L H Van Der Heijden
email: luuk.van_der_heijden@univ-lr.fr

J Rzeznik-Orignac

R Asmus

D Fichet

M Bréret

P Kadel

L Beaugeard

H Asmus

B Lebreton

How do food sources

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Introduction

Soft-bottom intertidal habitats are highly productive [START_REF] Underwood | Primary production by phytoplankton and microphytobenthos in estuaries[END_REF][START_REF] Hemminga | Seagrass ecology, First[END_REF], fueling complex food webs and providing food resources for a large diversity of top consumers like fish and birds (Day et al. 1989;[START_REF] Duarte | The fate of marine autotrophic production[END_REF] via trophic pathways going through meiofauna [START_REF] Chardy | Carbon flows in a subtidal fine sand community from the western English Channel: a simulation analysis[END_REF] and macrofauna [START_REF] Haubois | Ingestion rate of the deposit-feeder Hydrobia ulvae (Gastropoda) on epipelic diatoms: Effect of cell size and algal biomass[END_REF]. Meiofauna, here defined as metazoans associated with sediments with a size range from 40 to 500 µm, are considered as an important trophic component in the functioning of coastal ecosystems. They have an intermediate trophic position [START_REF] Leguerrier | Numerical analysis of the food web of an intertidal mudflat ecosystem on the Atlantic coast of France[END_REF][START_REF] Giere | Meiobenthology: the microscopic motile fauna of aquatic sediments, Second[END_REF]) and high secondary production (from 4 to 29 gC m -2 year -1 ) [START_REF] Escaravage | The distribution of meiofauna and its contribution to detritic pathways in tidal flats (Arcachon Bay, France)[END_REF][START_REF] Chardy | Carbon flows in a subtidal fine sand community from the western English Channel: a simulation analysis[END_REF]Danovaro et al. 2002) that often exceeds that of the macrofauna [START_REF] Giere | Meiobenthology: the microscopic motile fauna of aquatic sediments, Second[END_REF]. Meiofauna are connected to the microorganisms in the food web via the consumption of microalgae and bacteria [START_REF] Rzeznik-Orignac | Spatio-temporal structure of the nematode assemblages of the Brouage mudflat (Marennes Oléron, France)[END_REF][START_REF] Vafeiadou | Resource utilization and trophic position of nematodes and harpacticoid copepods in and adjacent to Zostera noltii beds[END_REF]) and to the food webs of macroconsumers especially via their consumption by small benthic fish and predatory invertebrates [START_REF] Coull | Role of meiofauna in estuarine soft-bottom habitats[END_REF]. Nematodes are the most dominant group in meiofauna; their food resources can be very specific, or in the contrary, relatively diverse, and their buccal cavity is very distinct based on these feeding types. This morphological trait has been used for decades to define feeding types [START_REF] Wieser | Die Beziehungen zwischen Mundhohlengestalt, Ernahrungsweise und Vorkommen bei freilebenden marinen Nematoden[END_REF][START_REF] Warwick | Nematode associations in the Exe Estuary[END_REF][START_REF] Heip | The ecology of marine nematodes[END_REF] and some more recent methods (i.e. feeding behavior experiments; [START_REF] Moens | Observations on the feeding ecology of estuarine nematodes[END_REF], trophic markers (Moens et al. 2005)) permitted to revise or precise the feeding types of some genera (e.g. assort of Metachromadora from the omnivores/predators to the epigrowth feeders).

Both, biotic and abiotic parameters, have been shown to affect meiofauna community structure.

Among biotic parameters, food availability in general [START_REF] Castel | Influence of seagrass beds and oyster parks on the abundance and biomass patterns of meio-and macrobenthos in tidal flats[END_REF][START_REF] Danovaro | Detritus-Bacteria-Meiofauna interactions in a seagrass bed (Posidonia oceanica) of the NW Mediterranean[END_REF], availability of specific food sources such as bacteria and microphytobenthos (Danovaro and Gambi 2002;[START_REF] Fonseca | Meiobenthic communities of seagrass beds (Zostera capricorni) and unvegetated sediments along the coast of New South Wales, Australia[END_REF], the quality of the food (Danovaro et al. 2002) and the deposition of organic material of pelagic origin [START_REF] Franco | Density, vertical distribution and trophic responses of metazoan meiobenthos to phytoplankton deposition in contrasting sediment types[END_REF]) have been identified as structuring parameters. Abiotic parameters such as sediment grain-size [START_REF] Steyaert | The importance of fine-scale, vertical profiles in characterising nematode community structure[END_REF][START_REF] Dupuy | Structure and functional characteristics of the meiofauna community in highly unstable intertidal mudbanks in Suriname and French Guiana ( North Atlantic coast of South America )[END_REF], habitat structure [START_REF] Fonseca | Meiobenthic communities of seagrass beds (Zostera capricorni) and unvegetated sediments along the coast of New South Wales, Australia[END_REF], oxygen gradients [START_REF] Josefson | Differential response of benthic macrofauna and meiofauna to hypoxia in the Gullmar Fjord basin[END_REF], intertidal water level [START_REF] Steyaert | Tidal migration of nematodes on an estuarine tidal flat (the Molenplaat, Schelde Estuary, SW Netherlands)[END_REF] as well as anthropogenic impacts [START_REF] Semprucci | Benthic communities along a littoral of the Central Adriatic Sea (Italy)[END_REF]) are influencing meiofauna community structure. Studies on meiobenthic communities in intertidal systems have been carried out mainly on single habitats, focusing on effects of abiotic and biotic parameters at the habitat scale [START_REF] Reise | Tidal flat ecology: An experimental approach to species interactions[END_REF][START_REF] Rzeznik-Orignac | Spatio-temporal structure of the nematode assemblages of the Brouage mudflat (Marennes Oléron, France)[END_REF][START_REF] Lebreton | Food sources used by sediment meiofauna in an intertidal Zostera noltii seagrass bed: A seasonal stable isotope study[END_REF]. Factors affecting the community structure of meiofauna at a larger scale [START_REF] Moens | Ecology of free-living marine nematodes[END_REF]) in intertidal habitats are still poorly known, as few studies have compared several habitats and/or different ecosystems.

The present study investigates whether meiobenthic communities, especially free-living nematodes and benthic copepods, differ among habitats with different characteristics (i.e. mudflats, seagrass beds and sandflats; influence of terrestrial organic matter inputs or not) to define which factors are structuring these communities. Particular attention is paid to the role of food sources, as they have been highlighted as important drivers of ecosystem functioning in intertidal habitats [START_REF] Asmus | The importance of grazing food chain for energy flow and production in three intertidal sand bottom communities of the northern Wadden Sea[END_REF][START_REF] Underwood | Primary production by phytoplankton and microphytobenthos in estuaries[END_REF][START_REF] Mclusky | The estuarine ecosystem: ecology, threats and management[END_REF]. Aim of this study is therefore to define if changes in biomass and quality of food sources can affect meiofauna community structure. If yes, the objective is to determine-among all available food sources (i.e. benthic microalgae, sediment organic matter (SOM), suspended particulate organic matter (SPOM) and bacteria)-what food sources play a role and how changes in these food sources influence meiofauna community structure. In this aim, a temporal sampling was donine in spring and autumn in several habitats to define the temporal and spatial variations of the food sources and those of meiofauna community structure.

Materials and methods

Study sites

Marennes-Oléron Bay

The Marennes-Oléron Bay is a semi-enclosed system, located in the middle of the western Atlantic coast of France, between the Oléron Island and the mainland (Fig. 1). The bay has a surface of 180 km 2 [START_REF] Gouleau | Short-and long-term sedimentation on Montportail-Brouage intertidal mudflat, Marennes-Oléron Bay (France)[END_REF] and is influenced by both oceanic and continental inputs, mainly from the Charente River (river input ranges from 15 to 500 m 3 s -1 ) [START_REF] Ravail | Incidence du débit de la Charente sur la capacité biotique du bassin ostréicole du bassin de Marennes-Oléron[END_REF]. Tides are semi-diurnal, with a period of emersion of ca. 6 h per tidal cycle and a tidal range from 0.9 to 6.5 m [START_REF] Kervella | Horizontal and vertical distribution of meiofauna on sandy beaches of the North Sea (The Netherlands, Belgium, France)[END_REF]. The system is dominated by intertidal bare mudflats (91 km 2 , i.e. 58%) which have a flat bottom slope (1:1000) and represent a large tidal area (up to 4.5 km wide) [START_REF] Gouleau | Short-and long-term sedimentation on Montportail-Brouage intertidal mudflat, Marennes-Oléron Bay (France)[END_REF].

The Brouage mudflat, located on the eastern part of the bay, is the most extensive mudflat (40 km 2 ) (Fig. 1). The sediment consists mainly of silt and clay particles (95%) with a size smaller than 63 μm [START_REF] Pascal | Seasonal variation in consumption of benthic bacteria by meio-and macrofauna in an intertidal mudflat[END_REF]. Seagrass beds, consisting of the species Zostera noltii [START_REF] Lebreton | Are epiphytes a significant component of intertidal Zostera noltii beds?[END_REF], are located on the western side of the bay, along the coast of the Oléron Island, which is more influenced by offshore waters than the eastern side [START_REF] Dechambenoy | Apport de la thermographie infrarouge aéroportée à la connaissance de la dynamique superficielle des estuaires (système Charente-Seudre-Anse de l' Aiguillon)[END_REF]. The seagrass meadow is stretched out over 15 km along the upper part of the flat, with an average width of 1.5 km [START_REF] Guillaumont | Utilisation de l'imagerie satellitaire pour des comparaisons spatiales et temporelles en zone intertidale[END_REF].

