Blanche Saint-Béat
email: blanche.saint-beat@univ-lr.fr

Christine Dupuy

Hélène Agogué

Alexandre Carpentier

Julien Chalumeau

Serena Como

Valérie David

Margot De Crignis

Jean-Claude Duchêne

Camille Fontaine

Margot De Crignis

Eric Feunteun

Katell Guizien

Hans Hartmann

Johann Lavaud

Sébastien Lefebvre

Christel Lefrancois

Clarisse Mallet

Hélène Montanié

Jean-Luc Mouget

Francis Orvain

Pascaline Ory

Pierre-Yves Pascal

Gilles Radenac

Pierre Richard

Alain Vézina

Nathalie Niquil

How does the resuspension of the biofilm alter the functioning of the benthos-pelagos coupled food web of a bare mudflat in Marennes-Oléron Bay (NE Atlantic)?

Keywords: intertidal mudflat, biofilm resuspension, stability, meta-ecosystem

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Introduction

The structure and the functioning of food webs affect the emergent properties and thus the stability of the ecosystem. As a consequence, describing the emergent properties of the ecosystem is a prerequisite for establishing their stability. The literature on the subject is diverse and can lead to controversial interpretations and conclusions. However, some trends can be observed such as equilibrium between two extremes that allows the ecosystem to act like a buffer to external perturbations. For instance, the coexistence of weak and strong interactions is assumed to bring stability to the ecosystem [START_REF] Mccann | Weak trophic interactions and the balance of nature[END_REF], or the asymmetry in the ecosystem ensures a higher stability [START_REF] Rooney | Structural asymmetry and the stability of diverse food webs[END_REF], or an ecosystem that presents a balance between Ascendency and redundancy is supposed to be more stable [START_REF] Ulanowicz | Some steps toward a central theory of ecosystem dynamics[END_REF]. Moreover, [START_REF]Fragile dominion: complexity and the commons[END_REF] proposed that a stable ecosystem pattern is composed of subsystems strongly intraconnected but weakly interconnected. At larger scale, this theory is transposable to the concept of the meta-ecosystem defined by [START_REF] Loreau | Meta-ecosystems: A theoretical framework for a spatial ecosystem ecology[END_REF] as a set of ecosystems connected by spatial flows. A set of ecosystems strongly intraconnected weakly interconnected thus form a stable meta-ecosystem. In this study, we propose to consider the benthos and the pelagos as systems connected by spatial flows at high tide to form a stable meta-system. We have used this concept in order to describe and to understand the effects of the benthos-pelagos coupling on the properties of the food webs and its consequences on the stability of the Brouage mudflat meta-system.

As bare intertidal mudflat, Brouage mudflat, is characterized by the development of a microbial biofilm at the surface of the sediments at diurnal low tide. This biofilm is usually mainly composed of brown micro-algae (diatoms) which constitute the microphytobenthos (Cariou-Le [START_REF] Gall | Monthly HPLC measurements of pigment concentration from an intertidal muddy sediment of Marennes-Oléron Bay, France[END_REF] and prokaryotes, all of them linked by a matrix of extracellular polymeric substances (EPS) [START_REF] Decho | Microbial biofilms in intertidal systems: an overview[END_REF]. The production of the biofilm is integrated to the benthic food web via the deposit feeders, especially Peringia ulvae [START_REF] Haubois | Ingestion rate of the deposit-feeder Hydrobia ulvae (Gastropoda) on epipelic diatoms: Effect of cell size and algal biomass[END_REF]Pascal et al., 2008a;[START_REF] Pascal | Seasonal variation in consumption of benthic bacteria by meio-And macrofauna in an intertidal mudflat[END_REF], via the facultative suspension feeders and via the meiofauna [START_REF] Pascal | Seasonal variation in consumption of benthic bacteria by meio-And macrofauna in an intertidal mudflat[END_REF]Pascal et al., 2008b;Pascal et al., 2008c). The meio-and macrofauna are not only involved in the regulation of the microphytobenthos biomass via the grazing, but also via the bioturbation and the biostabilisation of the sediment whose effects are coupled with physical factors (tides and swell) [START_REF] Blanchard | Dynamic behaviour of benthic microalgal biomass in intertidal mudflats[END_REF].

When the tidal flow arises, the microphytobenthos biomass decreases at the surface of the sediments [START_REF] Guarini | Dynamic model of the shortterm variability of microphytobenthic biomass on temperate intertidal mudflats[END_REF] for two reasons: i) the downward "migration" of motile diatoms into the sediments [START_REF] Consalvey | The ups and downs of life in a benthic biofilm: migration of benthic diatoms[END_REF][START_REF] Guarini | Dynamic model of the shortterm variability of microphytobenthic biomass on temperate intertidal mudflats[END_REF][START_REF] Herlory | Microstructure of microphytobenthic biofilm and its spatio-temporal dynamics in an intertidal mudflat (Aiguillon Bay, France)[END_REF][START_REF] Ni Longphuirt | Dissolved inorganic nitrogen uptake by intertidal microphytobenthos: nutrient concentrations, light availability and migration[END_REF] and ii) the resuspension of a part of the diatom stock into the water column. The resuspension of the microphytobenthic biofilm is controlled by a complex interaction between physical and biological forcing. The physical resuspension of the microphytobenthos depends on the bed shear stress which is induced by the tidal current and/or the wind-waves [START_REF] Blanchard | Properties of the dynamics of intertidal microphytobenthic biomass[END_REF][START_REF] De Jonge | Contribution of resuspended microphytobenthos to total phytoplankton in the EMS estuary and its possible role for grazers[END_REF]. The erodability of the sediment is strongly variable in space and in time [START_REF] Tolhurst | Small-scale temporal and spatial variability in the erosion threshold and properties of cohesive intertidal sediments[END_REF] and depends on biological factor modifying sediment properties such as macrofauna activities and microbial biofilm setting up [START_REF] Herman | Benthic community structure and sediment processes on an intertidal flat: results from the ECOFLAT project[END_REF][START_REF] Orvain | Interacting effects of Hydrobia ulvae bioturbation and microphytobenthos on the erodibility of mudflat sediments[END_REF]. The microphytobenthos resuspension also depends on biological factors such as the ageing of the biofilm [START_REF] Orvain | Interacting effects of Hydrobia ulvae bioturbation and microphytobenthos on the erodibility of mudflat sediments[END_REF] and the content of exopolysaccharids (EPS) (Orvain et al., present issue). When the microphytobenthic biofilm is in its exponential growth phase, it stabilizes the sediments and counteracts the bioturbation due to macrofauna which favors its resuspension [START_REF] Orvain | Interacting effects of Hydrobia ulvae bioturbation and microphytobenthos on the erodibility of mudflat sediments[END_REF]. In contrast, when the biofilm reaches its senescent phase, the roughness of the biofilm as well as bacterial biofilm degradation are enhanced and the mat is more easily resuspended [START_REF] Orvain | Interacting effects of Hydrobia ulvae bioturbation and microphytobenthos on the erodibility of mudflat sediments[END_REF]. The microphytobenthic diatom that are resuspended in the water column are integrated to the pelagic food web and can be ingested by suspension feeders, in the case of the Brouage mudflat especially by Crassostrea gigas, a cultivated species [START_REF] Riera | Isotopic determination of food sources of Crassostrea gigas along a trophic gradient in the estuarine bay of Marennes-Oleron[END_REF] and Cerastoderma edule [START_REF] Sauriau | Stable isotope evidence of benthic microalgae-based growth and secondary production in the suspension feeder Cerastoderma edule (Mollusca, Bivalvia) in the Marennes-Oléron Bay[END_REF].

In this study new in situ observations and experimentations were taken into account, especially on the resuspension of the biofilm (Orvain et al., present issue) and the consequences on the pelagic food web are evaluated by using a model describing trophic pathways. Erosion experiments allowed to determine the critical shear velocity necessary regarding the resuspension of benthic micro-organisms and resuspension rates (Dupuy et al., present issue) . The Lagrangian and Eulerian field surveys followed the future of the resuspended particles in the water column, respectively following the water mass or at a fixed point [START_REF] Guizien | Microorganism dynamics during a rising tide: Disentangling effects of resuspension and mixing with offshore waters above an intertidal mudflat[END_REF]. Moreover grazing experiments and viral lysis experiments (Montanié et al., present issue) were performed in order to determine the effect of resuspension of benthic organisms on the pelagic food web sensu largo (i.e. including virus).

