Florian Mermillod-Blondin

Magali Gérino

Sabine Sauvage

Michel Creuzé

Châtelliers

Influence of nontrophic interactions between benthic invertebrates on river sediment processes: a microcosm study

The main objective of this study was to measure the impact of benthic invertebrate diversity on river sediment processes. We quantified the effects of interactions between three taxa (asellids, chironomid larvae, and tubificid worms). The impacts of different taxa richness treatments were measured on sediment reworking, O 2 concentrations, bacterial abundances, and numbers of active bacteria in slow filtration sand-gravel columns. The coefficients of sediment reworking measured in multitaxa treatments were lower than those predicted from one-taxon treatments. The interactions among invertebrates also significantly reduced O 2 concentrations in sediments. These results were probably due to interactions between the different sediment structures produced by each taxon (tubes, macropores, and fecal pellets) that modified water flow and associated microbial activities in the interstitial habitat. The stimulation of aerobic microbial processes with two-and three-taxa treatments, whereas one-taxon treatments could increase or decrease O 2 consumption in columns, indicates that interactions among invertebrates limited the variability of the system functioning. We suggest that, beyond a small number of detritivorous taxa, a threshold effect on bioturbation process and microbial activities was produced by animals in the experimental system. Finally, the interactions between taxa played a significant role in microbial processes in the system studied.

Résumé : Le but de cette étude était de mesurer l'impact de la diversité des invertébrés sur le fonctionnement d'un milieu sédimentaire de rivière. Nous avons quantifié les effets des interactions entre trois taxons (aselles, larves de chironomes et oligochètes tubificidés). Les effets produits par différents traitements en invertébrés ont été mesurés sur le remaniement sédimentaire, les concentrations en O 2 , les abondances totales en bactéries et le nombre de bactéries actives dans des colonnes filtrantes remplies de sables et graviers. Les coefficients de remaniement sédimentaire mesurés dans les traitements avec plusieurs taxons étaient plus faibles que ceux estimés à partir des effets individuels produits par chaque taxon. Les interactions entre les taxons ont aussi produit une diminution significative des concentrations en O 2 dans les sédiments. Ces résultats étaient probablement liés à des interactions entre les structures sédimentaires produites par chaque taxon (tubes, macropores et pelotes fécales), lesquelles modifient les flux d'eau et les activités microbiennes dans l'habitat interstitiel. La stimulation des processus microbiens aérobies dans les traitements avec deux et trois taxons alors que les traitements avec un seul taxon peuvent stimuler mais aussi réduire la consommation en O 2 dans les colonnes indique que les interactions entre invertébrés limitent la variabilité dans le fonctionnement du système. Nous supposons que, au delà d'un nombre faible de taxons, un effet seuil est observé sur le processus de bioturbation et les activités microbiennes dans le système expérimental. En définitive, les interactions entre les taxons jouent un rôle significatif sur les processus microbiens dans le système étudié.

Introduction

In the last 10 years, an increasing number of studies have focussed on the effect of species diversity on ecosystem properties [START_REF] Naeem | Declining biodiversity can alter the performance of ecosystems[END_REF][START_REF] Loreau | Biodiversity and ecosystem functioning[END_REF][START_REF] Jonsson | Mechanisms behind positive diversity effects on ecosystem functioning: testing the facilitation and interference hypotheses[END_REF]. The impact of biodiversity on ecosystem processes depends on the functional characteristics of each species but also on the interactions among species present in the community [START_REF] Wardle | Biodiversity and plant litter: experimental evidence which does not support the view that enhanced species richness improves ecosystem function[END_REF][START_REF] Van Der Heijden | Mycorrhizal fungal diversity determines plant biodiversity, ecosystem variability and productivity[END_REF][START_REF] Loreau | Biodiversity and ecosystem functioning: current knowledge and future challenges[END_REF]. doi: 10.1139/F04-114 At the water-sediment interface in marine and lake systems, the effects of various benthic invertebrates on the bioturbation process and their influences on biogeochemical cycling have been widely studied [START_REF] Fukuhara | Enhancement of inorganic nitrogen and phosphate release from lake sediment by tubificid worms and chironomid larvae[END_REF][START_REF] Hansen | The impact of the polychaete Nereis diversicolor and enrichment with macroalgal (Chaetomorpha linum) detritus on benthic metabolism and nutrient dynamics in organic-poor and organic-rich sediment[END_REF][START_REF] Stief | Bioturbation effects of Chironomus riparius on the benthic N-cycle as measured using microsensors and microbiological assays[END_REF]. However, the impacts of assemblages of benthic invertebrates on nutrient fluxes have been analysed only very recently in marine sediments [START_REF] Emmerson | Detecting the effects of diversity on measures of ecosystem function: experimental design, null models and empirical observation[END_REF][START_REF] Emmerson | Consistent patterns and the idiosyncratic effects of biodiversity in marine ecosystems[END_REF][START_REF] Raffaelli | Biodiversity and ecosystem processes in shallow coastal waters: and experimental approach[END_REF]. These studies demonstrated that the diversity of functional traits (e.g., mode of bioturbation) strongly affects the ecosystem processes.

