Carinne Rigolet

Eric Thiébaut

Stanislas F Dubois
email: sdubois@ifremer.fr

Food web structures of subtidal benthic muddy habitats: evidence of microphytobenthos contribution supported by an engineer species

Keywords: Stable isotopes, Gregarious amphipod, Source contribution, Haploops, Benthic diatoms, Navicula

Stable C and N isotopic composition of benthic organisms was investigated to understand the effects of a gregarious tubiculous amphipod species (Haploops nirae) on benthic food web structures and test whether drastic changes in species composition cause changes in food web structure. The habitat engineered by this species was sampled and compared with the adjacent Amphiura filiformis muddy habitat in winter and summer. The isotopic signature of macro-and mega-faunal species associated with both habitats were analysed along with potential food sources at each sampling period. Similar food web structures for each habitat (and each season), with high δ 15 N ranges, spanning over 3 trophic levels, were reported although omnivory was less frequent in the Haploops habitat. The amphipod Haploops nirae appears to play a baseline role with the lowest δ 15 N values and δ 13 C values indicating that it primarily feeds on phytoplankton. Bayesian mixing models used to estimate the contributions of potential food sources to the diet of the species accounting for most of the biomass in each habitat showed that the epibionts covering tube mats are a key contribution to both habitats food webs. Identified as benthic diatoms, they may minimize inter-specific food competition between dominant suspension-feeders such as Polititapes virgineus or Haploops nirae within a habitat. This finding highlights the contribution of microphytobenthic producers to subtidal (~ 30 m deep) muddy habitats food webs, mucopolysaccharides composing Haploops tubes being suggested to support the growth of a heterotrophic diatom population. As an engineer species, Haploops acts as a facilitator for diatoms which ultimately sustain the secondary production of the Haploops habitat as well as the immediate surrounding habitats.

INTRODUCTION

Coastal zones are considered among the most productive marine systems in the world with both high ecological and economic values. They offer a wide variety of ecosystem services for humans but also sustain ecological processes of primary importance for marine wildlife such as nutrient regulation, carbon sequestration and detoxification of polluted waters (Constanza et al. 1997[START_REF] Gattuso | Carbone and carbonate metabolism in coastal aquatic ecosystems[END_REF][START_REF] Barbier | Revival capacity of diatoms in bivalve pseudofaeces and faeces[END_REF]. Coastal ecosystems are also known to support particularly species rich and abundant benthic communities, resulting in areas of high benthic secondary production which are of major importance for supporting nursery grounds and feeding areas for most coastal and many oceanic fish species [START_REF] Kopp | Spatial analysis of the trophic interactions between two juvenile fish species and their preys along a coastalestuarine gradient[END_REF]. The importance of coastal systems in sustaining marine wildlife has consequently required a much better knowledge of biological and ecological processes in coastal systems.

As reviewed in [START_REF] Duffy | The functional role of biodiversity in ecosystems: incorporating trophic complexity[END_REF], the trophic complexity within trophic levels (food chain width and variety of food sources) and across trophic levels (food chain length and omnivory) are essential elements to understand how biodiversity changes affect ecosystem functioning.

Yet the understanding of trophic dynamics and pathways in coastal systems remains partly unknown. Indeed, while the primary production in the open-ocean is only dominated by phytoplankton, a large variety of primary producers (e.g. macrophytes, salt marshes plants, mangroves, seagrass beds and microphytobenthos) contribute to the organic matter sedimentary pool available for benthic consumers in coastal ecosystems [START_REF] Evrard | Importance of phytodetritus and microphytobenthos for heterotrophs in a shallow subtidal sandy sediment[END_REF].

Unlike in shores and estuaries where benthic primary production through microphytopbenthos (MPB) play a key role in benthic food webs (Riera et al. 1996[START_REF] Dubois | Effects of oyster farming on macrofaunal assemblages associated with Lanice conchilega tubeworm populations: A trophic analysis using natural stable isotopes[END_REF][START_REF] Lefebvre | Seasonal dynamics of trophic relationships among co-occurring suspension-feeders in two shellfish-culture dominated ecosystems[END_REF], microphytobenthic primary producers are disregarded in subtidal benthic food webs, despite evidences that δ 13 C-enriched isotopic compositions of benthic suspension-and deposit-feeders cannot solely be explained by phytoplankton or macroalgae sources (e.g. [START_REF] Grall | Community structure and food web based on stable isotopes (δ 15 N and δ 13 C) analysis of a North Eastern Atlantic maerl bed[END_REF][START_REF] Carlier | Benthic trophic network in the Bay of Banyuls-sur-Mer (northwest Mediterranean, France): an assessment based on stable carbon and nitrogen isotopes analysis[END_REF]. Few studies however have emphasized the potential role of microphytobenthos in the trophic food webs of different coastal habitats such as estuarine systems, mudflats (Middleburg et al. 2000) or shallow sandy permeable sediments [START_REF] Evrard | Carbon and nitrogen flows through the benthic food web of a photic subtidal sandy sediment[END_REF].

Stable carbon and nitrogen isotope ratios have proven to be a powerful tool in evaluating feeding relationships and organic matter flows through marine food webs [START_REF] Dubois | Preface -Contributions to the 8th International Conference on Applications of Stable Isotope Techniques to Ecological Studies (ISOECOL)[END_REF]. As stable isotopes are frequently used to understand feeding behaviour and trophic relationships between species (Peterson & Fry 1987), they can also be used at the community scale to describe the overall structure of a food web such as food web length or trophic niche space [START_REF] Bearhop | Determining trophic niche width: a novel approach using stable isotope analysis[END_REF][START_REF] Cornwell | A trait-based test for habitat filtering: Convex hull volume[END_REF], Layman et al. 2007a), for example providing valuable information on the health of the system (Layman et al. 2007b[START_REF] Zambrano | Stable isotope variation of a highly heterogeneous freshwater system[END_REF]. However stable isotopes (mostly C and N) have been more

specifically used to disentangle trophic pathways in coastal systems by ranking the organic matter sources supporting food webs, and to provide estimates of food sources contributions to consumers' diet. When wisely used (but see recommendations in Fry 2013), the development of mixing models has greatly enhanced the power in isotopic tools [START_REF] Phillips | Source partitioning using stable isotopes: Coping with too many sources[END_REF][START_REF] Parnell | Source partitioning using stable isotopes: Coping with too much variation[END_REF].

Some benthic coastal systems are considered to have particularly high ecological value [START_REF] Wright | The concept of organisms as ecosystem engineers ten years on: Progress, limitations and challenges[END_REF][START_REF] Voultsiadou | The habitat engineering tunicate Microcosmus sabatieri (Roule, 1885) and its associated peracarid epifauna[END_REF], and these are often habitats which are physically modified by engineer species (as defined in [START_REF] Jones | Organisms as ecosystem engineers[END_REF]. Mangroves, seagrasses or coral reefs habitats have been shown not only to enhance the local biodiversity of benthic and pelagic species but also to modify the availability of resources for others species [START_REF] Berkenbusch | An examination of the spatial and temporal generality of the influence of ecosystem engineers on the composition of associated assemblages[END_REF]. Ecosystems engineers often are key species in ecosystem functioning. While their effects on the diversity of benthic communities have been widely investigated (see review and references in [START_REF] Reise | Coastal habitat engineers and the biodiversity in marine sediments[END_REF], only few studies have paid attention to the consequences of ecosystem engineers on the structure of the marine food

webs. Yet, ecosystem engineers can induce changes in the intensity of biological interactions (mainly predation-prey interactions) [START_REF] Irlandi | Modification of animal habitat by large plants: mechanisms by which seagrasses influence clam growth[END_REF] or can alter the food supply to other organisms (Allen & Williams 2003) thus having major consequences on flows of organic pathways in benthic coastal systems.