These refinements were incorporated in the framework of a trophic-flow model to better unravel the impact of the biofilm resuspension on the food web functioning by deciphering the contribution of each flow in the contribution in the functioning during high-tide phase. At high tide, the rise of the tide and the chemical/physical/biological processes associated with the increase of the water level on the Brouage mudflat create spatial flows linking benthic and pelagic parts. Consequently, it can be considered as a "meta-ecosystem" defined by [START_REF] Loreau | Meta-ecosystems: A theoretical framework for a spatial ecosystem ecology[END_REF] as a set of ecosystems which are linked by spatial biotic and/or abiotic flows across the ecosystem boundaries. We especially focused on one question: How does the resuspension of the microbial biofilm at high tide modifies the stability of ecosystem in respect to the metaecosystem pattern? We explored this question by comparing the network organization of two distinct food web models representing 2 scenarii of a summer situation. In the first model scenario, the hydrodynamic conditions were extreme and sufficient to induce the resuspension of the microphytobenthos (physical mass erosion, when bed shear stress BSS > 3 cm.s -1 on Brouage mudflat). In this case the velocity of current stays superior to the critical sinking velocity, thus no sedimentation is possible. While in the second case the physical forcing was too weak to induce any resuspension, only limited and extremely low erosion of chla (biological erosion when BSS < 3 cm.s -1 ) induced by the bioturbation of the macrofauna can be observed (Orvain et al., present issue). Moreover the settling of pelagic particles (organic or not) could occur and the sinking velocity enhanced by the pelletisation (Orvain et al., present issue). The missing flows of the food web (i.e. flows which were not measured in situ)

were estimated by the Monte Carlo Markov Chain Linear Inverse Modelling (MCMC-LIM) [START_REF] Van Den Meersche | xsample(): An R Function for Sampling Linear Inverse Problems[END_REF]. This mathematical method explores a solution space defined by constraints issued from in situ measurement and values issued from the literature. All solutions proposed by the MCMC-LIM were used to calculate several ecological network analysis (ENA) indices, describing the emergent properties of the ecosystem.

Material and Methods

The study area

The Brouage intertidal mudflat is located at the French Atlantic coast in the bay of Marennes-Oléron (figure 1). The bay covers 150 km² and the Brouage mudflat, at the eastern part of the bay, represents 68 km² at low tide. The averaged bottom slope is relatively flat (1:1000) and the tidal area is large (up to 4 km). The sediment consists of silt and clay particles (95% <63 µm) [START_REF] Pascal | Seasonal variation in consumption of benthic bacteria by meio-And macrofauna in an intertidal mudflat[END_REF]. The current speeds in the bay range from 0.2 to 0.6 m.s -1 and the bed shear stress from 1.5 to 4 Pa [START_REF] Bassoullet | Sediment transport over an intertidal mudflat: Field investigations and estimation of fluxes within the 'Baie de Marennes-Oleron' (France)[END_REF][START_REF] Hir | Characterization of intertidal flat hydrodynamics[END_REF]. The zone of interest is located in the middle of the Brouage mudflat and is characterized by a typical ridge and runnel bedform [START_REF] Gouleau | Short-and long-term sedimentation on Montportail-Brouage mudflat, Marennes-Oléron Bay, France[END_REF].

Inverse modelling

Two summer food web models were performed: the first one simulated a high-tide situation with massive suspension of micro-organisms in the water column (resuspension model), and the second one also at high tide, where the physical forcing was not sufficient to induce resuspension (sedimentation model). As a consequence, an insignificant quantity of particles is suspended via biological factors as macrofauna bioturbation and it counterparts by a strong sedimentation of organic matter.

The inverse modelling can be divided into 3 steps: (i) determine the species that compose the food web, and all possible flows between them. Twenty-one compartments were listed (Table 1) linked by 115 or 118 flows for the models with and without resuspension, respectively. (ii) determine the mass balance of each compartment and constrains flow values by in situ measurements. (iii)limite possible values of flows by biological constraints.

Considered compartments and flows

Resuspension and sedimentation

A portable erodimeter [START_REF] Guizien | Accounting for Rough Bed Friction Factors of Mud Beds as a Result of Biological Activity in Erosion Experiments[END_REF] was used to estimate the erosion shear stress of in situ cohesive sediments. An increased shear stress (by small steps, each timed to last about 10 minutes) was applied to the mud surface and the suspension of micro-organisms inhabiting in the sediment was monitored by changes in water column micro-organisms concentration.

From these experiments, it was defined that resuspension of the microorganisms and diatoms took place when the shear bed velocity was higher or equal to 3 cm.s -1 . Sedimentation could not occur at higher current velocity.

Sedimentation was only considered in the model without resuspension, because the lower current velocity (< 3 cm.s -1 ) allowed particles to settle down on the bottom during the slack water. The sedimentation rate was estimated from the formula D=Ws*C where D is expressed in mgC.m -².h -1 , C is the concentration of particles in the water column (mgC.m -3 ) and Ws the sinking velocity of particles (m.h -1 ) [START_REF] Krone | Flume studies of the transport of sediment in estuarine shaoling processes[END_REF]. Concentration of both pelagic bacteria and chlorophyll a were measured in situ. The minimal limit for the concentration of the particulate carbon corresponded to the pelagic particulate carbon produced during high tide. For defining its maximal limit we considered that the import of carbon into the water column was equal to the amount of the benthic particulate carbon resuspended in the water column and not consumed. Consequently, it was assumed that the particulate carbon present in the water column at high tide, when resuspension occurred was likely to be found in a similar quantity at high tide n+1.

The microphytobenthos

Primary production of the microphytobenthos is linked to the biomass of diatoms present in the biofilm and to light conditions [START_REF] Macintyre | Microphytobenthos: The ecological role of the 'secret garden' of unvegetated, shallow-water marine habitats. I. Distribution, abundance and primary production[END_REF]. During immersion, the turbidity of the overlying water, stopping the penetration of light [START_REF] Alpine | Phytoplankton growth rates in a light-limited environment[END_REF] inhibits benthic primary production [START_REF] Denis | Short-term variability of intertidal microphytobenthic production using an oxygen microprofiling system[END_REF][START_REF] Migné | Annual budget of benthic production in Mont Saint-Michel Bay considering cloudiness, microphytobenthos migration, and variability of respiration rates with tidal conditions[END_REF]. Moreover, just before the flood return, the diatoms move down into the sediment [START_REF] Herlory | Microstructure of microphytobenthic biofilm and its spatio-temporal dynamics in an intertidal mudflat (Aiguillon Bay, France)[END_REF][START_REF] Round | Persistent, vertical-migration rhythms in benthic microflora[END_REF]. In our models, no microphytobenthic production was considered.

The only input to the microphytobenthos compartment is thus an import of carbon which comes from the production of the previous diurnal low tide by the microphytobenthos, andwhich was not consumed during low tide. The import of carbon to the high tide corresponded to the export of carbon determined for microphytobenthos in a low tide model and was equal to 31.9 mgC.m -2 per high tide [START_REF] Saint-Béat | Key Features of Intertidal Food Webs That Support Migratory Shorebirds[END_REF]. Moreover the secretion of EPS, related to the activity of photosynthesis and migration [START_REF] Underwood | The importance of extracellular carbohydrate production by marine epipelic diatoms[END_REF], was supposed negligible at high tide [START_REF] Hanlon | Dynamics of extracellular polymeric substance (EPS) production and loss in an estuarine, diatom-dominated, microalgal biofilm over a tidal emersion-immersion period[END_REF]. This was confirmed by a survey of EPS concentration in a tidal mesocosm during a tidal cycle showing a fall of EPS concentration in the biofilm during high tide (Agogué et al., present issue) 9.7mgC of benthic diatoms were resuspended per high tide (Dupuy et al., present issue); they constitute a flow from the benthos to the pelagos that enhances the phytoplanktonic biomass after resuspension [START_REF] Brito | Does microphytobenthos resuspension influence phytoplankton in shallow systems? A comparison through a Fourier series analysis[END_REF][START_REF] Koh | Tidal resuspension of microphytobenthic chlorophyll a in a Nanaura mudflat, Saga, Ariake Sea, Japan: Flood-ebb and spring-neap variations[END_REF]. In this way, the microphytobenthos constitutes a food resource for the secondary producers in pelagic and benthic ecosystems [START_REF] Guarini | Dynamics of spatial patterns of microphytobenthic biomass: Inferences from a geostatistical analysis of two comprehensive surveys in Marennes-Oleron Bay (France)[END_REF][START_REF] Riera | Isotopic determination of food sources of Crassostrea gigas along a trophic gradient in the estuarine bay of Marennes-Oleron[END_REF][START_REF] Yoshino | Intertidal bare mudflats subsidize subtidal production through outwelling of benthic microalgae[END_REF].