In rivers, the exchange between the surface and the riverbed sediments (hyporheic zone) plays a key role in organic matter processing and nutrient cycling [START_REF] Grimm | Exchange between interstitial and surface water: implication for stream metabolism and nutrient cycling[END_REF][START_REF] Boulton | The functional significance of the hyporheic zone in streams and rivers[END_REF][START_REF] Morrice | Terminal electron accepting processes in the alluvial sediments of a headwater stream[END_REF]. [START_REF] Battin | A mixing model analysis of stream solute dynamics and the contribution of hyporheic zone to ecosystem function[END_REF] showed that hyporheic respiration accounts for 41% of whole-ecosystem respiration in White Clay Creek (Pennsylvania). [START_REF] Triska | The role of water exchange between a stream channel and its hyporheic zone in nitrogen cycling at the terrestrial-aquatic interface[END_REF] demonstrated that the nitrogen cycle in rivers was dependent on microbial processes occurring in bed sediments. [START_REF] Marmonier | Distribution of dissolved organic carbon and bacteria at the interface between the Rhône River and its alluvial aquifer[END_REF] also reported high efficiency of interstitial habitats in organic matter processing. Despite such functional importance of these sedimentary systems in organic matter processing and nutrient cycling in the hydrosystems, the impact of invertebrate diversity on biogeochemical processes occurring in river sediments (characterized by coarse particles and an advective flux of water) has been poorly studied [START_REF] Danielopol | Groundwater fauna associated with riverine aquifers[END_REF][START_REF] Boulton | The functional role of the hyporheos[END_REF].

It has, however, been demonstrated that benthic invertebrates may significantly affect physical, chemical, and microbial processes occurring in such river sediment habitats [START_REF] Danielopol | Groundwater fauna associated with riverine aquifers[END_REF][START_REF] Torreiter | The activity of oligochaetes in relation to their ecological role in slow filtration columns[END_REF]Mermillod-Blondin et al. 2000a, 2003). In a previous study [START_REF] Mermillod-Blondin | Functional diversity of bioturbation by three detritivorous hyporheic invertebrates: an experimental study in microcosms[END_REF], three invertebrate taxa typically associated with river sediment systems (asellids, chironomids, and tubificid worms) were described as three distinct ecosystem engineers (sensu [START_REF] Jones | Organisms as ecosystem engineers[END_REF]. Their activities in sediments were tested separately. They corresponded to three specific modes of bioturbation, producing different effects on sediment structure and microbial activities.

The aim of the present study was to test the influence of interactions among taxa (assemblages of the three taxa) on sediment properties and processes (sediment reworking, O 2 concentrations, and microbial characteristics). We used experimental microcosms with several treatments of different taxa richness combinations and measured the resulting processes. We predicted that the three benthic taxa would strongly interact in sediments by modifying the same physical habitat in different ways. Therefore, system functioning tested with a mixture of taxa was predicted to be different from that calculated by the summation of the individual roles of each taxon forming the mixture. To measure the impact of interactions between taxa, we used sediment reworking as an indication of the effect of taxa assemblages on the physical structure of sediments and dissolved O 2 concentration as an indicator of respiration in river sediments (with a constant flow rate, dissolved O 2 in pore water is determined by the distance from the O 2 source and the rate of O 2 consumption by microorganisms, [START_REF] Malard | Oxygen supply and the adaptations of animals in groundwater[END_REF]. The abundances of bacteria and electron transport system (ETS) ac-tive bacteria were measured to estimate the impact of invertebrates on biofilm characteristics.

Materials and methods

Experimental design

Experiments took place in gravel-sand filtration columns (50 cm high, 10 cm in diameter) (Fig. 1) modified from [START_REF] Torreiter | The activity of oligochaetes in relation to their ecological role in slow filtration columns[END_REF] and [START_REF] Griebler | Some applications for the DMSO-reduction method as a new tool to determine the microbial activity in watersaturated sediments[END_REF]. Each column was filled with sand (60-630 µm) previously incubated during 4 days with bacteria and cellulose powder (0.5% of sediment mass) as source of particulate organic matter at a final concentration equivalent to that measured in secondary channels of the Rhône River (Mermillod-Blondin et al. 2000b) and with calcinated (550 °C) fine gravel (4-5 mm) to a height of 40 cm. Constant masses of gravel (590 g) and incubated sand (210 g) were alternately added to obtain a heterogeneous interface with presence of zones where sand was trapped and zones with macropores. A water column (6-10 cm) was maintained at the surface of the sediment to simulate the overlying river water. Openings in the column at different depths permitted the collection of water and fine sediments with a syringe. Further details of the columns are given in Mermillod-Blondin et al. (2000a).