Sylt-Rømø Bight

The Sylt-Rømø Bight is a semi-enclosed basin, located in the north of Germany between the mainland of Germany and Denmark, the islands of Sylt (Germany) and of Rømø (Denmark) (Fig. 1). The bight consists of a mosaic of habitats, mainly intertidal and subtidal sandflats, seagrass beds, and mudflats [START_REF] Asmus | The importance of grazing food chain for energy flow and production in three intertidal sand bottom communities of the northern Wadden Sea[END_REF]. The bight has a coverage of 404 km 2 of which 135 km 2 are intertidal [START_REF] Asmus | The importance of grazing food chain for energy flow and production in three intertidal sand bottom communities of the northern Wadden Sea[END_REF] and the tidal gauge is 1.8 m on average [START_REF] Asmus | The importance of grazing food chain for energy flow and production in three intertidal sand bottom communities of the northern Wadden Sea[END_REF].

The Arenicola sandflat is the most represented intertidal habitat in the bay with 91 km 2 (67%) [START_REF] Asmus | Significance of suspension-feeder systems on different spatial scales[END_REF]. However, Zostera noltii seagrass beds expanded over the last years, from 15 km 2 in 1995 to 83 km 2 (i.e. 21% of the bay surface) in 2010 [START_REF] Reise | Seagrass recovery in the Northern Wadden Sea?[END_REF]. The intertidal bare mudflats are poorly represented in the bight, with an area of 4 km 2 (Fig. 1).

Field sampling and laboratory processes

Three intertidal habitats were studied in the two bays: a bare mudflat (mudflat MO) and a seagrass bed (seagrass MO) in the Marennes-Oléron Bay, and a bare mudflat (mudflat SR), a seagrass bed (seagrass SR) and a sandflat (sandflat SR) in the Sylt-Rømø Bight (Fig. 1). Based on knowledge already gained about these habitats in each of these five locations [START_REF] Leguerrier | Numerical analysis of the food web of an intertidal mudflat ecosystem on the Atlantic coast of France[END_REF][START_REF] Baird | Trophic dynamics of eight intertidal communities of the Sylt-Rømø Bight ecosystem, northern Wadden Sea[END_REF][START_REF] Pacella | Incorporation of diet information derived from Bayesian stable isotope mixing models into mass-balanced marine ecosystem models: A case study from the Marennes-Oléron Estuary, France[END_REF][START_REF] Saint-Béat | Key features of intertidal food webs that support migratory shorebirds[END_REF]) one sampling station that was the most representative for the studied habitat was selected.

Sediment grain size

Sediment samples (n = 3) were collected randomly with cores (ø = 4 cm) and were split into 0-1 cm and 1-5 cm depth fractions. Sediment grain size was determined by laser granulometry analyses for the sediment from the seagrass bed and mudflats (Mastersize 2000, Malvern Instruments Limited, United Kingdom). The freeze-dried sediment of these habitats was rehydrated before analyses in order to avoid the use of ultrasound to disaggregate the sediment. Sediment grain size of freeze-dried samples from the sandflat of the Sylt-Rømø Bight was determined by dry sieving. Samples were classified based on their median sediment grain size after [START_REF] Wentworth | A scale of grade and class terms for clastic sediments[END_REF].

Benthic primary producers, composite sources and bacteria

For all stations, sampling was carried out in two seasons, in spring (from May 17 to June 17, 2016) and in autumn (from November 14 to December 15, 2016), to compare a season with a high primary production with a season with a high load of detrital matter. Sampling was carried out during low tide, starting about one hour before lowest water level.

Samples for sediment organic matter (SOM), detritus, and seagrass leaves and roots (3 replicates) were taken using cores (ø = 19 cm) which were separated in two layers (a surface layer from 0-1 cm, a sub-surface layer from 1-5 cm). The collected material was gently sieved on a 500 µm mesh screen (1 mm for sandflat SR samples) after which the upper fraction was sorted in seagrass leaves, roots and detritus (brown to black colored leaves and roots). All samples were freeze-dried, then weighed (± 0.001 g). Carbon to dry weight ratios were determined on each sample: A small amount was ground to a fine and homogeneous powder using a ball mill (MM 400, Retsch, Germany) and carbon content was determined using an elemental analyzer (Flash EA 1112, Thermo Scientific, Italy). Subsamples were collected in the lower fraction (SOM), freeze-dried, and ground using a mortar and pestle. Chlorophyll a and organic carbon concentrations were determined in sub-samples by fluorimetry (TD-700, Turner Designs, USA) using the method of [START_REF] Lorenzen | A method for the continuous measurement of in vivo chlorophyll concentration[END_REF] and elemental analyses (Flash EA 1112, Thermo Scientific, Italy), respectively. Chlorophyll a values were converted into carbon biomass using the conversion ratio (47.63) provided by de Jonge (1980) and used as a proxy of fresh material of microalgae origin (i.e. microphytobenthos and trapped phytoplankton) in sediment, thereafter called "benthic microalgae". Carbonates were removed prior to elemental analyses by adding HCl at 2 mol.l -1 drop-bydrop on SOM samples until cessation of bubbling. Subsequently samples were dried at 60°C to constant weight, freeze-dried and ground again.

Bacterial biomass (3 replicates) was determined using cores (ø = 4 cm) which were separated in two layers (0-1 cm and 1-5 cm). These sediment layers were homogenized with a spatula and 2 mL of this material were mixed with 2 mL formalin (4%). Samples were deep frozen in liquid nitrogen and stored at -80°C. The number of bacteria was determined following the protocol of [START_REF] Lavergne | An efficient and rapid method for the enumeration of heterotrophic prokaryotes in coastal sediments by flow cytometry[END_REF]. Samples were diluted (1/2000) using sodium pyrophosphate (P-8010, Sigma Aldrich, France) and Tween 80 (P-1754, Sigma Aldrich, France), homogenized and then incubated at 4 °C during 30 minutes. Samples were then sonicated at 60 W during 30 seconds, after which 1 mL of the pre-diluted sample was stained with 20 µL of SYBR Green I (Invitrogen-11540746; 1/5000 final concentration) and incubated in the dark at room temperature for 15 minutes. Afterwards, 2 µL of calibration beads (1 µM) were added and the samples were analyzed using a flow cytometer (BD FACSCANTO II, BD Biosciences, USA). Bacterial densities were converted into carbon using the conversion factor (20 fgC cell -1 ) established by [START_REF] Lee | Relationships between biovolume and biomass of naturally derived marine bacterioplankton[END_REF].

Suspended particulate organic matter

Sampling of the suspended particulate organic matter (SPOM) was conducted by collecting samples of surface water during high tide, in the same timeframe as the benthic samples. Water was sampled at two stations (I and II) in the Marennes-Oléron Bay and three stations (III, IV and V) in the Sylt-Rømø Bight (Fig. 1). Station I, in the north of the Brouage mudflat, represented the water mass covering the mudflat MO station. Station II, near the inlet of the Marennes-Oléron bay, represented the water mass influencing the seagrass MO station. Station II is regularly sampled in the framework of the SOMLIT surveys (data provided by "Service d'Observation en Milieu Littoral, INSU-CNRS, La Rochelle station").

Water samples collected at stations III and IV represented water masses covering the sandflat SR (III), and the mudflat SR and seagrass SR stations (IV), respectively. Station V, near the entrance of the Sylt-Rømø Bight, represented the SPOM originating from the North Sea.

SPOM was sampled by collecting 10 to 100 mL of surface seawater which was pre-filtered on a 250µm mesh screen to eliminate large particles/organisms. Water samples were then filtered on precombusted (4 hours, 450°C) Whatman GF/F glass fiber filters (0.7 µm nominal porosity) under moderate vacuum and these filters were freeze-dried. Chlorophyll a content, used as a proxy of phytoplankton biomass, of the freeze-dried GF/F filters was measured by fluorimetry using the method of [START_REF] Lorenzen | A method for the continuous measurement of in vivo chlorophyll concentration[END_REF]. Phytoplankton biomass was determined by using the conversion factor (56.4) established by [START_REF] Lü | A field study on the conversion ratio of phytoplankton biomass carbon to chlorophylla in Jiaozhou Bay, China[END_REF].