Benthic bacteria

Biofilms (i.e an assemblage of benthic diatoms and bacteria) were reconstituted in a tidal mesocosm: the production and the biomass of the bacteria were measured during 5 days at low and high tides (Agogué et al., present issue). . The benthic bacterial production was estimated by tritiated thymidine incorporation [START_REF] Garet | Acid extraction of tritium label from bacterial DNA in clay sediment[END_REF], whichwas converted to numbers of cells using the ratio of 1.96*10 17 cells per mol of thymidine determined for this study. A mean increase of 24.3% of the bacterial production in comparison to the bacterial production at low tide was observed in mesocosm experiments.

The bacterial biomass was obtained from the mean cell volume calculated with Furhman"s formula (1981) and converted in carbon units from the formula 133.754*V 0.438 (Vin µm 3 ) [START_REF] Romanova | Relationships between the cell volume and the carbon content of bacteria[END_REF]. The carbon contain in a bacterium was thus estimated as equal to 79 fg C. cell -1 for a mean biovolume of 0.28 µm 3. During erosion experiments at the threshold velocity considered in this study, 58.2 mgC per high tide issued from the benthic bacteria were suspended (Dupuy et al., present issue). A part of the suspended bacteria were damaged or dead during the erosion process (40%) (Mallet et al., present issue), and thus they integrated the pelagic particulate organic carbon compartment. On the contrary the remaining suspended benthic bacteria, still active, were considered to increase the pelagic bacteria biomass (Mallet et al., present issue).

Infauna (meiofauna and macrofauna)

The microphytobenthos is the preferential resource of the benthic fauna but in case of unavailability of microphytobenthos, bacterivory becomes significant [START_REF] Pascal | Seasonal variation in consumption of benthic bacteria by meio-And macrofauna in an intertidal mudflat[END_REF]. At high tide, bacterivory was supposed to be higher than the low tide values: the values of low tide used in the previous model [START_REF] Saint-Béat | Key Features of Intertidal Food Webs That Support Migratory Shorebirds[END_REF] were thus integrated to high tide models as minimal values. The obligate as well as facultative suspension feeders were assumed to feed on particles from bacteria to mesozooplankton [START_REF] Self | Cross-phyletic patterns of particle selection by deposit feeders[END_REF][START_REF] Taghon | Optimal foraging by deposit-feeding invertebrates: Roles of particle size and organic coating[END_REF].

Phytoplankton

The primary production of the phytoplankton was estimated for six other sites in Marennes-Oléron Bay based on in situ measurement of chlorophyll a water concentration, salinity, temperature and depth profiles of light attenuation [START_REF] Struski | Preliminary estimate of primary production by phytoplankton in Marennes-Oléron Bay, France[END_REF]. The phytoplanktonic primary production is influenced by the erosion of sediment, limited light penetration and by the resuspension of benthic diatoms which likely participate to the phytoplanktonic production [START_REF] Macintyre | Microphytobenthos: The ecological role of the 'secret garden' of unvegetated, shallow-water marine habitats. I. Distribution, abundance and primary production[END_REF]. In order to prevent any bias in the estimation of the planktonic primary production, we constrained it by minimal and maximal primary production values measured in summer for the different stations of the bay. These stations are characterized by different hydrological conditions and are thus characterized by different resuspension and turbidity, and consequently by a different light penetration.

Constraining the phytoplanktonic production by a range of possible values allowed to adjust the probability density function for gross primary production according to the needs of the food web based on the situation considered (i.e. with or without resuspension).

Pelagic bacteria

The summer abundance and the summer production of the pelagic bacteria were measured in situ [START_REF] Ory | In situ structuring of virioplankton through bacterial exoenzymatic activity: Interaction with phytoplankton[END_REF]. Bacteria were counted by epifluorescence microscopy after being fixed with 0.02µm filtered formaldehyde (2% final concentration) and staining for 30 min with Sybr Green I (for more details see [START_REF] Noble | Use of SYBR Green I for rapid epifluorescence counts of marine viruses and bacteria[END_REF][START_REF] Ory | In situ structuring of virioplankton through bacterial exoenzymatic activity: Interaction with phytoplankton[END_REF]. The measurement of AMPase (V max ), which is considered as an indicator of the bacterial production, was used to estimate the bacterial production from the equation log BP = 0.9271*log Vmax + 5.3641 (r²=0.67,n=10,p=0.003). Pelagic bacteria being assumed to contain 16 fgC per cell [START_REF] Labry | The role of phosphorus on planktonic production of the Gironde plume waters in the Bay of Biscay[END_REF], the bacterial production was thus expressed in carbon.

Experiments on viral lysis were conducted to test the interactions between bacteria, heterotrophic nanoflagellates (HNF) and viruses in the water column. In artificial incubations, they were re-combined to mimic the field assemblage with respect to the natural viral to bacterial ratio (VRB) and the bacteria to flagellates ratios. Fractionation allowed creating experimental recombined treatments and then differentiating between the direct and indirect interactions of the presence/absence of the virus and HNF. Through in vitro experiments, bacterial losses induced by viruses and flagellates were estimated by comparing the reduction of the bacteria cell production (MBP) between the different experimental assemblages.

Benthic and pelagic viruses

Viral lysis was estimated from viral production (i.e. net increase of viral abundance divided by the time of the experiment) within 2 L bottle incubations in the presence or absence of benthic particulates in order to determine the effect of the biofilm resuspension on the viral lysis. Bacterial mortality due to viral lysis was calculated from the viral production divided by the burst-size viruses (i.e. number of viruses produced by a bacterium at burst-time) which was estimated as 33 in this study (Montanié, pers. comm.). The quantity of viruses produced per time unit was converted into carbon considering that one virus contains 0.2 fgC [START_REF] Magagnini | Viral abundance and distribution in mesopelagic and bathypelagic waters of the Mediterranean Sea[END_REF][START_REF] Suttle | Viruses in the sea[END_REF].

The viral lysis of benthic bacteria at high tide was considered to be similar to the one at low tide, thus we considered that 40 % of the bacterial production was lost by viral lysis [START_REF] Saint-Béat | Key Features of Intertidal Food Webs That Support Migratory Shorebirds[END_REF]. At high tide, the benthic viruses were suspended and integrated into the pelagic virus compartment. 1.29 mgC per high tide of virus per m² were resuspended in the water column at the critical shear bed velocity of 3cm.s -1 (Dupuy et al., present issue) determined by the erosion experiment (see above).

Ciliates and heterotrophic Nanoflagellates

The biomasses were expressed in carbon from a conversion factor of 19 µgC.µm-3 [START_REF] Putt | An experimentally determined carbon: volume ratio for marine "oligotrichous" ciliates from estuarine and coastal waters[END_REF]) by considering equivalent spherical diameter (ESD). The abundances and biomass of Ciliates (ESD < 50 µm) and heterotrophic nanoflagellates (2µm < ESD < 10µm) were estimated in situ. The seawater was filtered onto 0.8 µm Nucleopore black filter. The ciliates and nanoflagellates were enumerated by epifluorescence microscopy. Cells were first fixed by the glutaraldehyde and the paraformaldehyde and stained with the lugol 1% and DAPI respectively.