In South Brittany (France), a benthic community dominated by the gregarious tubicolous Ampeliscid Haploops nirae forms dense tube mats in two main coastal ecosystems, namely the Bay of Concarneau and the Bay of Vilaine [START_REF] Rigolet | Benthic control freaks: Effects of the tubiculous amphipod Haploops nirae on structural and functional diversity of benthic communities[END_REF]. As for many gregarious species, Haploops nirae is the most abundant species and formsdense populations, with average densities ranging between about 1 000 and 25 000 ind.m -² [START_REF] Rigolet | Life history and secondary production of the amphipod Haploops nirae (Kaim-Malka, 1976) in the Bay of Concarneau (South Brittany)[END_REF]. Whilst the presence of a Haploops community was reported during the first description of shallow benthic communities of South Brittany [START_REF] Glémarec | Les peuplements benthiques du plateau continental Nord-Gascogne[END_REF], recent acoustic surveys showed a significant increase in area of cover [START_REF] Ehrhold | The REBENT monitoring network, a spatially integrated, acoustic approach to surveying nearshore macrobenthic habitats: application to the Bay of Concarneau (South Brittany, France)[END_REF]. For example, the mapping of the Bay of Concarneau revealed a 5-fold increase in Haploops nirae habitat surface between 1963 (650 ha in [START_REF] Glémarec | Les peuplements benthiques du plateau continental Nord-Gascogne[END_REF]) and 2003(3680 ha in Ehrhold et al., 2006): Haploops are then colonizing the adjacent habitat. By investigating the consequences of this expansion on the benthic habitats, [START_REF] Rigolet | Benthic control freaks: Effects of the tubiculous amphipod Haploops nirae on structural and functional diversity of benthic communities[END_REF] showed that Haploops significantly modify sediment features (e.g. change in granulometry, increase in C and N organic content) and therefore deeply affect species diversity and benthic composition by creating a unusual and diverse macrofaunal assemblage. Analysis of species composition revealed that Haploops community shared very few species with surrounding soft-sediment communities. According to its effects on the physical and chemical properties of the sedimentary habitat, Haploops nirae can be defined as an ecosystem engineer [START_REF] Reise | Coastal habitat engineers and the biodiversity in marine sediments[END_REF]). However, very little is known about its direct effect on food web functioning (e.g.

resource availability) and whether changes in species composition can indirectly affect trophic pathways and food web structure. Several studies provided evidence that dense populations of Ampeliscids could play major roles in energy transfer in some marine systems, not only because they can affect benthic-pelagic coupling and have a major grazing impact on phytoplankton production (Grebmeier & Mc Roy 1989[START_REF] Rigolet | Group sweeping: Feeding activity and filtration rate in the tubicolous amphipod Haploops nirae (Kaim-Malka, 1976)[END_REF]) but also because they export a high production towards higher trophic levels [START_REF] Franz | Secondary production of the amphipod Ampelisca abdita Mills and its importance in the diet of juvenile winter flounder (Pleuronectes americanus) in Jamaica Bay, New York[END_REF][START_REF] Highsmith | Productivity of arctic amphipods relative to gray whale energy requirements[END_REF]. In this study, we sampled the diversity of benthic organisms associated with the Haploops habitat and the adjacent uncolonised Amphiura habitat over two seasons. We used stable C and N isotopes to investigate whether drastic changes in species composition also cause changes in food web structure and to identify the main trophic pathways and main food sources that support the macrofauna assemblages in these systems.

More specifically, we aim to address here the following questions: (1) how the expansion of an engineer species, i.e. Haploops nirae, influences the food web structure of benthic habitats and (2) how the presence of Haploops nirae controls the availability of food source within and among habitats.

MATERIALS AND METHODS

Study area and sampling strategy. This study was conducted in the Bay of Concarneau, situated in the Northern part of the Bay of Biscay (France) (Fig. 2). This area is sheltered by a succession of rocky islets and is characterized by soft-bottom substrata, spanning from muddy to muddy-sands, with a depth ranging from 15 to 35 m (Menesguen 1980[START_REF] Ehrhold | The REBENT monitoring network, a spatially integrated, acoustic approach to surveying nearshore macrobenthic habitats: application to the Bay of Concarneau (South Brittany, France)[END_REF]. The western part of the Bay (Mouton Islets and Glénan Islands) is composed of muddy sands and sandy muds with benthic community dominated by the echinoderm Amphiura filiformis. The central part of the bay -where currents are strongly reduced -is composed of pure mud supporting a dense population of the tubiculous amphipod Haploops nirae.

To investigate the effects of Haploops tube mats on benthic trophic structure, we compared the food web structure of the Haploops nirae habitat (hereafter Haploops habitat)

with the adjacent benthic and uncolonized Amphiura filiformis habitat (hereafter Amphiura habitat). Using a mapping survey of the seabed in the Bay of Concarneau based on geoacoustic approaches and complementary benthic biological grab samples [START_REF] Ehrhold | The REBENT monitoring network, a spatially integrated, acoustic approach to surveying nearshore macrobenthic habitats: application to the Bay of Concarneau (South Brittany, France)[END_REF], we selected 6 stations distributed along two north-south transects reflecting an inshoreoffshore gradient (Fig. 2). The six stations were sampled during two contrasting seasons: in winter (February 2010) when food inputs were particularly low and when the benthic fauna has a low metabolic activity, and in summer (August 2010) at the end of a period characterized by high supply of pelagic production in this area.

Samples collection and laboratory processes.

To investigate the trophic structure within each habitat (i.e. Haploops and Amphiura habitats), we collected the largest possible diversity of macrofaunal and megafaunal organisms for isotopic analyses at all stations of each community-transect and for the two seasons. Macrofauna (individual size: 1-10 mm) was sampled using a 0.1 m² Van Veen grab (at least 2 grabs per station) and megafauna (individual size > 10 mm) using a modified beam trawl (width = 2 m, sampled surface = ca. 2300 m²) [START_REF] Desaunay | Changes in occurrence and abundance of northern/southern flatfishes over a 20-year period in a coastal nursery area (Bay of Vilaine) and on the eastern continental shelf of the Bay of Biscay[END_REF] with one trawl at each station.

All collected benthic organisms were carefully sorted on board and kept frozen. In the laboratory, organisms were identified to the lowest taxonomic level (i.e. generally at the species level). Species names were checked using the World Register of Marine Species database (updated check 28/10/2013). Isotopic analyses were then performed on muscle tissue samples for megafauna and large macrofauna. The whole body (gut-dissected) was used for smaller species. In rare instances, several individuals were pooled to match the minimum weight required for stable isotope analyses. All samples were rinsed with Milli-Q water and freeze-dried. Three replicates were analyzed. For calcified organisms (crustaceans and echinoderms), a sub-sample was acidified (10% HCl) to remove any inorganic carbonates, then rinsed with distilled water and freeze-dried again for 13 C values (a sub-sample was left untreated for 15 N value). We used the mean isotopic value of organisms for each habitat by pooling organisms found in 2 or 3 stations of each transect. The mean isotopic value was considered to be a better estimate of the whole habitat and account for any spatial variability.