The experiments were performed in a constant-temperature room at l5 ± 0.5 °C with a 12 h light : 12 h dark cycle. The sediment of the column was kept in the dark for suppressing photoautotrophic growth. Dechlorinated drinking water, permanently aerated to maintain high O 2 concentrations, was continuously fed into the columns with a peristaltic pump. Before entering into the columns, the water was enriched with nitrate (to a nitrate concentration of 20 mg•L -1 ) and dissolved organic carbon (2 mg potassium acetate•L -1 ) with another peristaltic pump to support biofilm activity (Fig. 1). These concentrations of carbon and nitrate were equivalent to those measured in secondary channels of the Rhône River (Mermillod-Blondin et al. 2000b). The resulting infiltration rate was 2.0 ± 0.1 mL•min -1 (Darcy velocity 1.53 cm•h -1 ), which generated an interstitial water velocity of 3.75 cm•h -1 . This velocity corresponded to values measured in the hyporheic zone of streams [START_REF] Triska | The role of water exchange between a stream channel and its hyporheic zone in nitrogen cycling at the terrestrial-aquatic interface[END_REF][START_REF] Morrice | Terminal electron accepting processes in the alluvial sediments of a headwater stream[END_REF]. The first measurements were made 3 days after the beginning of the water flow after the hydraulic properties had stabilized.

As in previous studies (Mermillod-Blondin et al. 2000a, 2002), one experimental set consisted of four columns: three columns (animal columns) with the same combination of taxa and one column (without animals) acting as a control.

In the present study, we used seven experimental sets, giving eight treatments: (1) control without animals (seven columns), (2) 25 asellids (Asellus aquaticus, A treatment, three columns), (3) 50 chironomids (Prodiamesa sp., C treatment, three columns), (4) 100 tubificid worms (50 Limnodrilus spp. and 50 Tubifex sp., T treatment, three columns), (5) 25 asellids and 50 chironomids (A+C treatment, three columns), (6) 25 asellids and 100 tubificid worms (A+T treatment, three columns), ( 7) 50 chironomids and 100 tubificid worms (C+T treatment, three columns), and (8) 25 asellids, 50 chironomids, and 100 tubificid worms (A+C+T treatment, three columns). The tested densities were equivalent to 3250, 6500, and 13 000 individuals•m -2 for asellids, chironomids and tubificid worms, respectively. These densities approached those observed in natural gravel-sand sediments of braided channels of the Rhône River [START_REF] Fruget | L'aménagement du bas-Rhône. Évolution du fleuve et influence sur les peuplements de macro-invertébrés benthiques[END_REF][START_REF] Martinet | Le macrobenthos limnivore, descripteur des flux organiques liés aux sédiments : exemples dans diverses annexes fluviales du Rhône[END_REF]). The organisms were collected at three sites: a dead arm of the Rhône River about 20 km upstream of Lyon, the Semène River, and the Ardière River. For acclimation to the experimental conditions (sediment grain sizes and food), animals were maintained in the laboratory for more than 30 days before introduction into the microcosms. The first measurements (day 0) confirmed consistency in conditions among the four columns of each experiment before introduction of the invertebrates. After this, invertebrates were introduced in the overlying water of the columns.

Sediment reworking was estimated after 20 days of experimentation. O 2 concentrations and bacterial parameters were also measured on day 20. Dissolved O 2 concentrations were measured at five depths (5 cm above the sediment surface and 1, 5, 15, and 35 cm below the sediment surface), and bacterial parameters (bacterial abundance and numbers of ETS-active bacteria on fine sediment) were evaluated at four depths in the sediment (1, 5, 15, and 35 cm below the sediment surface).

Methods of analysis

Sediment reworking in the columns was quantified by the luminophore tracer technique [START_REF] Gérino | The effects of bioturbation on particle redistribution in Mediterranean coastal sediment -preliminary results[END_REF]). To reproduce the grain size heterogeneity of the sand present in the columns, we used two size fractions of luminophores (natural sediment particles dyed with fluorescent paint): 63-to 100-µm particles (pink) and 100-to 350-µm particles (yellow). On day 0 of each experiment, a mixture of the two fractions (0.5 grams per fraction) was deposited at the surface of the sediment of the four columns before invertebrate introduction. After a 20- day period, water was removed from all columns and columns were opened to allow sampling of sediment and luminophores. The top 2 cm of sediment was collected at 0.5cm intervals and the next 18 cm collected at 1-cm intervals. Each layer was homogenized and a 1-g subsample was dried at 50 °C for luminophore counting. The number of luminophores in each subsample was estimated using an ultraviolet light microscope and converted into grams of tracers per gram of dry sediment.