In the models, phytoplankton and pelagic bacteria are prey of the compartments ciliates and nanoflagellates, which are themselves the preys of ciliates. The nanoflagellates also potentially graze the viruses [START_REF] Bettarel | Low consumption of virus-sized particles by heterotrophic nanoflagellates in two lakes of the French Massif Central[END_REF][START_REF] Manage | Effect of heterotrophic nanoflagellates on the loss of virus-like particles in pond water[END_REF].

Mesozooplankton

Mesozooplankton (200µm < ESD < 400µm) play a central role in the pelagic food web in the Marennes-Oléron Bay [START_REF] Sautour | Importance of microzooplanktonic crustaceans in the coastal food chain: Bay of Marennes-Oleron, France[END_REF] and show a variable diet [START_REF] Vincent | Contribution of ciliated microprotozoans and dinoflagellates to the diet of three copepod species in the bay of Biscay[END_REF]. The resources of mesozooplankton, as confirmed by bottle grazer experiments, in the area of Brouage mudflat are phytoplankton, either directly or indirectly via ciliates and the heterotrophic nanoflagellates [START_REF] Azémar | Looking for general trends in trophic interactions among estuarine micro-and mesozooplankton[END_REF]. This diet can be completed by the ingestion of detritic matter [START_REF] David | The paradox high zooplankton biomass-low vegetal particulate organic matter in high turbidity zones: What way for energy transfer[END_REF].

The abundance and the biomass of the mesozooplankton were estimated during study period.

The mesozooplankton was sampled using a 200µm mesh WP2 net, preserved in buffered formaldehyde with Na(Bo 3 ) 4 (final concentration 5%) and counted under binocular microscope. The biomasses were expressed in carbon by multiplying the dry weight by 0.4 [START_REF] Simard | In situ twilight grazing rhythm during diel vertical migrations of a scattering layer of Calanus finmarchicus[END_REF]. These measurements were completed by bottle grazer experiments to test the effects of the biofilm suspension on the grazing of mesozooplankton. Water from the erodimeter (after erosion experiments) was mixed with filtered pelagic water (200 µm, in order to exclude mesozooplankton) in different proportions (20, 40 or 70%). This mix was transferred in 1.13 L Nalgene bottles and and incubated during 12 or 24 h in presence/absence (temoin) of mesozooplankton predators collected in situ. The resuspended biofilm in the water disturbs the trophic interactions of the mesozooplankton, especially through an inhibition of the grazing of phytoplankton and heterotrophic nanoflagellates by the mesozooplankton (Hartmann, pers. comm.). Thus in the model with the resuspension no flow between phytoplankton and nanoflagellates towards mesozooplankton was considered.

Grazing Fishes

Main species able to graze on mudflat at high tide are mullets (Liza ramada and Liza aurata).

Observed individuals arrive on the mudflat with an empty stomach, while they leave it with a full stomach (Carpentier et al., present issue). Thus the mullets were considered as a vector of carbon export. Since, the abundance of individuals going about the Brouage mudflat at high tide could not be measured in situ, grazing traces left by mullets on mudflat were considered as a proxy of their grazing pressure. Presence of traces was estimated from pictures of one square meter quadrats (expressed by surface of sediment removed by mullets by square meter). In addition, experiments on the feeding behavior of mullets were conducted in mesocoms to assess the volume of sediment ingested per individual at each tide (Como et al., present issue)). The coupling with field pictures finally allowed estimating the density of fishes per square meter.

Imports and exports

For all benthic compartments, we considered that the production during the previous low tide was not totally consumed, thus imports of carbon from the diurnal low tide were taken into account. These import values corresponded to the mean export values of the low tide model [START_REF] Saint-Béat | Key Features of Intertidal Food Webs That Support Migratory Shorebirds[END_REF]. Export was considered for both models regarding the microphytobenthos and the macrofauna, while export was considered only in the model without resuspension regarding the benthic particulate carbon and the dissolved particulate carbon. For the pelagic compartment, no import from the open sea was considered in the model with resuspension and an import from the open sea of pelagic particulate carbon was considered in the model without resuspension. We supposed that the production of the pelagos was totally consumed during the high tide, thus no export from the bay to the open sea of carbon was considered except for the pelagic particulate carbon, when the suspension occurred.

Equations

The second step characterizes the mass balances of each compartment (listed in Table S1) and flows measured in the field. These two elements (i.e. mass balance and equations) were written within an equation: 𝐴 * 𝑥 = 𝑏 where 𝑥 was vector that contained possible flows, the matrix 𝐴 expressed the mass balance and the field observation as a combination of coefficients of the carbon flows and the vector 𝑏 contained value of mass balances and values of known flows [START_REF] Vézina | Construction of flow networks using inverse methods[END_REF]. The mass balances correspond to the report of inputs and outputs for each compartment of the food web. By default, a compartment is considered to be at the equilibrium (i.e. a constant biomass). Concerning the model with suspension we needed to consider standing stock of the benthic particulate carbon and the benthic dissolved carbon in deficit. Indeed, for these two compartments, there was a net change in mass equal to resuspension term for particulate carbon and equal to the minimal value necessary to the running of model for the dissolved organic carbon. In the case without resuspension, we considered that the biomass loss of pelagic bacteria was equal to the value of the sedimentation. The other sets of equations corresponded to the values of flows, which were measured in situ.

Inequalities

At the last step, some biological constraints were added to the mass balances and flow values.

These constraints were obtained from the literature and limit the possible solutions of flows to realistic values. The information was added to the model with the inequality: 𝐺 * 𝑥 ≤ ℎ, where 𝑥 remains the vector containing flows, 𝐺 is a matrix that contains the coefficients of the biological constraints and the vector ℎ is composed of values of biological constraints [START_REF] Vézina | Construction of flow networks using inverse methods[END_REF]. For the benthic compartments the set of inequalities of the low tide model [START_REF] Saint-Béat | Key Features of Intertidal Food Webs That Support Migratory Shorebirds[END_REF] was used. When the constraints corresponded to a value of flows, the value of this constraint was updated according to the time of high tide (8h). The inequalities for the pelagic compartments were grouped in the Table 2.

The sedimentation flows values were limited, considering two different sinking velocities, which is different according to the particle size [START_REF] De La Rocha | Factors influencing the sinking of POC and the efficiency of the biological carbon pump[END_REF]. The minimal sinking velocity corresponded to the sinking velocity of a single isolated particle. We considered a minimal sinking velocity of 0.05 m.d -1 for free bacteria [START_REF] Lapoussière | Role of free-living and particleattached bacteria in the recycling and export of organic material in the Hudson bay system[END_REF], 0.25 m.d -1 for chlorophyll a [START_REF] Lapoussière | Role of free-living and particleattached bacteria in the recycling and export of organic material in the Hudson bay system[END_REF] and 2.32 m.d -1 for particulate carbon [START_REF] Burns | In situ measurement of the settling velocity of organic carbon particles and 10 species of phytoplankton[END_REF]. The maximal sinking velocity referred to the sinking velocity of "marine snow", that are defined as organic aggregates with a diameter > 0.5 mm [START_REF] Alldredge | Characteristics, dynamics and significance of marine snow[END_REF]. The maximum sinking velocity chosen for this study was 16 m.d -1 [START_REF] Turner | Zooplankton fecal pellets, marine snow and sinking phytoplankton blooms[END_REF]. We considered that pelagic dissolved carbon can fall with the aggregates formed by "marine snow" and represents one third of the total carbon in aggregates [START_REF] Alldredge | Interstitial dissolved organic carbon (DOC) concentrations within sinking marine aggregates and their potential contribution to carbon flux[END_REF].

