS community: assemblage classification and species composition

As stressed previously by numerous authors, the soft-bottom communities of the Mediterranean Sea are characterized by their high species richness [START_REF] Bakalem | Les peuplements des sables fins de la côte Alg erienne. Th ese de Doctorat d'Etat, option Oc eanographie Biologique[END_REF]Grimes, 2010 and references therein). In this study, the diversity of chosen sites and the longterm monitoring of certain sites increased the probability of sampling a large collection of species; in this way, the species composition of our sites reaches a total of 521 species. This value is high for a single shallow community that is known to show moderate species richness (see the publications of Mass e in the 1970s). Nevertheless, our overall list contains several categories of species:

-Species characteristic of the WSFS, which can be found in what can be considered ' a reference state' i.e. the lists of Mass e's stations sampled in the late 1960s. Those species are numerically dominant at one or several locations (see Appendix C), including the polychaetes: Owenia fusiformis, Prionospio caspersi, and Ditrupa arietina; the bivalves: Spisula subtruncata, Lucinella divaricata, Thracia phaseolina, Tellina fabula, and Chamelea gallina; the amphipods Ampelisca tenuicornis and A. brevicornis (which were also described as dominant in [START_REF] Bayed | Variations interannuelles de la macrofaune benthique des substrats meubles de la baie de l'Oued Laou (Mer d'Alboran, Maroc)[END_REF], and the tanaid Apseudopsis latreillii for the Italian sites. 6) for each station group (see Table 5).

-Species which are characteristic of one site (locality) showing some differences with fauna from surrounding regions, such as the polychaetes Aricidea (Acmira) catherinae, Spiophanes reysii, and the bivalve Loripinus fragilis along the Italian coast, the amphipod Ampelisca sarsi and the ophiurid Amphiura chiajei along the Algerian coast, the polychaetes Aricidea (Strelzovia) suecica et Paraexogone hebes and the bivalve Ensis ensis along the Spanish coast and the polychaete Euclymene oerstedii along the French coast. -Species which respond to an increase in fine particles and organic matter in the sediment (tolerant and opportunistic species belonging to EG-III, EG-IV and EG-V which are not found on WSFS under normal conditions). This increase of fine particles on the WSFS is well documented (see [START_REF] Bakalem | Les peuplements des sables fins de la côte Alg erienne. Th ese de Doctorat d'Etat, option Oc eanographie Biologique[END_REF][START_REF] Grimes | Les peuplements macrobenthiques des substrats meubles algériens: organisation et structure[END_REF]and also Bayed and Bazairi, 2008); it corresponds to natural river discharges in some areas such as the Bay of Algiers or recent human modifications that increase the sedimentation of fine particles on the sea bottom as in the Djendjen area. -Species which are numerically dominant at one or several locations (see Appendix C), including the polychaetes: Owenia fusiformis, Prionospio caspersi, and Ditrupa arietina; the bivalves: Spisula subtruncata, Lucinella divaricata, Thracia phaseolina, Tellina fabula, and Chamelea gallina; the amphipods Ampelisca tenuicornis and A. brevicornis, and the tanaid Apseudopsis latreillii for the Italian sites. This latter species also shows high abundances along the Mediterranean Spanish coast (Valencia) (De-la-Ossa-Carretero et al., 2010), but its response to increased organic matter remains a matter of debate. Dense populations of the polychaete Ditrupa arietina have recently been observed on the French and Spanish Catalan coast, with high abundances recorded at Banyuls-sur-Mer and in the Blanes area, as well as at Bejaia and Arzew along the Algerian coast. For example, the species was rare or absent during the late 1960s [START_REF] Guille | Bionomie benthique du plateau continental de la côte catalane française. II-Les communaut es de la macrofaune[END_REF][START_REF] Guille | Bionomie benthique du plateau continental de la côte catalane française IV-Densit es, biomasses et variations saisonni eres de la macrofaune[END_REF]. Following a peak of abundance in the 1990s and up until the middle of the years 2000 (Gr emare et al., 1998b;Labrune et al., 2007a,b;Sard a et al., 1999;2001), the populations decreased dramatically [START_REF] Labrune | Comparison of ecological quality indices based on benthic macrofauna and sediment profile images: a case study along an organic enrichment gradient off the Rhône River[END_REF]Romero-Rodriguez et al., 2016). In his review of the occurrence and ecology of dense populations of Ditrupa arietina, Hartley (2014) pointed out that dense populations are found in areas where the sea bed is periodically disturbed by internal wave action. The WSFS are known to be located in shallow waters under high-energy hydrodynamic conditions, so any rapid increase in the population of D. arietina, which is an annual species, could be linked to an episode of higher energy.

