# Mesoscale assemblages of fish and megainvertebrates as evidence of benthiscapes on continental shelves

Thomas Outrequin, Hervé Le Bris, Jacques Grall, Pascal Laffargue

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Thomas Outrequin, Hervé Le Bris, Jacques Grall, Pascal Laffargue. Mesoscale assemblages of fish and megainvertebrates as evidence of benthiscapes on continental shelves. Journal of Marine Systems, 2025, 250, 1-20 p./104088. 10.1016/j.jmarsys.2025.104088. hal-05168167

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Mesoscale assemblages of fish and megainvertebrates as evidence of benthiscapes on continental shelves

Thomas Outrequin<sup>a, *, 1</sup>, Hervé Le Bris<sup>a</sup>, Jacques Grall<sup>b</sup>, Pascal Laffargue<sup>a</sup>

ARTICLE INFO

Keywords:

Continental shelf

Bay of Biscay

Benthos

Demersal

Environmental forcings

Spatial structure

A B S T R A C T

Despite the relatively small proportion of ocean surface they represent, continental shelf ecosystems are among the most productive in the world. Located at the interface between terrestrial and marine environments, these habitats are structured by strong environmental forcings, especially on the sea bottom. A clear understanding of the spatial distribution of these habitats, along with knowledge on the composition and functioning of their associated communities, is essential for fisheries management and ecosystem conservation. Here, we used data from yearly EVHOE otter trawl surveys (2008–2020) to characterize the spatial structuration of benthic communities of the entire continental shelf of the Bay of Biscay (France), and to investigate the potential environmental drivers of these patterns. Two separate biological components were studied: epibenthic megainvertebrates and benthо-demersal fish. Clustering analyses identified seven assemblages from species composition of stations. Each assemblage was denominated according to its geographical location. We detected a strong correlation between the spatial organization of the different assemblages identified for fish and megainvertebrates, providing evidence for broad-scale spatial structuration of benthic habitats—benthiscapes—in this shelf ecosystem. The most influential environmental variables were identified as bottom temperature, sediment type, and primary production. Patterns in certain structural parameters, such as biomass, revealed possible spatial differences in ecological functioning. For example, we observed a drop in biomass from the coast to the central part of the shelf, followed by an increase in biomass near the edge of the Armorican shelf. These patterns reflect major large-scale processes (river inputs versus shelf-break upwelling) structuring the entire Bay of Biscay ecosystem. A comparative analysis revealed that the biological features and functioning observed in this study are shared with other European continental shelves. In addition to improving our knowledge of benthic environments, studies such as this one can promote improvements in ecosystem-based management and marine spatial planning of a fast-changing ecosystem under multiple anthropogenic stresses.

**1. Introduction**

Continental shelves occupy a small proportion of the total ocean surface (review in Hall, 2002) but are among the most productive eco-
systems in the world (Costanza et al., 1997). In addition, the ecosystem services they provide are among the most valuable (e.g., carbon storage, fisheries, biogeochemical cycling) (Costanza et al., 1997; Longhurst et al., 1995; Raffaelli et al., 2003). Within these environments, the benthic domain is of considerable importance: it is responsible for a significant proportion of marine biological production, especially the part exploited by humans (Hall, 2002), it makes an essential

contribution to benthо-pelagic coupling, and it represents a major
reservoir for terrestrial carbon storage (Zhang et al., 2021). The spatial
location of these ecosystems—at the interface between terrestrial and
marine environments—places them under strong pressure from natural
processes that vary in time and space, and exposes them to extensive
anthropogenic influences (Halpern et al., 2008; Worm et al., 2006).
These intense and dynamic environmental conditions structure the
habitats and ecological niches present and determine how species are
brought together and interact with one another (Guimerà et al., 2010).

Due to their long (multi-year) lifespans and wide spatial dispersion,
assemblages of epibenthic megainvertebrates (i.e., invertebrates larger

<footnote>ª DECOD (Ecosystem Dynamics and Sustainability), INRAE, Institut Agro, IFREMER, Rennes, Nantes, France</footnote>

<footnote>ᵇ OSU UAR 313 IUEM, Univ Brest, CNRS, IRD, 29280 Plouzané, France</footnote>

<footnote>* Corresponding author.
E-mail address: thomas.outrequin@institut-agro.fr (T. Outrequin).</footnote>

<footnote n="1">¹ Postal address: 65 rue de Saint-Brieuc, Rennes, 35,000, France</footnote>
than 1 cm living on the seabed) and benthodemersal fish (fish species living on or near the seabed) can provide useful information on the structure of the benthic ecosystem and its response to environmental drivers across large spatial and temporal scales. This approach—investigating the spatial structure of ecosystems through the analysis of various benthic components—has been previously applied to different European shelves (Bremner et al., 2006; Daan et al., 2005; Neumann et al., 2016; Sánchez et al., 2008). While some recent studies have considered epibenthic megainvertebrates and benthodemersal fish separately (Reiss et al., 2010; Sánchez et al., 2008), many older studies did not (Colloca et al., 2004; Ellis et al., 2011; Kaiser et al., 2004; Rees et al., 1999), relying instead on the hypothesis that these two communities share common biological traits and could therefore be grouped together.

Whether grouped together or separately, knowledge on the spatial structuration of these two benthic communities can provide a better understanding of the meso-scale (from 10 to 100 km) functioning of shelf ecosystems (Hooper et al., 2005; Shojaei et al., 2021) in the face of local and/or global environmental change. Moreover, this type of work can help to identify ecologically coherent spatial entities which can then be integrated into management efforts (e.g., Bell et al., 2021), with particular benefits for the ecosystem-based management of fisheries (Garcia et al., 2003).

Our study focuses on the Bay of Biscay (France) as a representative case study of a North Atlantic temperate continental shelf. This zone features an extensive shelf that stretches far offshore until finally breaking towards the abyssal plain (review in Borja et al., 2019). In its coastal section, it faces numerous anthropogenic pressures (eutrophication, pollution) and is the site of major fishing activity, which targets benthodemersal species in particular (Quemmerais-Amice et al., 2020). Additionally, this area is now of growing interest for the development of technologies for renewable marine energy production (Fofack-Garcia et al., 2023; Taormina et al., 2018).

The first overview of benthic communities in the Bay of Biscay and the Celtic Sea is credited to Le Danois (1948); although descriptive and not quantitative, this study provided an initial picture of the overall structure of communities from the shelf to the slope. Macrobenthic communities of the Bay of Biscay were further characterized in the north by Glémarec (1969) and in the south by Lagardère (1973), who both described spatial structuration that was mainly determined by a thermal depth gradient and sedimentary substrate characteristics. A partial revisit in the north in 2001–2002 revealed notable changes in the macrobenthic fauna as the result of strong fishing pressure (Hily et al., 2008). At the scale of the entire bay, work by Souissi et al. (2001) provided a characterization of the main assemblages of benthodemersal fish. To date, though, there has not been a wide-scale study to generate a standardized picture of the larger benthic communities of the entire Bay of Biscay. In particular, there is a need for the identification of the main patterns and environmental forcings that structure communities, especially with respect to a joint analysis of fish and invertebrate fauna.

The aim of this study was to assess the spatial structuration of the benthic ecosystem of the continental shelf of the Bay of Biscay, specifically by analyzing communities of epibenthic megainvertebrates and benthodemersal fish and their relationships with environmental forcings. For this, we made use of the large, homogenous dataset available thanks to the recurrent international bottom trawl survey "EVHOE" (Laffargue et al., 2020). Our work was carried out in two steps:

1. Identify and characterize assemblages of epibenthic megainvertebrates and benthodemersal fish and their spatial distribution within the study area.

2. Analyze the links between the spatial patterns observed and the environmental forcings that structure the identified assemblages.

## 2. Materials and methods

### 2.1. Study area

This study covers the entire French part of the continental shelf of the Bay of Biscay, extending from the island of Sein to the canyon of Capbreton (Fig. 1). The present work considers both coastal and deep circalittoral areas along the margin of the continental shelf, with bathymetry ranging from 16 to 200 m in depth. This continental shelf is composed of a broad northern shelf (Armorican shelf, ~180 km wide) and a narrower southern shelf (Aquitaine shelf, ~90 km wide) (Koutsikopoulos and Le Cann, 1996).

The shelf is characterized by numerous complex seasonal hydrographic processes: slope-shelf exchanges, northern slope fronts, southern coastal upwellings, and high terrigenous contributions, mainly from the Loire, Vilaine, and Gironde Rivers (Akpinar et al., 2020; Borja et al., 2019; Charria et al., 2013; Koutsikopoulos and Le Cann, 1996; Labry et al., 2001; Loyer et al., 2006).

### 2.2. Biological data

The observations used in this study were recorded as part of the EVHOE (Evaluation Halieutique Ouest de l'Europe) survey undertaken within the framework of IBTS surveys of European fisheries (ICES, 2017). The EVHOE survey took place each year between October and December. Samples were collected using an otter trawl with a large vertical opening (GOV, Grande Ouverture Vertical) and a 20-mm mesh size cod-end (Laffargue et al., 2020). Each haul lasted 30 min at a speed of about 4 knots and covered about 0.07 km². The dataset covers the period 2008–2020 (except 2017, which was incomplete due to cruise damage), with a total of 685 stations sampled.

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Study area: continental shelf of the Bay of Biscay. Gray dots represent sampling points from EVHOE survey used in the analysis. Red frame divides the shelf into the northern Armorican shelf and the southern Aquitaine shelf. 100 and 200 m isobaths are delineated (gray lines) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).</figcaption></figure>
Whenever possible, sampled organisms were sorted to the lowest
possible taxonomic level on board, with verification in the laboratory for
some specimens, if necessary. Species names were assigned as per the
World Register of Marine Species (WoRMS, 2022).

Only epibenthic megainvertebrates and benthodemersal fish were
retained for these analyses. “Megainvertebrates” refers here to organ-
isms with a size greater than 1 cm that can live on or in the substrate and
that need to move on the substrate to perform a vital and/or repro-
ductive function. This excluded from the analysis large swimming
cephalopods (e.g., *Loligo* sp., *Illex* sp.) and gelatinous taxa. Colonial or-
ganisms (from the groups Bryozoa, Hydrozoa, and Ascidiacea) were
uncountable on-board, and thus were also removed from the analyses.
The epibenthic megainvertebrates component will be further referred to
as “benthos”.

As we were particularly interested in benthodemersal fish, we excluded species whose lifecycles and activities are mainly related to pelagos. The benthodemersal ichtyofauna component will be further referred to as “fish”.

2.3. *Environmental variables and data*

We selected a total of seven major environmental variables that are assumed to have a structuring effect on benthic communities.

The coupled hydrodynamic-ecosystem 3D model NEMO-NEMOVAR-
ERSEM was used to obtain data for a large number of environmental
variables at each haul station (EU-Copernicus., 2020b; EU-Copernicus.,
2020a). Those variables were: chlorophyll concentration, mixed layer
depth, nitrate concentration, dissolved oxygen concentration, pH,
phytoplanktonic carbon, phosphate concentration, primary production,
salinity, and temperature. Monthly averaged environmental data were
gridded with a resolution of 0.111° x 0.067° (cells of about 7 km of
horizontal and vertical resolution). Considering the distribution of sta-
tions and the distance sampled per haul (about 3.5 km), we judged that
this resolution was appropriate for the spatial scale of the study. With
the exception of primary production, we selected the surface and near-
bottom values for each of these variables. For net primary production,
we considered the maximum value in the water column for each station
and each month. Data were integrated over the three years preceding
each sampling event, since the life expectancy for the majority of taxa
considered here is expected to be at least three years. For each available
environmental variable and each year of the dataset, we calculated the
minimum, maximum, standard deviation, and mean values. The values
assigned to each station were then averaged over the three years prior to
the observation date. In this way, we obtained an initial set of 72
environmental variables and derived calculations for each sampling
station.