All potential sources of organic matter available for the benthic fauna were collected for isotopic analyses. Terrestrial inputs of organic matter (TOM) are very limited in this area as it receives freshwater inputs from small rivers only (i.e. mean monthly discharge < 1 m 3 s -1 ).

Because of a reduced light input, subtidal soft-bottom systems are commonly assumed to host very low benthic primary production, especially in muddy turbid waters (Le [START_REF] Loc | Benthic community and food web structure on the continental shelf of the Bay of Biscay (North Eastern Atlantic) revealed by stable isotopes analysis[END_REF]. Consequently, most of the organic matter (OM) available for benthic primary consumers was supposed to originate from the suspended particulate organic matter (POM)

(composed mainly of phytoplankton) sedimenting from upper water layers and from the sediment organic matter (SOM). However, close observations of Haploops tubes revealed that the surface of tubes was covered with brown epibionts (hereafter EPI) especially during spring blooms, identified as benthic diatoms mats that belong to the genus Navicula, (identification Siano R., pers. com.) (Fig. 1). Benthic diatoms attached to Haploops tubes

were also considered as a potential food source. In addition, macroalgal fragments and detritus originating from adjacent rocky islets (i.e. Glenan and Moutons) were caught in some trawls and were also considered as a potential food source for benthic organisms, even though sampled stations are below the photic limit required by attached macroalgae to grow (ca. 35m deep).

Freshwater was collected at one site located at the mouth of one small river for TOM.

Marine surface water was collected using Niskin bottles for POM analysis at stations along an inshore-offshore transect (Fig. 2) in between the two habitats which shared the same water masses. Water samples were prefiltered on a 100 µm mesh to remove small animals such as zooplankton and then filtered on pre-combusted GFF filters (4h, 550°C). Half of GFF filters were acidified with 10N HCl fumes to remove traces of inorganic carbonates. Sediment organic matter (SOM) was sampled at each of the 6 stations using a Reineck corer to extract undisturbed sediment core: from each core, 2 sub-cores were extracted to analyse both the isotopic composition of the organic matter and the chlorophyll a and phaeopigments content.

For the 3 Haploops stations (H1, H2 and H3, Fig. 2), the surface of several tubes were gently scrapped with a razor blade under a dissecting scope and collected with distilled water. For both SOM and EPI samples, a sub-sample was acidified (10% HCl) to remove inorganic carbonates ( 13 C) and the rest was left untreated ( 15 N). Molar C:N ratio were calculated from the %C and %N values obtained from acidified and untreated sediment samples, respectively.

Chlorophyll a and phaeopigments content were estimated using the monochromatic technique [START_REF] Lorenzen | Determination of chlorophyll and phaeopigment spectrophotometric equations[END_REF] as described in [START_REF] Aminot | Hydrologie des écosystèmes marins. Paramètres et analyses[END_REF]. Finally, brown, red and green stranded macroalgae were collected from trawls in summer only, as very little macroalgae was noticed during winter.

Isotopic analyses. After freeze-drying, each sample was ground to a homogeneous powder and 1 mg was weighed in tin capsules for isotopic analyses. The isotopic composition of carbon (δ 13 C) and nitrogen (δ 15 N) was then measured with a stable isotope ratio mass spectrometer Finnigan MAT Delta Plus, operating in continuous-flow method, coupled to an elemental analyser Carlo Erba NC2500 (Cornell University, Stable Isotope Laboratory, New York). Isotopic ratios for carbon and nitrogen were expressed using the standard δ notation according to the following equation:

δX= ((R sample /R reference ) -1) × 1000 (‰)
where X= 13 C (carbon) or 15 N (nitrogen) and R= 13 C/ 12 C for carbon and 15 N/ 14 N for nitrogen.

The reference for carbon was Vienna Pee Dee Belemnite (VPDB, δ 13 C=0 ‰) and for nitrogen was atmospheric nitrogen (δ 15 N=0‰). The analytical precision was 0.2 ‰ for both nitrogen and carbon.

Data analyses. To provide a detailed description of the structure of the food web associated with both habitats, macrofaunal and megafaunal species were classified into five trophic groups: suspension-feeders (SF), surface deposit-feeders (SDF), subsurface deposit-feeders (SSDF), omnivores-carnivores (C-O) and grazers (G). Trophic information on species were gathered from several scientific publications and from the Biological Traits Information

Catalogue developed by the Marine Life Information Network (www.marlin.ac.uk/biotic).

The nitrogen isotope ratio is commonly used to estimate the trophic position of aquatic consumers [START_REF] Cabana | Comparison of aquatic food chains using nitrogen isotopes[END_REF][START_REF] Vander Zanden | Comparing trophic position of freshwater littoral fish species using stable nitrogen isotopes (δ 15 N) and literature dietary data[END_REF][START_REF] Post | Using stable isotopes to estimate trophic position: Models, methods, and assumptions[END_REF] and organisms tend to fractionate nitrogen positively by 3-4 ‰ [START_REF] Minagawa | Stepwise enrichment of 15 N along food chains: Further evidence and the relation between δ 15 N and animal age[END_REF][START_REF] Post | Using stable isotopes to estimate trophic position: Models, methods, and assumptions[END_REF]. In this context, Vander [START_REF] Vander Zanden | Variation in δ 15 N and δ 13 C trophic fractionation: Implications for aquatic food web studies[END_REF] showed that the δ 15 N baseline used to determine the trophic level of consumers is of primary importance. They calculated that the error variance in consumer trophic position was three times larger when using primary producers rather than primary consumers as baseline indicator organisms. In this perspective, we considered primary consumers as time (season) integrators of the primary producer isotopic variability and the primary consumer Haploops nirae that feeds on phytoplankton [START_REF] Rigolet | Group sweeping: Feeding activity and filtration rate in the tubicolous amphipod Haploops nirae (Kaim-Malka, 1976)[END_REF] was used as isotopic baseline. As a result, consumer trophic position can be calculated using the following formula:

Trophic Level = (δ 15 N consumerδ 15 N mean Haploops )/3.4 + 2 where 3.4 ‰ is the assumed mean 15 N trophic-enrichment factor according to [START_REF] Minagawa | Stepwise enrichment of 15 N along food chains: Further evidence and the relation between δ 15 N and animal age[END_REF]. Primary, secondary and tertiary consumers were thus designated as the second, third and fourth trophic levels respectively.