The effect of invertebrates on particle transport was estimated using the diffusion-advection-nonlocal model in nonsteady-state conditions [START_REF] Mugnai | Bioturbation experiments in the Venice lagoon[END_REF]. This model links a nonlocal component to a diffusion-advection model [START_REF] Officer | Interpretation procedures for the determination of sediment parameters from time-dependent flux inputs[END_REF][START_REF] Gérino | Quantitative estimation of biodiffusive and bioadvective sediment mixing: in situ experimental approach[END_REF]. Biological diffusive transport (diffusion (D), square centimetres per year) was defined as omnidirectional transport in the sediment column and is analogous to molecular diffusion in water. Biodiffusion spreads tracers in the sediment column [START_REF] Guinasso | Quantitative estimates of biological mixing rates in abyssal sediments[END_REF] by the exchange of small amounts of material with adjacent parcels of sediment. Most depositfeeding activities result in the vertical transport of material, which cannot be fitted with diffusional models. Biologically mediated vertical transports account for advective transport and nonlocal transport. Bioadvection (V, centimetres per year) is created by head-down deposit-feeders like worms that feed in the deeper sediment and egest fecal pellets at the surface (conveyor belt organisms). This feeding mode causes an accelerated rate of sediment and pore water burial within the feeding zone. Nonlocal transport results from largely open burrows into which surface particles may fall, and this type of vertical transport is much more rapid than bioadvection. This nonlocal mixing is modeled as a removal function that simulates the deposition of surface material (expressed in grams of transported tracer per day) in a deposition zone. The basic equation is

(1) ∂ ∂ = ∂ ∂ - ∂ ∂ + - C z t t D C z t z V C z t z K z t R z t ( , ) ( , ) ( , ) ( , ) ( , ) 2 2
where C is the normalized tracer concentration, t is time (years), z is depth (centimetres), D is the diffusive mixing rate (square centimetres per year), V is the advective transport rate (centimetres per year), R is the removal function that determines the mass of tracer (grams per day) removed from the surface and K is the injection function of the nonlocal transport that simulates tracer inputs (grams per day) into the injection zone of the sediment column; K e is a constant parameter (per day) estimated from the model, and depths Z min and Z max represent the upper and lower limits of the injection zone, respectively. The nonlocal transport is thus quantified by a flux of sediment removed from the surface. In this case, R(z,t) = 0 for z > 0 and R(z,t) = K(z,t) (Z max -Z min ) for z = 0 and K(z,t) = K e for z ∈Z min ,Z max ] and

K(z,t) = 0 for z ó [Z min ,Z max ].
Luminophores were added as pulse input at the surface of the sediment at the beginning of each experiment so that the model was applied under non-steady-state conditions. Thus, model eq. 1 was used with the upper boundary condition of an instantaneous source of unit strength (maximal C of tracer) at z = 0 at t = 0, a lower boundary C → 0 at z → ∞, and ini-tial condition C = 0 at z > 0. The general solution was given by [START_REF] Officer | Interpretation procedures for the determination of sediment parameters from time-dependent flux inputs[END_REF] as

(2) C z t Dt z Vt Dt ( , ) exp ( ) = - - ⎡ ⎣ ⎢ ⎤ ⎦ ⎥ 1 4 2 π - ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ + ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ - + V D Vz D erfc z Vt Dt R t K t 2 2 exp 4 e e
with R e = K e (Z max -Z min )

and

erfc x t t x ( ) = - - ∫ 1 2 e d 2 π 0
where c is a normalized concentration relative to unit input.

The model allows the calculation of the theoretical tracer concentration given suitable values of the parameters D, V, Z min , Z max , and K e . These parameters were obtained from profiles that produced the best fit with the experimental data using the least squares method. O 2 concentration was measured with the technique developed by Mermillod-Blondin et al. (2000a). At each depth, 30 mL of water was collected in a N 2 sparged bottle. This water circulated at a rate of 20 mL•min -1 through an Orbisphere-type respirometer chamber equipped with a Clarke-type electrode connected to an Orbisphere oxymeter (model 3600). This procedure allowed the measurement of dissolved O 2 concentrations in the samples without contact with atmospheric O 2 .

The total number of bacteria was estimated by epifluorescence after 4′,6-diamidino-2-phenylindole (DAPI) staining [START_REF] Porter | The use of DAPI for identifying and counting aquatic microflora[END_REF]. Wet sediment (0.3-0.5 g) was suspended in 40 mL of filtered (on a 0.2-µm acetate membrane) distilled water and 10 mL of a solution of pyrophosphate (0.002 mol•L -1 final concentration) by sonication (Bioblock Vibracell 72041; for 180 s, power 2, duty cycle 50%). One millilitre of the suspension was filtered through a GF/D Whatman filter (mean pore size 2.7 µm) to eliminate the coarsest particles. Staining was performed by addition of 0.1 mL of DAPI solution (100 µg•L -1 ) to 1 mL of the filtrate. After 10 min of incubation, the suspension was filtered on a GTBP Millipore membrane (mean pore size 0.2 µm), which was then washed, air-dried, and mounted in low-fluorescence immersion oil on a microscope glass slide. Bacterial counts were performed using an epifluorescence microscope (Zeiss microscope fitted with a 200-W mercury burner and a Zeiss 365-nm excitation filter). At least 30 randomly selected fields were counted per slide. Results were expressed as numbers of bacteria per gram of dry sediment.