Network analysis

From the 500,000 solutions estimated by MCMC-LIM, seven ecological network analysis (ENA) indices were calculated. These indices allow assessing the structure and the functioning of the two food webs. The magnitude of cycling within the system was described by the Finn cycling Index (i.e. FCI). This index represents the fraction of flows involved in the cycling [START_REF] Finn | Measures of ecosystem structure and function derived from analysis of flows[END_REF]. A cycle represents a series of transfers between components in an ecosystem beginning and ending in the same compartment without going through the same compartment twice. The FCI is estimated by the ratio Tc/TST, where TST is the total system throughput (i.e. sum of all flows) and Tc the amount devoted to cycling. Various global indices describe the developmental and organizational state of the ecosystem [START_REF] Ulanowicz | Growth and Development[END_REF]. The TST measures the activity of the whole ecosystem. The TST can be considered as the total power generated within the system [START_REF] Baird | Assessment of spatial and temporal variability in ecosystem properties of the St Marks National Wildlife Refuge, Apalachee Bay, Florida[END_REF]. The AMI value is indicative of the specialization of flows in the network [START_REF] Ulanowicz | Quantitative methods for ecological network analysis[END_REF]. The probability of flows between two compartments increases with the AMI value, and thus with the specialization of flows. The Ascendency (A) which represents the state of organization within the ecosystem [START_REF] Ulanowicz | Growth and Development[END_REF], is described as the product of the TST and the average mutual information (i.e. AMI). The development capacity (DC) is defined as the upper limit of Ascendency. The relative Ascendency is the ratio A/DC and estimates the proportion of the network that is organized and thus efficient. The (DC -A) difference estimates the inefficient part of the network, corresponding to the overheads (i.e. overheads on imports, exports and dissipation) and redundancy, that measures the uncertainty associated to the presence of multiple or parallel pathways among the compartments [START_REF] Ulanowicz | Symmetrical overhead in flow networks[END_REF]. The internal Ascendency (A i ) and internal development capacity (DC i ) refer to internal exchanges alone and exclude the exogenous flows. These indices were estimated using MATLAB © routine written by Carole Lebreton and Markus Schartau (GKSS Research Centre, Geesthacht, Germany) to calculate the index value for every solution estimated by the LIM-MCMC.

Statistical test

Since the distribution of the data did not follow a normal distribution a non-parametric test was used. The significance of the differences between the indices calculated for both networks with and without resuspension was controlled by the Wilcoxon test (α = 0.01). The tested hypothesis was that the two data sets were issued from a continuous distribution with equal medians. Statistical tests is possible because using the LIM-MCMC technique on 500,000 solutions, 500,000 values of each ENA index were also calculated, as consequence we can use statistical tests which are not usually possible in such a context of food web modelling using static methods and at this level of functional diversity.

Results

Flow values

Some differences in flow values between the two conditions (i.e. with or without suspension) were observed (Table S2). The pelagic primary production was higher with resuspension. On the whole, consumption rates remained the same irrespective of the condition, except for the bacterivory of heterotrophic nanoflagellates (doubled with resuspension), the bacterivory of nematodes (consumption without resuspension was 7 times higher than the value during resuspension), as well as herbivory of deposit-feeders (about twice higher without resuspension than with resuspension) and consumption on nematodes by grazing fishes that both doubled without suspension. The exudation of DOC by benthic bacteria increased without suspension contrary to the exudation of DOC by pelagic bacteria that was five times higher during resuspension. The mortality of phytoplankton (i.e. phyTOppc) was higher when resuspension occurred. The egestion of nematodes without suspension was twice the egestion during resuspension. The export of carbon from benthic compartment was higher without suspension.

Compartment activities

Significant differences appeared according to the condition considered (Figure 2). The benthic activity was stimulated by the sedimentation of micro-organisms of the water column. In contrast, the resuspension of micro-organisms inhabiting in the sediment stimulated the pelagic activity.

Several pelagic compartments were affected by the resuspension of micro-organisms. The phytoplankton (phy), the pelagic non-living compartments (i.e. particulate compartment (ppc) and dissolved organic carbon (pdc)) as well as the pelagic bacteria (bcp) had a higher activity in the case of resuspension. On the contrary the microphytobenthos, the benthic bacteria, the benthic non-living compartments were more active when sedimentation occurred.

Diet and consumption

Herbivory tended to represent a more important part in the pelagic system (Table 3). In contrast, bacterivory was higher in the benthic compartment. The resuspension of the microorganisms to the water column had an effect on bacterivory, herbivory and the ratio between them. In the pelagic part, the resuspension favored the bacterivory. On the contrary, the herbivory was favored in the case without resuspension. The herbivory and bacterivory in benthos were both higher without resuspension.

The ratios showed that the herbivory was dominant in the pelagos. Nevertheless, when resuspension occurred, the decline in the ratio (almost divided by 2) was due to a fall of herbivory as well as a rise in bacterivory. A fall in the ratio was observed regarding the benthos due to a decline in the bacterivory lower than in herbivory.

None of the compartment was affected by the resuspension (Figure 3). Conversely, the diet of nematodes was drastically altered during the resuspension phase (B). Whatever the resuspension occurred or not, the contribution of the microphytobenthos as a food item for nematodes did not change (about 15%). On the contrary, benthic bacteria and detritus contributed to nematodes diet almost equally in the case without resuspension (40 and 30%, respectively), while it mainly shifted to benthic particulate (about 80%) in resuspension situation.

The detritus contributed only slightly to the diet of deposit feeders. This group fed mainly on microphytobenthos and benthic bacteria. The contribution of each of these two groups changed with the resuspension. While the consumption on the benthic bacteria remained the same between resuspension and sedimentation phases, it corresponded to a higher contribution to the deposit-feeder diet during resuspension.

The facultative suspension feeders fed on the planktonic and benthic species. The contribution of the phytoplankton to the diet of this group was higher when the resuspension occurred.

Consequently, the microphytobenthos was merely consumed. In contrast, in the case without resuspension, the microphytobenthos contributed to 40% of the consumption and the phytoplankton for 20%. The contribution of pelagic and particulate carbon tended to be lower with the resuspension.

The resuspension tended to have some consequences on the diet of the heterotrophic nanoflagellates and mesozooplankton (Figure 4). The diet of the heterotrophic nanoflagellates was more diversified during resuspension because of the contribution of virus (vrp). The contribution of pelagic bacteria (bcp) was three times higher in the case of resuspension.

Consequently the contribution of the phytoplankton (phy) decreased. The most affected compartment was the mesozooplankton. Without resuspension the mesozooplankton fed equally on the phytoplankton, heterotrophic nanoflagellates, pelagic particulate carbon (ppc) and ciliates. During the resuspension, the consumption on both heterotrophic nanoflagellates and phytoplankton was inhibited.

ENA indices

Significant differences between values of the indices of the two situations were observed (Figure 5). The TST was about 3 times higher when the resuspension occurred. The Ascendency followed the same trend. However, a lower organization of the system (i.e. lower AMI value) was observed. The higher relative Ascendency, coupled with a lower AMI value observed in the case of resuspension, suggested a lower diversity of flows. The overheads based on the imports, exports, respiration and redundancy of the system were lower during resuspension. The internal normalized Ascendency tended to be similar for the two situations.

The proportion of flows involved in the cycling (i.e. FCI value) was lower during resuspension event.

Discussion

Activity of the benthic and the pelagic compartments

The benthos was more active than the pelagos regarding simulation without resuspension; in contrast the pelagic activity was higher during resuspension. This fact can be explained by a large import of particulate carbon to the pelagic particulate compartment. Without this import of pelagic carbon, the pelagic activity was equal to the benthic activity during resuspension.

The higher benthic activity was previously shown in the model of the Brouage mudflat food web. In fact, the higher activity of the benthos was observed irrespective of the model considered in previous studies by regarding annual budget [START_REF] Leguerrier | Modeling the impact of oyster culture on a mudflat food web in Marennes-Oléron Bay (France)[END_REF] or by deciphering seasonal budgets [START_REF] Degré | Comparative analysis of the food webs of two intertidal mudflats during two seasons using inverse modelling: Aiguillon Cove and Brouage Mudflat, France[END_REF]. However, in our model, the ranking of the compartments was modified. The main difference concerned the benthic bacteria and the microphytobenthos. The benthic bacteria dominated the ecosystem; they were followed by the benthic non-living detritus compartments. Surprisingly, the microphytobenthos was ranked only 8 th . The difference with previous models is the time-scale considered: a mean year [START_REF] Leguerrier | Modeling the impact of oyster culture on a mudflat food web in Marennes-Oléron Bay (France)[END_REF] or a mean month [START_REF] Degré | Comparative analysis of the food webs of two intertidal mudflats during two seasons using inverse modelling: Aiguillon Cove and Brouage Mudflat, France[END_REF] which is in dramatic contrast to the small scale mean immersion period integrated in our model. During immersion, because of darkness due to sediment burying, the microphytobenthos production was null [START_REF] Blanchard | Analyse conceptuelle du système de production primaire microphytobenthique des vasières intertidales[END_REF]. As a consequence, the carbon input to this compartment exclusively originated from the photosynthesis performed during the previous diurnal low tide, which might explain its 8 th rank in our model. The first pelagic compartments were ranked 4 th and 5 th and corresponded to the phytoplankton and the dissolved organic carbon, respectively.