Meiofauna

Meiofauna was sampled for taxonomic composition, density (ind. m -2 ) and biomass (gC m -2 ) estimations at the same time as benthic primary producers and composite sources. Three replicates were collected randomly in a ten square meters area using cores (ø = 4 cm). Cores were separated in two layers (0-1 cm and 1-5 cm) to study the changes of meiofauna communities related to sediment depth. Samples were sieved with a 500-µm mesh to eliminate macrofauna and large detritus. Meiofauna was extracted from the sediment using Ludox HS-40 (Sigma Aldrich, France) (de Jonge and Bouwman 1977) and retrieved with a 40-µm mesh sieve [START_REF] Higgins | Diversity and evenness: a unifying notation and its consequences[END_REF]. From each replicate, 100 nematodes were haphazardly taken with fine tweezers (#55, Dumont, Switzerland), mounted on slides in anhydrous glycerol to prevent dehydration [START_REF] Seinhorst | A rapid method for the transfer of nematodes from fixative to anhydrous glycerin[END_REF]) and examined microscopically. The taxonomic composition of the nematode community was determined at least to the genera level based on these 100 individuals according to the identification keys of Platt andWarwick (1983, 1988), and [START_REF] Warwick | Taxonomic distinctness and environmental assessment[END_REF]. Density was calculated for each genera as number of individuals per m 2 (ind. m -2 ) and relative abundance as the percent composition of each genera relative to the total number of organisms.

Species diversity indices were calculated on 100 individuals for each replicate at the five habitats for the two seasons. Community structure of the other meiofauna groups was not determined due to low densities. Nematodes were allocated into four trophic groups according to [START_REF] Wieser | Die Beziehungen zwischen Mundhohlengestalt, Ernahrungsweise und Vorkommen bei freilebenden marinen Nematoden[END_REF] as follows:

selective deposit-feeders (1A), non-selective deposit-feeders (1B), epigrowth feeders (2A), and omnivores/predators (2B). Various nematode genera, including at least one of each from the 4 different feeding groups, were selected for determination of individual dry weight and carbon content. Between 10 and 200 nematodes from the same genera were picked for dry weight determination, freeze-dried and weighed (± 1 µg) using a microbalance (ME 5, Sartorius, Germany). Organic carbon composition was determined using an elemental analyzer (Flash EA 1112, Thermo Scientific, Italy). Mean biomass of species-specific nematodes and benthic copepods were used to convert the density of nematodes into grams of carbon per m 2 and dry weight per m 2 (Supplementary material, Appendix 1).

Data treatment and statistical methods

All analyses were conducted using the R software (R Core [START_REF] Team | R: A language and environment for statistical computing[END_REF]) and all hypotheses were tested at p < 0.05 level. Normality of the data was tested using Shapiro-Wilks tests. Homogeneity of variance was checked using Bartlett and Levene tests. Average hierarchical clusters (data dissimilarity, Bray-Curtis distance) were realized to discriminate groups within the food sources and within the meiofauna trophic structure using the vegan package [START_REF] Ndaro | Soft-bottom fauna with emphasis on nematode assemblage structure in a tropical intertidal lagoon in Zanzibar, eastern Africa: I. spatial variability[END_REF]) and pvclust package [START_REF] Suzuki | Hierarchical clustering with p-values via multiscale bootstrap resampling[END_REF]. ANOVA followed by Tukey post-hoc test were used to compare sediment characteristics, meiofauna densities, biomass and diversity indices: Hill's diversity N1 (Hill 1973) and species evenness J' (Formula in supplementary material, Appendix 2). Comparisons between sediment layers were conducted on meiofauna densities (ind. m -3 ) and biomasses (gC m -3 ) expressed in volume of sediment. Comparisons between seasons and habitats were done on meiofauna densities, biomasses and diversity indices from combined layers (expressed per surface area). Environmental variables have been analyzed through a preliminary Principal Components Analysis (PCA) to visualize the differences between habitats based on these variables. In a subsequent step copepods, other meiofauna taxa (ostracods, foraminifers and polychaetes) and feeding types of nematodes were compared with Bray-Curtis similarity redundancy relationship analyses (RDA) made with the vegan package (Oksanen et al.

2017

). The relative abundance of copepods, other meiofauna (ostracods, foraminifers and polychaetes) and different feeding types of nematodes (dependent variables) were linked to environmental variables (independent variables) that were selected after stepwise forward procedure. Multicollinearity was tested for the explanatory variables using the vif.cca function in the vegan package (values over 10 indicated collinearity; [START_REF] Ndaro | Soft-bottom fauna with emphasis on nematode assemblage structure in a tropical intertidal lagoon in Zanzibar, eastern Africa: I. spatial variability[END_REF]. The significance of the model was tested using a two-way ANOVA.

Results

Sediment grain size

Sediment of the mudflats and seagrass beds consisted of a mixture of sand (29 to 64%), silt (29 to 58%) and clay (7 to 15%), while the sandflat consisted mainly of sand (98%) (Fig. 2). Sediments were classified as silt for mudflat MO, mudflat SR (autumn) and seagrass MO (autumn), very fine sand for -9 -seagrass SR, mudflat SR (spring) and seagrass MO (spring) and medium sand for sandflat SR. The mud content within the different layers of the same habitat was relatively similar, with a variation lower than 2%. The sediment grain-size of mudflat MO was lower than all other habitats with a median sediment grain-size of 32 ± 4.3 µm, and the grain-size of sandflat SR was higher than all other habitats with a median sediment grain-size of 407 ± 16.3 µm (Supplementary material, Appendix 3). (16.7%, seagrass bed MO, autumn). In the seagrass beds, leaves and roots were occurring with biomass ranging from 1.6 (0.5%, seagrass SR, autumn) to 15.0 gC m -2 (5.1%, seagrass MO, spring) and 0.3 (0.1%, seagrass SR, autumn) to 24.4 gC m -2 (8.2%, seagrass MO, autumn). Bacteria represented the lowest biomass in all habitats with 0.6 (0.9%, sandflat SR, autumn) to 20.1 gC m -2 (4.5%, mudflat SR, spring). Some differences of food source biomass could be observed between the surface and the sub-surface -10 -layers, always with higher biomass in the surface layer when they occurred: detrital matter in mudflat SR in spring and in seagrass MO in autumn, bacteria in mudflat SR in spring, and benthic microalgae in all habitats in all seasons except for mudflat SR and sandflat SR in autumn (two-way ANOVA with Tukey post-hoc test, highest p-value 0.032).

The average hierarchical cluster of dissimilarities highlighted that food source composition in the sandflat SR was different than in the other habitats (Fig. 3; A), due to the much lower biomass of SOM in sandflat SR (29.2 gC m -2 ) than in mudflats and seagrass beds (256.6 gC m -2 ; two-way ANOVA with Tukey post-hoc test, p < 0.001, Fig. 3C andE). Benthic microalgae biomass was much higher in sandflat SR (54.2 gC m -2 ) than in mudflat SR (27.8 gC m -2 ), mudflat MO (10.5 gC m -2 ) and seagrass MO (22.6 gC m -2 , two-way ANOVA with Tukey post-hoc test, p < 0.001). Food source composition in both mudflats and both seagrass beds of the two bays appeared to be similar as highlighted by the clustering but some variations could be pointed out based on two-way ANOVA. Biomass of bacteria in spring was higher in mudflat SR (13.5 gC m -2 ) compared to mudflat MO (3.9 gC m -2 ), seagrass MO (4.2 gC m -2 ), seagrass SR (6.9 gC m -2 ) and sandflat SR (1.8 gC m -2 , two-way ANOVA with Tukey post-hoc test, p < 0.001). The presence of detrital matter in mudflat SR versus its absence in mudflat MO discriminated both mudflats as well. Lastly, biomass of leaves and roots in spring were higher in the Marennes-Oléron Bay (39.4 gC m -2 ) compared to the Sylt-Rømø Bight (6.4 gC m -2 , two-way ANOVA with Tukey post-hoc test, p < 0.001).

Some differences occurred at the seasonal scale as well, with higher biomass of primary producers in spring when a difference occurred. Such a pattern was observed for benthic microalgae in sandflat SR, seagrass SR and seagrass MO, and for seagrass leaves in seagrass bed MO (two-way ANOVA with Tukey post-hoc test, p < 0.001). Bacteria biomass of mudflat SR and seagrass SR were also higher in spring compared to autumn (two-way ANOVA followed by Tukey post-hoc test, p < 0.001). About detrital matter, seasonal changes followed an opposite pattern when they occurred, with an increase in biomass of detritus from spring to autumn in mudflat SR and seagrass MO (two-way ANOVA with Tukey post-hoc test, p < 0.001). SOM biomass was stable between spring and autumn (two-way ANOVA with Tukey post-hoc test, lowest p-value 0.44).

Regarding quality of potential food sources in the different habitats, they varied from being partly detrital in sandflat SR (57 to 63%) to being almost completely made out of detrital matter in mudflat MO (from 93 to 99%). Carbon: chlorophyll a (C:chl a) ratios of SOM were higher in mudflat MO (1201.4), mudflat SR (681.8) and seagrass MO (504.6) compared to seagrass SR (221.4) and sandflat SR (72.3; two-way ANOVA with Tukey post-hoc test, p < 0.001, Table 1). SOM had much higher C:chl a ratios in the sub-surface layer than in the surface layer (two-way ANOVA with Tukey post-hoc test, p < 0.001).