To complete the set of environmental variables, additional data were
selected regarding bottom energy and substrate. Bottom stress, which
represents the velocity of the water movements experienced by benthic
organisms due to currents and waves, has been found to be an important
variable structuring the diversity of benthic communities (Reiss et al.,
2010). Here, this was quantified as the 90th percentile of the kinetic
energy at 1 m from the seabed, with a horizontal resolution of 1/36°
(about 3 km). Since the NEMO-NEMOVAR-ERSEM model does not
include wave components, we obtained data on the kinetic energy due to
currents from the MANGA500 model, available on the EMODnet plat-
form (Accensi and Maisondieu, 2015), and data on the kinetic energy
due to waves from the WAVEMATCH III model (Caillaud et al., 2016).
Data were averaged between 2010 and 2015. We assumed that there
were no strong temporal variations in these two variables (energy due to
currents and energy due to waves). These two types of energy were
summed into a single quantitative “bottom energy” variable.

We performed a preliminary analysis to avoid over-correlation
among variables. From the initial set of variables, we selected five
quantitative descriptors corresponding to key ecological processes:

*   **Standard deviation of surface salinity** (*sd_Sal*'), assumed to reflect the proximity to river plumes with variable seasonal inputs.

*   **Standard deviation of bottom seafloor temperature** ('sd_Temp'), reflecting the climatic gradation of the benthic domain (infralittoral, coastal circalittoral, and deep circalittoral) (Chassé and Glémarec, 1976).

*   **Bottom dissolved oxygen minimum** (min_Oxy'), reflecting the input at the air/water interface and the use by living organisms (respiration, bacterial organic mineralization).

*   **Kinetic energy at the seabed** (q90_Energy'). This variable has a direct influence, through the physical pressure exerted, on the ability of a species to colonize or live in a given habitat, as well as an indirect influence on the trophic capacity of an area through organic matter resuspension and export for example.

*   **Maximum net primary production in the water column** (max-NetPP'), averaged over the year. This highlighted areas with high net primary production rates, notably below the water surface in offshore areas.

Sources and computations methods of these five quantitative descriptors are summarized in Appendix Table A1.

To this subset of environmental descriptors, we added a qualitative
variable, from a map furnished by defined by the French Marine Hy-
drographic and Oceanographic Service (SHOM, 2021), representing
substrate type, following the categories: mud, silt, fine sand, sand,
gravel, and rock. Substrate categories were viewed as indicative of
overall environmental conditions, with low current and high deposition
rates in muddy areas, and strong hydrological conditions in coarser
areas.

This set of six descriptors (five quantitative and one qualitative) was
found to strongly reflect the variability in the original data (Gower's
dissimilarity, Spearman's correlation: 79.3 %, Mantel permutation test
significance: *p*-value <0.001). Maps of the average spatial distribution of
each variable are available in Appendix (Fig. A2). Pearson's correlation
between the 5 continuous variables are available in a corregram in
Appendix (Fig. A3) to which we have added depth to help with inter-
pretation of the results.

2.4. *Biological data analysis*

Each analysis was conducted separately on benthos and fish. Analyses were performed using the vegan, stats, and cluster packages in R 4.3.1 software (Oksanen et al., 2022; R core team., 2023). Only species with an occurrence equal to or greater than 5 % were retained, and abundance matrices were double square-root transformed to avoid the distorting effect of heavily abundant species. Bray-Curtis dissimilarity matrices were generated for fish and benthos and used for hierarchical clustering with Ward linkage. The number of clusters was determined by analyzing explained variability at different dendrogram cutoff levels (Borcard et al., 2018b), using the ‘elbow criterion’ to identify the point beyond which marginal gains in explained variability diminished (Figs. A4 and A5). Additionally, the geographical coherence of clustered stations was considered. This analysis was carried out independently for fish and benthos. Clusters that were faunistically distinct from others were denoted “assemblages”. The significance of the clustering was assessed using a Permutational Analysis of Variance (PERMANOVA). Since the dataset spanned a 12-year period, we also tested for a yearly effect to evaluate the variability explained by inter-annual differences in comparison to the cluster effect. This analysis was conducted using the adonis2 function from the vegan package, and results are given in Appendix (Table A6).

Spatial overlap between benthos and fish assemblages was first
assessed visually and then quantified using a Spearman's correlation
between the two dissimilarity matrices. This correlation was tested with
a Mantel permutation test.

Each cluster was taxonomically characterized using the IndVal
method from the labdsv package. Indices reflecting structural parameters—biomass (kg.km⁻²), abundance (thousands of individuals per km²) (10³ ind.km⁻²), and Shannon diversity—were used to identify spatial patterns in both benthos and fish clusters. Calculations of observed taxonomic richness (per haul) and Pielou's evenness are available in the Appendix (Tables A7 and A8). Pairwise Wilcoxon tests were performed to assess differences between clusters with respect to the structural indices. We also compared the overall distribution of the structural parameters of each assemblage per station by performing Pearson correlations between invertebrates and fish for each parameter (biomass, abundance, and diversity). In order to balance the weight of the different assemblages depending on the number of stations represented, we calculated median correlations based on 1000 random selections of 26 stations per cluster. For this analysis, we only kept the stations (444 stations out of a total of 685) with a similar cluster attribution for the two biological components.

We performed distance-based redundancy analyses (db-RDAs) on Bray-Curtis dissimilarity matrices to explicitly explore the relationships between species distributions and environmental variables (Borcard et al., 2018a). Variance partitioning was performed to assess the importance of each environmental variable in determining the structure of benthic assemblages. By combining the results of the variance partitioning with canonical ordination, we were able to identify relationships among the identified assemblages as well as relationships between the assemblages and environmental variables.

# 3. Results

## 3.1. Spatial organization of the clusters

In total, 141 fish and 342 benthos taxa were recorded during the sampling period. Of these, 47 fish and 77 benthos taxa were considered

to be non-rare species (occurrence >5 %) and were retained for the hierarchical clustering. This procedure identified seven clusters within each of the two communities (corresponding dendrograms are in Appendix Figs. A4 and A5). For both fish and benthos, clustering patterns were spatially coherent (Fig. 2) and significant as based on the permutation analysis of variance (PERMANOVA: F = 34.7 with *p*-value <0.001 and F = 29.5 with *p*-value <0.001 for benthos and fish, respectively). The clusters accounted for five times more variability than the inter-annual effect (Appendix Table A6).

The north coast cluster was mostly located in shallow depths (median depth of 32 m for benthos and 26 m for fish) and close to estuarine areas (Loire, Vilaine, and Gironde) on the Armorican shelf and the northern part of the Aquitaine shelf (Fig. 2). The south coast cluster was found mainly in shallow waters in the southern portion of the Aquitaine shelf at a median depth of 32 and 35 m for benthos and fish, respectively. Moving offshore, the shallow north coast cluster was followed by the inner shelf cluster (median depth of 89 and 64 m for benthos and fish, respectively) and then by a cluster associated with the Grande Vasière (median depth of 102 and 100 m, respectively, for benthos and fish), a well-characterized muddy area that hosted distinct assemblages of both fish and benthos. Further offshore, the southern outer shelf, located on the Aquitaine shelf, was also characterized by unique assemblages of both benthos and fish (median depth of 146 and 127 m, respectively), as were the northern middle shelf (respective median depths of 128 and 123 m) and the northern outer shelf (respective median depths of 146 and 143 m). The southern portion of the Aquitaine shelf appeared to be predominantly structured into two clusters: the south coast and the southern outer shelf.

It was evident from this analysis that there are strong spatial convergences between the benthos and fish communities. Indeed, in a comparison of the fish and benthos components (Bray-Curtis dissimilarity matrices), we detected a significant overall Spearman correlation of 68 % (Mantel statistic R: 0.6763, *p* < 0.001, 10,000 permutations).

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Clustering of megainvertebrates and benthodemersal fish. Each colour represents one cluster. Left panel (Triangular points) represent invertebrate samples; right panel (square points) represent fish. 100 and 200 m isobaths are delineated (gray lines).</figcaption></figure>
## 3.2. Description of the assemblages

The main groups of benthos were Malacostraca (93 taxa), Gastro-poda (45 taxa), Bivalvia (37 taxa), Polychaeta (30 taxa), Anthozoa (26 taxa), Asteroidea (19 taxa), Echinoidea (17 taxa), Ophiuroidea (16 taxa), Cephalopoda (10 taxa), Holothuroidea (10 taxa), and Ascidiacea (9 taxa). Most occurrent and abundant species in each cluster are given in Appendix (Tables A9 and A10).

We used an indicator species analysis (IndVal) to identify the most representative species within each cluster (Fig. 3). The north coast cluster was represented by coastal species such as the hermit crab *Pagurus bernhardus*, the crab *Liocarcinus holsatus*, and the polychaete *Aphrodita aculeata*. Although the south coast cluster shared many species with the north coast, but was less faunistically typed with the sea anemone *Calliactis parasitica* and the crab *Polybius henslowii* as the most characteristic species. Species in the inner shelf cluster were less indic-ative than those of the other clusters; here, the highest indicator values were assigned to the pennatulid *Pteroeides griseum*, the sea anemone *Adamsiapalliata* associated with the hermit crab *Pagurus prideaux*, but these species were all also observed in several other clusters. The Grande Vasière cluster appeared to be highly characterized by the presence of Norway lobster (*Nephrops norvegicus*), the shrimp *Crangon allmanni*, and the squat lobster *Munida rugosa*. Instead, the northern middle shelf was mostly defined by the deep-water crinoid *Leptometra celtica*, and the northern outer shelf by the starfish *Porania (Porania) pulvillus* and the polychaete *Hyalinoecia tubicola* (Fig. 3). Finally, the southern outer shelf cluster was characterized by the brittlestar *Ophiura ophiura* and the anemone *Actinauge* sp., even those these taxa were also shared with other clusters.

With respect to fish, the taxa represented were mainly Gadiformes (22 taxa), Perciformes (20 taxa), Pleuronectiformes (18 taxa), Sparidae (12 taxa), Gobiiformes (11 taxa), Rajiformes (8 taxa), Labridae (7 taxa), and Syngnathiformes (7 taxa).

Several species were identified in multiple clusters: the dogfish *Scyliorhinus canicula*, the European hake (*Merluccius merluccius*), the red gurnard (*Chelidonichtys cuculus*), and the thickback sole (*Microchirus*

variegatus). The coastal, Grande Vasière, and outer shelf clusters appeared to be more faunistically defined than the northern middle shelf or inner shelf clusters, with more-characteristic species: the bogue (*Boops boops*), greater weever fish (*Trachinus draco*), and the striped red mullet (*Mullus surmuletus*) for the southern coast; the whiting (*Merlangius merlangus*) and the common sole (*Solea solea*) for the northern coast; the goby *Lesueurigobius friesii* for the Grande Vasière; the lesser silver smelt (*Argentina sphyraena*) and the ray *Leucoraja naevus* for the northern outer shelf; and the megrim *Lepidorhombus boscii* and two gurnards, *Eutrigla gurnardus* and *Trigla lyra*, for the southern outer shelf.