Numerous approaches to solve isotopic mixing models and quantify the relative contributions of primary producers to species diets have been developed (see review in [START_REF] Layman | Applying stable isotopes to examine food-web structure: an overview of analytical tools[END_REF]. Conventional linear mixing models such as IsoError and IsoSource [START_REF] Phillips | Source partitioning using stable isotopes: Coping with too many sources[END_REF][START_REF] Phillips | Converting isotope values to diet composition: The use of mixing models[END_REF]) calculate the contribution of food resources to a consumer's diet using a set of mass-balance equations. However, such mixing models cannot incorporate uncertainties and variations in the isotopic compositions of prey and consumers and most importantly, the outputs from these models represent a range of feasible solutions, with no quantification as to which solutions are most likely [START_REF] Parnell | Source partitioning using stable isotopes: Coping with too much variation[END_REF]. Quite recently, Bayesian mixing models have been proposed to circumvent many limitations of the linear models. They are also based on a set of linear equations that use Bayesian statistical techniques to identify the relative contributions of different sources but they allow variability in input terms. Furthermore, outputs from the Bayesian models are in the form of true probability distributions and not a list of all feasible solutions [START_REF] Parnell | Source partitioning using stable isotopes: Coping with too much variation[END_REF]). In the present study, we ran Bayesian mixing models with the package SIAR (Stable Isotope Analysis in R) of the free software R, version 2.11.1 (R Development Core Team 2010) using default parameters. For a more comprehensive output, the model was performed only on species that account for the greatest part of the benthic biomass (i.e. 80 %), i.e. 6 species on the Amphiura habitat and 5 species on the Haploops habitat.Biomass data (not presented here) were obtained using grab samples collected in each habitat, from which macrofauna (> 1 mm) was identified and weighed (ash-free dry weight). The relative contribution of each species to the total habitat biomass was then calculated and used to select species (Rigolet et al, in prep). The model was run for summer isotopic compositions only as it has been showed that mixing models results from winter period in temperate regions were irrelevant since turnover in organisms' tissues was close to zero [START_REF] Leal | Stable isotopes (δ 13 C, δ 15 N) and modelling as tools to estimate the trophic ecology of cultivated oysters in two contrasting environments[END_REF]) and food assimilation ultimately negligible. Two variables (δ 13 C and δ 15 N) were used and three sources (POM, SOM and EPI) were considered for the computation of the model, as a posteriori knowledge of isotopic signature and relative importance of macroalgae and TOM sources were likely too low to be integrated in the mixing model and would otherwise increase uncertainties in model outputs [START_REF] Fry | Alternative approaches for solving underdetermined isotope mixing problems[END_REF].

Mixing models require an a priori estimate of the enrichment in δ 15 N and δ 13 C values between preys and predators. Variations in fractionation values contributed to the major part of the error variance in mixing model outputs [START_REF] Phillips | Source partitioning using stable isotopes: Coping with too many sources[END_REF][START_REF] Fry | Alternative approaches for solving underdetermined isotope mixing problems[END_REF]). While it is commonly assumed that the δ 15 N and the δ 13 C values of a consumer are typically enriched by 3.4 ‰ and 1 ‰ respectively [START_REF] Minagawa | Stepwise enrichment of 15 N along food chains: Further evidence and the relation between δ 15 N and animal age[END_REF], recent reviews indicated that fractionation values are affected by multiple factors, such as food quality, tissues turnover, environmental conditions and even taxonomic group [START_REF] Vander Zanden | Variation in δ 15 N and δ 13 C trophic fractionation: Implications for aquatic food web studies[END_REF][START_REF] Vanderklift | Sources of variation in consumer-diet delta 15 N enrichment: A meta-analysis[END_REF][START_REF] Mccutchan | Variation in trophic shift for stable isotope ratios of carbon, nitrogen, and sulfur[END_REF]. [START_REF] Caut | Variation in discrimination factors (Δ 15 N and Δ 13 C): the effect of diet isotopic values and applications for diet reconstruction[END_REF] reviewed carbon and nitrogen isotopic discrimination factors from the literature and suggested linear models to estimate discrimination factors according to the signature of food sources. We calculated a trophic enrichment factor (TEF) for each source following the equations provided by [START_REF] Caut | Variation in discrimination factors (Δ 15 N and Δ 13 C): the effect of diet isotopic values and applications for diet reconstruction[END_REF] for invertebrates. We used source-specific TEF for the POM (4.04 ± 0.20 ‰ for δ 15 N and 2.18 ± 0.11‰ for δ 13 C), for the SOM (3.32 ± 0.12 ‰ for δ 15 N and 1.68 ± 0.02 ‰ for δ 13 C) and for the EPI (3.03 ± 0.05 ‰ for δ 15 N and 0.05 ± 0.07 ‰ for δ 13 C). For secondary consumers, trophic enrichment factors were multiplied by the number of trophic levels between the basal resources and the consumer, as classically used for community approaches (e.g. [START_REF] Phillips | Converting isotope values to diet composition: The use of mixing models[END_REF][START_REF] Colombo | Trophic structure of vermetid reef community: High trophic diversity at small spatial scales[END_REF]. Beca use we used a Bayesian approach, we were able to insert variability (standard deviation) in TEF values as well [START_REF] Parnell | Source partitioning using stable isotopes: Coping with too much variation[END_REF].

RESULTS

Food sources

Isotopic ratios of potential organic matter sources for the two transects and the two seasons are listed in Chlorophyll a and phaeopigments analyses revealed a much higher total content of primary producer pigments in the Haploops habitat than in the adjacent Amphiura habitat irrespective of the season (Table 2). More specifically, even though phaeopigments are about one order of magnitude higher than Chl. a, a significant content of Chl. a was found in

Haploops sediments in winter. The C:N ratio was lower in the Haploops habitat suggesting a higher quality of the available organic matter for benthic consumers.

Benthic habitat food web structures

The δ 13 C and δ 15 N values displayed by the benthic fauna for the two habitats in winter and summer are represented in Fig. 3A and3B, respectively (mean isotopic compositions of each species are provided as supplement, S1 for winter and S2 for summer). Graphically, three potential food sources (i.e. SOM, POM and EPI) are likely to support the primary consumers in the two communities. A few species showed particular isotopic signatures: in winter the sea slug Aplysia punctata (Fig. 3A) displayed very 13 the distribution between the two seasons. This was most noticeable for the Amphiura habitat though (Fig. 4).

When pooled into trophic functional groups, organisms' isotopic signatures revealed that the two habitats have very similar trophic structure. Indeed, the average values of both δ 13 C and δ 15 N for each trophic groups did not show significant differences between the two communities (MANOVA, p>0.05), irrespective of the season. Trophic groups are characterized by large within-group variations in δ 13 C values (Table 3). Suspension-feeders (SF) spanned consistently over the same ranges in δ 13 C values (4.44‰ to 4.85‰), not only between the two seasons but also between the two communities. For the two seasons and the two communities, carnivores-omnivores had the same mean δ 13 C range (4.47‰ to 4.87‰), except in the Haploops habitat in winter where carnivores-omnivores exhibited a larger δ 13 C range (i.e. 6.38 ‰). Unlike other trophic groups, surface deposit feeders (SDF) had no clear pattern in δ 13 C ranges and varied from 1.82 ‰ up to 4.51 ‰. The Haploops community is characterized for both seasons by a narrower δ 13 C range in subsurface deposit-feeders (SSDF) (mean 1.25‰) as compared to bare sediment community (mean 3.64‰). Variations in δ 15 N among trophic groups showed that subsurface deposit feeders values are consistently above TL = 3 and largely overlapping with carnivores-omnivores. The largest community variation is due to surface deposit-feeders in winter, exhibiting higher δ 15 N value in Haploops habitat (TL > 3) than in Amphiura habitat (TL < 3).

Comparison of isotopic values of species shared by the two communities

A total of 28 (36%) and 17 (18%) species were found in both habitats in winter and summer, respectively. Stable isotopes ratios of common species of the 2 habitats are represented in the same biplots (Fig. 5). Overall, the majority of species falls within the range of the 1:1 ratio, for the two seasons. However, some species exhibited isotopic differences between the two habitats. For example, in winter the δ 15 N value of the predators Eunice

Contribution of food sources to benthic food webs

Biomass data revealed that 5 and 6 species accounted for more than 80 % of the benthic fauna biomass in summer in the Haploops and Amphiura habitats, respectively. When plotted along with isotopic compositions, biomass revealed a complete different picture of the food web for each habitat (Fig. 6). In the Amphiura habitat, the suspension-feeders Amphiura seemed to be the largest contributors, with the smallest uncertainties for most suspensionfeeders. The phytoplankton contribution (POM) is however associated with large 95% confidence intervals but also significantly contributes to suspension-feeders' diets such as

Haploops nirae in the Haploops habitat or Dosinia lupinus and Chamelea striatula in the Amphiura habitat. The predator Sthenelais boa seemed to feed primarily on organisms which feed themselves primarily on POM. The contribution of organic matter from the sediment seemed to be less obvious from SIAR outputs, as it is also associated with large uncertainties.