The number of bacteria with an active ETS (ETS-active bacteria) was measured using 5-cyano-2,3-ditolyltetrazolium chloride staining [START_REF] Rodriguez | Use of a fluorescent redox probe for direct visualization of actively respiring bacteria[END_REF]. Wet sediment (0.3-0.5 g) was incubated with a 1.38 mg 5-cyano-2,3-ditolyltetrazolium chloride•L -1 solution (final concentration) for 3 h at 20 °C and then sonicated using the same procedure as for DAPI with a dilution in 20 mL of pyrophosphate solution. Five millilitres of the suspension was processed and slides pre-pared using the procedure for DAPI samples. Bacteria were counted with an epifluorescence microscope fitted with a 200-W mercury burner, a blue 420-nm excitation filter (Zeiss model BP 390-420), and a 540-nm barrier filter (Zeiss model LP 540). At least 30 randomly selected fields were counted per slide. Results were expressed as numbers of bacteria per gram of dry sediment.

At the end of each experiment, the living invertebrates were enumerated at three layers (0-10, 10-20, and 20-40 cm depth) of each column. The numbers of collected invertebrates were used to estimate the animal mortality and to provide information about the vertical distribution of invertebrates in columns.

Data treatment

Effect of each animal treatment

The data obtained in control columns of the seven experiments were combined in the statistical analyses. The sediment reworking coefficients obtained from the eight treatments were ln(x + 1) transformed to homogenize variances. Differences in transformed reworking coefficients were tested using a oneway analysis of variance (ANOVA). Bonferroni-Dunn post hoc tests were performed if significance was detected to determine which treatments differed. For each column, the O 2 concentrations measured in sediments were expressed as percentages of the value measured in the overlying water (= dissolved O 2 in water supplied to the column, mean values ± SD: 5.92 ± 0.24 mg•L -1 ). In the analyses, the total number of bacteria and the number of ETS-active bacteria were log transformed. The effects of animals on O 2 concentrations and numbers of bacteria measured on day 20 were compared using a two-way ANOVA with treatment and depth as main effects (Statistica 5 TM ; Statsoft, Tulsa, Oklahoma). Bonferroni-Dunn post hoc tests were performed to determine which treatments differed. Numbers of living invertebrates found at the end of the experiments were expressed as percentages. We compared the percentages of living animals between animal treatments using a one-way ANOVA. The numbers of animals found at the three sampling layers were also expressed as percentages. We tested the taxa and depth effects on invertebrate distributions using a two-way ANOVA with taxa and depth as main effects. Variables expressed as percentages (microbial variables) were arcsine transformed [START_REF] Sokal | Biometry. 3rd ed[END_REF].

Impact of mixture in comparison with individual effects (measure of interactions)

Comparisons of effects of two-or three-taxa mixtures with effects of invertebrates in isolation were made by comparing sediment reworking coefficients, O 2 concentrations, and microbial variables in animal and control columns. Using single-taxon effects, we used an additive model similar to those described by Raffaelli et al. (2003, p. 137) against which we compared observed multitaxa effects. The interactions were measured as the differences between the observed effects with two-or three-taxa mixtures and the predicted effects of mixtures calculated by all combinations of the effects of the taxon in isolation (individual effects). For example, in the case of the A+C treatment, values calculated using the additive model (n = 9, 3 columns × 3 columns) = E(A) n=3 + E(C) n=3 were compared with observed values (n = 3) = E(A+C) n=3 , where E(A) and E(C) are asellid effect and chironomid effects in one-taxon experiments and E(A+C) is the value measured in the A+C experiment. With this method, we expected that, in the absence of interactions, the effects of 50 chironomids and the effects of 25 asellids would be simply added in the treatment with 50 chironomids + 25 asellids. Using this additive model, we assumed that effects measured in mixtures were linked to interactions between taxa rather than a simple effect of animal density. Because the impact of each invertebrate taxon in sediment was linked to its life mode [START_REF] Mermillod-Blondin | Functional diversity of bioturbation by three detritivorous hyporheic invertebrates: an experimental study in microcosms[END_REF], the type of effect (production of tubes or galleries, sediment reworking mode) produced by a taxon in sediment was not expected to be strongly affected by its density. For O 2 concentrations and microbial variables, observed and predicted values were compared using a two-way ANOVA with observed-predicted and depth as main effects. For coefficients of sediment reworking, observed and predicted values were compared using Student t tests. A significant (negative or positive) difference indicates that interactions between taxa modified the system functioning because the effects of the two or three taxa were not reported in mixture treatments. In addition, it could be observed that the dominant effect of one taxon in mixtures can lead to the detection of significant interactions. Thus, the effects of mixtures were compared with the effects of each taxon forming the mixture to determine if one taxon present in the mixture drove the system functioning. Statistical tests used to compare mixture effects and one-taxon effects were the same as those used for observed-predicted comparisons.