For both benthos and fish, the north coast clusters had on average the highest biomass and abundance (Fig. 4, summarized values given in Appendix Tables A7 and A8) over the study period (2008–2020). Within this cluster, benthos had an average biomass of 249.75 kg/km2 and an average abundance of 20.29 10³ ind.km⁻², and fish had an average of 1302.27 kg/km2 of biomass and an abundance of 20.27 10³ ind.km⁻².

When considering the distribution of biomass from the coast to offshore areas, we detected an increase in benthos biomass from the inner shelf to the northern outer shelf cluster (47.06 kg/km2 to 132.56 kg/km2, respectively) (Fig. 4). A similar pattern was observed for abundance (ranges from 4.18 10³ ind.km⁻² to 17.41 10³ ind.km⁻²), with a peak on the northern middle shelf (20.68 10³ ind.km⁻², Fig. 4). The northern coastal, middle, and outer shelf clusters of benthos had significantly higher biomass than the others (pairwise Wilcoxon test). High values for biomass in the northern middle shelf cluster were mainly due to the presence of *Leptometra celtica* and the octopus *Eledone cirrhosa*, while biomass patterns in the northern outer shelf appeared to be strongly influenced by the crab *Cancer pagurus* and the holothurian *Parastichopus regalis*. Regarding diversity, values of Shannon's index were constant on the shelf and significantly higher close to the shelf edge (northern outer shelf). Benthos diversity was significantly higher in the northern clusters than in the southern clusters (south coast and southern outer shelf).

For fish, the same global patterns in biomass and abundance were observed, but with peak abundance occurring in the Grande Vasière

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** The five most indicative taxa of each of the seven clusters, as determined from indicator values (IndVal analysis). The warmer the colour of the cell, the higher the indicator value (blue = low; red = high) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Distribution of structural indices for each spatial cluster of benthos (left) and fish (right). Biomass (in kg.km⁻²) and abundance (in thousands of individuals. km⁻²) are log-transformed. Within each panel, boxplots with the same letters were not significantly different following a pairwise Wilcoxon test. For visual convenience, outliers are removed from the figure.</figcaption></figure>

cluster. On the inner shelf, average fish biomass and abundance were 693.75 kg/km² and 11.17 10³ ind.km⁻², respectively. These values increased in the direction of the northern outer shelf cluster, with the highest values found for the Grande Vasière cluster (962.49 kg/km² and 22.86 10³ ind.km⁻², respectively). This cluster, along with the north coast cluster, had significantly higher fish abundance than the others. The main species responsible for the high abundances and biomass observed in these areas were the pouting (*Trisopterus luscus*) for the north coast and the hake (*Merluccius merluccius*) for the Grande Vasière. Values of Shannon diversity and Pielou's evenness were more stable across the shelf for fish than for benthos (Fig. 4, and Appendix Tables A7 and A8), and larger-scale diversity patterns from coast-to-offshore were less clear. However, we did observe that the Grande Vasière was home to a significantly less diverse assemblage of fish than the outer shelf clusters.

For both benthos and fish, biomass and abundance were always significantly higher in the Grande Vasière cluster than in the inner shelf cluster. However, the benthos in the northern outer shelf cluster had significantly higher biomass and abundance than those in the Grande Vasière cluster (Fig. 4). Among all the clusters, patterns of biomass and abundance were 56 % and 60 % correlated, respectively, between benthos and fish communities.

## 3.3. Role played by environmental forcings

### 3.3.1. Influence of environmental forcings on benthos

With respect to the benthos abundance data, the distance-based redundancy analysis (db-RDA) explained 15.49 % of the total variance. The first two axes of the analysis explained 76.69 % of the fitted variation and 11.88 % of the total variation in benthos clusters (Fig. 5). The results highlighted a coast-to-offshore gradient, along the first axis, with a higher range in seafloor temperature and net primary production in coastal areas. The second axis was more related to substrate categories. The southern coast cluster had higher bottom energy than the north coast cluster, where bottom energy was more variable. The Grande Vasière cluster, which featured the muddiest conditions in the entire area, stood out from the others along axis 2. The inner shelf and southern outer shelf were associated with the widest ellipses, which was likely a function of their lower faunistic specificity (see above) and greater environmental variability. The inner shelf cluster seemed to represent a transitional zone between the coastal and offshore clusters, and did not appear to feature any clearly discriminating environmental characteristics of its own. Offshore areas of the northern shelf were mainly characterized by fine sand, rocky substrate, and low variation in bottom temperature. Finally, the substrate of the northern middle shelf cluster was coarser than that of the northern outer shelf.
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Distance-based redundancy analysis for benthos abundance at EVHOE sampled stations. Ellipses represent the 95 % confidence interval. Together with substrate type, five environmental drivers were considered: annual minimum dissolved oxygen at the seafloor (min_Oxyg), annual standard deviation of sea surface salinity (sd_Sal), annual standard deviation of the temperature at the seafloor (sd_Temp), annual maximum net primary production in the water column (max_NetPP), and 90th percentile of average bottom kinetic energy (q90_Energy).</figcaption></figure>

Variance partitioning analysis quantified the proportion of variability explained by each of the six environmental descriptors (Appendix Fig. A11). Among these, the most influential factors were seafloor temperature (standard deviation), substrate type, and the maximum net primary production (average). Coastal clusters were also distinguished from the others based on differences in seafloor bottom temperature (standard deviation), maximum net primary production (average), and bottom energy. Surface salinity (standard deviation) was higher only for the northern coast cluster, reflecting the terrigenous influence in that location.

### 3.3.2. *Influences of environmental forcings on fish*

For the fish abundance dataset, the redundancy analysis constrained 22.42 % of the total variance. The first two axes explained 80.4 % of the fitted variation and 18 % of the total variance (Fig. 6). Overall, redundancy analysis was able to explain more of the variation in fish distributions (22.42 %) than it was for benthos (15.49 %). As for benthos, the first axis corresponded to a bathymetric gradient, and the second axis corresponded to a substrate gradient, despite a less distinct Grande Vasière compared to benthos. The north coast cluster was associated with an increasing range (standard deviation) of salinity, while the south coast cluster was linked with stronger bottom energy. Instead, both coastal clusters were associated with stronger net primary production. However, the overall separation of the coastal clusters from the other assemblages was less clear for benthos than for fish. As observed for benthos, the inner shelf cluster seemed to represent a transitional state between the coastal and offshore clusters. Likewise, the northern middle

shelf cluster (characterized by a coarse substrate) was located in an intermediate position between the Grande Vasière, the northern outer shelf, and the southern outer shelf (Figs. 5 and 6). Variance partitioning analysis quantified the proportion of variability explained by each of the six environmental descriptors (Appendix Fig. A12). The forcings influencing fish clusters were seafloor temperature (standard deviation), maximum net primary production (average), and bottom energy (90th quantile). Compared to benthos communities, fish were less influenced by substrate but more influenced by bottom energy.

To sum up, the coastal clusters corresponded to areas with a higher range of seafloor temperatures and higher primary production. The south coast cluster was located in a sandier and more energetic area than the north coast cluster. The cluster associated with the Grande Vasière was in a muddy area with low bottom energy. The inner shelf represented a transitional zone between the coastal and offshore areas, while the northern middle shelf bridged the gap between the Grande Vasière and the northern outer shelf.

With respect to substrate, that of the northern middle shelf was coarser than that of the Grande Vasière and the northern outer shelf. The north and south outer shelf clusters were distinguished from each other with more variable environmental condition for the southern shelf.
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** Distance-based redundancy analysis for fish abundance at EVHOE sampled stations. Ellipses represent the 95 % confidence intervals. In addition to substrate type, five environmental drivers were considered: annual minimum dissolved oxygen at the seafloor (min_Oxyg), annual standard deviation of sea surface salinity (sd_Sal), annual standard deviation of the temperature at the seafloor (sd_Temp), annual maximum net primary production in the water column (max_NetPP), and 90th percentile of average bottom kinetic energy (q90_Energy).</figcaption></figure>

# 4. Discussion

## 4.1. *Definition of assemblages as a tool for understanding the structure and functioning of continental shelf ecosystems*

There is a pressing need for improved characterizations of the spatial structure and functioning of marine environments (Pittman et al., 2021), especially in the context of marine ecosystem quality assessment and management (Lavialle et al., 2023). The present work—a meso-scale (10–100 km) study of the spatial distribution of epibenthic mega-invertebrates and benthodemersal fish on a continental shelf—represents an important step forward in this regard. In particular, it provides complementary perspectives to the previously published EUNIS typology of the Bay of Biscay, which was mainly based on macrofauna features (Bajjouk et al., 2015).

Our study identified seven clusters of epibenthic megainvertebrates and benthodemersal fish that were biologically coherent and spatially well-delineated and, thus, represented distinct assemblages distributed within the larger study site. A limited number of environmental forcings (e.g., hydrological features, estuarine influence, sediment type) were identified as the main factors responsible for structuring the identified patterns within each biological community (benthos or fish). Despite the biological differences between benthodemersal fish and epibenthic invertebrates, we found a high degree of similarity in the spatial distribution of the different assemblages within the two communities (correlation 68 %), which exhibited similar coast-to-offshore patterns.

Any attempt to understand the ecosystem functioning of continental shelves based on the definition of biological assemblages is likely to be influenced by the choice of methodology. The spatial structures resulting from a given analysis can be highly dependent on the sampling strategy and the nature of the data recorded (e.g., sampling gear, spatial resolution, sampling period). In this case, the equipment used was more appropriate for sampling benthodemersal fish than benthic invertebrates. Because the sampling gear poorly penetrates the soft bottom, only larger epibenthic organisms can be properly sampled. In addition, although our study relies on data from several years (2008–2020), sampling was conducted in only a single month of the year (October). However, the absence of any clear seasonal fluctuation in epifaunal megabenthic communities in comparable environments (e.g., Hinz et al., 2004) gave us confidence that the overall structure described in our study was very likely stable over an annual scale. Moreover, we checked that each of the identified clusters remained distinct from the others over the 12 years of our study, and detected no strong inter-annual variations in the observed spatial distributions. This hypothesis was also supported by a recent report that spatial patterns were stable and strongly dominant compared to temporal variations in shaping the beta-diversity of benthodemersal fish in the Bay of Biscay and the Celtic Sea (Eme et al., 2022).
## 4.2. Spatial distribution of assemblages and environmental drivers on the continental shelf of the Bay of Biscay

The spatial structures identified here followed a pattern of coast-to-offshore succession, and were marked by a clear difference between the Armorican shelf in the north and the Aquitaine shelf in the south.