The resuspension of the microbial biofilm stimulated the activity of the phytoplankton, the non-living compartments (detritus) and the pelagic bacteria. This stimulation was not only the consequence of the input of new matter in the water column. The phytoplankton showed a higher gross primary production when the resuspension of the microbial biofilm occurred. Pelagic primary production was not imposed to the model; it was only constrained by minimal and maximal values of pelagic production found for different hydrological conditions in Marennes-Oléron Bay. The higher production was somewhat surprising because the resuspension also generates a high turbidity and the reduction of the light penetration which dramatically reduces phytoplankton photosynthesis [START_REF] Billerbeck | Benthic photosynthesis in submerged Wadden Sea intertidal flats[END_REF][START_REF] Porter | Effect of tidal resuspension on benthic-pelagic coupling in an experimental ecosystem study[END_REF]. When buried in the sediments, the microphytobenthos can remove nutrients from the overlying water and the sediment pore water [START_REF] Macintyre | Microphytobenthos: The ecological role of the 'secret garden' of unvegetated, shallow-water marine habitats. I. Distribution, abundance and primary production[END_REF]. Such activity tends to reduce the nutrient fluxes from the sediments to the water column which can limit the phytoplankton production [START_REF] Sarker | Contribution of benthic microalgae to the whole water algal biomass and primary production in Suo Nada, the Seto Inland Sea, Japan[END_REF][START_REF] Sloth | Resilience of Pelagic and Benthic Microbial Communities to Sediment Resuspension in a Coastal Ecosystem, Knebel Vig, Denmark[END_REF]. When the microphytobenthos resuspension occurs, the nutrient fluxes to the water column increase which has a positive effect on the phytoplankton production even if light is limiting [START_REF] Porter | Effect of tidal resuspension on benthic-pelagic coupling in an experimental ecosystem study[END_REF]. Paradoxically, the import of microphytobenthic diatoms to the water column contributes itself positively to the production of the phytoplankton [START_REF] Macintyre | Microphytobenthos: The ecological role of the 'secret garden' of unvegetated, shallow-water marine habitats. I. Distribution, abundance and primary production[END_REF].

By enriching water in particulate and dissolved carbon, which sustained a higher bacterial activity as previously reported [START_REF] Cotner | Intense winter heterotrophic production stimulated by benthic resuspension[END_REF][START_REF] Poremba | Tidal impact on planktonic primary and bacterial production in the German Wadden Sea[END_REF][START_REF] Sloth | Resilience of Pelagic and Benthic Microbial Communities to Sediment Resuspension in a Coastal Ecosystem, Knebel Vig, Denmark[END_REF], the resuspension also stimulated heterotrophic production.

On the contrary, the settling of pelagic micro-organisms to the bottom of the water column had smaller but significant consequences on the benthic compartments. In our model, the input of pelagic fresh matter to the benthos increased the stock of available carbon for higher trophic levels but it did not affect the production of the different compartments. The higher activity observed for the benthic compartments (mpb, bdc, bcb, bpc) (Figure 2) was linked to the deposit of pelagic diatoms, dissolved carbon, pelagic bacteria and particulate carbon respectively at the surface of the sediments. The photosynthetic production of the pelagic diatoms settled at the bottom of the water column during immersion is more probably limited by the penetration of light than by the nutrient availability. It is supposed that, in contrast to the light reaching the surface of the sediments [START_REF] Billerbeck | Benthic photosynthesis in submerged Wadden Sea intertidal flats[END_REF][START_REF] Macintyre | Microphytobenthos: The ecological role of the 'secret garden' of unvegetated, shallow-water marine habitats. I. Distribution, abundance and primary production[END_REF], nutrients are often not a limiting factor for the photosynthesis in the intertidal sediments [START_REF] Migné | In situ measurements of benthic primary production during emersion: seasonal variations and annual production in the Bay of Somme (eastern English Channel, France)[END_REF][START_REF] Serôdio | Modelling the primary productivity of intertidal microphytobenthos: time scales of variability and effects of migratory rhythms[END_REF]. The high turbidity reported in the bay of Marennes-Oléron [START_REF] Raillard | An ecosystem box model for estimating the carrying capacity of a macrotidal shellfish system[END_REF] likely stops the light penetration to the sediment surface and strongly impairs the photosynthesis of pelagic diatoms. The effect of the settling on the benthic bacterial production is more obvious. The bacterial production depends on the substrate supply including organic carbon [START_REF] Sander | Factors controlling Bacterial Production in Marine and Freshwater Sediments[END_REF]. For instance in Kiel Bight, the settling of detritus stimulates the benthic bacterial production [START_REF] Meyer-Reil | Benthic response to sedimentation events during autumn to spring at a shallow water station in the Western Kiel Bight[END_REF]). Consequently, it is very likely that the benthic bacterial production is stimulated in the Brouage mudflat during sedimentation.

Herbivory versus bacterivory

The massive resuspension event impacted the pelagic microbial food web. It stimulated the bacterivory of the pelagos as reported before for heterotrophic nanoflagellates [START_REF] Garstecki | Effects of Experimental Sediment Resuspension on a Coastal Planktonic Microbial Food Web[END_REF]. A higher quantity of carbon flowed through the virus (viral lysis was doubled) and consequently more virus were consumed by heterotrophic nanoflagellates. The increase of pelagic bacterial abundance and production doubled the bacterivory rate of heterotrophic nanoflagellates. In contrast, the resuspension tended to decrease the pelagic herbivory. In spite of a higher gross primary production and a direct input of benthic diatoms biomass to the water column, the phytoplankton was integrated to a lower proportion to the pelagic food web. Indeed, the grazing of phytoplankton by the mesozooplankton had been shown to be inhibited during catastrophic erosion event (Hartmann, pers.comm.). Consequently, our models showed that a lower part of the phytoplankton was consumed in the model with resuspension, inducing a greater mortality of this comportment. This result of the models (i.e. higher phyTOppc when resuspension occurs) is coherent with previous results of resuspension experiments [START_REF] Porter | Effect of tidal resuspension on benthic-pelagic coupling in an experimental ecosystem study[END_REF]. It thus appears that in conditions of resuspension, the phytoplankton indirectly participated to the enrichment of the water column in dissolved organic carbon and to the pelagic bacterial production.

The bacterivory dominated the benthic compartment during immersion irrespective of the resuspension. At high tide, benthic diatoms moves down the sediment and cannot perform photosynthesis because of the absence of light [START_REF] Cartaxana | Physiological versus behavioral photoprotection in intertidal epipelic and epipsammic benthic diatom communities[END_REF]. Hence, the benthic food web must be sustained by the input of carbon coming from the photosynthetic production of the previous emersion and by the bacterial production. In our model, we supposed that the meiofauna and the macrofauna showed a constant consumption along the day irrespective of immersion/emersion periods. This hypothesis was based on the assumption that the meiofauna and the deposit feeders had alternative resources since the microphytobenthos was not sufficient to sustain their respective metabolism. Detritus (van Oevelen et al., 2006) bacteria were possible alternative resource for benthic fauna [START_REF] Pascal | Seasonal variation in consumption of benthic bacteria by meio-And macrofauna in an intertidal mudflat[END_REF][START_REF] Van Oevelen | The trophic significance of bacterial carbon in a marine intertidal sediments: Results of an in situ stable isotope labeling study[END_REF]. Some isotopic analyses at the Brouage mudflat showed that the benthic detritus contributes to 11% in maximum to the deposit-feeders consumption (Richard, comm. pers.). We supposed that the alternative feeding resource was the benthic bacteria (see result section .