Therefore, our results show that each zone has a particular taxonomic richness and specific dominant species (Table 5).

Quality status of the WSFS in the western part of the Mediterranean Sea

Apart from Djendjen, all the situations show a high or good Ecological Status (Table 4). WSFS occur under high-energy hydrodynamic conditions and are relatively resistant to human pressures, while anthropogenic fine particles and OM are probably prevented from accumulating because of the hydrodynamic regime. However, some tolerant and opportunistic species could be present due to temporary inputs of nutrients from rivers, oueds or urban discharges. All the indices yield the same patterns; moreover, H' is sensitive to dominant species and shows lower values in the case of high abundances due to one or two strongly dominant species such as at Banyuls, site A at Blanes in 1994, the Mass e Prado station and Pisa C. The benthic indices appear unable to discriminate between locations or between historic and present-day compositions of the macrofauna in such shallow habitats.

Furthermore, abundance values and their response to environmental change appear to be better indicators of human pressures on such shallow macrobenthic habitats. As shown in our data, there are extreme difference of abundances between sites with very high abundances of S. subtruncata at both Spanish and Italian sites. At 'original communities', Bou Ismail Bay and other Algerian sites, Prado or Camargue, the abundances are <1000 ind.m 2 ; there is probably little input of nutrients and low Primary Production at these localities. The main characteristic of the WSFS is their low population density but also an equitable distribution of individuals amongst species, with values ranging from 200e300 to 1000 ind.m 2 . In a long-term comparison (1967e1968; 1996) of the benthic composition of two stations in Prado Bay, Mass e (1996,1998) showed that the improvement in water quality of the bay had a positive effect on the species richness of the WSFS, but the drastic reduction of freshwater and organic matter inputs had a negative effect on abundances and biomass of the macrobenthos. Mass e (2000) compared the general patterns of the macrofauna of the WSFS between 1966 and 1996 at five sites, respectively Faraman, Fos, Verdon, Prado and Bandol. He showed that the abundance of the macrobenthos was mainly sensitive to the effect of well identified human actions, such as the deviation of the river in Prado Bay and the increase of nutrient inputs from the Rhône at the nearby Faraman site. He also highlighted a decline of abundance of crustaceans at the studied sites, especially amphipods, and considerable fluctuations and alternations in the relative abundance of molluscs and echinoderms.

Thus, the high abundances are mainly due to two phenomenon: 1) high settlement of a very limited number of species such as the bivalve Spisula subtruncata (EG-I, life-span of 2 y), the polychaetes Owenia fusiformis (EG-II, life-span of 2e5 y) and Ditrupa arietina (EG-I, life-span of 2 y), and the phoronid Phoronis psammophila (EG-II, life-span of 2 y); 2) changes in freshwater inputs due to natural hydrological variations controlled by climatic factors, or human actions such as improvements in water quality or the diversion of rivers. For example, the abundances of WSFS in the Bay of Algiers are higher than those observed along other parts of the Algerian coast, owing to the freshwater inputs of the main oueds and urban discharges into the Bay of Algiers.

Biological traits of life

An approach allowing us to classify the situations in a different way, in terms of Traits of Life, shows a clear separation of the Djendjen data points from the other sites. This is highlighted by the abundances of species which reflect the presence of tolerant and opportunistic species having specific Traits of Life. The Italian and Spanish stations with high abundances of Owenia fusiformis and Spisula subtruncata can also be distinguished by analyses of the biological traits of life (Fig. 4).

The subdivision of the species into seven functional groups is of great interest in this context (Table 6).