The northern coast assemblages are situated in a shallow circalittoral area (around 30 m deep) and are influenced by river (Loire and Gironde) plumes, as evident from their higher variability in surface salinity in the canonical ordination. Communities of both fish and invertebrates featured species that are characteristic of coastal muddy areas, such as the crab *Liocarcinus holsatus* and the flatfish *Solea solea* (Koutsikopoulos et al., 1989). With respect to coastal fish, the main indicator species observed in our study (e.g., *Pomatoschistus minutus* and *Merlangius merlangus*) were the same as those described by Souissi et al. (2001). River plumes are a major structuring factor of the Bay of Biscay as they are significant drivers of primary production (Lunven et al., 2005), leading to high species abundance, biomass, and secondary production in coastal areas (Guillaud et al., 2008; Saulnier et al., 2019). Along the northern coast of the Bay of Biscay, this terrestrial influence is also retrieved in coastal macrofaunal assemblages (Dutertre et al., 2013). Another explanation for the greater taxonomic diversity (Stein et al., 2014) observed in this area could be the high number and variety of small-scale habitats utilized by epibenthic communities (Vasquez et al., 2015).

Southern coastal assemblages are also located in shallow circalittoral areas (around 30 m deep). They do not benefit from the influence of plumes but are subjected to high-velocity bottom currents, which is reflected in the mud-free nature of the substrate. Thus, both currents and substrate type were found to have a significant structuring effect on the fish and benthos assemblages in this region; as an example, the fish community was characterized by the presence of species such as the lesser weever fish (*Echiichthys vipera*), which is known to burrow in clear-sandy substrate (Wheeler, 1969). Similar benthо-demersal fish species were identified by Souissi et al. (2001) within this area in October 1990. Such "sandy habitat" was observed by Lagardère (1973) and Monbet (1972) for macrobenthos communities along the Aquitaine shelf, which were also structured along a bathymetric gradient. Compared to the northern coast, the abundance and biomass of both fish and benthos were 10 times lower in the south, possibly the result of a lower nutrient supply due to a weaker river influence. To our knowledge, our study represents the first large-scale study on epibenthic megainvertebrates (with the exception of decapods) along the coast of the Bay of Biscay, and sheds new light on the benthic communities of this area.

Further offshore, in the central part of the continental shelf, the inner shelf assemblages appeared to represent a transitional zone between shallow and deeper areas. This area was characterized by a high degree of environmental variability, which likely promoted the development of assemblages composed of habitat generalists rather than specialists.

The north central area of the shelf—the Grande Vasière—hosted a unique assemblage of benthos, which appeared to be associated with the characteristic muddy substrate of this zone. Instead, the fish community of this area was less distinctive, probably reflecting the fact that these assemblages are less dependent on substrate type. Among the species observed here were typical burrowers such as the crustacean decapod *N. norvegicus* or the goby L. *friesii* (Ellis et al., 2013). The high amounts of benthos and fish biomass we found were expected, as this area is of great importance to the fishing industry; it benefits from high spring bloom production (Guillaud et al., 2008) and high quantities of detrital matter (Le Loc'h et al., 2008).

Offshore on the Armorican shelf, the northern middle shelf hosted assemblages of fish and benthos that were less well defined faunistically and environmentally than their neighbors, and can therefore be viewed as transitional between the Grande Vasière and the deeper areas near the shelf edge. Glémarec (1969) described this area as a barrier with coarser

substrate that limits the offshore extent of the Grande Vasière. As in the present study, he identified sandy communities with high densities of *Leptometra celtica* as observed also in our study, marking a transition between the "Grande Vasière" muds and bathyal muds further offshore.

The northern outer shelf assemblages are located in the deep circalittoral area (around 145 m deep). This layer is caused by the presence of a frontal zone (Koutsikopoulos and Le Cann, 1996) where the interaction of barotropic tide with the slope results in surface-water mixing (Simpson and Sharples, 2012). This promotes the upwelling of cold, nutrient-rich waters from great depths and thus feeds strong primary production (Sharples et al., 2009). The influence of this frontal structure was previously observed in a study of zooplanktonic secondary production on the northern shelf of the Bay of Biscay (Vandromme et al., 2014). In the context of our study, this phenomenon would be more likely to affect the structural parameters (biomass, abundance, and diversity) of invertebrate communities, and to a lesser extent those of the benthо-demersal ichthyofauna. Indeed, here we found that, compared to the assemblages further from edge, the outer shelf boasted significantly higher invertebrate biomass, confirming previous observations by Le Loc'h (2004). This suggests that the shelf break upwelling in this area is a major driver structuring the shelf-edge ecosystem from plankton to benthos.

Despite their proximity to the shelf break, the southern outer shelf assemblages differ from the northern ones and do not benefit from the upwelling observed on the Armorican shelf break (Koutsikopoulos and Le Cann, 1996). They are located in a deep circalittoral area (around 135 m deep) and are less faunistically distinct than communities on the northern outer shelf. Prior to our study, the only types of epibenthic megainvertebrates that had been described in this area were the decapod crustaceans (Lagardère, 1973). Here, we observed that the widespread brittle-star *Ophiura ophiura*, known to have no sediment preference (Boos et al., 2010), was the most indicative species. For fish, the species with the highest indicator value was the flatfish *Lepidorhombus boscii*, which was also proposed to be indicative (as a 'secondary species') in this area 20 years earlier (Souissi et al., 2001).

The benthic assemblages identified in our study are distributed along main environmental forcings commonly observed on continental shelves (e.g., Bland et al., 2019; Lacharité et al., 2016; Reiss et al., 2010). A main coastal to offshore environmental gradient reflects differences with, for example, higher variability (e.g., bottom temperature) or higher food resources (e.g., primary production) in coastal habitats. Moreover, benthic communities are also distributed according to substrate types, from finer sediments (silts and mud) to coarser substrates (sands and gravels) associated with increasing bottom energy (currents and waves). Additionally, the proximity of the main estuaries combined with a south-north current, results in a higher availability of organic matter in the northern coastal area, which likely results in the observed biomass contrast of coastal assemblages.

Regarding anthropogenic influence, our study focused on deeper, circalittoral habitats, which are less affected by coastal pollution (e.g., Le Bris and Glémarec, 1995; Quemmerais-Amice et al., 2020). Abrasion by bottom trawling is a major human-induced pressure distributed over a large proportion of the studied area (ICES, 2024; Quemmerais-Amice et al., 2020) and known to significantly affects benthic communities in the Bay of Biscay (Beauchard et al., 2021). This distribution is heterogeneous and depends, among other things, on the fishery resources targeted and their links with specific habitats and associated benthо-demersal communities. In this context, disentangling the effects of environmental variables from those related to fishing pressure on the distribution of benthic communities is a challenge (e.g., Beauchard et al., 2021). This is especially true for the central mudflat ("Grande Vasière"), corresponding to some of the most intense fishing pressure in the Bay of Biscay, and where fishing pressure variations lead to small spatial scale differences in benthic community composition (Mérillet et al., 2018). However, these differences were not detected at the spatial scale of our study, the entire 'Grande Vasière' habitat being assigned to a
single spatial entity. At this scale, it is common to observe greater variability explained by natural factors rather than anthropogenic ones, as described by Mérillet et al. (2019) for benthodemersal fish in the Celtic Sea. Furthermore, the literature shows that the response to fishing pressure is much more efficiently detected when analyses are based on the functional traits of species (e.g., Beauchard et al., 2021) rather than their taxonomic identity.

### 4.3. Spatial structuration of meso-scale benthic ecosystems across European continental shelves

The main benthic features identified in our study can be found in other areas of the North Atlantic European continental shelf system, and reflect certain common characteristics of benthic ecosystem functioning. Although assemblages may differ in terms of faunal composition, similar key species are found in comparable habitats in all European areas. The convergence of spatial structures between the various ecosystem components—mega- and macroinvertebrates and benthodemersal fish—has also been widely reported, and seems to originate from the effects of common driving factors.

Shallow coastal assemblages similar to the north coast assemblages of our study are observed in the Celtic Sea, the English Channel, and the North Sea, which all feature indicator species similar to those found in the shallower area of our study site, such as the crustaceans *Pagurus bernhardus* and *Liocarcinus holsatus* (Ellis et al., 2013; Hinz et al., 2004; Reiss et al., 2010) and the fish *Solea solea*, *Trisopterus luscus*, and *Merlangius merlangus* (Vaz et al., 2007) or *Buglossidium lutem* (Reiss et al., 2010). The presence and abundance of these species reflect terrigenous influences, specifically related to the proximity of estuaries (Ruiz-Castillo et al., 2019; Vaz et al., 2007).

Along the Welsh coastline (Kaiser et al., 2004) or in the south of the North Sea (Ellis et al., 2011), surveys have identified fish species with affinity for sandy substrate, such as *Echiichthys vipera* (Reiss et al., 2010) which here was detected as an indicator species of the south coast assemblage in the Bay of Biscay. As in the present study, assemblages with this species on the Welsh coastline were also located in areas with high tidal amplitude (Horrillo-Caraballo et al., 2021) and therefore high bottom energy.

Further offshore, previous studies of other European shelves have identified assemblages that appear to be similar to the transitional ones we identified between shallow and deeper areas of the Bay of Biscay. For example, Ellis et al. (2013) described a broad mid-shelf invertebrate assemblage in the Celtic Sea that included the polychaete *Hyalinoecia tubicola* and the crab *Liocarcinus holsatus*. In the same area, Mérillet et al. (2019) reported a broad mid-shelf fish assemblage with widespread species like the fish *Trisopterus minutus*. These widespread assemblages do not appear to have been shaped by any clearly identified physical processes and, given their faunal composition, can be considered transitional groups.

Assemblages typical of muddy areas corresponding to the Grande Vasière of our study site also appear to be common along European shelves, for example in the North Sea near to the English coast (Rees et al., 1999) and in the Celtic Sea off the Bristol Channel (Ellis et al., 2013; Rees et al., 1999). These mud-associated faunas appear to be located in areas where strong primary production occurs during spring blooms (González-Gil et al., 2022; Seguro et al., 2019).

Similarly, the outer shelf assemblages found in our study also have their counterparts on other European shelves. Species that were found here to be representative of northern outer shelf assemblages in the Bay of Biscay (e.g., *Lepidorhombus whiffiagonis*, *Ophiotrix luetkeni*, or *Argentina* sp.) are commonly found along shelf edges in other European areas like the Celtic Sea (Ellis et al., 2013; Mérillet et al., 2019). These areas are all under the influence of shelf-break upwellings which are known to enhance local biological production (Kossack et al., 2023). This influence can also extend to the more internal assemblages of the continental shelf, for example through high densities of crinoids such as

*Leptometra* sp., as found in deep areas of the Mediterranean shelf (Colloca et al., 2004).

### 4.4. From spatial assemblages to “benthiscapes”: Towards ecological modeling and ecosystem-based management

The correspondence of spatial distributions among different components of benthic communities is a common feature of continental shelves. For example, in the Celtic Sea similar spatial patterns were noted for assemblages of epifaunal invertebrates and fish (Ellis et al., 2013; Mérillet et al., 2019). Farther to the north, the benthic communities of the North Sea shelf were found to be structured into mesoscale assemblages comparable, to some extent, to those in the Bay of Biscay, with a strong correlation between communities of epifaunal invertebrates and demersal fish as well as infauna (Reiss et al., 2010). Beyond the European shelves of the North Atlantic, close similarities in mesoscale community structure have also been reported for demersal fish and epibenthic invertebrates in the Mediterranean Sea, in the Gulf of Lions (Gaertner et al., 1999) and along the Italian shelf break (Colloca et al., 2004). Even in regions with stronger environmental gradients, such as the Barents Sea, communities of benthic invertebrates and fish were strongly associated with each other, and co-varied with depth and the ice/temperature gradient (Johannesen et al., 2017). This high degree of spatial similarity between epifaunal invertebrates and fish does not necessarily imply direct interactions (e.g., trophic linkages) between these ecological components. Instead, it may be the result of the strong convergence of intense environmental drivers at the scale of continental shelves that shape and structure different biological communities in similar ways.