DISCUSSION

In shallow subtidal zones, amphipod populations are common and can generate extensive tube mats that considerably increase local habitat complexity (Bellan-Santini and Dauvin, 1988;[START_REF] Franz | Secondary production of the amphipod Ampelisca abdita Mills and its importance in the diet of juvenile winter flounder (Pleuronectes americanus) in Jamaica Bay, New York[END_REF]Sudo and Azeta, 1996;Göransson, 2002).

Engineer species in general and tubiculous species in particular are known to affect species composition of soft-bottom communities [START_REF] Reise | Coastal habitat engineers and the biodiversity in marine sediments[END_REF]). [START_REF] Rigolet | Benthic control freaks: Effects of the tubiculous amphipod Haploops nirae on structural and functional diversity of benthic communities[END_REF] showed that the presence of Haploops nirae in the Bay of Concarneau (South Brittany) affects the local biodiversity and the associated species assemblages. By modifying local sedimentary features,

Haploops individuals create conditions for an unusual species assemblage to settle and develop, with species only found within Haploops community, including species new to science [START_REF] Myers | A new species of amphipod, Photis inornatus sp nov (Corophiidea, Photidae) from a 'Haploops community' in Brittany[END_REF]). To investigate whether drastic changes in species composition also cause changes in food web structure, we sampled the diversity of benthic organisms associated with the Haploops habitat and the adjacent uncolonised Amphiura habitat over two seasons.

Our results first showed that the overall isotopic food web structure was similar between the two habitats, since the isotopic spaces occupied by the species in the δ 13 C-δ 15 N biplots largely overlap. The distribution of species over a continuum of three trophic levels appears to be a common feature in temperate subtidal macrobenthic ecosystems. Indeed, Le [START_REF] Loc | Benthic community and food web structure on the continental shelf of the Bay of Biscay (North Eastern Atlantic) revealed by stable isotopes analysis[END_REF] for the muddy Nephrops habitat in the Bay of Biscay, [START_REF] Grall | Community structure and food web based on stable isotopes (δ 15 N and δ 13 C) analysis of a North Eastern Atlantic maerl bed[END_REF] for the north-eastern Atlantic maerl beds habitat or [START_REF] Carlier | Benthic trophic network in the Bay of Banyuls-sur-Mer (northwest Mediterranean, France): an assessment based on stable carbon and nitrogen isotopes analysis[END_REF] for a variety of softsediment habitats of the north-western Mediterranean Bay of Banyuls also reported food web structure of benthic macrofauna with three trophic levels. We also showed here that isotopic values for both primary and secondary consumers spread over a large range of δ 13 C values.

Such a large range of δ 13 C values within primary consumers is also considered a common feature of continental shelf communities [START_REF] Hobson | A stable isotope (δ 13 C, δ 15 N) model for the North Water food web: implications for evaluating trophodynamics and the flow of energy and contaminants[END_REF][START_REF] Loc | Benthic community and food web structure on the continental shelf of the Bay of Biscay (North Eastern Atlantic) revealed by stable isotopes analysis[END_REF], where a larger number of food sources are available. And yet, the δ 13 C ranges for subtidal shallow benthic habitats seem to remain stable between -14‰ and -22‰ for the whole macrofauna community [START_REF] Grall | Community structure and food web based on stable isotopes (δ 15 N and δ 13 C) analysis of a North Eastern Atlantic maerl bed[END_REF][START_REF] Carlier | Benthic trophic network in the Bay of Banyuls-sur-Mer (northwest Mediterranean, France): an assessment based on stable carbon and nitrogen isotopes analysis[END_REF][START_REF] Loc | Benthic community and food web structure on the continental shelf of the Bay of Biscay (North Eastern Atlantic) revealed by stable isotopes analysis[END_REF]. However, we have excluded species exhibiting unusual isotopic signatures from very specific diets, e.g. specialists of very 13 C-depleted red macroalgae such as Aplysia punctata (Carefoot 1985) or species which host symbiotic sulphur-oxydising bacteria in their gill tissue such as Thyasira flexuosa [START_REF] Dufour | Symbiont abundance in thyasirids (Bivalvia) is related to particulate food and sulphide availability[END_REF]). This depletion is likely due to the significant contribution of carbon fixation by symbionts in their food source [START_REF] Dando | Varying nutritional dependence of the thyasirid bivalves Thyasira sarsi and T. equalis on chemoautotrophic symbiotic bacteria, demonstrated by isotope ratios of tissue carbon and shell carbonate[END_REF]. It should be noted that between-habitat variations in organisms' isotopic compositions are not only due to intraspecific trophic plasticity, but are likely to be constrained within food sources' isotopic compositions at the ecosystem level.

Several studies reported that ampeliscidae can constitute the major prey for some toppredators marine mammals and finfishes [START_REF] Franz | Secondary production of the amphipod Ampelisca abdita Mills and its importance in the diet of juvenile winter flounder (Pleuronectes americanus) in Jamaica Bay, New York[END_REF][START_REF] Highsmith | Productivity of arctic amphipods relative to gray whale energy requirements[END_REF][START_REF] Cui | Feeding ecology of dominant groundfish in the northern Bering Sea[END_REF]. To our knowledge, few studies have paid special attention to the position of the ampeliscidae in a food web, and even less use stable isotopes to ascertain this. Haploops nirae occupied here the lowest δ 13 C and δ 15 N position in the isotopic biplot. Ampeliscidae always exhibit the lowest δ 15 N value in food web structures determined with stable isotopes, as showed from the muddy bottoms of the southern Tyrrhenian Sea (Western Mediterranean)

for Ampelisca spp. [START_REF] Fanelli | Trophodynamics of suprabenthic fauna on coastal muddy bottoms of the southern Tyrrhenian Sea (western Mediterranean)[END_REF]) to the South Orkneys island (Antarctic Peninsula) for A. richardsoni [START_REF] Nyssen | Trophic position of Antarctic amphipodsenhanced analysis by a 2-dimensional biomarker assay[END_REF]. We suggest here that this amphipod family in general, despite laboratory observations reporting strong trophic plasticity and changes in foraging behaviour (e.g. Mills 1967), can be used as isotopic trophic baseline for coastal marine and estuarine systems. This provides a viable alternative to the current practice of using long-live primary consumers like mussels as a trophic baseline [START_REF] Cabana | Comparison of aquatic food chains using nitrogen isotopes[END_REF].

Despite the strong bioturbating tube-building activity (over 10 000 tubes.m -2 , Rigolet et al. 2012) and the active grazing activity [START_REF] Rigolet | Group sweeping: Feeding activity and filtration rate in the tubicolous amphipod Haploops nirae (Kaim-Malka, 1976)[END_REF], Haploops individuals do not appear to affect the higher trophic levels. As suggested by [START_REF] Mills | The keystone-species concept in ecology and conservation[END_REF], the feeding activities of dense species greatly affect community features (such as sediment characteristics)

but do not necessarily have direct trophic effects on other species. Similarly, Jones et al.