Previous studies on the bacterivory rates of the nematodes, the foraminifera and Peringia ulvae in the Brouage mudflat indeed showed that bacteria can constitute a significant alternative resource to the microphytobenthos under some conditions (Pascal et al., 2008a;Pascal et al., 2008b;Pascal et al., 2008c). When there was no resuspension, the herbivory and the bacterivory were both stimulated, although the bacterivory was stimulated in a larger proportion due to the higher activity of the nematodes. The bacterivory thus remained dominant in the benthic compartment irrespective of the physical forcing.The Sylt-Rømø Bight in the North of Germany is composed of a mosaic of habitats including a mudflat. Its benthic food web is based on microphytobenthos and macrophytes. In this benthic ecosystem, the herbivory dominates the bacterivory [START_REF] Baird | Energy flow of a boreal intertidal ecosystem, the Sylt-Rømø Bight[END_REF]. Indeed, herbivory is more than two times higher than bacterivory. Thus the ratio herbivory/bacterivory of the Sylt-Rømø Bight displays an opposite tendency than the ratio estimated for the Brouage mudflat. This opposite tendency may be linked to the fact that this ratio was obtained from the food web for the whole bay on a long term. As a consequence, habitats with high and low primary production were associated, thus the available carbon issued from the primary production was more important. Moreover, the food web considered in [START_REF] Baird | Energy flow of a boreal intertidal ecosystem, the Sylt-Rømø Bight[END_REF] represents a mean day as a consequence the difference between low tide and high tide was not visible and the effect of the season was not considered contrary to present simulations, which focused on summer conditions and high tide only In this study, that considered the summer period, little carbon issued from the primary production at low tide was available. The consideration of the food web for the whole year at the Brouage mudflat should abate the seasonal difference and change the trend of herbivory/bacterivory ratio.

Functioning of the Brouage food web

For comparing the functional indices from our model to others, we took care of selecting ecosystem models that coupled the pelagic and benthic compartments. Moreover the nonliving compartments must be separated from bacteria, otherwise the ENA indices would be biased [START_REF] Johnson | The effects of aggregation on the performance of the inverse method and indicators of network analysis[END_REF]. Values of relative Ascendency and internal relative Ascendency were in general higher to those previously reported. Relative Ascendency ranged from 33.4 [START_REF] Monaco | Comparative ecosystem trophic structure of three U.S. mid-Atlantic estuaries[END_REF] to 49.5 for the Chesapeake Bay [START_REF] Baird | The Comparative Ecology of Six Marine Ecosystems[END_REF] and the internal Ascendency from 31.2 for the Delaware [START_REF] Monaco | Comparative ecosystem trophic structure of three U.S. mid-Atlantic estuaries[END_REF] to 44.1 for the Sundays Bay [START_REF] Scharler | A comparison of selected ecosystem attributes of three South African estuaries with different freshwater inflow regimes, using network analysis[END_REF]. The cycling estimated without resuspension was higher to the FCI (i.e. Finn Cycling Index) estimated in the Sylt-Rømø Bight (= 17.2%) and close to the value of Sundays Estuary [START_REF] Scharler | A comparison of selected ecosystem attributes of three South African estuaries with different freshwater inflow regimes, using network analysis[END_REF]. The differences between our study and previous ones are most probably based on the shorter time scale in our models.

In the aforementioned studies, the ecosystems considered are estuaries which are subjected to the tidal rhythm and thus which are controlled by strong temporal/physical forcing. It was demonstrated how physical parameters can influence the ecological properties described by the ENA indices [START_REF] Niquil | The effect of physical drivers on ecosystem indices derived from ecological network analysis: Comparison across estuarine ecosystems[END_REF]. The consequences of physical forcing like the resuspension are smoothed when the considered networks use flows averaged over a mean day, more representative of normal conditions without waves. Additionally, the biological processes change according to the immersion and the emersion periods which impacts the carbon budget [START_REF] Migné | Annual budget of benthic production in Mont Saint-Michel Bay considering cloudiness, microphytobenthos migration, and variability of respiration rates with tidal conditions[END_REF]. Consequently, when emersion and immersion are considered separately (i.e. short time scale), it allows more precisely deciphering the biological and physical processes that control the functioning of the food web.

When the massive resuspension of the microbial biofilm occurred, the enrichment of the water column by the benthic particulate carbon (i.e. 3.584gC.m -2 .per high tide) mainly supported the increased activity of the whole system and the decreased organization of the network decreased. The higher value of Ascendency (2*10 4 mgC.m -2 .per high tide against 0.6*10 4 mgC.m -2 .per high tide during sedimentation) was the consequence of a higher TST (10000 mgC.m -2 .per high tide during resuspension and 3500 mgC.m -2 .per high tide during sedimentation), in spite of a drop down in the specialization of the trophic way (measured by AMI)This is a characteristic observation for a so-called "pulse eutrophication", an intermittent increase of organic matter supply combined with physical factors [START_REF] Patrício | Ascendency as an ecological indicator: A case study of estuarine pulse eutrophication[END_REF].

Moreover a high value of Ascendency derived from a very high TST, could disturb the internal stability of the system [START_REF] Ulanowicz | Some steps toward a central theory of ecosystem dynamics[END_REF]. The lower overheads suggest a lower resistance to the perturbation as proposed by [START_REF] Ulanowicz | Some steps toward a central theory of ecosystem dynamics[END_REF]. Thus the resuspension decreased the resistance of the system to a perturbation. We propose that the massive resuspension event in the Brouage mudflat could be defined as a "pulse eutrophication" event that regularly disturbs the meta-ecosystem. On the contrary the biological erosion coupled with a high sedimentation tended to reduce the perturbation in the meta-system.

During the massive resuspension, the lower proportion of cycling (i.e. lower value of FCI) coupled with a high internal relative Ascendency (close to 50%) can be explained by the limited integration to the planktonic food web of the non-living carbon suspended in the water column. Indeed, in spite of a higher pelagic bacterial production and a higher detritivory, a low quantity of carbon was recycled. As a consequence, almost all the carbon suspended in the water column (i.e. 3.534 gC.m -2 .per high tide) was exported. Note that the export value was not constrained in the model, thus this value reflected a real property of the network. The larger the difference between the cycling and the internal relative Ascendency, the less organized and more under pressure a system would be [START_REF] Baird | Trophic dynamics of eight intertidal communities of the Sylt-Rømø Bight ecosystem, northern Wadden Sea[END_REF][START_REF] Baird | The Comparative Ecology of Six Marine Ecosystems[END_REF].

Thus the Brouage mudflat was less organized (confirmed by lower value of AMI) and submitted to a higher stress during the resuspension. A similar relation (i.e. great difference) between FCI and internal relative Ascendency was found for the mussel-bed in the Rømø-Sylt Bight [START_REF] Baird | Trophic dynamics of eight intertidal communities of the Sylt-Rømø Bight ecosystem, northern Wadden Sea[END_REF] and in an upwelling area [START_REF] Baird | The Comparative Ecology of Six Marine Ecosystems[END_REF]. [START_REF] Baird | The Comparative Ecology of Six Marine Ecosystems[END_REF] brought a significant nuance to the stress sense, by the distinction between ecosystems which are under physically or chemically pressure. Indeed, the two constraints do not refer to the same time scale. A chemical stress is in general recent and it has an exogenous origin to the considered ecosystem. In contrast, physical perturbations are older and the ecosystem can have adapted to it. These differences explain how a low cycling value can be coupled with a high internal relative Ascendency [START_REF] Baird | The Comparative Ecology of Six Marine Ecosystems[END_REF]. During the massive resuspension phase, and as expected, the Brouage mudflat obviously showed the characteristics of a system that is physically perturbed.