The strong convergence of the spatial distributions of mega-invertebrates and benthodemersal fish in the Bay of Biscay provides evidence for the existence of different benthic habitats characterized by different substrates and hydrological processes, as well as shelf-scale environmental gradients (coast-to-offshore and south-to-north).

We propose defining these benthic habitats as “benthiscapes”. In our study, these habitats appear to be consistent with the “hydrological landscapes” described by Planque et al. (2004), which appear to structure both the benthic and the pelagic systems. In the pelagic domain, the main factors shaping the spatial structure of biological communities are the influences of freshwater, water column stratification, and bottom temperature (Petitgas et al., 2018), similar to many of the processes highlighted in the present work. These commonalities strengthen the “meta-ecosystem” vision of the continental shelf proposed by Petitgas et al. (2018). The benthic system is also known to be strongly connected to water column processes (Piepenburg et al., 1997), and the biological components of benthic ecosystems are fundamental to the transfer of matter from the water column to sediments (Zhang et al., 2021). These components are in turn fundamentally shaped by the physical structure of continental shelves, particularly in terms of nutrient sources. Previous work by Ruzicka et al. (2018) defined a few of the major physical processes structuring various shelf ecosystems from the inner to the mid- and outer shelf. Similarly, the benthiscapes identified in our study reflect spatially determined alterations in ecosystem functioning that go beyond simple taxonomy-based structures. Here, these structural patterns have been described very briefly in terms of differences in ecological parameters such as biomass gradients. The spatial variability we observed raises the question of differences in trophic functioning between the different benthiscapes, which is all the more important given the correlations we observed (mostly with respect to abundance and biomass) between communities of invertebrates and fish. Such patterns suggest that ecological functioning varies spatially in the same way for both benthos and fish; further investigation of this issue will improve our understanding of organic matter fluxes at the scale of the continental shelf.

One application of the concept of benthiscapes—benthic habitats characterized by unique ecological assemblages, distinct functions, and
potentially differing responses to natural and anthropogenic pressures—could be in the development and improvement of strategies for ecosystem-based management. The definition of spatially coherent benthic entities, based on this study and others, could assist in efforts to monitor seafloor biodiversity and understand and manage anthropogenic perturbation at the scale of continental shelves (e.g., Lauria et al., 2020). In this context, the benthiscapes described in our study could therefore be considered as one of the fundamental structures to develop tools for spatial planning. However, our knowledge of seafloor ecosystem functioning in the Bay of Biscay remains incomplete (Galparsoro et al., 2014), and further studies are needed to fill the gaps on the different biological communities of this area. The need is especially pressing given the significance of the natural resources of the Bay of Biscay (e.g., sand extraction, offshore wind farms, fishing for benthic and demersal species), which are already heavily utilized and likely to be increasingly important in the future (Boussarie et al., 2023; Lavialle et al., 2023).

## 5. Conclusion

This study investigated the overall spatial structure of ecological communities on the continental shelf of the Bay of Biscay through the identification and characterization of assemblages of epibenthic megainvertebrates and benthodemersal fish. These assemblages were faunistically similar to communities described on other European continental shelves. The spatial delineation of biological communities into distinct assemblages was highly similar between fish and invertebrates, suggesting that the continental shelf ecosystem is made up of different spatial habitats that are predominantly structured by environmental drivers such as bottom temperature, sediment type, bottom energy, and primary production. Here, we propose the term “benthiscapes” for such habitats.

The benthiscapes characterized in this study differed from each other with respect to multiple structural parameters, particularly biomass, which was highest at the coast and close to the continental slope. This

suggests that the trophic functioning of this ecosystem is dependent not only on terrestrial inputs from major rivers but also on the influence of offshore upwelling.

The concept of benthiscapes could be particularly useful for applications in the areas of ecosystem-based management and marine spatial planning. Further characterization of these spatially distinct meso-scale entities could lead to an improved understanding of continental shelf ecosystems and the essential ecosystem functions they provide.

### CRediT authorship contribution statement

**Thomas Outrequin:** Conceptualization, Writing – original draft. **Hervé Le Bris:** Supervision, Writing – review & editing. **Jacques Grall:** Supervision, Writing – review & editing. **Pascal Laffargue:** Supervision, Writing – review & editing.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data availability

Data will be made available on request.

### Acknowledgements

This work was supported by a PhD grant from the project BenTroph, co-funded by the Brittany region and IFREMER. The authors wish to thank each surveys' crew, as well as technicians involved in the sampling at sea during surveys and in laboratory. Authors also would like to express their gratitude to Lindsay Higgins for her insightful proofreading, and to the anonymous reviewers for their meticulous feedback, which has significantly enhanced the quality of this manuscript.

## Appendix A. Appendix

<table><caption>**Table A1**
Summary of environmental variables analyzed.</caption><thead><tr><th>Variable name</th><th>Short name</th><th>Method of calculation</th><th>Time period averaged</th><th>Position in the water</th><th>References</th></tr></thead><tbody><tr><td>Bottom energy</td><td>q90_Energy</td><td>90th percentile</td><td>2010–2015</td><td>1 m from the seabed</td><td>(Accensi and Maisondieu, 2015; Caillaud et al., 2016)</td></tr><tr><td>Dissolved oxygen</td><td>min_Oxyg</td><td>Minimum</td><td>year-2; year-1; year of sample</td><td>Near-seafloor value</td><td>(EU-Copernicus., 2020a)</td></tr><tr><td>Net primary production</td><td>Max_NetPP</td><td>Mean</td><td>year-2; year-1; year of sample</td><td>Depth where monthly maximum value was reached</td><td>(EU-Copernicus., 2020a)</td></tr><tr><td>Salinity</td><td>sd_Sal</td><td>Standard deviation</td><td>year-2; year-1; year of sample</td><td>Surface of water column</td><td>(EU-Copernicus., 2020b)</td></tr><tr><td>Seafloor temperature</td><td>Sd_Temp</td><td>Standard deviation</td><td>year-2; year-1; year of sample</td><td>Seafloor</td><td>(EU-Copernicus., 2020b)</td></tr></tbody></table>
<figure><img src="image_9.png" /><figcaption>**Fig. A2.** Spatial map of environmental variables used. Continuous variables extracted from NEMO-NEMOVAR-ERSEM model are averaged over the study period (2008–2020).</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. A3.** Correlogram of environmental variables: annual minimum dissolved oxygen at the seafloor (min_Oxyg), annual standard deviation of sea surface salinity (sd_Sal), annual standard deviation of the temperature at the seafloor (sd_Temp), annual average of maximum net primary production in the water column (max_NetPP), 90th percentile of average bottom kinetic energy (q90_Energy) and bathymetry (Depth). Indicated correlations values were significant (*p*-value <0.05).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. A4.** Clustering dendrogram and associated inertia (top left panel) for epibenthic megainvertebrates. Red horizontal line is the cutoff level chosen (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Fig. A5. Clustering dendrogram and associated inertia for benthodemersal fish. Red horizontal line is the cutoff level chosen. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

Table A6

Results from PERMANOVA for both fish and benthos. Df: Degrees of freedom; SumOfSqs: Sum of Squares; R2: R-squared. This table gather two independent analysis performed on both biological components.

<table><thead><tr><th></th><th></th><th>Df</th><th>SumOfSqs</th><th>R2</th><th>F</th><th>Pr(&gt;F)</th></tr></thead><tbody><tr><td rowspan="4">Benthos</td><td>Cluster</td><td>6</td><td>49.285</td><td>0.20030</td><td>29.4703</td><td>0.001</td></tr><tr><td>Year</td><td>11</td><td>10.859</td><td>0.04413</td><td>3.5416</td><td>0.001</td></tr><tr><td>Residual</td><td>667</td><td>185.911</td><td>0.75557</td><td></td><td></td></tr><tr><td>Total</td><td>684</td><td>246.055</td><td>1.00000</td><td></td><td></td></tr><tr><td rowspan="4">Fish</td><td>Cluster</td><td>6</td><td>56.014</td><td>0.22765</td><td>34.6977</td><td>0.001</td></tr><tr><td>Year</td><td>11</td><td>10.581</td><td>0.04300</td><td>3.5751</td><td>0.001</td></tr><tr><td>Residual</td><td>667</td><td>179.460</td><td>0.72935</td><td></td><td></td></tr><tr><td>Total</td><td>684</td><td>246.055</td><td>1.00000</td><td></td><td></td></tr></tbody></table>

Table A7

Summary of structural indices for epibenthic megainvertebrate communities (average (standard-deviation)).

<table><thead><tr><th>Index</th><th>South coast</th><th>North coast</th><th>Inner shelf</th><th>Grande Vasière</th><th>Northern middle shelf</th><th>Northern outer shelf</th><th>Southern outer shelf</th></tr></thead><tbody><tr><td>Shannon's diversity</td><td>1.86 (0.8)</td><td>2.69 (0.6)</td><td>2.66 (0.65)</td><td>2.64 (0.54)</td><td>2.8 (0.91)</td><td>3.08 (0.67)</td><td>1.76 (0.85)</td></tr><tr><td>Pielou's evenness</td><td>0.72 (0.18)</td><td>0.64 (0.14)</td><td>0.7 (0.17)</td><td>0.62 (0.13)</td><td>0.61 (0.2)</td><td>0.65 (0.14)</td><td>0.46 (0.23)</td></tr><tr><td>Taxonomic richness</td><td>7.5 (3.71)</td><td>20.26 (8.21)</td><td>15.64 (6.47)</td><td>20.09 (4.86)</td><td>24.83 (6.5)</td><td>27.8 (5.73)</td><td>16.71 (7.11)</td></tr><tr><td>Abundance (ind.10<sup>3</sup>.km<sup>-2</sup>)</td><td>1.87 (4.45)</td><td>20.29 (52.5)</td><td>4.23 (5.9)</td><td>8.3 (7.11)</td><td>19.18 (41.64)</td><td>20.28 (34.49)</td><td>18.31 (34.04)</td></tr><tr><td>Biomass (kg.km<sup>-2</sup>)</td><td>54.36 (173.55)</td><td>249.75 (555.39)</td><td>43.81 (74.95)</td><td>78.15 (51.36)</td><td>91.75 (119.64)</td><td>132.14 (132.46)</td><td>96.82 (157.1)</td></tr></tbody></table>
<table><caption>**Table A8**
Summary of structural indices for epi-benthodemersal fish communities (average (standard-deviation)).</caption><thead><tr><th>Index</th><th>South coast</th><th>North coast</th><th>Inner shelf</th><th>Grande Vasière</th><th>Northern middle shelf</th><th>Northern outer shelf</th><th>Southern outer shelf</th></tr></thead><tbody><tr><td>Shannon's diversity</td><td>2.12 (0.67)</td><td>2.09 (0.61)</td><td>1.95 (0.84)</td><td>1.79 (0.69)</td><td>2.13 (0.6)</td><td>2.02 (0.54)</td><td>2.2 (0.66)</td></tr><tr><td>Pielou's evenness</td><td>0.6 (0.17)</td><td>0.5 (0.14)</td><td>0.52 (0.21)</td><td>0.43 (0.15)</td><td>0.51 (0.14)</td><td>0.52 (0.13)</td><td>0.56 (0.16)</td></tr><tr><td>Taxonomic richness</td><td>12.42 (4.41)</td><td>19.07 (4.6)</td><td>14.44 (4.8)</td><td>18.24 (3.67)</td><td>18.15 (2.99)</td><td>15.16 (3.14)</td><td>15.53 (3.53)</td></tr><tr><td>Abundance (ind.10<sup>3</sup>, km<sup>-2</sup>)</td><td>4.91 (5.88)</td><td>20.27 (23.28)</td><td>11.17 (16.91)</td><td>22.86 (29.76)</td><td>13.1 (9.68)</td><td>22.6 (136.18)</td><td>7.84 (6.58)</td></tr><tr><td>Biomass (kg.km<sup>-2</sup>)</td><td>480.1 (728.79)</td><td>1302.27 (1391.75)</td><td>693.75 (914.57)</td><td>962.49 (554.5)</td><td>945.35 (564.44)</td><td>1039.93 (832.23)</td><td>760.45 (555.91)</td></tr></tbody></table>