The first group distinguishes species that are characteristic of the WSFS, including numerous bivalves such as Callista chione, Chamelea gallina, Spisula subtruncata and Tellina fabula, as well as the polychaete families Oweniidae and Spionidae; most of these dominant and characteristic taxa are suspension feeders that underline the fact that the fauna in such benthic hydrodynamic environments depend on the quality of the water column. Pristine WSFS occur mainly in oligotrophic zones with low macrofauna abundances [START_REF] Mass E | Trente ans d'am enagements dans la baie du Prado (golfe de Marseille) comment r eagit la macrofaune des sables fins?[END_REF][START_REF] Mass E | Cons equences a long terme de travaux d'am enagements littoraux sur la macrofaune des sables fins de deux stations de la baie du Prado (M editerran ee nord-occidentale, golfe de Marseille[END_REF][START_REF] Mass E | Long-term changes in sand-bottom macrofauna along the coast of Provence (northwest Mediterranean Sea)[END_REF]. As a response to the eutrophication of the coastal zone, mainly due to the increase of nutrients supplied by freshwater inputs, there is an enrichment of the fauna with the introduction of indicative sentinel species and an increase in the abundance of macrofauna [START_REF] Bellan | Pollution et peuplements benthiques sur substrat meuble dans la r egion de Marseille[END_REF][START_REF] Bakalem | Benthic indices and ecological quality of shallow Algeria fine sand community[END_REF][START_REF] Bayed | Variations interannuelles de la macrofaune benthique des substrats meubles de la baie de l'Oued Laou (Mer d'Alboran, Maroc)[END_REF][START_REF] Mass E | Long-term changes in sand-bottom macrofauna along the coast of Provence (northwest Mediterranean Sea)[END_REF].

Groups FG2 and Fg7 are characterized by coarse sand species mainly comprised of polychaetes of small size (FG2) or large size (FG7). Groups FG3 and FG6 are made up of crustaceans (especially the amphipods Ampelisca spp. Bathyporeia spp. and Urothoe spp.) which are characteristic of WSFS, with high mobility and able to swim in the water column at night (diurnal vertical migration). These species are also sensitive to pollution [START_REF] Dauvin | Response of the different indices to the diverse human pressures[END_REF][START_REF] Dauvin | Polychaete/Amphipod ratios: an approach to validating simple benthic indicators[END_REF]. Group FG4 is characterized by opportunistic species found in sediments enriched in organic matter, probably indicating a temporary or permanent organic pollution. It is clear that these species are absent at stations with low macrofauna abundances, but are well represented at the Djendjen site. Finally, group FG6 is comprised of 14 polychaetes found mainly in muddy fine sand and which occur in areas enriched by fine particles. In this way, the analyses based on Biological Traits of Live highlight the presence of three groups of species: i) typical characteristic species of WSFS such as groups FG1, FG3 and FG7; ii) indicator species reflecting enrichment of fine particles and organic matter (groups FG4 and FG5), and iii) coarse sand species which are accessorily found on fine sand (groups FG2 and FG7).