(1997) suggested that many engineer species physically affect their environment in numerous ways but that not all of the changes have important (positive or negative) ecological consequences. Studies that compare food webs of engineered habitat with adjacent homogeneous bare sediments using stable isotopes are scarce (but see for example [START_REF] Dubois | Effects of oyster farming on macrofaunal assemblages associated with Lanice conchilega tubeworm populations: A trophic analysis using natural stable isotopes[END_REF]). Yet, [START_REF] Baeta | Eutrophication and trophic structure in response to the presence of the eelgrass Zostera noltii[END_REF] investigated whether the occurrence of the eelgrass Zostera marina changed the benthic and pelagic food web structures in comparison with uncolonized sediment. They showed no major difference in the structure of the benthic food web, which is consistent with our results in a different system. This result has strong implications in terms of trophic niche partitioning: as composition of species assemblages associated with Haploops habitat drastically differs from those in adjacent sediments [START_REF] Rigolet | Benthic control freaks: Effects of the tubiculous amphipod Haploops nirae on structural and functional diversity of benthic communities[END_REF], it means that each species or each group of species uniquely found in the Haploops habitat (ca. 70% of the species) has a trophic-equivalent species or group of species in the adjacent uncolonized sediments. As a result, deep changes in diversity and species composition do not always reflect changes in food web structure, even though the physical environment is remarkably affected by an engineer tubiculous species.

As measured with the δ 15 N ranges, the length of the benthic food chain in the Haploops habitat did not differ from the length in the adjacent Amphiura habitat. Both food chains spanned over a continuum of almost three trophic levels, defined according to an isotopic baseline [START_REF] Post | Using stable isotopes to estimate trophic position: Models, methods, and assumptions[END_REF]. Food web length in aquatic systems influences ecosystem functioning and food-web stability [START_REF] Long | Food chain length and omnivory determine the stability of a marine subtidal food web[END_REF] but it is also recognized that longer food webs are predicted in more productive systems because more energy can be transferred between trophic levels [START_REF] Morin | Food web architecture and population dynamics: Theory and empirical evidence[END_REF]. Secondary production is not significantly different between Haploops habitats and surrounding Amphiura habitats [START_REF] Rigolet | Life history and secondary production of the amphipod Haploops nirae (Kaim-Malka, 1976) in the Bay of Concarneau (South Brittany)[END_REF], further suggesting a difference in food chain length would be unlikely. However, it is worth noticing differences in the frequencies distributions of δ 15 N compositions between the Amphiura and the Haploops habitats. The bi-modal shape of the distribution of the Haploops habitat revealed that food chain levels between primary and secondary consumers are well discriminated in Haploops habitat, possibly indicating less trophic plasticity and less diet variations at each consumer level, as well as more omnivorous species in the Amphiura habitat, with potential consequences on the food-web properties of each habitat. The influence of complexity and omnivory on the food-web dynamics was intensively debated during the last decades with contradictory results: recent empirical and theoretical developments have reported that omnivory in general and omnivorous links between species tend to have a stabilizing effect on the food web dynamics when trophic interactions are weak [START_REF] Emmerson | Weak interactions, omnivory and emergent food-web properties[END_REF]. With this in mind, we could hypothezised that Haploops food web, largely dominated in terms of abundance and biomass by a gregarious small tubiculous species, is less stable than the Amphiura food web and would ultimately take more time to return to equilibrium, should large changes in food sources occur.

In coastal shallow systems, the biomass and the secondary production of benthic consumers originate from a diversity of sources [START_REF] Peterson | Stable isotopes as tracers of organic matter input and transfer in benthic food webs: A review[END_REF]. For example, in the Bay of Concarneau, many sources are potentially available for the benthic fauna, such as particulate organic matter (POM) and sedimented organic matter (SOM), terrestrial inputs of organic matter (TOM) as well as macroalgae detritus from surroundings hard bottoms. It is commonly assumed that the subsurface POM (used as a proxy for phytoplankton) directly or indirectly (after sedimentation and resuspension) is the major contribution to the benthic primary consumers in subtidal environments [START_REF] Grall | Community structure and food web based on stable isotopes (δ 15 N and δ 13 C) analysis of a North Eastern Atlantic maerl bed[END_REF][START_REF] Loc | Benthic community and food web structure on the continental shelf of the Bay of Biscay (North Eastern Atlantic) revealed by stable isotopes analysis[END_REF]). However, a growing body of literature recently focused on subtidal benthic primary production in subtidal sedimentary habitats (see review in Cahoon 1999). For example, [START_REF] Grippo | Spatial variation in basal resources supporting benthic food webs revealed for the inner continental shelf[END_REF] investigated the microphytobenthos as potential support of benthic food web in Louisiana shoals (Gulf of Mexico) and revealed that subtidal sandy dunes (peaking between 5 and 10 m depth) are supporting a secondary benthic production during spring and summer times. In temperate shallow subtidal sedimentary systems of the Wadden Sea (ca. 2 m depth), 13 Clabelling in situ experiences revealed the pivotal role of autochtonous MPB production vs.

allochtonous phytoplankton in benthic consumers diets [START_REF] Evrard | Importance of phytodetritus and microphytobenthos for heterotrophs in a shallow subtidal sandy sediment[END_REF]) and within the bulk MPB, benthic consumers preferentially select benthic diatoms and benthic cyanobacteria [START_REF] Evrard | Carbon and nitrogen flows through the benthic food web of a photic subtidal sandy sediment[END_REF]). An extensive survey of the subtidal MPB production was carried out in the Bay of Brest (Brittany, France) between 5 and 13 meters deep [START_REF] Ni-Longphuirt | Primary production and spatial distribution of subtidal microphytobenthos in a temperate coastal system, the Bay of Brest, France[END_REF]).

These authors highlighted that the MPB production represented up to 20% of total primary production, with the highest MPB biomass in bare muddy sediments. Our results from sediment analyses showed that even during the winter season, and despite a higher turbidity due to wave activity, Haploops sediment contains Chl. a. This probably indicates benthic primary production in Haploops sediments, supported by a low C:N ratio close to what was expected for marine microalgae (i.e. C:N = 6.6 ; [START_REF] Redfield | The influence of organisms on the composition of seawater[END_REF]. Visual observations showed that Haploops tube can be a physical support for epiphytes in general and benthic diatoms mats in particular (Navicula sp.) (Fig. 1). The δ 13 C value from the tube epibionts falls perfectly within the expected value of microphytobenthos in summer (ca. -13‰) (Riera & Richard 1996[START_REF] Leal | Stable isotopes (δ 13 C, δ 15 N) and modelling as tools to estimate the trophic ecology of cultivated oysters in two contrasting environments[END_REF][START_REF] Grippo | Spatial variation in basal resources supporting benthic food webs revealed for the inner continental shelf[END_REF]). However in winter (higher turbidity and lower luminosity), tube scrapings revealed a δ 13 C value similar to the sedimented organic matter, suggesting an absence of growing mats of primary producers on Haploops tubes. Yet SIAR Bayesian mixing models incorporate isotope and fractionation uncertainty in the development of posterior probability distributions of sources contributions. As a result, model outputs include a margin of error that can be high when too many food sources are incorporated or when consumer's isotopic composition do not perfectly fall within the isotopic space defined by selected food sources [START_REF] Fry | Alternative approaches for solving underdetermined isotope mixing problems[END_REF]. Since SOM and POM are not well isotopically discriminated in this study, results of the mixing model are associated with high uncertainty for those two food sources. Epibionts (benthic diatoms) exhibited very significantly enriched δ 13 C value (-13.19 ± 0.34 ‰) and their contribution to the diet to the species with the most enriched δ 13 C values is undoubted. The species with the highest δ 13 C value (-11.64 ± 0.2 ‰) is indeed the micrograzer Geitodoris planata, likely feeding solely on the epibionts of Haploops tubes. Also, many species can benefit from this food source, especially deposit-feeders which abundances are indeed greater in the Haploops habitat [START_REF] Rigolet | Benthic control freaks: Effects of the tubiculous amphipod Haploops nirae on structural and functional diversity of benthic communities[END_REF], or suspension-feeding species if benthic diatoms are resuspended.