<table><caption>**Table A9**
Main species of epibenthic megainvertebrates occurrence and average relative abundance within each benthiscapes (SC, NC, IS, GV, NMS, NOS, SOS). Bold values highlight species with at least 40 % of occurrence, or that account for 75 % of the cumulative abundance in a given benthicscape.</caption><thead><tr><th rowspan="2">Phylum</th><th rowspan="2">Species</th><th colspan="2">SC</th><th colspan="2">NC</th><th colspan="2">IS</th><th colspan="2">GV</th><th colspan="2">NMS</th><th colspan="2">NOS</th><th colspan="2">SOS</th></tr><tr><th>O</th><th>N</th><th>O</th><th>N</th><th>O</th><th>N</th><th>O</th><th>N</th><th>O</th><th>N</th><th>O</th><th>N</th><th>O</th><th>N</th></tr></thead><tbody><tr><td rowspan="10">Annelida</td><td>A.aculeata</td><td>0</td><td>&lt;0.05</td><td><strong>64.89</strong></td><td><strong>3.59</strong></td><td>27.55</td><td>0.59</td><td>39.53</td><td>0.41</td><td>20.71</td><td>0.12</td><td>7.14</td><td>&lt;0.05</td><td>5.88</td><td>&lt;0.05</td></tr><tr><td>H.tubicola</td><td>2.17</td><td>0.14</td><td>8.51</td><td>0.18</td><td>38.78</td><td>1.75</td><td>9.3</td><td>&lt;0.05</td><td><strong>74.75</strong></td><td>3.29</td><td><strong>98.21</strong></td><td><strong>16.58</strong></td><td><strong>41.18</strong></td><td>1.3</td></tr><tr><td>Laetmonice sp.</td><td>0</td><td>&lt;0.05</td><td>1.06</td><td>&lt;0.05</td><td>1.02</td><td>&lt;0.05</td><td>2.33</td><td>&lt;0.05</td><td>17.17</td><td>0.07</td><td><strong>47.32</strong></td><td>0.57</td><td>7.84</td><td>&lt;0.05</td></tr><tr><td>A.glaber</td><td>0</td><td>&lt;0.05</td><td>8.51</td><td>0.07</td><td>11.22</td><td>0.28</td><td><strong>76.74</strong></td><td>0.88</td><td>6.06</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>1.96</td><td>0.1</td></tr><tr><td>C.pagurus</td><td>2.17</td><td>&lt;0.05</td><td>19.15</td><td>0.09</td><td>25.51</td><td>0.38</td><td><strong>58.14</strong></td><td>0.26</td><td><strong>58.08</strong></td><td>0.34</td><td><strong>80.36</strong></td><td>0.61</td><td>27.45</td><td>0.11</td></tr><tr><td>C.crassicornis</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>14.29</td><td>0.53</td><td><strong>76.74</strong></td><td>2.47</td><td>30.3</td><td>0.3</td><td>6.25</td><td>&lt;0.05</td><td>13.73</td><td>0.22</td></tr><tr><td>C.allmanni</td><td>0</td><td>&lt;0.05</td><td>7.45</td><td>0.35</td><td>8.16</td><td>0.11</td><td><strong>89.53</strong></td><td><strong>16.89</strong></td><td>21.72</td><td>0.7</td><td>8.04</td><td>0.22</td><td>1.96</td><td>&lt;0.05</td></tr><tr><td>C.crangon</td><td>0</td><td>&lt;0.05</td><td><strong>48.94</strong></td><td>2.57</td><td>3.06</td><td>0.32</td><td>1.16</td><td>&lt;0.05</td><td>0.51</td><td>0.05</td><td>0.89</td><td>&lt;0.05</td><td>1.96</td><td>&lt;0.05</td></tr><tr><td>D.bonnieri</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>1.02</td><td>0.22</td><td><strong>40.7</strong></td><td>2.42</td><td>11.62</td><td>0.09</td><td>9.82</td><td>0.07</td><td>3.92</td><td>0.07</td></tr><tr><td>G.rhomboides</td><td>2.17</td><td>0.05</td><td>23.4</td><td>0.58</td><td>18.37</td><td>0.75</td><td><strong>58.14</strong></td><td>0.64</td><td>13.64</td><td>0.09</td><td>0</td><td>&lt;0.05</td><td>5.88</td><td>0.09</td></tr><tr><td>L.dorsettensis</td><td>2.17</td><td>&lt;0.05</td><td>30.85</td><td><strong>2.65</strong></td><td>20.41</td><td>0.41</td><td>5.81</td><td>0.08</td><td>29.8</td><td>0.17</td><td><strong>43.75</strong></td><td>0.34</td><td>9.8</td><td>&lt;0.05</td></tr><tr><td>L.depurator</td><td>2.17</td><td>&lt;0.05</td><td><strong>56.38</strong></td><td>3.34</td><td><strong>60.2</strong></td><td>5.07</td><td><strong>90.7</strong></td><td><strong>10.49</strong></td><td><strong>50.51</strong></td><td>2.2</td><td>34.82</td><td>0.94</td><td>9.8</td><td>0.33</td></tr><tr><td>L.holsatus</td><td>19.57</td><td>1.18</td><td><strong>87.23</strong></td><td><strong>16.45</strong></td><td>16.33</td><td>0.9</td><td>32.56</td><td>0.55</td><td>9.09</td><td>0.6</td><td>3.57</td><td>&lt;0.05</td><td>1.96</td><td>0.12</td></tr><tr><td rowspan="10">Arthropoda</td><td>L.typicus</td><td>0</td><td>&lt;0.05</td><td>1.06</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>12.79</td><td>0.14</td><td>10.1</td><td><strong>0.88</strong></td><td>8.04</td><td>0.19</td><td>0</td><td>&lt;0.05</td></tr><tr><td>M.tuberculatus</td><td>0</td><td>&lt;0.05</td><td>2.13</td><td>&lt;0.05</td><td>7.14</td><td>0.22</td><td>31.4</td><td>0.38</td><td><strong>56.57</strong></td><td>2.08</td><td><strong>76.79</strong></td><td>1.5</td><td>21.57</td><td>0.18</td></tr><tr><td>M.tenuirostris</td><td>0</td><td>&lt;0.05</td><td>27.66</td><td>0.35</td><td><strong>46.94</strong></td><td>1.55</td><td><strong>58.14</strong></td><td>0.42</td><td><strong>66.67</strong></td><td>0.85</td><td><strong>68.75</strong></td><td>0.81</td><td>23.53</td><td>0.36</td></tr><tr><td>M.rugosa</td><td>2.17</td><td>&lt;0.05</td><td>8.51</td><td>0.09</td><td>6.12</td><td>0.42</td><td><strong>86.05</strong></td><td><strong>10.95</strong></td><td>29.8</td><td>1.08</td><td>15.18</td><td>0.06</td><td>7.84</td><td>0.13</td></tr><tr><td>N.norvegicus</td><td>2.17</td><td>&lt;0.05</td><td>9.57</td><td>0.64</td><td>19.39</td><td>2.5</td><td><strong>97.67</strong></td><td><strong>21.08</strong></td><td>13.64</td><td>0.59</td><td>1.79</td><td>&lt;0.05</td><td>3.92</td><td>0.41</td></tr><tr><td>P.bernhardus</td><td>32.61</td><td>1.78</td><td><strong>71.28</strong></td><td>1.34</td><td>5.1</td><td>0.31</td><td>4.65</td><td>0.05</td><td>1.01</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>3.92</td><td>0.42</td></tr><tr><td>P.prideaux</td><td>13.04</td><td>0.34</td><td>38.3</td><td>1.9</td><td><strong>69.39</strong></td><td><strong>14.15</strong></td><td>36.05</td><td>0.75</td><td><strong>91.92</strong></td><td><strong>10.63</strong></td><td><strong>91.07</strong></td><td>7.36</td><td><strong>47.06</strong></td><td>1.91</td></tr><tr><td>P.henslowii</td><td>21.74</td><td><strong>10</strong></td><td>39.36</td><td>2.7</td><td>10.2</td><td>0.44</td><td>5.81</td><td>&lt;0.05</td><td>4.04</td><td>&lt;0.05</td><td>1.79</td><td>&lt;0.05</td><td>3.92</td><td>0.74</td></tr><tr><td>P spinosus</td><td>0</td><td>&lt;0.05</td><td>9.57</td><td>0.18</td><td>13.27</td><td>0.27</td><td><strong>79.07</strong></td><td>1.71</td><td>34.34</td><td>0.32</td><td><strong>40.18</strong></td><td>0.32</td><td>3.92</td><td>&lt;0.05</td></tr><tr><td>Processa spp.</td><td>0</td><td>&lt;0.05</td><td>20.21</td><td>0.21</td><td>6.12</td><td>0.09</td><td><strong>72.09</strong></td><td>2.73</td><td>27.27</td><td>0.57</td><td>16.07</td><td>0.06</td><td>11.76</td><td>0.05</td></tr><tr><td>S.scalpellum</td><td>15.22</td><td>1.33</td><td>22.34</td><td>0.76</td><td>33.67</td><td>0.87</td><td>16.28</td><td>0.77</td><td>39.39</td><td><strong>3.61</strong></td><td>20.54</td><td>0.57</td><td>25.49</td><td>1.19</td></tr><tr><td>S.membranacea</td><td>2.17</td><td>0.27</td><td>18.09</td><td>0.3</td><td>15.31</td><td>0.52</td><td><strong>53.49</strong></td><td>2.92</td><td>13.13</td><td>0.28</td><td>2.68</td><td>&lt;0.05</td><td>15.69</td><td>0.35</td></tr><tr><td rowspan="2">Chordata</td><td>Ascidia sp.