Concluding remarks

Our study is focused on the shallow Biocoenosis of Well Sorted Fine Sands (WSFS) in zones with relatively high-energy hydrodynamic conditions along the Algerian, French, Italian and Spanish coasts of the western Basin of the Mediterranean Sea. We compare the structure, ecological status and diversity of the macrofauna of the WSFS, with a view to examining the effects of human pressures on the state of this shallow macrobenthic community. In spite of several types of pressure in the four studied areas (Table 1), such as river inputs, pollution by urbanization, heavy metals and hydrocarbons, only one site at Djendjen along the Algerian coast shows a low-quality status. All the other situations show a good or a high quality status according to the seven benthic indices employed in thus study. These indices appear unable to discriminate the response of WSFS communities arising from human pressures; by contrast, the total abundances of the macrofauna could serve as an indicator of dysfunction, since an increase of abundances of opportunistic species could be due to the influence of river and organic matter inputs. The change from oligotrophic (normal situation) to eutrophic conditions (with enhanced nutrient and organic matter inputs) is followed by an increase of abundances and the presence of sentinel or indicative species reflecting organic pollution. Moreover, this situation can be reversible as shown by Mass e (1996,1998) in the case of Prado Bay in the Gulf of Marseilles, with a decrease in the total abundance of macrofauna after the cessation of continual freshwater inputs or in the case of the Besos River off Barcelona associated with the reduction of organic inputs [START_REF] Cardell | Spatial changes in sublittoral soft-bottom Polychaete Assemblages due to river inputs and sewage discharges[END_REF]. This indicates that the WSFS communities have a high resilience. This ecological property has been highlighted in the context of sand extraction in shallow subtidal zones of Algeciras Bay (southern Mediterranean Sea, Spain), in the northern Adriatic Sea [START_REF] Simonini | The effects of sand extraction on the macrobenthos of a relict sands area (Northern Adriatic Sea): results 12 months post-extraction[END_REF] and along the Catalan coast [START_REF] Sard A | Changes in the dynamics of shallow sandy-bottom assemblages due to sand extraction in the Catalan Western Mediterranean Sea[END_REF]. In Algeciras Bay, surveys conducted over five years following dredging operations showed that the macrofauna was beginning to be re-established only one month after the cessation of dredging. After two years, there was a confusing biological impoverishment, while after four years, the population was largely re-established. For the Northern Adriatic Sea [START_REF] Simonini | The effects of sand extraction on the macrobenthos of a relict sands area (Northern Adriatic Sea): results 12 months post-extraction[END_REF], one year after the extraction, the recolonization of the macrofauna at the impacted stations was at an advanced stage, while on the Catalan coast, two years after the cessation of dredging activities, the abundances recovered but biomasses were still high [START_REF] Sard A | Changes in the dynamics of shallow sandy-bottom assemblages due to sand extraction in the Catalan Western Mediterranean Sea[END_REF]. Similarly, in a study of the effect of sediment discharge (100,000 m 3 over a period of one week) on benthic communities in the Tyrrhenian coastal zone, including the shallow WSFS habitat, no changes were observed after the dumping operations, even though it took place only about 5 nautical miles from the shallow biocoenosis [START_REF] De Biasi | Monitoring of marine macrobenthic communities at a dumping site: are cause/effect relationships clear?[END_REF]. As discussed above, this habitat is located in shallow waters associated with high-energy environments that are continually subjected to natural physical perturbations. This ensures the selection of species adapted to living in clean sand intensely reworked by a combination of strong wave action and swell, implying that the community is able to recolonize this environment rapidly after the cessation of anthropogenic pressures. This high resilience maximizes the capacity of the characteristic species of this biocoenosis to recover rapidly after undergoing pressures. The high capacity of recovery of Mediterranean WSFS contrasts with the low resilience of typical Mediterranean communities under the combined effects of anthropogenic and climatic factors, such as observed in the case of Posidonia oceanica meadows [START_REF] Bourcier | Long-term changes (1954 to 1982) in the benthic macrofauna under the combined effects of anthropogenic and climatic action (example of one Mediterranean Bay)[END_REF] and coralligenous reefs [START_REF] Gatti | Coralligenous reefs state along anthropized coasts: application and validation of the COARSE index, based on a rapid visual assessment (RVA) approach[END_REF]. In agreement with [START_REF] Gatti | Coralligenous reefs state along anthropized coasts: application and validation of the COARSE index, based on a rapid visual assessment (RVA) approach[END_REF], who criticized the use of coralligenous reef state as an indicator of water quality, because it responds primarily to physical pressures (sedimentation, temperature, mechanical damage, etc.), our study on the WSFS not only shows similar evidence but also a high resilience after the cessation of anthropogenic pressures.

However, our analyses only take into account of the abundance of the species, and are mainly concerned with the dominant species. It is well known that shallow, non-vegetated, soft-bottom habitats of the western Mediterranean are generally poor in biomass and show a low productivity (1e3 g dry weight m À2 ) (Ben-Tuvia, 1983), except near large rivers (e.g., the Rhone or the Ebro) or major cities (e.g. Marseilles or Barcelona), where values of up to 18.4 g dry weight m À2 have been recorded (Mass e, 1972 for the Gulf of Fos, France), or in enclosed areas or coastal lagoons [157.2 g dry weight m À2 for the Camargue area (Mass e, 1972), and 66.0 g dry weight m À2 in Alfacs bay of the Ebro Delta [START_REF] Martin | Distribution patterns and trophic structure of soft-bottom polychaete assemblages in a north-western Mediterranean shallow-water bay[END_REF]] where biomass and productivity tend to increase. Previous studies have systematically documented the dominance of annelid polychaetes, as well as bivalve species, both in terms of abundance and biomass of species. More recent studies [START_REF] Albertelli | Macrobentos y par ametros ambientales en fondos arenosos del Mar de Liguria, Italia. VII Simposio Ib erico de Estudio del Bentos Marino[END_REF] in the Ligurian Sea (Italy); Gr emare et al., 1998aGr emare et al., , 1998b, in the bay of Banyuls (France); Sard a et al. (1999) in the Cove of Portbou (Spain)] propose that most of the macroinfauna remains dominated by increasing numbers of small polychaetes which disappear a few months after their settlement. However, the biomass of large bivalves is lower, and there is a clear tendency for a decrease in the mean individual macroinfaunal biomass. This shift probably reflects a habitat that is being continuously stressed by anthropic disturbances (see Sard a and Fluvi a, 1999 for a review of the Costa Brava). The evidence from the bay of Blanes suggests that the absence of large organisms could be associated with the continuous disturbance and stress caused by bivalve fisheries in this area. This selective pressure on larger invertebrates means that the populations of the affected assemblages are mainly structured by annual species, which may explain the recurrent pattern found in their seasonal dynamics (Sard a et al., 1999). We consider that, at present, this could be a general pattern in shallow soft-bottom habitats of the north western Mediterranean coastal zones, which could be analysed in the future using the biomass data available for this part of the Mediterranean.