Results

Effects of animal treatments

The luminophore profiles obtained at the end of the experiments showed that the average percentage of tracer measured at the sediment surface (0-0.5 cm layer) was the highest in the control columns (87%) and varied from 62% to 38% in invertebrate treatments (Fig. 2). With the exception of chironomid treatment, the profiles were characterized by a rapid decrease of tracer with depth, indicating a biodiffusive reworking of sediment. With chironomids, similar concentrations of luminophore were measured in the 0-0.5 and 0.5-1 cm layers. This result was attributed to a nonlocal transport of the tracer from the surface to the 0.5-1 cm layer. Model data fit experimental data well (Fig. 2). The coefficients of sediment reworking from model simulations are shown in Table 1. No bioadvection was observed in the treatments. The biodiffusion coefficient varied significantly among treatments (Fig. 3; Table 2). With the exception of the A and the A+T treatments, all invertebrate assemblages significantly increased the diffusion of surface sediment within the porous media (Table 3). No significant difference in nonlocal transport was measured among the different treatments (Table 2). However, C and A+C treatments had significantly higher nonlocal transport rates than the control (Table 3).

Whatever the treatment, O 2 concentration decreased with depth in all columns and this decrease was the highest in the top 5 cm of sediment (Fig. 4; Table 2). Significant decreases in total number of bacteria and number of ETS-active bacteria with depth were also observed in columns (Figs. 5 and6; Table 2). For all variables, significant differences were measured among treatments. The A and the C treatments significantly increased the O 2 concentrations (Table 3). In contrast, the other animal treatments significantly reduced O 2 concentrations in the sediment. These significant differences depended on depth (two-way ANOVA, interaction taxa-depths) (Table 2), as most animal treatments had significantly higher effects in the top 5 cm of sediments. The same patterns were observed in number of ETS-active bacteria: all animal treat-ments significantly increased the number of active bacteria except the A treatment, which had no significant effect, and the C treatment, which reduced this bacterial variable in sediments. Like ETS-active bacteria, the total abundance of bacteria significantly decreased with depth and was different among treatments. The A treatment and C treatments had no significant effect on this variable. However, unlike results obtained with O 2 measurements and abundances of ETSactive bacteria, all of the other treatments did not increase the number of bacteria in the sediment (A+C and C+T treatments had no significant effects on this variable (Table 3)).

The results also showed that the variabilities among the effects of one-taxon treatments were higher than those obtained among multitaxa treatments (Table 3). For O 2 concentrations, the three one-taxon treatments did not have the same effect (increase or decrease of O 2 concentrations), whereas the same response of the system (a decrease of O 2 concentrations) was measured in all two-and three-taxa treatments. All two-and three-taxa treatments also produced a significant increase in the number of ETS-active bacteria. In contrast, with one taxon, increases or decreases in ETS- Note: Bonferroni-Dunn test with n = 3 for invertebrate treatments and n = 7 for the control. 0, no significant difference; +, significant increase; -, significant decrease owing to the invertebrates. Animal treatments: A, 25 asellids; C, 50 chironomids; T, 100 tubificid worms; A+C, 25 asellids + 50 chironomids; A+T; 25 asellids + 100 tubificid worms; C+T, 50 chironomids + 100 tubificid worms; A+C+T, 25 asellids + 50 chironomids + 100 tubificid worms. active bacteria abundances were observed depending on the taxon tested.

Estimation of the interactions between taxa

For both diffusion and nonlocal transport, the reworking coefficients measured in mixtures were in the range of values measured in one-taxon treatments (Fig. 3). However, the measured coefficients in mixtures tended to be lower than the predicted coefficients calculated from individual effects of the taxa. Significantly lower coefficients of biodiffusion were measured in A+T and A+C+T treatments in comparison with predictions. The nonlocal transports of surface sediment measured in the A+C and A+C+T treatments were significantly lower than those predicted.

Whatever the invertebrate mixture, all treatments produced significantly higher decreases in O 2 concentration than those predicted with the additive model (Fig. 7a). The effects of the A+C treatment decreased O 2 concentrations in columns, whereas A and C treatments had the opposite effect. Therefore, the mixture of asellids and chironomids created a significantly different effect than expected and the single-taxon effects (two-way ANOVA, measured data in mixture versus measured data in one-taxon treatments, p < 0.01). For the mixtures with tubificid worms, the O 2 concentrations were always lower at 1 cm depth in the mixture than in one-taxon treatments (two-way ANOVA, measured data versus calculated data and measured data in one-taxon treatments, p < 0.01) and significant differences between predicted and observed values in mixtures were detected (Fig. 7a).