Variation in C:chl a ratios between spring and autumn were not observed (two-way ANOVA, lowest p-value 0.33, Table 1). Proportions of SOM that were originating from fresh microalgae (i.e. microphytobenthos and trapped phytoplankton) ranged from 3.6 (mudflat MO, spring) to 71.1% (sandflat SR, autumn, Table 1). These proportions were much higher in sandflat SR (66.6%) and seagrass SR (22.0%) than in mudflat SR (7.0%) and mudflat MO (4.0%, two-way ANOVA with Tukey post-hoc test, Table 1). Quantities of SPOM ranged between 0.32 and 0.54 gC L -1 . Minor variations between habitats observed within SPOM are reported in the supplementary material (Appendix 4).

Table 1. Carbon: chlorophyll a ratios (mean ± standard deviation, n = 3) and percentage of carbon originating from fresh microalgae (mean ± standard deviation, %, n = 3) in sediment organic matter (SOM) in the mudflats, seagrass beds and sandflat in the Marennes-Oléron Bay (MO) and the Sylt-Rømø Bight (SR) in spring and autumn. Letters displayed after mean values indicated grouping of similar samples (two-way ANOVA with Tukey post-hoc tests, p < 0.05).

Meiofauna community structure

The meiofauna community was dominated by nematodes with relative abundance ranging from 41.7% (sandflat SR, surface layer, spring) to 100% (mudflat MO, sub-surface layer, spring), except for the surface layer of sandflat SR where copepods dominated (55.5%, Fig. 4). Four other meiofauna taxa occurred in the different habitats: copepods (0 to 55.5%), foraminifera (0 to 7.4%), polychaetes (0 to 5.3%), and ostracods (0 to 0.4%).Total meiofauna density was higher in the surface layer than in the subsurface layer in all habitats (two-way ANOVA with Tukey post-hoc test, highest p-value 0.033). Mean meiofaunal density of both layers combined ranged from 1.7*10 5 ind. m -2 (mudflat SR, autumn) to 3.4*10 6 ind. m -2 (mudflat MO, autumn, Appendix 5). Meiofauna densities (both layers combined) were similar between spring and autumn, except in seagrass SR where densities were higher in spring (two-way ANOVA with Tukey post-hoc test, highest p-value 0.020, Table 2).

Total biomass of nematodes (both layers combined) ranged from 0.053 gC m -2 (1.6*10 5 ind. m -2 , mudflat SR, autumn) to 1.541 gC m -2 (2.9*10 6 ind. m -2 , mudflat MO, spring, Fig. 3F). Nematode density and biomass were higher in surface layers than in the sub-surface layers (two-way ANOVA with Tukey post-hoc test, highest p-value 0.042). Nematodes from the surface layers dominated the contribution to the total biomass in all habitats, with lowest proportions in seagrass SR (54.0%) and mudflat SR (55.4%) and higher proportions in sandflat SR (82.3%), mudflat MO (83.3%) and seagrass SR (95.7%). At the habitat scale, nematode density and biomass were much higher in mudflat MO (density: 3.0*10 6 ind. m -2 ; biomass: 1.257 gC m -2 ) than in all other habitats (two-way ANOVA with Tukey post-hoc test, p < 0.001, Total biomass of benthic copepods, which were merely present in the surface layers, ranged from 0.003 gC m -2 (1.0*10 4 ind. m -2 , mudflat SR, autumn) to 0.110 gC m -2 (1.7*10 5 ind. m -2 , mudflat MO, spring, Fig. 3F). Copepod biomass of the mudflat MO (0.095 gC m -2 ) was much higher than in the mudflat SR (0.006 gC m -2 ), while copepod density was higher in the seagrass MO than in the seagrass SR and mudflat SR (two-way ANOVA with Tukey post-hoc test, p < 0.001). Densities and biomass of benthic copepods were similar in spring and autumn (two-way ANOVA with Tukey post-hoc test, lowest p-value 0.178, Table 2). Praeacanthonchus (2A, 4%). Four genera represented 70% of the community in the sub-surface layer of all habitats combined: Terschellingia (1A, 28%), Sabatieria (1B, 25%), Spirinia (2A, 13%), and Metachromadora (2A, 6%). The surface layer had a higher number of genera (from 3 to 9 genera, mean:

5.8) that represented 75% of the community compared to the sub-surface layer (from 2 to 9 genera, mean:

3.4) for all habitats. The ten most dominant genera per habitat, season and sediment layer are presented in the supplementary material (Appendix 6). Species richness varied from 8 to 28, Hill's diversity from 2.3 to 19.1, and species evenness from 0.41 to 0.88 (Table 3). Species richness, diversity and evenness decreased with depth at all habitats (two-way ANOVA, p < 0.001, Table 4). Species richness and evenness were higher at the seagrass MO than at the mudflat SR and the seagrass SR (two-way ANOVA with Tukey post-hoc test, p < 0.001). Hills diversity was higher at the seagrass MO than at all other habitats (two-way ANOVA with Tukey post-hoc test, p < 0.001). There was no difference in diversity and evenness between seasons (two-way ANOVA, lowest p-value 0.126, Table 4). Layer (in cm) 0-1 1-5 0-1 1-5 0-1 1-5 0-1 1-5 0-1 1-5 0-1 1-5 0-1 1-5 0-1 1-5 0-1 1-5 0-1 1-5 Species richness 22 11 21 13 23 12 15 13 16 11 16 12 24 19 23 14 20 12 21 18

Hill's diversity (N1) 14 5 12 6 13 4 9 5 7 3 6 4 15 12 14 8 11 6 12 7

Species evenness (J') 0.9 0.6 0.8 0.7 0.8 0.6 0.8 0.6 0.7 0.5 0.6 0.6 0.8 0.9 0.8 0.8 0.8 0.7 0.8 0.7 Community compositions, based on proportions of benthic copepods and of trophic groups of nematodes, were similar between the two mudflats (besides the very large difference of biomass), as highlighted by the average hierarchical cluster analysis (p < 0.05, Fig. 3B). The sandflat SR and the two seagrass beds had similar communities as well. The difference of trophic structures between the mudflats and the other habitats was related to the higher proportions of selective deposit-feeders (1A) and of nonselective deposit-feeders (1B) in the mudflats. The mudflat MO was dominated by genera belonging to non-selective deposit feeders (36%, 0.579 gC m -2 ), epigrowth feeders (34%. 0.525 gC m -2 ), and selective deposit feeders (25%, 0.071 gC m -2 ) with Terschellingia (1A, 22%, 0.055 gC m -2 ), Comesa (1B, 14%, 0.082 gC m -2 ) and Ptycholaimellus (2A, 13%, 0.048 gC m -2 ) being the most common genera (Fig. 3D). At the mudflat SR, the non-selective deposit feeders dominated (55%), and the most abundant genera were Sabatieria (1B, 43%, 0.042 gC m -2 ) and Terschellingia (1A, 13%, 0.004 gC m -2 ). The seagrass bed SR was dominated by epigrowth feeders with the genera Spirinia (2A, 46%, 0.225 gC m -2 ) and Chromadorita (2A, 16%, 0.026 gC m -2 ). The seagrass bed MO was dominated by various genera belonging to epigrowth feeders (31%, 0.165 gC m -2 ) and non-selective deposit feeders (29%, 0.061 gC m -2 ). There, the biomass of nematodes was dominated by the epigrowth feeder genera Metachromadora (7%, 0.035 gC m -2 ). The sandflat SR was dominated by genera belonging to epigrowth feeders (35%, 0.023 gC m -2 ) and nonselective deposit feeders (19%, 0.040 gC m -2 ), with the genera Microlaimus (2A, 15%, 0.006 gC m -2 ) and Daptonema (1B, 13%, 0.028 gC m -2 ) being the most common.

Relationships between environmental parameters and meiofauna trophic structure

The PCA on the environmental variables explained 63.2% of the total variation (Axis 1: 39.9%, axis 2: 23.3%, Fig. 5A). Axis 1 highlights a positive relationship between SPOM, benthic microalgae and sediment grain size. At the opposite, when biomass of benthic microalgae, SPOM and sediment grain size were low, quantities of SOM were higher, as well as bacteria and detritus biomass, and carbon:chlorophyll a ratios of SOM. Axis 2 demonstrates that biomass of bacteria increased with biomass of SOM, SPOM, and benthic microalgae. Sandflat SR is defined by larger sediment grain size, higher biomass of benthic microalgae and of SPOM, as highlighted on axis 1. Axis 1 also discriminates seagrass SR from mudflat SR, mudflat MO and seagrass MO (autumn), the later ones being characterized with lower biomass of benthic microalgae and SPOM and/or higher carbon:chlorophyll a ratios of SOM, and higher biomass of SOM and detritus. In spring axis 2 discriminates well the mudflat and seagrass bed in the Marennes-Oléron Bay compared to the Sylt-Rømø Bight, which could be due to higher biomass of bacteria (mudflat SR) and/or benthic microalgae (seagrass SR). In terms of temporal variations, axis 1 highlights low variation in environmental variables between spring and autumn except for seagrass MO, which could be due to a significant increase in biomass of detritus from spring to autumn in this habitat.

Axis 2 highlights a stronger temporal variation (Fig. 5A), especially in seagrass SR, seagrass MO and mudflat SR. For seagrass SR this could be due to a decrease in bacteria, benthic microalgae, SPOM and SOM biomass from spring to autumn, as well as an increase in detrital matter, while changes in seagrass MO may be related to opposite patterns. For mudflat SR variations could be explained by the decrease in biomass of benthic microalgae and bacteria and/or the increase in detritus from spring to autumn.