Fig. 1 .

 1 Fig. 1. Locations of the studied sites in the western basin of the Mediterranean Sea.

  abundances and species richness according to the 28 selected situations, considering all the 521 species found at the different sites, and then the 108 top ten species found at each site. The results show very high levels of resemblance between the assemblages identified by the different analyses. As an example, the HCA and MDS for the matrix of abundances after a Log(xþ1) transformation of the 108 top ten species and 28 situations (Figs.2 and 3) show the identification of five groups of stations. The first group (A) is represented by the Banyuls site, which is isolated from the other clusters. Group B includes the seven Spanish situations, while the third group (C) brings together the two Djendjen situations plus the Arzew sites (Algeria). Group D is made up of 12 situations and can be sub-divided into three sub-groups: D1 with the five historic situations in the Marseilles area, D2 with the six Italian situations and D3 with the Bay of Algiers. Group E comprises four Algerian situations plus the Camargue site, corresponding to a low faunal

Fig. 2 .

 2 Fig. 2. Dendrogram using group average method for clustering, showing the BrayeCurtis similarity index based on abundance [with log (x þ 1) transformation] using the 108 top ten species and the 28 situations matrix.

Fig. 4 .

 4 Fig. 4. Ordination plot of the 28 situations by factorial correspondence analysis (FCA) based on abundance-weighted biological traits for axes 1 and 2 (upper part). Factorial maps of the five main biological traits contributing to station variability (lower part). For abbreviations, see Table 2.

Fig. 5 .

 5 Fig. 5. Proportion of the seven functional groups (see Table6) for each station group (see Table5).

Table 1

 1 Main characteristics of the benthic communities and human pressures at the sampling sites. Information on the sampling design is also indicated for each sampling station.

	Location	Date Sampling's	Stations Number of	Depth	Sampling	Origin of changes and Human pressure
	of the sites		season		stations or	(m)
					temporal
					observations
	Marseilles	1967	Spring	Bandol	3	5e11 Brett's hydraulic pump	Reference state from Mass e (1970, 1971a,b,c); Mass e, 1971d
	area,	e1968	(April to	Faraman 3
	France		June)	Fos	1
				Prado	2
				Verdon	2
	Camargue	2011 March	Camargue 19	2.5	0.1 m 2 Day grab	Two main human pressures, at the east the input of the Rhône
	area,		eApril			e13		River, and in the west forbidden trawling's near the coast in
	France							shallow waters
	Banyuls-	1994 May	53	1	10	0.1 m 2 Van Veen grab	Sediment instability with a reduction of silt between reference
	sur-							situation in 1968 and 1994, and occurrence of high temperature
	mer,							(Gr emare et al., 1998a,b)
	France
	Blanes	1992	All year	2	Biweekly to	15	0.17 m 2 Van Veen grab	Station A. Urbanization (Blanes town)
	(Girona,	e1996			monthly			Station B. Riverine inputs (Tordera River)
	Spain)
	Tuscany,	2005 September Pisa	18	5e10 0.1 m 2 Van Veen grab	Anthropogenic pressure due to organic and nutrients input of
	Italy	2005 September Grosseto 18			Arno river (Pisa) and natural heavy metals since the presence of
		2006 May	Pisa	18			the most important mineral deposits of the whole Tuscany
		2006 May	Grosseto 18			(Grosseto).
	Algerian	1984	One year	Algiers	4	8e10 0.1 m -2 Van Veen grab (for Algiers	Sewage, urbanization (Fetzara, Jijel, Bou Ismail)
	coast	e85					and Jijel) and 0.1 m -2 Smith	Hydrocarbons, sewage, heavy metal, urbanization (Arzew, Bejaia,
		1985 January	Arzew	8	10e20	McIntyre grab in other sites	Algiers Bay)
		1983 December Bejaia	6	9e25
		1988 August	Bou	9	1 0 e25
				Ismail
		1986 July	Jijel	19	10e21
		1990 February	Skikda	8	13e25
	Algerian	1997 July	Djendjen 38	5e19 0.1 m -2 Van Veen grab	New harbour in a zone without industrialization and pollution
	coast	2009 December Djendjen 21	8e22 0.1 m -2 Van Veen grab

Table 2

 2 Biological traits and modalities of species selected for the biological traits analysis. Abb. Abbreviations used in the Fig.4.