Interestingly, while investigating isotopic signatures of benthic invertebrates in a coastalestuarine gradient, Koop et al. ( 2013) noticed an unexpected enrichment in organisms' δ 13 C isotopic composition only in several offshore stations (Bay of Biscay, France). These stations were actually colonized by Haploops sp. and our findings suggest a contribution of MPB to the macrofauna diet.

Our understanding of food web structure can be very different depending on whether one considers all the species or only the species that account for most of the biomass.

Those species play a key functional role in trophic webs [START_REF] Villéger | New multidimensional functional diversity indices for a multifaceted framework in functional ecology[END_REF]. From the species contributing the most to the total biomass in each community, Bayesian mixing models revealed that epibionts can contribute to 50% to the diet of suspension-feeders such as

Polititapes virgineus or Amphiura filiformis, while other suspension-feeders rely mainly on particulate organic matter (Haploops nirae), suggesting differences in food selectivity among filter-feeding organisms but also a limited competition within the suspension-feeders among the Haploops habitat. The bivalve P. virgineus is only found in Haploops habitat and represents one of the largest biomass. [START_REF] Mackenzie | Ampelisca amphipod tube mats may enhance abundance of northern quahogs Mercenaria mercenaria in muddy sediments[END_REF] similarly reported high biomass of bivalves Mercenaria mercenaria in Ampelisca mats of the southeastern Raritan Bay (New Jersey). We suggest that the Haploops habitat offers a broader diversity and quantity in food sources, and that the Haploops tubes act as a substrate for MPB species. These results are supported by several mesocosm experiments designed to disentangle impacts of species assemblages and environmental characteristics on MPB biomass and production. For example, [START_REF] Dyson | Influence of macrofaunal assemblages and environmental heterogeneity on microphytobenthic production in experimental systems[END_REF] showed that the sedimentary patchiness in organic matter enrichment affects macrofaunal (diversity and abundance) distribution which in turn affects MPB production. With similar experiments, [START_REF] Hicks | Impact of biodiversity-climate futures on primary production and metabolism in a model benthic estuarine system[END_REF] showed that changes in grazing invertebrate assemblages are likely to influence MPB biomass. These investigations highlighted that in complex systems, such as natural habitats engineered by a tubiculous species, changes in species composition could fundamentally influence the resource provision and its utilisation, and ultimately the equilibrium between autotrophic and heterotrophic production.

Benthic organisms from adjacent habitats largely benefit from benthic primary production in Haploops sediments, as the overall isotopic space is similar between the two habitats (see convex hull of species isotopic composition in Figs. 2 and3) and the isotopic δ 13 C signature of the SOM in adjacent sediments did not show enriched δ 13 C values. Despite very low benthic primary production in the adjacent Amphiura habitat, resuspended microbenthic algae are the major component of the diet of the most dominant species in the bare sediment, the amphiurid Amphiura filiformi, and significantly support the secondary production of the bare sediment. Haploops habitats -and potentially all large tube mats in muddy coastal systems -appear to be food exporters to adjacent communities. This study reveals that benthic habitat engineered by species potentially have strong trophic connections with surrounding other habitats and can -because of their biological activity -export primary production. This supports the definition of engineer species not only as species modifying diversity and species assemblages but as species with the capacity to directly or indirectly control resources for others species (Allen & Williams 2003).

FIGURES CAPTIONS

  C-depleted values(-29.41 ‰ and -30.81 ‰ in both Amphiura and Haploops habitats) and appeared to rely on 13 C-depleted red macroalgae (such as Palmaria palmata, Hypoglossum hypoglossoides and Dasya corymbifera). The polychaete Nereis sp. (δ 13 C = -24.71 ‰), seemed to partly feed on 13 Cdepleted red macroalgae too. In summer, the nudibranch Geitodoris planata displayed 13 Cenriched values (Fig.3B) in both habitats (δ 13 C = -11.83 ‰ and -11.64 ‰ in Amphiura and Haploops habitat, respectively). The bivalve Thyasira flexuosa (not plotted in Figs.2 and 3) displayed strongly depleted δ 13 C (-26.40 ‰) and δ 15 N values (-0.3 ‰). The absolute frequencies of δ 13 C and δ 15 N compositions displayed by all sampled species in the two habitats in winter and summer are represented in Fig. 4. Apart from a few species with particular isotopic values (namely Aplysia punctata, Nereis sp., Thyasira flexuosa and Geitodoris planata), the frequency distributions exhibited a similar range in δ 13 C values, with winter compositions spanning from -21.63 ‰ (Psammechinus miliaris) to -14.75 ‰ (Aphrodita aculeata) in Amphiura habitat and from -20.48 ‰ (Schistomeringos rudolphii) to -14.86 ‰ (Buccinum undatum) in the Haploops habitat. In summer, δ 13 C isotopic values displayed the same pattern with values ranging from -20.91 ‰ (Ampelisca spinipes) to -13.98 ‰ (Philine aperta) in the Amphiura habitat and from -20.43 ‰ (Ampelisca typica) to -14.26 ‰ (Luidia ciliaris) in the Haploops habitat. Frequency distributions of δ 15 N values exhibited close variations in the two habitats as well, ranging from 7.17 ‰ (Ampharete finmarchica) to 14.43 ‰ (Glycera unicornis) in winter in the Amphiura habitat and from 7.02 ‰ (Haploops nirae) to 13.28 ‰ (Maja squinado) in the Haploops habitat. In summer, δ 15 N values showed almost identical variations with δ 15 N values ranging from 6.93 ‰ (Cultellus pellucidus) to 13.45 ‰ (Luidia ciliaris) in the Amphiura habitat and from 6.40 ‰ (Timoclea ovata) to 15.24 ‰ (Glycera unicornis) in the Haploops community. The distributions of δ 15 N values appeared to be bi-modal in winter and summer for the Haploops habitat, but less structured for the Amphiura habitat. As for the distributions of δ 13 C values, calculations of the Kurtosis index revealed a decrease between winter and summer for the two habitats (7.4 to 4.1 for the Amphiura habitat and 3.4 to 3.3 for the Haploops habitat), indicating a sign of flattening in

  vittata and Liocarcinus pusillus and the suspension-feeder Pecten maximus were lower in the Haploops habitat. Conversely, the predator Inachus dorsettensis and the surface depositfeeder Terebellides stroemi exhibited higher δ 15 N values in Haploops habitat. As for winter δ 13 C values, common species tend to be 13 C-enriched in the Haploops habitat, as evidenced by the two suspension-feeders Pecten maximus and Anapagurus hyndmanni as well as the deposit-feeder Oestergrenia digitata and the predators Eunice vittata and Natatolana neglecta. The same pattern is observed in summer. Several species showed enriched δ 15 N values in the Haploops habitat (Scoletoma fragilis and Sabellaria spinulosa) while others are (Orbinia cuvieri and Maldane glebifex) were 13 C-depleted in the Haploops habitat while the suspension-feeder Crepidula fornicata displayed slightly enriched δ 13 C values in the Haploops habitat.