The redundancy analyses (RDA) represented 51.8% of the total variation and 81.1% of the constrained variation (Fig. 5B). Fresh seagrass material was excluded from the RDA due to collinearity after stepwise forward selection of constraints (vif.cca > 10). Axis 1 explains 33.0% of the total variation and 51.8% of the constrained variation. Axis 2 explains 18.8% of the total variation and 29.4% of the constrained variation. Both axes significantly explain the model, which highlights relationships between differences of communities and environmental parameters, particularly biomass of benthic microalgae, SPOM, detritus, SOM, carbon:chlorophyll a ratios and sediment grain size. Patterns highlighted by axis 1 about environmental parameters were similar as those highlighted in the PCA: there was a positive relationship between benthic microalgae and SPOM biomasses, and sediment grain size. At the opposite, when benthic microalgae, SPOM and detritus biomasses were low, the carbon:chlorophyll a ratio was higher, as well as biomasses of SOM, bacteria and detritus. Differences of meiofauna trophic structure within habitats are mainly explained by sediment grain-size, carbon:chlorophyll a ratio of SOM, and biomasses of SOM, SPOM, detritus, benthic microalgae, and bacteria (Fig. 5). Selective deposit feeders A regression using nematode feeding type data from all habitats highlighted that the ratios between densities of epigrowth feeders and densities of non-selective deposit feeders increased when benthic microalgae biomass increased (F[1, 8] = 10.4, p = 0.01, r 2 = 0.61, Fig. 6). The relative abundance of selective deposit feeders was positively correlated to the ratio between biomass of fresh organic material (i.e. benthic microalgae and seagrass leaves and roots) and biomass of detrital material (i.e. SOM and detritus) (F[1, 8] = 23.1, p < 0.01, r 2 = 0.74, Fig. 7). (n = 3)

Discussion

Habitat description

The different habitats compared in this study are characterized by a broad range of abiotic and biotic factors. This provides a good framework to determine how changes in these parameters, particularly potential food resources, may influence meiofauna community structure. Biomass of benthic microalgae was much higher in habitats with a larger sediment grain size (sandflat SR and seagrass SR) as already observed in the Marennes-Oléron Bay [START_REF] Guarini | Dynamics of spatial patterns of microphytobenthic biomass: inferences from a geostatistical analysis of two comprehensive surveys in Marennes-Oléron Bay (France)[END_REF], and benthic microalgae therefore represented a much higher proportion of the SOM in these habitats. This pattern is quite unusual, as habitats with large sediment grain size generally support less microphytobenthos biomass than habitats with small sediment grain size [START_REF] Cammen | Annual bacterial production in relation to benthic microalgal production and sediment oxygen uptake in an intertidal sandflat and an intertidal mudflat[END_REF][START_REF] Jong | Dynamics and distribution of microphytobenthic chlorophyll-a in the Western Scheldt estuary (SW Netherlands)[END_REF]. Estimations of benthic microalgae biomass were based on chlorophyll a measurements and therefore included trapped fresh phytoplankton. Trapped phytoplankton represents a lower proportion of fresh microalgae material than benthic microphytobenthos, although some variations can occur depending on seasons and locations [START_REF] Asmus | Benthische und pelagische Primärproduktion und Nährsalzbilanz. Eine Freilanduntersuchung im Watt der Nordsee[END_REF][START_REF] Lebreton | Food sources used by sediment meiofauna in an intertidal Zostera noltii seagrass bed: A seasonal stable isotope study[END_REF].

Microphytobenthos community structure and its production may change as well depending on habitats: muddy sediments are usually dominated by large epipelic diatoms [START_REF] Thornton | Factors affecting microphytobenthic biomass, species composition and production in the Colne Estuary (UK)[END_REF][START_REF] Azovsky | Spatio-temporal variability of micro-and meiobenthic communities in a White Sea intertidal sandflat[END_REF]) whereas small epipsammic diatoms are the main taxa in sandy habitats [START_REF] Asmus | The microphytobenthos of Königshafen-spatial and seasonal distribution on a sandy tidal flat[END_REF][START_REF] Azovsky | Spatio-temporal variability of micro-and meiobenthic communities in a White Sea intertidal sandflat[END_REF]). Benthic microalgae production can be very high in muddy sediments (i.e. 372 gC m - 2 yr -1 in mudflat MO) [START_REF] Pinckney | Modeling the annual production of intertidal benthic microalgae in estuarine ecosystems[END_REF][START_REF] Leguerrier | Numerical analysis of the food web of an intertidal mudflat ecosystem on the Atlantic coast of France[END_REF] and, under favorable conditions, epipelic diatoms can double their standing stock on a daily basis [START_REF] Underwood | Recovery of intertidal benthic diatoms after biocide treatment and associated sediment dynamics[END_REF]. All these changes of biomass, community structure and production rates likely affect the use of this food resource by meiofauna.

Biomass of SOM was much lower in sandflat SR than in seagrass beds and mudflats. This is likely related to the high benthic microalgae production in bare mudflats [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] and to the smaller sediment grain size in mudflats and seagrass beds [START_REF] Bergamaschi | The effect of grain size and surface area on organic matter, lignin and carbohydrate concentration, and molecular compositions in Peru Margin sediments[END_REF]. A decrease in sediment grain size generally leads to an increase in SOM [START_REF] Bergamaschi | The effect of grain size and surface area on organic matter, lignin and carbohydrate concentration, and molecular compositions in Peru Margin sediments[END_REF] as fine particles enhance adsorption of organic compounds [START_REF] Mayer | Relationships between mineral surfaces and organic carbon concentrations in soil and sediments[END_REF]. Decrease of sediment grain size is also associated to lower hydrodynamics which increases sedimentation of organic matter particles [START_REF] Frontalini | Biodiversity trends of the meiofaunal and foraminiferal assemblages of Lake Varano (southern Italy)[END_REF]) and of finer sediment. The presence of seagrass beds, reducing hydrodynamics and enhancing sedimentation as well [START_REF] Leduc | A multi-method approach for identifying meiofaunal trophic connections[END_REF], also likely increased the load of SOM. As a result, even if SPOM was relatively similar between habitats, the presence of meadows may influence the biomass and the nature of the organic matter available to meiofauna between unvegetated and vegetated habitats [START_REF] Machás | Sources of organic matter in Ria Formosa revealed by stable isotope analysis[END_REF]. The quality of the organic matter available to consumers was also correlated with sediment grain size, with SOM dominated by fresh organic matter in the sandflat and being of much lower quality in habitats made of fine sediments (seagrass bed and mudflats), where loads of detrital matter were higher.

Loads of detritus were high in seagrass beds, the largest ones being measured in seagrass MO, probably due to the much higher biomass of seagrass roots in this habitat compared to seagrass SR. The higher load of detrital matter in mudflat SR compared to mudflat MO is much likely related to the proximity between the mudflat and the seagrass bed in the Sylt-Rømø Bight, highlighting that seagrass material can be exported to bare systems. The presence of seagrass in intertidal systems may also indirectly affect the composition of the detrital organic matter available to meiofauna and meiofauna distribution in the sediment [START_REF] Marbà | Seagrass beds and coastal biogeochemistry[END_REF]). The complex root structure of seagrass beds indeed plays a role for the amount of fine particles and detritus associated with the shallow oxic zone in vegetated sediments [START_REF] Wetzel | Effects of hypoxia and anoxia on meiofauna: A review with new data from the Gulf of Mexico[END_REF], as well as in the release of oxygen and dissolved organic matter by the roots [START_REF] Pedersen | Oxygen dynamics in the rhizosphere of Cymodocea rotundata[END_REF]). Nevertheless, seagrass material is generally poorly used by most of the consumers as fresh or as detrital material [START_REF] Danovaro | Detritus-Bacteria-Meiofauna interactions in a seagrass bed (Posidonia oceanica) of the NW Mediterranean[END_REF][START_REF] Vizzini | The role and contribution of the seagrass Posidonia oceanica (L.) Delile organic matter for secondary consumers as revealed by carbon and nitrogen stable isotope analysis[END_REF]. The presence of seagrass and detritus as well as smaller sediment grain size also affected the abundance of bacteria within the studied habitats. Much lower biomass of bacteria was observed in habitats with lower quantities of detrital matter and SOM and in habitats with coarser sediment (i.e. sandflat), likely due to the reduced surface for bacteria colonization [START_REF] Dale | Bacteria in intertidal sediment: Factors related to their distribution[END_REF][START_REF] Mayer | Relationships between mineral surfaces and organic carbon concentrations in soil and sediments[END_REF]).