	Biological	N	Abb.	Modality	Definition
	trait
	Larval development		plk	Planktotrophic	Planktonic larvae feeding on plankton
			lec	Lecithotrophic	Planktonic larvae feeding on yolk
			dde	Direct development	No planktonic larvae
	Life span		short	Short	<2 years
			med	Medium	2-5 years
			long	Long	>5 years
	Mobility		swim	Swimmer	Adults actively swim in water column
			walk	Walker	Adults capable of extensive movement at sediment surface
			craw	Crawler	Adults with limited movements at sediment surface
			burr	Burrower	Endofauna that moves in the sediment
			sess	Sessile	Non mobile adults (attached, limited to a tube or a burrow)
			bur_sw	Burrower & Swimmer	Diel migrations
			wlk_sw	Walker & Swimmer	Diel migrations
	Living position		epi	Epifauna	Live at the surface of the sediment
			endo	Endofauna	Live in the sediment
	Habit		tdwe	Tube-dweller	Adults builds tube
			bdwe	Burrow-dweller	Adults lives in burrows (temporary or permanent)
			free	Free-living	Adults not limited by a structure
	Bioturbation		nobio	No bioturbation	Do not induce sediment displacement
			surf_dep	Surface deposition	Surface displacement
			up_conv	Upward conveyor	Displacement of particles from depth to surface
			dn_conv	Downward conveyor	Displacement of particles from surface to depth
			dif_mix	Diffusive mixing	Small-scale displacement
	Trophic groups		nsdf	Non-specific deposit feeder	Feeds on particles at sediment surface and within the sediment
			sdf	Surface deposit feeder	Feeds on particles at sediment's surface
			ssdf	Sub-surface deposit feeder	Feeds on particles within the sediment
			suspf	Suspension feeder	Feeds on particles within the water column
			carn	Carnivorous	Feeds on live prey
			omn	Omnivorous	Generalist feeder
			mixt	Mixtes	Feeds on particles in the water column and at sediments' surface
	Ecological groups (AMBI)		eg1	Sensitives	Only present in unpolluted areas
			eg2	Indifferent	Always present at low densities
			eg3	Tolerant	More abundant in slightly enriched areas
			eg4	Second-order opportunists	Present in unbalanced conditions
			eg5	First-order opportunists	Proliferate in reduced sediment
	Substratum affinity		mud	Mud	Particles <63 mm are present or dominant
			smud	Sandy mud	50% to < 90% sand, mud remainder
			msand	Muddy sand	10% to < 50% sand; mud remainder
			fsand	Fine clean sand	>90% sand, median 0.125 to < 0.250 mm
			csand	Coarse clean sand	>90% sand, median 0.500 to < 1.000 mm
	Maximal size		vsmal	Very small	<10 mm
			smal	Small	10 to 20 mm
			med_size	Medium	21 to 100 mm
			larg	Large	>100 mm

Table 3

 3 Summary of the results of the four cluster analyses obtained for 520 species and the 108 top ten species based on Log(xþ1) transformed abundances (Bray-Curtis similarity) and Presence/Absence of the species (Sørensen index).

	Transformation	Log(xþ1 abundance)	Log(xþ1 abundance)	Presence/Absence	Presence/Absence
	Matrix (species x situations) 520 Â 28	108 Â 28	520 Â 28	108 Â 28
	Group I	Banyuls	Banyuls	Banyuls	Banyuls
	Group II	Blanes (7)	Blanes (

Table 5

 5 Species (among 108 top ten species) contributing to 80% of the abundance of the five groups of stations (A to E, first column), showing stations forming the five groups.