  filiformis, Thyone fusus, Dosinia lupinus and Chamelea striatula made up for the bulk of the benthic biomass. The subsurface deposit-feeder Maldane glebifex and the surface depositfeeder Aspidosiphon muelleri accounted in a lesser extent for the biomass in this habitat. In the Haploops habitat, the amphipod Haploops nirae and the bivalve Polititapes virgineus each accounted for 35 % of the biomass of the benthic fauna. The other main suspensionfeeder Turritella communis as well as the surface deposit-feeder Aspidosiphon muelleri and the predator Sthenelais boa accounted for the remaining benthic biomass in the Haploops habitat.Boxplots showing results of the Bayesian mixing models computed on species that accounted for 80% of the biomass in summer are shown in Fig.7. Overall, the 2 communities exhibited similar patterns in food sources contributions. Even though large uncertainties hampered accurate quantifications, epiphytes (represented by benthic diatoms Navicula sp.)

a

  low Chl. a concentration indicates a residual growing population of microalgae. Haploops tubes are made of mud, with pseudofeces and mucus secreted by individuals[START_REF] Rigolet | Group sweeping: Feeding activity and filtration rate in the tubicolous amphipod Haploops nirae (Kaim-Malka, 1976)[END_REF]) and potentially offer a high quality habitat of polysaccharides for benthic diatom to grow. Benthic diatoms (including Navicula species) can compensate for low nutrients or luminosity with adaptations such as mixotrophy[START_REF] Cloern | Phytoplankton community ecology: principles applied in San Francisco Bay[END_REF][START_REF] Ross | Phytoplankton motility and the competition for nutrients in the thermocline[END_REF]. For example,Barillé & Cognie (2000) showed that growth of benthic diatoms was stimulated by oyster pseudofaeces, enriching sediment in mucopolysaccharides which diatom can exploit. Carbohydrates derived from Haploops tubes can likely be used by diatoms to grow under limited light condition.

Fig. 1 .

 1 Fig. 1. Top picture: close up of several Haploops tubes, protruding a few centimetres above

Fig. 2

 2 Fig. 2

  Fig. 3 920

Fig. 5

 5 Fig. 5

Fig. 6

 6 Fig. 6

  Fig. 7

1015

  Supplement S2. Mean (± SD) isotopic compositions (δ 13 C and δ 15 N) of all species sampled in summer for both Amphiura and Haploops habitats. TG = Trophic Group, n = number of samples, TP = Trophic Position calculated after Post 2002 using Haploops nirae as a trophic baseline. Information on trophic groups gathered from the Biological Traits Information Catalogue developed by the Marine Life Information Network, as C-O = Carnivorous and/or 1020 Omnivorous, SF = Suspension-Feeders, SDF = Surface Deposit-Feeders, SSDF = Sub-Surface Deposit-Feeders, G = Grazers or micro-Grazers. Species names were checked using world register of marine species database on November 28 th 2013

Table 1 .

 1 The δ 13 C of potential food sources were well discriminated and varied from -28.58 ‰ (TOM) to -21.14 ‰ (SOM) in winter, and from -34.29 ‰ (Dasya corymbifera, a red alga) to -13.19 ‰ (epibionts on Haploops tubes) in summer. The potential food sources displayed a δ 13 C range of 7.4 ‰ in winter which increased to 21.1‰ in summer mainly due to the 13 C-enrichment of benthic diatoms collected on the Haploops tubes

	in winter, it was strongly 13 C-enriched in summer. The δ 15 N values of potential food sources
	spanned from 3.06 ‰ to 7.67 ‰ in winter and from 4.14 ‰ to 7.21 ‰ in summer.

(epibionts) and the 13 C-depletion of benthic red macroalgae (Table

1

). Terrestrial inputs (TOM) δ 13 C values were depleted in 13 C in winter (-28.58 ‰) but were not sampled in summer (dry season) when the riverine flow was at its lowest. As for the phytoplankton (POM), δ 13 C values recorded in winter (-24.06 ‰) and summer (-23.73 ‰) did not significantly differ (t-test, p = 0.626). The isotopic signature of macroalgae displayed groupspecific isotopic ratios (δ 13 C) with strongly 13 C-depleted red algae (Palmaria palmata, Hypoglossum hypoglossoides, Dasya corymbifera) and more 13 C-enriched brown (Laminaria sp., Dictyota dichotoma) and green (Ulva sp.) macroalgae. Isotopic ratios of sedimented organic matter (SOM) exhibited the same values between bare sediments stations (sandy mud Amphiura habitat) and muddy Haploops habitat for both seasons (t-test, p = 0.508 in winter and p = 0.060 in summer). Therefore, all isotopic values for SOM were averaged into one single average isotopic value for the SOM. The isotopic signature of Haploops epibionts varied significantly among seasons (t-test, p < 0.001). While it is close to the SOM signature

Table 1 .

 1 Stable nitrogen and stable carbon isotope values (mean and standard deviation) of the potential organic matter (OM) sources in the Bay of Concarneau, during winter and summer times.NA indicates that the sources were not sampled, either because the occurrence was very limited (in the case of macroalgae in winter), or because the water level was too low (in the case of riverine outputs in summer).

	Winter	Summer

Table 2 .

 2 Chlorophyll a (µg.g -1 ), phaeopigments (µg.g -1 ) and sedimented organic matter C:N ratios in sediments of the two habitats (Amphiura and Haploops) for the two seasons (winter and summer). Mean values (±SD) are calculated for the 3 stations in each habitat. Feeders, G = Grazers or micro-Grazers. Species names were checked using world register of marine species database on November 28 th 2013

			Winter		Summer
		Amphiura	Haploops	Amphiura	Haploops
	Chlorophyll a (µg.g -1 )	0.22 (± 0.15)	1.06 (± 0.44)	1.67 (± 0.24)	2.04 (± 0.11)
	Phaeopigments (µg.g -1 )	8.74 (± 2.85)	26.25 (± 4.48)	7.99 (± 3.70)	18.03 (± 2.14)
	C:N ratio	7.12 (± 0.58)	5.90 (± 0.17)	7.36 (± 0.71)	5.76 (± 0.17)

Acknowledgements. We would like to thank J.D. Gaffet, A. Brind'Amour, P. Laffargue, X.

Caisey and the crew of the R.V. Thalia for help at sea and providing logistic support. D.

Delmas is thanked for analytical help of sediment. We also thank C. Gelpi for English grammar editings. Three reviewers greatly improved the quality of the manuscript. S.F.D was funded by a grant from the EC2CO National program on Coastal Environments and by the Total Foundation for Biodiversity and the Sea and C.R. was funded by a PhD grant of Region Bretagne. LITERATURE CITED Allen BJ, Williams SL (2003) Native eelgrass Zostera marina controls growth and reproduction of an invasive mussel through food limitation. Mar Ecol Prog Ser

unique diet (see Fig. 3), Thyasira flexuosa, Aplysia punctata and Nereis sp. were excluded from the calculations. n = number of species.