Meiofauna community structure: spatial and temporal variations

Meiofauna density and biomass varied highly within habitats and ecosystems, with lowest and highest values observed in mudflat SR (density: 0.17*10 6 ind. m -2 , biomass: 0.057 gC m -2 ) and mudflat MO (density: 3.36*10 6 ind. m -2 , biomass: 1.598 gC m -2 ), respectively. Meiofauna in all habitats were dominated by nematodes, followed by benthic copepods, as typically observed in intertidal habitats [START_REF] Armonies | Synoptic patterns of meiofaunal and macrofaunal abundances and specific composition in littoral sediments[END_REF][START_REF] Rzeznik-Orignac | Spatio-temporal structure of the nematode assemblages of the Brouage mudflat (Marennes Oléron, France)[END_REF][START_REF] Leduc | Small-scale effect of intertidal seagrass (Zostera muelleri) on meiofaunal abundance, biomass, and nematode community structure[END_REF]. Data comparisons (Table 5) highlight that density and biomass of nematodes and benthic copepods measured in this study are in the range of those previously measured but, more generally, that these values can be highly variable between different soft-bottom habitats as well as within similar habitats [START_REF] Heip | The ecology of marine nematodes[END_REF][START_REF] Castel | Influence of seagrass beds and oyster parks on the abundance and biomass patterns of meio-and macrobenthos in tidal flats[END_REF]Kotwicki et al. 2005b).

In terms of community structure, proportions of benthic copepods increased from habitats with a lower complexity (mudflats: 6.2%) to habitats with a higher complexity caused by a higher seagrass biomass (seagrass beds: 15.5%) or by larger sediment grain size (sandflat: 38.4%). Larger proportions of copepods in the sandflat could be explained by a higher feeding efficiency in sandier sediment, probably due to a better accessibility to benthic microalgae and a better ability of these grazers to move in sand [START_REF] De Troch | Does sediment grain size affect diatom grazing by harpacticoid copepods?[END_REF]). Higher proportions of benthic copepods in seagrass bed MO may be related to a more developed seagrass bed and therefore a more complex habitat, providing a larger diversity of ecological niches [START_REF] Ndaro | Soft-bottom fauna with emphasis on nematode assemblage structure in a tropical intertidal lagoon in Zanzibar, eastern Africa: I. spatial variability[END_REF][START_REF] De Troch | Zonation and structuring factors of meiofauna communities in a tropical seagrass bed (Gazi Bay, Kenya)[END_REF]. The presence and development stage of seagrass beds also affected nematode diversity as species richness was higher in the most dense seagrass bed (Marennes-Oléron) between the two studied ones. Hill's diversity was also higher in seagrass MO than in all other habitats, which could be partially linked to habitat complexity as well [START_REF] Mc | Effects of biological disturbance on diversity and structure of meiobenthic nematode communities[END_REF][START_REF] Steyaert | The importance of fine-scale, vertical profiles in characterising nematode community structure[END_REF][START_REF] Giere | Meiobenthology: the microscopic motile fauna of aquatic sediments, Second[END_REF]. Proportions of omnivore/predatory nematodes increased as well when complexity increased due to larger sediment grain size as previously observed in sandflats of several other coastal ecosystems [START_REF] Warwick | Nematode associations in the Exe Estuary[END_REF][START_REF] Steyaert | The importance of fine-scale, vertical profiles in characterising nematode community structure[END_REF]. This could be explained by the increase of foraging efficiency of predators with an increase in sediment grain size [START_REF] Gallucci | Can field distributions of marine predacious nematodes be explained by sediment constraints on their foraging success?[END_REF].

If some patterns could be observed between meiofauna community structure and habitat type, direct relationships between biomass of food sources and density and/or biomass of nematodes and benthic copepods could not be observed in this study. This is very likely related to the diversity of feeding types observed in nematodes (Moens et al. 2005b;[START_REF] Vafeiadou | Resource utilization and trophic position of nematodes and harpacticoid copepods in and adjacent to Zostera noltii beds[END_REF]) and, to a lower extent, in benthic copepods [START_REF] Cnudde | Trophodynamics of intertidal harpacticoid copepods based on stable isotopes and fatty acid profiling[END_REF], confirming the importance of approaches carried out at the scale of the feeding type, as this was done for nematodes in this study.

Within these different habitats nematode density, biomass and diversity were similar between spring and autumn, except for higher nematode density in seagrass SR and higher nematode biomass in mudflat MO in spring. No temporal variation was observed for meiofauna community structure. The lack of a clear temporal pattern between spring and autumn [START_REF] Escaravage | The distribution of meiofauna and its contribution to detritic pathways in tidal flats (Arcachon Bay, France)[END_REF]Danovaro and Gambi 2002;[START_REF] Rzeznik-Orignac | Spatio-temporal structure of the nematode assemblages of the Brouage mudflat (Marennes Oléron, France)[END_REF]) could be partly explained by low temporal variation in parameters strongly defining meiofauna community structure (i.e. food sources and sediment grain size; Moens et al.

2013).

Table 5. Density (ind. m -2 * 10 6 ) and biomass (in gC m -2 and/or g dry weight m -2 ) of nematodes and benthic copepods in mudflats, seagrass beds (Zostera spp.) and sandflats from this study and from the Benthic microalgae and sediment organic matter are important drivers of nematode community structure Changes in benthic microalgae biomass influenced the community structure of nematodes: relative abundance of epigrowth feeders, relying on benthic microalgae, were reduced when biomass of this food source decreased. Such a relationship between biomass of benthic microalgae and density of epigrowth feeders had already been observed in a subtidal seagrass meadow (Danovaro and Gambi 2002) and during grazing experiments (Rzeznik-Orignac and Fichet 2012). A relationship was also observed between biomass of benthic microalgae and relative abundances of benthic copepods and omnivores/predators.

However, this relationship may be partly mediated by sediment grain size as benthic microalgae biomass (this study) and relative abundance of benthic copepods increase when sediment get coarser [START_REF] Hicks | The ecology of marine meiobenthic harpacticoid copepods[END_REF]). Nevertheless, the much higher biomass of benthic microalgae in the sandflat compared to SOM suggests that consumers rely--at least partly--directly (i.e. copepods) or indirectly (i.e.

omnivores/predators) on benthic microalgae in this habitat. Such a dependence to benthic microalgae as a food source has been demonstrated for benthic copepods [START_REF] Leduc | A multi-method approach for identifying meiofaunal trophic connections[END_REF].

At a more global scale (i.e. whole meiofauna, all habitats combined), no general pattern could be observed between benthic microalgae biomass and those of nematodes and benthic copepods. Some other parameters should probably be taken into account to better consider relationships between this food resource and consumers. Benthic microalgae can indeed reach very high production rates and is a food source of high quality [START_REF] Cebrián | Patterns in the fate of production in plant communities[END_REF]. Intense grazing by meiofauna may affect benthic microalgae as well.

The high primary production of benthic microalgae in mudflat MO (372 gC m -2 yr -1 ; [START_REF] Leguerrier | Numerical analysis of the food web of an intertidal mudflat ecosystem on the Atlantic coast of France[END_REF] and high grazing pressure on this food source by the large biomass of nematodes [START_REF] Blanchard | Measurement of meiofauna grazing rates on microphytobenthos: is primary production a limiting factor?[END_REF]) could explain the lack of relationship between benthic microalgae and meiofauna biomass within these habitats.

Quality is of high importance as well: Moens et al. (1999b) suggested that meiofauna migrates actively towards 'optimal' food sources. In addition to epigrowth feeders, several studies demonstrated that deposit feeders rely on microalgae [START_REF] Moens | Observations on the feeding ecology of estuarine nematodes[END_REF][START_REF] Rzeznik-Orignac | Spatio-temporal structure of the nematode assemblages of the Brouage mudflat (Marennes Oléron, France)[END_REF][START_REF] Vafeiadou | Resource utilization and trophic position of nematodes and harpacticoid copepods in and adjacent to Zostera noltii beds[END_REF] likely due to the better quality of microalgae. A relationship between density of nematodes belonging to these feeding groups and microalgae has been observed (Moens et al. 1999a). In particular, grazing experiments using 14 C labelled diatoms indicated the dependence of a non-selective deposit feeding nematode (Daptonema), an epigrowth feeding nematode (Metachromadora) and benthic copepods on microalgae (Rzeznik-Orignac and Fichet 2012). Buccal cavities of epigrowth feeders are appropriate for consumption of benthic microalgae, due to the presence of a dorsal tooth which is used to pierce diatoms [START_REF] Wieser | Die Beziehungen zwischen Mundhohlengestalt, Ernahrungsweise und Vorkommen bei freilebenden marinen Nematoden[END_REF][START_REF] Moens | Observations on the feeding ecology of estuarine nematodes[END_REF]. [START_REF] Vafeiadou | Resource utilization and trophic position of nematodes and harpacticoid copepods in and adjacent to Zostera noltii beds[END_REF], using stable isotopes, confirmed that nonselective deposit feeding nematodes, epigrowth feeding nematodes and benthic copepods can largely rely on benthic microalgae in sparsely covered seagrass beds. The non-selective deposit feeders can therefore be considered as opportunistic feeders, which may change feeding strategies depending on the availability of food sources [START_REF] Moens | Observations on the feeding ecology of estuarine nematodes[END_REF][START_REF] Rzeznik-Orignac | Stable isotope analysis of food source and trophic position of intertidal nematodes and copepods[END_REF]). As a result, benthic microalgae are often considered as the principal food source for nematodes and benthic copepods (Moens et al. 2005a;[START_REF] Rzeznik-Orignac | Stable isotope analysis of food source and trophic position of intertidal nematodes and copepods[END_REF][START_REF] Maria | The importance of biological interactions for the vertical distribution of nematodes in a temperate ultra-dissipative sandy beach[END_REF].