		Species contributing to 80% (Decreasing order)	Stations
	A	Magelona mirabilis	Chaetozone sp.	Alger
		Scoletoma impatiens	Aponuphis bilineata	Arzew
		Chamelea gallina	Prionospio caspersi	Camargue
		Urothoe grimaldii	Urothoe pulchella	Faraman
		Spisula subtruncata	Nephtys hombergii	Fos
		Owenia fusiformis	Bathyporeia guilliamsoniana
		Sigalion mathildae	Paradoneis harpagonea
		Pariambus typicus	Euclymene oerstedi
		Phoronis psammophila	Lumbrineris latreilli
	B	Spisula subtruncata	Owenia fusiformis	Bejaia
		Spiophanes reyssi	Thracia phaseolina	Blanes92B
		Ditrupa arietina	Ampelisca brevicornis	Blanes93B
		Apseudopsis latreillii		Banyuls
				Verdon
				PisaA
				PisaB
				PisaC
	C	Loripinus fragilis	Bathyporeia guilliamsoniana	Bou Ismail
		Apseudopsis latreillii	Paradialychone filicaudata	Jijel
		Tellina fabula	Ampelisca brevicornis	Skikda
		Mediomastus sp.	Sigalion mathildae	Bandol
		Autonoe spiniventris	Cylichna cylindracea	GrossetoA
		Spisula subtruncata	Thracia phaseolina	GrossetoB
		Prionospio caspersi	Perioculodes longimanus	GrossetoC
		Paradoneis armata	Tellina tenuis
		Scoletoma impatiens
	D	Heteromastus filiformis	Monocorophium acherusicum	Djendjen97
		Lumbrineris coccinea	Lumbrineris latreilli	Djendjen09
		Amphiura chiajei	Owenia fusiformis
		Trichobranchus glacialis	Mediomastus sp.
		Corbula gibba	Scolelepis (Parascolelepis) tridentata
		Scoletoma impatiens	Orbinia latreillii
		Abra alba	Aricidea (Strelzovia) suecica
	E	Owenia fusiformis	Spisula subtruncata	Blanes92A
		Phoronis psammophila	Lucinella divaricata	Blanes93A
				Blanes94A
				Blanes95A
				Blanes96A
				Prado

Table 2 .

 2

corresponding colours of the WFD. Most of the values indicate that the macrobenthos shows good or high Quality Status, e.g. all the values of AMBI, IQ. and PD., and all the values of BO2A and BPOFA values except for Djendjen97 (presence of high abundance of the opportunistic polychaete Heteromastus filiformis, which was also present in 2009 but at lower abundance than in 1997). ITI also gives a degraded status for the Djendjen97 situation. ITI highlights the dominance of suspension feeders when the status is normal, and deposit-feeders when the status is perturbed; thus, most of the Spanish, Algerian and Grosseto situations are classified as perturbed, in a similar way to Bandol and Faraman for the Marseilles area, while the others are classified as normal. The Shannon diversity index H' is very sensitive to the dominance of a single or a few species as observed at Banyuls (Ditrupa arietina), Pisa B and C (Spisula subtruncata), Blanes 93A, 94A and 96A (Owenia fusiformis and Spisula subtruncata), Prado (Phoronis psammophila) and

Table 6

 6 Biological trait profiles of species belonging to the seven functional groups (FG) on Fig.5(BT: Biological Traits of the 108 top ten species).