In contrast, the interaction between taxa did not provide significant differences in bacterial abundances (total numbers and numbers of ETS-active bacteria) (Figs. 7b and7c), except the interaction between chironomids and tubificids, which significantly increased the number of ETS-active bacteria in sediments (two-way ANOVA, measured data versus calculated data, p < 0.05). High variability in bacterial counting clearly limited statistical power. For instance, although the A+C treatment tended to increase the number of ETSactive bacteria at 5 and 15 cm depth, while calculated values showed the opposite trend, no significant difference between measured and predicted values was observed.

Estimated mortality and distribution of invertebrates at the end of the experiments

Whatever the invertebrate assemblage tested, the percentage of living invertebrates varied from 77% to 92% and was not significantly different among treatments (one-way ANOVA, p = 0.11) (Table 4). The number of invertebrates found at each layer of sediment showed that most animals (>90%) were observed in the first 10 cm of sediment (two-way ANOVA, depth effect, p < 0.001). Furthermore, no significant differences were measured in the vertical distribution of each taxon among treatments.

Discussion

Individual effects of the three taxa

In the river sediments, the three taxa showed significantly different bioturbation activities: asellids had no significant effect on surface sediment distribution, tubificid worms increased the biodiffusion of surface particles, and chironomids enhanced both biodiffusion and nonlocal transport of surface sediment in the porous media. As described in a previous study [START_REF] Mermillod-Blondin | Influence of macroinvertebrates on physico-chemical and microbial processes in hyporheic sediments[END_REF], the pore water flow in sediment columns was characterized by a 50% mobile water phase in macropores (where water flow took place by advection), a 50% immobile phase in sandy zones (where water flow took place by diffusion), and an exchange coefficient of 0.15•h -1 between the mobile and immobile phases. Several studies [START_REF] Molz | Simulation of microbial growth dynamics coupled to nutrient and oxygen transport in porous media[END_REF][START_REF] Murphy | The influence of physical heterogeneity on microbial degradation and distribution in porous media[END_REF][START_REF] Thullner | Interaction between water flow and spatial distribution of microbial growth in a two-dimensional flow field in saturated porous media[END_REF] have shown that microbial activities are linked to O 2 and nutrient transport in porous media. Thus, the modification of hydrodynamics by bioturbation was expected to affect microbial characteristics in the experimental system. As a consequence, the distinct bioturbation processes performed by the three taxa may have modified hydrodynamics and nutrients availability for microorganisms in sedi-ments [START_REF] Mermillod-Blondin | Functional diversity of bioturbation by three detritivorous hyporheic invertebrates: an experimental study in microcosms[END_REF]. The nonlocal transport of surface sediment produced by chironomids was due to the presence of tubes inserted into the sediments. The O 2 measurements showed that these tubes opened macropores through the sediment, which favoured a rapid transfer of O 2 to the deeper layers by concentrating the flow through pores (reducing the contact between mobile and immobile phases of water flow). This effect on water transport reduced the nutrient availability for bacteria in sandy zones and decreased microbial activity outside these pores, as indicated by the decrease in the number of ETS-active bacteria in chironomid treatment. The lack of surface sediment reworking by asellids was probably linked to their use of sediments restricted to the subsurface zone (1-3 cm), as demonstrated by [START_REF] Mermillod-Blondin | Functional diversity of bioturbation by three detritivorous hyporheic invertebrates: an experimental study in microcosms[END_REF]. Despite a low redistribution of surface sediment by asellids, the sediment reworking in subsurface might have created macropores that concentrated the downward flow of water. As supposed for chironomid action, this change in hydrodynamics would reduce the contact time between water and the sandy zone and lead to the increase in O 2 concentrations measured in the foraging zone of asellids (0-5 cm below the sediment surface). In one-taxon experiments, the effect of tubificid worms on O 2 concentration was the opposite of that of the other two taxa. In contrast with asellids and chironomid larvae, which created macropores that limited the exchange of infiltrated water with sandy zones, tubificid worms built a dense network of galleries and ejected fecal pellets at the sediment surface (Mermillod-Blondin et al. 2001), producing increased exchanges between mobile and immobile phases of water, which stimulated microbial activities in the sediment.