Benthic microalgae followed an opposite pattern than detrital material (SOM and detritus), with decreasing biomass of benthic microalgae when detrital material increased. SOM biomass was stable between spring and autumn, as well as between surface and sub-surface layers, but large variations were observed between the different habitats (i.e. much lower biomass in sandflat SR). Relative abundance of non-selective deposit feeders, closely related to detrital matter based on the RDA, decreased when biomass of SOM was lowered. Higher SOM quality in the surface sediment layers could lead to higher biomass, density and diversity of nematodes in the surface layer compared to the sub-surface layer. As a result, the important role of SOM is very likely related to its very high amount, as well as variations in quality between the studied systems. A relationship between SOM and density of non-selective deposit feeders was also recognized in a subtidal seagrass meadow (Danovaro and Gambi 2002), while a grazing experiment indicated that non-selective and selective deposit feeding nematode species assimilated predominantly SOM and bacteria [START_REF] Tietjen | Feeding behaviour of marine nematodes[END_REF].

As a result, SOM and benthic microalgae appear to be important biotic factors driving the nematode community structure and, a fortiori, the composition of meiofauna assemblages as nematodes represent a large proportion of the meiofauna in coastal ecosystems [START_REF] Armonies | Synoptic patterns of meiofaunal and macrofaunal abundances and specific composition in littoral sediments[END_REF][START_REF] Rzeznik-Orignac | Spatio-temporal structure of the nematode assemblages of the Brouage mudflat (Marennes Oléron, France)[END_REF][START_REF] Leduc | Small-scale effect of intertidal seagrass (Zostera muelleri) on meiofaunal abundance, biomass, and nematode community structure[END_REF].

Role of bacteria as a food source?

Selective deposit feeders had much higher relative abundances in habitats with larger loads of detritus and SOM compared to those dominated by fresh material. In addition, multivariate analyses indicated that biomass of bacteria were higher when loads of SOM increased. These results highlight a probable relationship between bacteria biomass and relative abundance of selective deposit feeders, endorsing that consumers from this trophic group rely on bacteria [START_REF] Moens | Observations on the feeding ecology of estuarine nematodes[END_REF], which develop on detrital particles (Day et al. 1989;[START_REF] Danovaro | Detritus-Bacteria-Meiofauna interactions in a seagrass bed (Posidonia oceanica) of the NW Mediterranean[END_REF], or dissolved organic matter [START_REF] Moens | Observations on the feeding ecology of estuarine nematodes[END_REF]. The use of bacteria as food resources has been confirmed by stable isotope studies demonstrating that selective deposit feeders can rely on sulfide-oxidizing bacteria [START_REF] Vafeiadou | Resource utilization and trophic position of nematodes and harpacticoid copepods in and adjacent to Zostera noltii beds[END_REF], such specific diets being related to their small buccal cavity [START_REF] Wieser | Die Beziehungen zwischen Mundhohlengestalt, Ernahrungsweise und Vorkommen bei freilebenden marinen Nematoden[END_REF][START_REF] Moens | Observations on the feeding ecology of estuarine nematodes[END_REF]. The lack of direct relationship between abundances of selective deposit feeders and biomass of bacteria, as in [START_REF] Semprucci | Benthic communities along a littoral of the Central Adriatic Sea (Italy)[END_REF], might be due to the influence of additional abiotic parameters (i.e. oxic conditions [START_REF] Jensen | Meiofaunal abundance and vertical zonation in a sublittoral soft bottom, with a test of the Haps corer[END_REF]) and sediment grain size [START_REF] Steyaert | The importance of fine-scale, vertical profiles in characterising nematode community structure[END_REF])), the reliance of bacteria on a large diversity of substrates [START_REF] Boschker | Limited coupling of macrophyte production and bacterial carbon cycling in the sediments of Zostera spp. meadows[END_REF], and high bacterial production which is only partially grazed (< 28%, Pascal et al.

2009).

Multivariate analyses demonstrated as well a relationship between relative abundances of nonselective deposit feeders and biomass of detritus and, to a lower extent, those of bacteria and SOM.

Nevertheless, no direct relationship was observed between relative abundances of non-selective deposit feeders and bacteria, SOM or detritus biomass, which is likely related to the more opportunistic feeding behavior of these nematodes [START_REF] Moens | Observations on the feeding ecology of estuarine nematodes[END_REF][START_REF] Rzeznik-Orignac | Stable isotope analysis of food source and trophic position of intertidal nematodes and copepods[END_REF]. The shape of the buccal cavity of non-selective deposit feeders allows them to rely on a much larger diversity of food sources: benthic microalgae, SOM, bacteria and seagrass detritus, as demonstrated by stable isotope studies [START_REF] Rzeznik-Orignac | Stable isotope analysis of food source and trophic position of intertidal nematodes and copepods[END_REF][START_REF] Maria | The importance of biological interactions for the vertical distribution of nematodes in a temperate ultra-dissipative sandy beach[END_REF][START_REF] Vafeiadou | Resource utilization and trophic position of nematodes and harpacticoid copepods in and adjacent to Zostera noltii beds[END_REF]) and observations of feeding behavior [START_REF] Moens | Observations on the feeding ecology of estuarine nematodes[END_REF]. Biomass of fresh seagrass matter, which varied highly between habitats in this study, could also not be linked to variations in non-selective deposit feeders. However, the lack of relation here is expected since this resource is poorly used by these nematodes, and, to a larger extent, to meiofauna in general, due to its low nutritional value and high lignocellulose content [START_REF] Vizzini | The role and contribution of the seagrass Posidonia oceanica (L.) Delile organic matter for secondary consumers as revealed by carbon and nitrogen stable isotope analysis[END_REF]. This was confirmed by [START_REF] Materatski | A comparative analysis of benthic nematode assemblages from Zostera noltii beds before and after a major vegetation collapse[END_REF] who observed no changes in nematode density before and after a seagrass collapse.

Effect of sediment depth on meiofauna communities: influence of oxic conditions

Nematode and benthic copepod densities and biomasses decreased from surface to sub-surface layers in all habitats, which is a common pattern in seagrass beds and unvegetated habitats [START_REF] Castel | Influence of seagrass beds and oyster parks on the abundance and biomass patterns of meio-and macrobenthos in tidal flats[END_REF][START_REF] Lebreton | Analyse de la structure et du fonctionnement du reseau trophique d'un herbier par approche multi-traceurs: tracage isotopique naturel et profils acides gras[END_REF][START_REF] Leduc | Small-scale effect of intertidal seagrass (Zostera muelleri) on meiofaunal abundance, biomass, and nematode community structure[END_REF]. Benthic microalgae, which have the highest biomass in the surface layer, might influence the vertical distribution and migration of nematodes and benthic copepods [START_REF] Moens | Ecology of free-living marine nematodes[END_REF]), particularly nematode epigrowth feeders which specifically rely on benthic microalgae and which were very abundant in all habitats. Differences in abiotic parameters, such as oxygen depletion towards deeper layers, could affect as well the vertical distribution of meiofauna [START_REF] Steyaert | The importance of fine-scale, vertical profiles in characterising nematode community structure[END_REF][START_REF] Vanaverbeke | Changes in morphometric characteristics of nematode communities during a spring phytoplankton bloom deposition[END_REF], as anoxic conditions are much less favorable to nematodes and benthic copepods.

Besides influencing biomass and densities, anoxic conditions affect diversity and community structure as well. Diversity of nematodes clearly decreased with depth in all the studied habitats, which was observed in other studies [START_REF] Steyaert | The importance of fine-scale, vertical profiles in characterising nematode community structure[END_REF][START_REF] Leduc | Small-scale effect of intertidal seagrass (Zostera muelleri) on meiofaunal abundance, biomass, and nematode community structure[END_REF]. Terschellingia spp. and Sabatieria spp. had a higher relative abundance in sub-surface layers than in surface layers, related to their higher tolerance regarding hypoxic conditions [START_REF] Steyaert | Responses of intertidal nematodes to short-term anoxic events[END_REF]). Terschellingia spp. and Sabatieria spp. are also known to feed on sulfide-oxidizing bacteria that occur in these hypoxic zones [START_REF] Semprucci | Free-living marine nematodes as bioindicators: past, present and future perspectives[END_REF][START_REF] Vafeiadou | Resource utilization and trophic position of nematodes and harpacticoid copepods in and adjacent to Zostera noltii beds[END_REF].

Conclusion

Benthic microalgae and SOM are two important drivers in the functioning of soft-bottom coastal food webs as changes in these food resources affect the two most dominant nematode trophic groups (i.e. nonselective deposit feeders, considered as more opportunistic, and epigrowth feeders, more specifically related to benthic microalgae). This is likely related to the high biomass of these food sources, as well as the quality and high production rates of benthic microalgae [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][START_REF] Cebrián | Patterns in the fate of production in plant communities[END_REF]. Therefore, this study highlights the existence of some relationships between composition of food sources and the trophic structure of the meiofauna community, when studied at the scale of trophic groups. On the contrary, no clear relationship was observed between bacterial biomass and density of selective deposit feeders (bacterivores; 1A), despite a relationship between SOM biomass and relative abundances of selective deposit feeders. This lack of relationship may be related to the high production rates of bacteria and the influence of abiotic factors, especially oxic conditions. Approaches such as trophic markers (e.g. stable isotope and/or fatty acid analyses) would be very complementary to the determination of feeding types based on the morphology of the buccal cavity. Such tools would provide a more comprehensive picture of food sources really assimilated by meiofauna and highlight how the use of food sources by meiofauna may change depending on availability. Combining trophic marker data with biomass and production data in food web models would then provide a much more complete overview of the functioning of coastal food webs.