</td><td>2.17</td><td>&lt;0.05</td><td>18.09</td><td>0.41</td><td>27.55</td><td>1.06</td><td>5.81</td><td>0.06</td><td>39.39</td><td>0.67</td><td><strong>47.32</strong></td><td>0.79</td><td>25.49</td><td>0.32</td></tr><tr><td>Actinauge</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>8.16</td><td>0.75</td><td>2.33</td><td>&lt;0.05</td><td><strong>57.07</strong></td><td>0.96</td><td>20.54</td><td>0.08</td><td><strong>72.55</strong></td><td>12.23</td></tr><tr><td rowspan="10">Cnidaria</td><td>Actiniaria</td><td>8.7</td><td>3.25</td><td>32.98</td><td>0.52</td><td>27.55</td><td>1.09</td><td>19.77</td><td>0.14</td><td><strong>52.53</strong></td><td>1</td><td>9.82</td><td>&lt;0.05</td><td><strong>43.14</strong></td><td>0.37</td></tr><tr><td>A.palliata</td><td>10.87</td><td>0.33</td><td>39.36</td><td>1.67</td><td><strong>69.39</strong></td><td><strong>13.72</strong></td><td>34.88</td><td>0.77</td><td><strong>86.87</strong></td><td><strong>7.94</strong></td><td><strong>91.07</strong></td><td><strong>6.28</strong></td><td>37.25</td><td>0.6</td></tr><tr><td>C.parasitica</td><td><strong>45.65</strong></td><td>8.05</td><td><strong>71.28</strong></td><td>4.73</td><td>35.71</td><td>1.05</td><td>10.47</td><td>0.06</td><td>7.07</td><td>&lt;0.05</td><td>0.89</td><td>&lt;0.05</td><td>11.76</td><td>0.06</td></tr><tr><td>A.placenta</td><td>0</td><td>&lt;0.05</td><td>1.06</td><td>&lt;0.05</td><td>19.39</td><td>0.36</td><td>1.16</td><td>&lt;0.05</td><td><strong>78.28</strong></td><td>3.05</td><td><strong>91.96</strong></td><td>2.85</td><td>37.25</td><td>0.19</td></tr><tr><td>A.petasus</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>3.06</td><td>&lt;0.05</td><td>2.33</td><td>&lt;0.05</td><td>30.3</td><td>0.54</td><td><strong>59.82</strong></td><td>3.73</td><td>21.57</td><td>0.13</td></tr><tr><td>A.rubens</td><td><strong>89.13</strong></td><td><strong>36.48</strong></td><td><strong>89.36</strong></td><td><strong>20.64</strong></td><td><strong>60.2</strong></td><td><strong>5.68</strong></td><td>24.42</td><td>0.83</td><td>21.21</td><td>0.5</td><td>3.57</td><td>&lt;0.05</td><td>7.84</td><td>0.1</td></tr><tr><td>A.irregularis</td><td><strong>69.57</strong></td><td><strong>13.31</strong></td><td><strong>82.98</strong></td><td>7.53</td><td><strong>73.47</strong></td><td><strong>7.94</strong></td><td><strong>58.14</strong></td><td>2.06</td><td><strong>94.44</strong></td><td>3.4</td><td><strong>62.5</strong></td><td>0.99</td><td><strong>92.16</strong></td><td>5.31</td></tr><tr><td>G.acutus</td><td>0</td><td>&lt;0.05</td><td>1.06</td><td>&lt;0.05</td><td>4.08</td><td>0.06</td><td>0</td><td>&lt;0.05</td><td>27.27</td><td>0.4</td><td><strong>75.89</strong></td><td>1.19</td><td>25.49</td><td>0.9</td></tr><tr><td>L.celtica</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>8.16</td><td>0.21</td><td>3.49</td><td>0.17</td><td><strong>68.69</strong></td><td><strong>22.59</strong></td><td><strong>67.86</strong></td><td><strong>10.48</strong></td><td><strong>56.86</strong></td><td>2.1</td></tr><tr><td>L.sarsii</td><td>0</td><td>&lt;0.05</td><td>4.26</td><td>&lt;0.05</td><td>12.24</td><td>0.12</td><td>5.81</td><td>0.14</td><td><strong>56.06</strong></td><td>0.82</td><td><strong>73.21</strong></td><td>1.09</td><td><strong>60.78</strong></td><td>1.17</td></tr><tr><td>O.nigra</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>8.08</td><td>0.08</td><td><strong>52.68</strong></td><td>3.15</td><td>0</td><td>&lt;0.05</td></tr><tr><td>O.luetkeni</td><td>0</td><td>&lt;0.05</td><td>3.19</td><td>0.06</td><td>23.47</td><td>0.88</td><td>4.65</td><td>&lt;0.05</td><td><strong>44.44</strong></td><td><strong>3.57</strong></td><td><strong>88.39</strong></td><td>7.14</td><td>33.33</td><td>0.61</td></tr><tr><td>O.ophiura</td><td>39.13</td><td>6.52</td><td><strong>73.4</strong></td><td><strong>8.79</strong></td><td><strong>71.43</strong></td><td><strong>7.64</strong></td><td>27.91</td><td>0.73</td><td><strong>82.32</strong></td><td><strong>6.11</strong></td><td><strong>89.29</strong></td><td><strong>4.47</strong></td><td><strong>98.04</strong></td><td><strong>49.89</strong></td></tr><tr><td>P.regalis</td><td>4.35</td><td>0.19</td><td>4.26</td><td>&lt;0.05</td><td>5.1</td><td>0.09</td><td>3.49</td><td>&lt;0.05</td><td><strong>45.96</strong></td><td>0.38</td><td><strong>54.46</strong></td><td>1.22</td><td>23.53</td><td>0.19</td></tr><tr><td>P.pulvillus</td><td>0</td><td>&lt;0.05</td><td>1.06</td><td>&lt;0.05</td><td>5.1</td><td>0.16</td><td>8.14</td><td>0.06</td><td>80.3</td><td>5.62</td><td><strong>100</strong></td><td><strong>13.58</strong></td><td>3.92</td><td>&lt;0.05</td></tr><tr><td>S.rosea</td><td>0</td><td>&lt;0.05</td><td>4.26</td><td>&lt;0.05</td><td>6.12</td><td>0.18</td><td>13.95</td><td>0.09</td><td><strong>72.73</strong></td><td>2.83</td><td><strong>95.54</strong></td><td><strong>5.55</strong></td><td>3.92</td><td>0.12</td></tr><tr><td>T.subinermis</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>3.06</td><td>0.09</td><td>1.16</td><td>&lt;0.05</td><td>20.71</td><td>0.08</td><td><strong>59.82</strong></td><td>0.29</td><td>9.8</td><td>0.09</td></tr><tr><td>A.opercularis</td><td>6.52</td><td><strong>2.34</strong></td><td>22.34</td><td><strong>2.47</strong></td><td>7.14</td><td>0.13</td><td>4.65</td><td>&lt;0.05</td><td>1.01</td><td>&lt;0.05</td><td>0.89</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td></tr><tr><td>E.cirrhosa</td><td>2.17</td><td>&lt;0.05</td><td>12.77</td><td>0.12</td><td><strong>61.22</strong></td><td>2.84</td><td><strong>86.05</strong></td><td>2.04</td><td><strong>69.7</strong></td><td>0.97</td><td><strong>66.07</strong></td><td>0.36</td><td><strong>52.94</strong></td><td>1.57</td></tr><tr><td>G.rubrum</td><td>4.35</td><td>0.16</td><td>11.7</td><td>0.37</td><td>23.47</td><td>2.89</td><td>8.14</td><td>0.05</td><td>40.4</td><td>1.7</td><td>35.71</td><td>1.34</td><td><strong>60.78</strong></td><td>6.47</td></tr><tr><td>L.perspicua</td><td>0</td><td>&lt;0.05</td><td>2.13</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>8.59</td><td>0.11</td><td><strong>50</strong></td><td>0.99</td><td>13.73</td><td>0.1</td></tr><tr><td rowspan="7">Mollusca</td><td>P.aperta</td><td>6.52</td><td>0.18</td><td>32.98</td><td><strong>1.27</strong></td><td>3.06</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>0.89</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td></tr><tr><td>S.lignarius</td><td>0</td><td>&lt;0.05</td><td>22.34</td><td>0.38</td><td>29.59</td><td>0.77</td><td>9.3</td><td>&lt;0.05</td><td><strong>43.94</strong></td><td>0.56</td><td><strong>49.11</strong></td><td>0.4</td><td><strong>66.67</strong></td><td>1.07</td></tr><tr><td>S.elegans</td><td>4.35</td><td>0.92</td><td>20.21</td><td>0.31</td><td><strong>64.29</strong></td><td>3.72</td><td>8.14</td><td>&lt;0.05</td><td><strong>70.2</strong></td><td>0.76</td><td><strong>71.43</strong></td><td>0.62</td><td>29.41</td><td>0.4</td></tr><tr><td>S.officinalis</td><td><strong>47.83</strong></td><td>6.01</td><td><strong>74.47</strong></td><td>3.8</td><td>7.14</td><td>0.14</td><td>1.4</td><td>&lt;0.05</td><td>2.53</td><td>&lt;0.05</td><td>1.79</td><td>&lt;0.05</td><td>3.92</td><td>&lt;0.05</td></tr><tr><td>S.orbignyana</td><td>2.17</td><td>0.15</td><td>5.32</td><td>&lt;0.05</td><td>21.43</td><td>0.35</td><td><strong>52.33</strong></td><td>0.53</td><td><strong>49.49</strong></td><td>0.42</td><td><strong>61.61</strong></td><td>0.37</td><td>37.25</td><td>0.25</td></tr><tr><td>Sepiolidae</td><td>2.17</td><td>0.31</td><td><strong>59.57</strong></td><td>2.14</td><td>37.76</td><td>2.36</td><td><strong>96.51</strong></td><td><strong>11.52</strong></td><td><strong>69.7</strong></td><td>3.03</td><td><strong>83.93</strong></td><td>1.36</td><td>37.25</td><td>0.4</td></tr><tr><td>Porifera</td><td>Suberites sp.</td><td>0</td><td>&lt;0.05</td><td>9.57</td><td>0.08</td><td>9.18</td><td><strong>0.75</strong></td><td>6.98</td><td>0.06</td><td>4.04</td><td>&lt;0.05</td><td>7.14</td><td>&lt;0.05</td><td>1.96</td><td>0.06</td></tr></tbody></table>
<caption>**Table A10**</caption>