When massive resuspension did not occur, the Brouage mudflat was characterized by a high specialization (i.e. high AMI) and by a relative Ascendency close to 60%. Such value illustrates a state closed to the equilibrium between the efficient and the fraction of the network that has not yet been organized [START_REF] Bodini | Towards a sustainable use of water resources: a whole-ecosystem approach using network analysis[END_REF]; it is based on redundancy in the imports, the exports, the dissipation and on internal redundancy [START_REF] Baird | Energy flow of a boreal intertidal ecosystem, the Sylt-Rømø Bight[END_REF]. The equilibrium between both parts (relative Ascendency and overheads) is supposed to bring sustainability to the ecosystem [START_REF] Ulanowicz | Quantifying sustainability: Resilience, efficiency and the return of information theory[END_REF]; the inefficient part being used as a reserve that brings the necessary flexibility for the ecosystem sustainability.

Moreover the high internal relative Ascendency is a strong sign that the system is relatively mature [START_REF] Baird | The Comparative Ecology of Six Marine Ecosystems[END_REF]. The lower difference between internal relative Ascendency and FCI supposed a higher organization and a less disturbed system [START_REF] Baird | Trophic dynamics of eight intertidal communities of the Sylt-Rømø Bight ecosystem, northern Wadden Sea[END_REF]. Hence, without massive resuspension of the microbial biofilm, the Brouage system seems to be relatively mature and stable.

Conclusion: the stability of the Brouage meta-system

As defined by [START_REF] Loreau | Meta-ecosystems: A theoretical framework for a spatial ecosystem ecology[END_REF], a meta-ecosystem corresponds to the different ecosystems which are linked together by spatial flows of energy and matter. The rise of the tide and the chemical/physical/biological processes, which are associated with the increase of the water level on the mudflat can be considered as spatial flows. Here, we considered two different events according to the hydrodynamical conditions: 1) the massive resuspension of benthic matter in the water column 2) the sedimentation of pelagic matter on the mudflat sediments associated to a biological resuspension induced by macrofauna activities. As described above, their respective impact on the functioning of the benthic and the pelagic food webs strongly differs. The massive resuspension tends to disturb the Brouage meta-system while the sedimentation stabilizes it. These opposite consequences can be explained by the difference in the intensity of the flows. When the massive resuspension occurs, the sum of flows from the sediments to the water column was strong (about 3654 mgC.m -2 per immersion) while during the sedimentation, it was only 10%of the flow during resuspension. The interaction between the benthic and the pelagic compartments also appeared weaker during sedimentation than during massive resuspension. As suggested by [START_REF]Fragile dominion: complexity and the commons[END_REF], a highly modular system (composed of strongly connected sub-systems which are connected by weak links) is a stable system. This concept could be transposed to the meta-ecosystem. We observed that the sedimentation constitutes a weak link between the two subsystems benthos and pelagos. In contrast the massive resuspension constitutes a strong link between benthos and pelagos. To conclude the stabilizing pattern of Levin is observed when sedimentation occurs and not during massive resuspension event. This conceptual step appears essential for the better understanding of (meta-)ecosystem structure and functioning in order to improve our prediction for their sustainability.

Despite its visible destabilizing effect, the massive resuspension brought some benefits to the Brouage meta-system. It stimulated the pelagic microbial food web by increasing both phytoplanktonic and bacterial production, and by stimulating bacterivory. Because of the coupling of beneficial and destabilizing effects, massive resuspension show features characteristic of an intermediate disturbance (reviewed in [START_REF] Shea | Moving from pattern to process: coexistence mechanisms under intermediate disturbance regimes[END_REF]). An intermediate disturbance can be defined as an event that alters the specific niche availability, for instance by removing the biomass or changing the nutrient availability, while it maintains the general biodiversity [START_REF] Shea | Moving from pattern to process: coexistence mechanisms under intermediate disturbance regimes[END_REF]. A complementary and extensive study of the long term massive resuspension frequency and its consequences on the Brouage meta-system would allow to confirm the intermediate disturbance hypothesis. Table 3: Mean values of herbivory and bacterivory in the benthos and the pelagos according to the resuspension of the biofilm. HT -1 = per High Tide. These values correspond to the mean and the standard deviation calculated from the 500,000 iterations calculated by the inverse analyses. For each compartment (i.e. benthos and pelagos), values were significantly different with or without resuspension (Wilcoxon test, p<0.05) according to the condition considered.

Tables

Figures captions

Figure 1: Map of the study site: the Brouage mudflat within the Marennes-Oléron Bay. The activity corresponds to the sum of inflows and is expressed in mgC.m -2 per high tide. A) corresponds to the sum of the activity of all compartments for the benthos and the pelagos according to the 2 modeled situations. B) and C) refer to the activity of each compartment of the benthos and of the pelagos. Compartments are sorted by top down activity in order to determine which ones of the compartments have higher contribution to the activity of the whole ecosystem according to the situation considered (i.e. sedimentation or resuspension).

See table 1 for the abbreviations of the compartments.

  2.2.4. Calculation of solutionsThe generated matrices(A, b, G and h) define a multi-dimensional space delimiting possible solutions of the flows (x). The MCMC-LIM mirror (Van den Meersche et al., 2009) was used to sample through that solution space in an attempt to map it completely. The MCMC-LIM, based on the mirror technique defined by Van Den Meersche et al.(2009) and updated for LIM use by van Oevelen et al. (2010), calculates several solutions and allows a direct characterization of the uncertainty. This modelling technique brings the advantage of calculating a range of possible values for each flow (i.e. a probability density function). For each model (with resuspension and without resuspension), 500, 000 iterations with a jump of 0.5 were calculated. The length of jump and the number of iterations were determined to cover the solution space as completely as possible. In this study the simulations were realized with a MATLAB © translation conceived by Alain Vézina and Lauriane Campo of the R-CRAN project package LIM-Solve created by Van den Meersche et al. (2009).

Figure 2 :

 2 Figure 2: Histogram presenting the activity of the compartments composing the food web.

Figure 3 :

 3 Figure 3: Diet of the meiofauna and macrofauna. A) Top diagrams refer to the case without resuspension and B) down diagrams refer to the case where the resuspension occurred. The contribution corresponds to the fraction that represents the consumption on a prey in comparison to the total consumption. The contribution of each species to the compartment consumption was estimated from the mean value of each flow.

Figure 4 :

 4 Figure 4: Diet of heterotrophic nanoflagellates (hnf), ciliates (cil) and mesozooplankton (mes). A) top diagrams refer to the simulation without resuspension and B) down diagrams refer to the simulation with resuspension. The contribution corresponds to the fraction that represents the consumption on a specific compartment in comparison to the total diet. The contribution of each species to the compartment consumption was estimated from the mean value of each flow.

Figure 5 :

 5 Figure 5: Boxplots displaying the values of different ENA indices: the total SystemThroughput (TST), the Ascendency, the overheads, the relative Ascendency (A/DC), the Average mutual Information (AMI), the internal relative Ascendency (Ai/DCi) and the Finn Cycling index (FCI). The indices were calculated from the 500,000 solutions coming from the MCMC-LIM method. Red crosses correspond to outliers. Medians of all these indices were significantly different for the two seasons (Wilcoxon test, H 0 was rejected, p-value < 0.01).

Figure 1

 1 Figure 1

Table 1 :

 1 List of compartments and used abbreviations.

	Compartments	Abbreviations
	Benthos
	Microphytobenthos	mpb
	Benthic bacteria	bcb
	Meiofauna	mfb
	Nematodes	nem
	Deposit-feeders	dep
	Suspension-feeders	sus
	Facultative suspension-feeders	suf
	Omnivorous species	omn
	Carnivorous species	car
	Benthic viruses	vrb
	Benthic particulate carbon	bpc
	Benthic dissolved carbon	bdc
	Pelagos
	Phytoplankton	phy
	pelagic bacteria	bcp
	Heterotrophic nanoflagellates	hnf
	Cilliates	cil
	Mesozooplankton	mes
	Grazing fishes	gfi
	Pelagic viruses	vrp
	Pelagic particulate carbon	ppc
	Pelagic dissolved carbon	pdc

Acknowledgements

This work was supported by the French ANR (National Research Agency) through the VASIREMI Project "Trophic significance of microbial biofilms in tidal flats" (Contract ANR-06-BLAN-0393-01). Blanche Saint-Béat's PhD work was financed by the CNRS and Région Poitou-Charentes.