Compliance with ethical standards

Figure 1 .

 1 Figure 1. Sampling stations of the mudflats, seagrass beds and sandflat in the Marennes-Oléron Bay (MO) and the Sylt-Rømø Bight (SR) along the European coast. Pelagic sampling stations, where samples for suspended particulate organic matter were taken, are indicated by roman numbers in both Marennes-Oléron Bay (I-II) and Sylt-Rømø Bight (III-V).

Figure 2 .

 2 Figure 2. Ternary diagram presenting the sediment composition of the mudflats, seagrass beds and sandflat in the Marennes-Oléron Bay (MO) and the Sylt-Rømø Bight (SR) in spring (grey symbols) and autumn (black symbols).Separation by size for clay (< 3.9 µM), silty (3.9 -63 µM), and sandy (63 -2000 µM) sediment. (n = 3)

Figure 3 .

 3 Figure 3. Composition of potential benthic food sources for meiofauna and of meiofauna in the mudflats, seagrass beds and sandflat in the Marennes-Oléron Bay (MO) and the Sylt-Rømø Bight (SR) in spring and autumn. A) Hierarchical average cluster of dissimilarity for the proportional composition of potential food sources. B) Hierarchical average cluster of dissimilarity for proportional composition of meiofauna (nematodes and benthic copepods). Approximately unbiased (AU) probability values (p-values) by multiscale bootstrap resampling are under each node and clusters (AU >= 95%) are highlighted with grey rectangles. C) Proportional composition of potential food sources. D)

Figure 4 .

 4 Figure 4. Proportional composition of meiofauna in the mudflats, seagrass beds and sandflat in the Marennes-Oléron Bay (MO) and the Sylt-Rømø Bight (SR) in spring and autumn and in each sediment layer (0-1cm/1-5cm). Densities (%) are indicated when > 7.5%. (n = 3)

  and non-selective deposit feeders occurred mainly within habitats where biomasses of SOM, detritus and bacteria where high (i.e. sub-surface layers of mudflat SR in both seasons, sub-surface layers of seagrass MO in both seasons, and sub-surface layers of mudflat MO in autumn). Epigrowth feeders dominated in habitats where benthic microalgae were an important driving factor (i.e. surface layers of mudflat MO in both seasons, sub-surface layers of mudflat MO in spring, surface and sub-surface layers of seagrass SR in both seasons, and sub-surface layers of sandflat SR in both seasons). Omnivore/predatory nematodes, benthic copepods and other meiofauna taxa occurred in habitats with high biomass of benthic microalgae and SPOM, as well as large sediment grain-size (i.e. surface and sub-surface layers of sandflat SR in both seasons and surface layers of seagrass MO in both seasons, and surface layers of mudflat SR in both seasons).

Figure 5 .

 5 Figure 5. (A) Principal component analyses for the environmental factors (arrows) plotted for the mudflats, seagrass beds and sandflat in the Marennes-Oléron Bay (MO) and the Sylt-Rømø Bight (SR) in spring (closed symbols) and autumn (open symbols). (B) Redundancy relationship analyses of environmental factors (arrows) selected with stepwise forward procedure plotted against the different groups of meiofauna (benthic copepods, trophic groups of nematodes and other meiofauna taxa) and the surface (grey symbols) and sub-surface layers (black symbols) of the mudflats, seagrass beds and sandflat in the Marennes-Oléron Bay (MO) and the Sylt-Rømø Bight (SR) in spring (closed symbols) and autumn (open symbols). Abbreviations: OTHER = other meiofauna (i.e. polychaetes, foraminifers, and ostracods); SPOM = suspended particulate organic matter; SOM = sediment organic matter; 1A = nematodes, selective deposit feeders; 1B = nematodes, non-selective deposit feeders; 2A = nematodes, epigrowth feeders; 2B = nematodes, omnivores/predators.

Figure 6

 6 Figure 6 Relationship between the biomass of benthic microalgae and the ratio between densities of epigrowth feeders (2A) and densities of non-selective deposit feeders (1B) in the mudflats, seagrass beds and sandflat in the Marennes-Oléron Bay (MO) and the Sylt-Rømø Bight (SR) in spring (grey symbols) and autumn (black symbols).

Figure 7 .

 7 Figure 7. Relationship between the ratio of fresh organic material to detrital material and the relative abundance of selective deposit feeders (1A) in the mudflats, seagrass beds and sandflats in the Marennes-Oléron Bay (MO) and the Sylt-Rømø Bight (SR) in spring (grey symbols) and autumn (black symbols). (n = 3)

Table 2 ,

 2 Appendix 5). Nematode biomass in the seagrass beds ranged from 0.146 (seagrass MO, autumn) to 0.459 gC m -2 (seagrass SR, spring). Nematode biomass of the mudflat SR and sandflat SR ranged from 0.054 (mudflat SR, autumn) to 0.150 gC m -2 (mudflat SR, spring). Some temporal variations occurred as well, with higher nematode densities in spring in seagrass SR and higher nematode biomass in spring in mudflat MO (two-way ANOVA with Tukey post-hoc test, highest p-value 0.005, Table2).

Table 2 .

 2 Results of the two-way ANOVA for differences in total meiofauna, nematode, and benthic copepod density and nematode and benthic copepod biomass across habitats and seasons.

	Source		Total meiofauna	Nematodes	Benthic copepods
		df	F-ratio	P > F	F-ratio	P > F	F-ratio	P > F
	Density
	Habitat	4	363.0	< 0.001	385.8	< 0.001	12.7	< 0.001
	Season	1	6.3	0.020	9.6	0.005	1.9	0.178
	Habitat:season 4	8.8	< 0.001	9.1	< 0.001	2.8	0.052
	Error	20
	Biomass
	Habitat	4			216.2	< 0.001	29.01	< 0.001
	Season	1			34.5	< 0.001	0.47	0.503
	Habitat:season 4			11.7	< 0.001	2.14	0.113
	Error	40

Table 3 .

 3 Number

	Mudflat MO	Mudflat SR	Seagrass SR	Seagrass MO	Sandflat SR
	Spring Autumn Spring Autumn Spring Autumn Spring Autumn Spring Autumn

of nematode genera, Hill's diversity (N1), and species evenness (J') in the mudflats, seagrass beds and sandflat in the Marennes-Oléron Bay (MO) and the Sylt-Rømø Bight (SR) in spring and autumn. (n = 3)

Table 4 .

 4 Results of the two-way ANOVA for differences in species richness, Hill's diversity (N1), and species evenness (J') between habitats, sediment layers and season in the mudflats, seagrass beds and sandflat in the Marennes-Oléron Bay and the Sylt-Rømø Bight.

	Source		Species richness	Hill's diversity	Species evenness
		df	F-ratio	P > F	F-ratio	P > F	F-ratio	P > F
	Layer	1	88.3	< 0.001	149.6	< 0.001	55.9	< 0.001
	Site	4	11.0	< 0.001	29.0	< 0.001	16.2	< 0.001
	Season	1	0.1	0.802	2.4	0.126	0.0	1.000
	Layer:site	4	1.5	0.214	4.75	0.003	4.5	0.005
	Layer:season	1	6.4	0.015	3.3	0.078	0.6	0.452
	Site:season	4	3.1	0.026	1.9	0.134	0.3	0.878
	Layer:site:season 4	2.3	0.079	2.5	0.062	2.3	0.077
	Error	38

df = degree of freedom; F = F statistic; P = probability. Values in bold indicate a significant difference (α ≤ 0.05)

Acknowledgements

This study forms part of the Ph.D. thesis of L. van der Heijden. This work was financially supported by the University of La Rochelle through a grant provided by the 'Ministère de l'Enseignement Supérieur et de la Recherche'. This work was supported by the CNRS through the CNRS research chair provided to B. Lebreton. Financial support was provided by the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, as well as the University of La Rochelle (Mobility for doctoral students) and the German Academic Exchange Service (DAAD: short-term research grant, 2017, no. 57314023). We acknowledge P. Pineau, B. Hussel, S. Horn, M. Burgdorf, M. Paar and N. Lachaussée for their support during fieldwork, and G. Guillou and C. Labrune for their support in the analyses. We thank Q. Bernier for his help in sample processing and analysis. Finally, L. van der Heijden thanks the members of his Ph.D. committee, M. Graeve, N. Niquil and G. Blanchard, for their guidance, as well as two anonymous reviewers whose comments greatly improved the manuscript.

Conflict of interest

All authors declare that they have no conflict of interest.

Ethical statement

All applicable international, national and/or institutional guidelines for the sampling, care and use of organisms were followed.