Main species of benthodemersal fishes occurrence and average relative abundance within each benthiscapes (SC, NC, IS, GV, NMS, NOS, SOS). Bold values highlight species with at least 40 % of occurrence, or that account for 75 % of the cumulative abundance in a given benthicscape.

<table><thead><tr><th rowspan="2">Class</th><th rowspan="2">Species</th><th colspan="2">SC</th><th colspan="2">NC</th><th colspan="2">IS</th><th colspan="2">GV</th><th colspan="2">NMS</th><th colspan="2">NOS</th><th colspan="2">SOS</th></tr><tr><th>O</th><th>N</th><th>O</th><th>N</th><th>O</th><th>N</th><th>O</th><th>N</th><th>O</th><th>N</th><th>O</th><th>N</th><th>O</th><th>N</th></tr></thead><tbody><tr><td rowspan="20">Actinopterygii</td><td>A.sphyraena</td><td>0</td><td>&lt;0.05</td><td>5.17</td><td>&lt;0.05</td><td>50</td><td>4.05</td><td>78.38</td><td>2.57</td><td>95.48</td><td>8.93</td><td>98.67</td><td>32.65</td><td>89.09</td><td>12.84</td></tr><tr><td>A.imperialis</td><td>10.77</td><td>0.36</td><td>15.52</td><td>0.19</td><td>50</td><td>2.13</td><td>31.53</td><td>0.18</td><td>92.9</td><td>1.42</td><td>92</td><td>1.8</td><td>29.09</td><td>0.83</td></tr><tr><td>A.laterna</td><td>29.23</td><td>0.84</td><td>75.86</td><td>1.59</td><td>25</td><td>0.57</td><td>68.47</td><td>0.93</td><td>20.65</td><td>0.17</td><td>3.33</td><td>0.05</td><td>71.82</td><td>2.25</td></tr><tr><td>B.boops</td><td>93.85</td><td>23.02</td><td>46.55</td><td>1.23</td><td>13.89</td><td>1.02</td><td>0.9</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>11.82</td><td>0.18</td></tr><tr><td>B.luteum</td><td>26.15</td><td>1.05</td><td>77.59</td><td>3.26</td><td>16.67</td><td>0.3</td><td>5.41</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>5.45</td><td>0.15</td></tr><tr><td>C.lyra</td><td>49.23</td><td>3.63</td><td>86.21</td><td>2.17</td><td>94.44</td><td>7.98</td><td>92.79</td><td>1.45</td><td>94.84</td><td>1.27</td><td>30</td><td>0.14</td><td>71.82</td><td>1.41</td></tr><tr><td>C.maculatus</td><td>4.62</td><td>&lt;0.05</td><td>3.45</td><td>&lt;0.05</td><td>11.11</td><td>0.17</td><td>72.97</td><td>0.54</td><td>80</td><td>0.95</td><td>53.33</td><td>0.32</td><td>41.82</td><td>0.6</td></tr><tr><td>C.macrophthalma</td><td>3.08</td><td>&lt;0.05</td><td>5.17</td><td>&lt;0.05</td><td>8.33</td><td>&lt;0.05</td><td>55.86</td><td>0.46</td><td>21.94</td><td>0.13</td><td>0</td><td>&lt;0.05</td><td>23.64</td><td>0.54</td></tr><tr><td>C.cuculus</td><td>1.54</td><td>&lt;0.05</td><td>1.72</td><td>&lt;0.05</td><td>66.67</td><td>2.18</td><td>71.17</td><td>0.59</td><td>98.06</td><td>3.74</td><td>99.33</td><td>3.61</td><td>66.36</td><td>3.23</td></tr><tr><td>C.ramada</td><td>41.54</td><td>2.13</td><td>41.38</td><td>0.78</td><td>8.33</td><td>0.11</td><td>1.8</td><td>&lt;0.05</td><td>0.65</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>4.55</td><td>&lt;0.05</td></tr><tr><td>C.conger</td><td>16.92</td><td>0.18</td><td>72.41</td><td>0.79</td><td>41.67</td><td>0.24</td><td>68.47</td><td>0.31</td><td>78.06</td><td>0.47</td><td>69.33</td><td>0.26</td><td>42.73</td><td>0.22</td></tr><tr><td>C.labrax</td><td>32.31</td><td>0.57</td><td>70.69</td><td>0.41</td><td>52.78</td><td>5.48</td><td>46.85</td><td>0.58</td><td>3.87</td><td>&lt;0.05</td><td>0.67</td><td>&lt;0.05</td><td>10</td><td>0.13</td></tr><tr><td>D.cuneata</td><td>52.31</td><td>1.55</td><td>67.24</td><td>1.93</td><td>8.33</td><td>0.11</td><td>0.9</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>1.82</td><td>&lt;0.05</td></tr><tr><td>E.vipera</td><td>60</td><td>4.03</td><td>22.41</td><td>0.18</td><td>63.89</td><td>3.52</td><td>1.8</td><td>&lt;0.05</td><td>9.68</td><td>0.08</td><td>0.67</td><td>&lt;0.05</td><td>0.91</td><td>&lt;0.05</td></tr><tr><td>E.cimbrius</td><td>0</td><td>&lt;0.05</td><td>3.45</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>50.45</td><td>0.27</td><td>6.45</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>1.82</td><td>&lt;0.05</td></tr><tr><td>E.gurnardus</td><td>4.62</td><td>&lt;0.05</td><td>20.69</td><td>0.08</td><td>19.44</td><td>0.29</td><td>46.85</td><td>0.3</td><td>44.52</td><td>0.3</td><td>20</td><td>0.09</td><td>75.45</td><td>1.93</td></tr><tr><td>G.argenteus</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>17.12</td><td>0.54</td><td>43.23</td><td>1.41</td><td>46.67</td><td>1.29</td><td>30.91</td><td>3.9</td></tr><tr><td>L.boscii</td><td>1.54</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>1.8</td><td>&lt;0.05</td><td>14.19</td><td>&lt;0.05</td><td>31.33</td><td>0.11</td><td>65.45</td><td>1.61</td></tr><tr><td>L.whiffiagonis</td><td>0</td><td>&lt;0.05</td><td>1.72</td><td>&lt;0.05</td><td>16.67</td><td>0.16</td><td>53.15</td><td>1.09</td><td>89.68</td><td>1.81</td><td>100</td><td>3</td><td>69.09</td><td>1.44</td></tr><tr><td>L.friesii</td><td>1.54</td><td>&lt;0.05</td><td>15.52</td><td>0.53</td><td>8.33</td><td>0.08</td><td>73.87</td><td>2.62</td><td>9.03</td><td>0.07</td><td>1.33</td><td>&lt;0.05</td><td>7.27</td><td>&lt;0.05</td></tr><tr><td>L.budegassa</td><td>1.54</td><td>&lt;0.05</td><td>1.72</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>5.41</td><td>&lt;0.05</td><td>67.74</td><td>0.39</td><td>85.33</td><td>1.78</td><td>7.27</td><td>&lt;0.05</td></tr><tr><td>L.piscatorius</td><td>10.77</td><td>0.12</td><td>1.72</td><td>&lt;0.05</td><td>5.56</td><td>0.06</td><td>58.56</td><td>0.31</td><td>69.68</td><td>0.42</td><td>66.67</td><td>0.8</td><td>68.18</td><td>0.63</td></tr><tr><td>M.merlangus</td><td>26.15</td><td>0.36</td><td>22.41</td><td>&lt;0.05</td><td>41.67</td><td>0.3</td><td>81.08</td><td>0.35</td><td>60.65</td><td>0.18</td><td>59.33</td><td>0.24</td><td>32.73</td><td>0.17</td></tr><tr><td>M.merluccius</td><td>67.69</td><td>3.26</td><td>87.93</td><td>11.36</td><td>77.78</td><td>1.58</td><td>45.95</td><td>0.85</td><td>2.58</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>1.82</td><td>&lt;0.05</td></tr><tr><td>M.variegatus</td><td>7.69</td><td>0.05</td><td>94.83</td><td>7.34</td><td>91.67</td><td>14.11</td><td>100</td><td>54.7</td><td>100</td><td>35.83</td><td>99.33</td><td>13.95</td><td>100</td><td>43.18</td></tr><tr><td>M.surmuletus</td><td>86.15</td><td>8.15</td><td>22.41</td><td>0.06</td><td>38.89</td><td>0.6</td><td>88.29</td><td>2.15</td><td>74.19</td><td>1.23</td><td>38.67</td><td>0.29</td><td>61.82</td><td>1.12</td></tr><tr><td>P.acarne</td><td>41.54</td><td>2.71</td><td>67.24</td><td>0.92</td><td>44.44</td><td>1.09</td><td>9.01</td><td>&lt;0.05</td><td>7.74</td><td>&lt;0.05</td><td>10</td><td>&lt;0.05</td><td>21.82</td><td>0.09</td></tr><tr><td>P.blennoides</td><td>0</td><td>&lt;0.05</td><td>15.52</td><td>0.42</td><td>0</td><td>&lt;0.05</td><td>0.9</td><td>&lt;0.05</td><td>0.65</td><td>&lt;0.05</td><td>2</td><td>&lt;0.05</td><td>1.82</td><td>&lt;0.05</td></tr><tr><td>P.minutus</td><td>9.23</td><td>0.17</td><td>0</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>51.35</td><td>0.35</td><td>69.68</td><td>0.3</td><td>45.33</td><td>0.16</td><td>17.27</td><td>0.15</td></tr><tr><td>S.solea</td><td>21.54</td><td>0.19</td><td>62.07</td><td>6.79</td><td>22.22</td><td>0.27</td><td>27.03</td><td>0.47</td><td>14.19</td><td>0.21</td><td>8</td><td>&lt;0.05</td><td>4.55</td><td>&lt;0.05</td></tr><tr><td>S.cantharus</td><td>81.54</td><td>5.77</td><td>46.55</td><td>0.44</td><td>77.78</td><td>5.35</td><td>92.79</td><td>2.81</td><td>99.35</td><td>4.68</td><td>98.67</td><td>9.95</td><td>100</td><td>8.65</td></tr><tr><td>T.draco</td><td>98.46</td><td>28.71</td><td>94.83</td><td>1.97</td><td>27.78</td><td>0.23</td><td>33.33</td><td>0.06</td><td>15.48</td><td>&lt;0.05</td><td>2</td><td>&lt;0.05</td><td>18.18</td><td>0.07</td></tr><tr><td>T.luscus</td><td>35.38</td><td>8.19</td><td>63.79</td><td>1.81</td><td>27.78</td><td>1.07</td><td>0</td><td>&lt;0.05</td><td>3.23</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>3.64</td><td>&lt;0.05</td></tr><tr><td>T.minutus</td><td>23.08</td><td>2.39</td><td>37.93</td><td>1.5</td><td>47.22</td><td>7.3</td><td>0.9</td><td>&lt;0.05</td><td>3.87</td><td>&lt;0.05</td><td>0</td><td>&lt;0.05</td><td>10</td><td>0.64</td></tr><tr><td>Z.faber</td><td>18.46</td><td>0.15</td><td>100</td><td>50.03</td><td>50</td><td>2.04</td><td>63.96</td><td>1.37</td><td>66.45</td><td>2.19</td><td>39.33</td><td>1.03</td><td>58.18</td><td>5.9</td></tr><tr><td>L.naevus</td><td>1.54</td><td>&lt;0.05</td><td>44.83</td><td>3.44</td><td>55.56</td><td>34.79</td><td>97.3</td><td>21.76</td><td>99.35</td><td>31.61</td><td>76</td><td>27.2</td><td>47.27</td><td>3.31</td></tr><tr><td>Elasmobranchii</td><td>S.canicula</td><td>44.62</td><td>1.88</td><td>36.21</td><td>0.08</td><td>72.22</td><td>1.02</td><td>76.58</td><td>0.57</td><td>80.65</td><td>0.58</td><td>66</td><td>0.53</td><td>63.64</td><td>0.88</td></tr></tbody></table>
<figure><img src="image_13.png" /><figcaption>**Fig. A11.** Variance partitioning of the effect of each environmental variables in shaping benthos communities. “Physical” variables correspond to temperature, and current; “chemical” variables are oxygen, salinity, and net primary production.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>**Fig. A12.** Variance partitioning of the effect of each environmental variables in shaping fish communities. "Physical" variables correspond to temperature and current; "chemical" variables are oxygen, salinity, and net primary production.</figcaption></figure>

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