	Dominant BT of the species Species		Comments
	FG1 Long lifespan	Abra alba	Scolaricia typica	Group dominated by mollusc assemblage and with species characteristic of the
	Sessile	Callista chione	Scolelepis (Parascolelepis)	WSFS
	Suspension feeders	Chamelea gallina	tridentata
	Mud substrate	Digitaria digitaria	Solen marginatus
		Ditrupa arietina	Spio martinensis
		Echinocardium cordatum	Spiochaetopterus costarum
		Echinocardium	Spiophanes bombyx
		mediterraneum	Spiophanes reyssi
		Ensis ensis	Spisula subtruncata
		Galathowenia oculata	Tellina fabula
		Loripinus fragilis	Tellina pulchella
		Lucinella divaricata	Tellina tenuis
		Mactra stultorum	Terebella lapidaria
		Myriochele heeri	Thracia phaseolina
		Owenia fusiformis	Trichobranchus glacialis
		Pharus legumen
		Phoronis psammophila
	FG2 Walker	Actiniidae	Exogone verugera	Mainly species of coarse sediment (small polychaetes)
	Crawler	Amphiura chiajei	Myrianida prolifera
	Free living	Branchiostoma lanceolatum	Paradialychone filicaudata
	No bioturbation	Chone duneri	Parexogone hebes
	Coarse sand substrate	Chone infundibuliformis	Protodorvillea kefersteini
		Cylichna cylindracea	Sphaerosyllis taylori
		Diogenes pugilator	Spio decoratus
		Euclymene oerstedi	Syllis sp.
		Euclymene santandarensis
	FG3 Very small	Ampelisca brevicornis	Lembos websteri	All species amphipods
	Burrower-Swimmer	Ampelisca sarsi	Monocorophium
	Walker-Swimmer	Ampelisca spinipes	acherusicum
		Ampelisca tenuicornis	Pariambus typicus
		Autonoe spiniventris	Photis longicaudata
	FG4 Conveyers (up/down)	Anobothrus gracilis	Corbula gibba	Opportunistic species (found in sediment enriched in organic matter)
	EG 5	Aphelochaeta marioni	Heteromastus filiformis
	Mud and sandy mud	Axinulus eumyarius	Mastobranchus trinchesii
	substrates	Capitella capitata	Mediomastus sp.
		Capitella minima	Notomastus (Clistomastus)
		Caulleriella bioculata	lineatus
		Chaetozone sp.	Pseudopolydora antennata
	FG5 Mud substrate	Aponuphis bilineata	Orbinia latreillii	Non-conventional BT assemblage dominated by small polychaetes mainly from
	Up conveyers	Aricidea (Acmira) catherinae	Paradoneis armata	Paraonidae and Spionidae families
	Sub-surface deposit	Aricidea (Strelzovia) suecica	Paradoneis harpagonea
	feeders	Aponuphis brementi	Prionospio caspersi
		Laonice cirrata	Prionospio cirrifera
		Magelona minuta	Prionospio malmgreni
		Magelona mirabilis	Scoloplos (Scoloplos)
			armiger
	FG6 Very small	Apseudopsis latreillii	Siphonoecetes	All crustacean species including 12 amphipods characteristic of the WSFS
	Burrower-Swimmer	Atylus sp.	(Siphonoecetes) sabatieri
	Walker-Swimmer	Bathyporeia guilliamsoniana	Urothoe brevicornis
	Diffusive mixing	Bathyporeia pilosa	Urothoe elegans
		Paramysis (Longidentia)	Urothoe grimaldii
		helleri	Urothoe intermedia
		Perioculodes longimanus	Urothoe poseidonis
		Pseudocuma longicorne	Urothoe pulchella
		Siphonoecetes
		(Centraloecetes) dellavallei
	FG7 Walker	Glycera capitata	Lumbrineris latreilli	Large carnivorous polychaetes with coarse sand affinity
	Large size	Glycera tridactyla	Nephtys hombergii
	Walker-swimmer	Glycera unicornis	Scoletoma impatiens
	Up convoyer	Hilbigneris gracilis	Sigalion mathildae
	Sub-surface deposit	Lumbrineris coccinea	Sigambra tentaculata
	feeders

Acknowledgements

The authors thank the MEDISCAN programme for the Camargue Data, the Italian Research was funded by University of Pisa (Lardicci 308/2004) with the technical help of the Environmental Protection Regional Agency of Tuscany (ARPA Toscana). The authors thanks M. Carpenter for the English revision, and both reviewers for their very useful comments and suggestions on the first draft of this paper.

Appendix A

Threshold values between the several ecological statuses considered for the selected benthic indicators: AMBI, BO2A, BPOFA, H', IQ, PD, and ITI (see [START_REF] De-La-Ossa-Carretero | A comparison of two biotic indices, AMBI and BOPA/BO2A, for assessing the Ecological Quality Status (EcoQS) of benthic macro-invertebrates[END_REF][START_REF] Dauvin | Response of the different indices to the diverse human pressures[END_REF][START_REF] Dauvin | Polychaete/Amphipod ratios: an approach to validating simple benthic indicators[END_REF][START_REF] Bellan | Pollution indices[END_REF].

Appendix B

Groups of species used for measuring the Environmental Quality Index (IQ) and the Pollution-Degradation Index (PD) (From Bellan, 2007).

Appendix C

Total number of individuals for the 20 top species, with their numerical rank in the 28 situations, with mean total density of the macrobenthos per m 2 in the 28 situations. > 20 indicates that the species in not among the 20 top species and 0 indicates the absence of the species at the site. (Ban: Banyuls-sur-Mer; Bl92A, Bl92B, Bl93A, Bl93B, Bl94A, Bl95A, Bl96A: Blanes, years 1992e1996, sites A & B; Fara: Faraman; Verd: Verdon; GrA, GrB, GrC: Grosseto, sites