Effect of interactions on system functioning

The results of sediment reworking and O 2 measurements support the hypothesis that the mixture effect was not the sum of the individual effects of the taxa tested in the mixture. This lack of additive process was not due to the dominance of certain taxon in mixtures because mixture treatments always produced lower O 2 concentrations than predicted and observed in one-taxon treatments. These results suggest that interactions between taxa have strong effects on the functioning of the water-sediment interface. The sediment reworking induced by the assemblage of two taxa was mostly lower than the predicted one calculated with the additive model. These significant interactions measured between taxa on bioturbation processes probably resulted from a spatial competition among the three taxa, which lived and foraged in the same habitat (>90% of animals were found in the first 10 cm of sediment). As a consequence, these modifications of sediment reworking affected water transport and associated microbial activities. For example, O 2 concentration, which is an indicator of aerobic microbial activity in the controlled system, showed that interaction in bioturbation processes affected microbial respiration.

The interaction between asellids and chironomid larvae (A+C treatment) strongly affected microbial respiration. The association of the two taxa produced an opposite effect (O 2 decrease in sediments) compared with the A and C treatments (O 2 increase). The sediment reworking experiments showed that nonlocal transport was significantly lower in the assemblage than expected with the additive model. As chironomid tubes were responsible for nonlocal transport, this result suggests that burrowing by asellids destroyed the tubes produced by chironomids (Fig. 8). Therefore, this interaction may have limited the occurrence of continuous macropores in the sediment matrix, which was observed when each taxon was tested separately. The bioturbated zone produced by the assemblage of chironomids and asellids acted as a finesediment filter, increasing the dispersion of the water in the heterogeneous sediment and stimulating O 2 utilization by bacteria in the top 5 cm of sediment (according to O 2 decreases). The opposite effects observed in the A+C treatment in comparison with the A and the C treatments indicate that interactions between taxa can mask the specific role of each taxon. As a consequence, the assessment of the interaction between taxa is necessary to determine the role of invertebrate assemblages in river sediment processes.

As in the A+C treatment, the interactions between chironomids and tubificid worms strongly increased the aerobic microbial respiration (decrease in O 2 ) in the first centimetre of sediment. At this depth, O 2 concentration and ETS-active bacteria were the opposite of the effects observed with C and T treatments and the effects predicted by the additive model. Although there were no significant differences in sediment reworking between observed and predicted effects of the C+T treatment, the mean coefficient of nonlocal transport was twofold lower than the measured coefficient with chironomids and the predicted value. Tubificids produce fecal pellets at the water-sediment interface and we speculate that, in assemblages, fecal pellets deposited by worms filled the macropores produced by chironomid larvae, limiting the continuity of tubes. Therefore, the transport of surface particles within sediments and the rapid downward transfer of O 2 through macropores were reduced. The colonization of the fecal pellet surface by bacteria [START_REF] Yingst | The role of bioturbation in the enhancement of microbial turnover rates in marine sediments[END_REF] may also have stimulated aerobic respiration in the top 5 cm of sediment.

The A+T and A+C+T treatments also indicated that interaction between taxa affected the bioturbation process (measured coefficients of surface sediment reworking were lower than those predicted) and resulted in increased O 2 respiration. Table 4. Numbers of living invertebrates found on day 20 in the three animal columns of each animal treatment at three sediment layers.

As explained previously, interactions between built structures (tubes, macropores, galleries, and fecal pellets) may explain the functioning of river sediments (Fig. 8). However, biogenic structure interaction may not be the only explanation. Despite the fact that the distribution of each taxon was not strongly modified by the presence of another taxon, the behaviour of one taxon (type and intensity of bioturbation) may be affected by another taxon and could also modify system processes.

In contrast with coefficients of sediment reworking and O 2 measurements, few significant interactions were measured among taxa on microbial variables. This result is probably linked to the high variability in bacterial counting but may also reflect the competing effects of invertebrates on bacteria [START_REF] Riisgard | Irrigation and deposit feeding by the lugworm Arenicola marina, characteristics and secondary effects on the environment -a review of current knowledge[END_REF]: the fauna grazes on biofilm and reduces the bacteria number but also stimulates bacterial growth rates, increasing the bacterial biomass (gardening effect) and the respiration rate of the microbial community.

Impact of taxa addition on system functioning

The results indicated that one-taxon treatments might increase (A and C treatments) or decrease (T treatment) O 2 concentrations in the sediment as a result of the different activities of fauna (burrowing and moving) in the porous media. In contrast, all two-and three-taxa treatments decreased O 2 concentration and stimulated the number of ETS-active bacteria in the sediment. This lower variability measured among two-taxa than among one-taxon treatments was mostly due to the interaction between asellids and chironomids, which created a complete shift in respiration process. Therefore, the fact that interactions between taxa can reduce the variability in several processes supports the assumption that functioning variability decreases with taxa number [START_REF] Naeem | Biodiversity enhances ecosystem reliability[END_REF]. The similar modifications of microbial processes observed with two and three taxa can also be of great importance for the stability of ecosystem functioning. This result suggests that addition of a new taxon did not significantly modify the functioning of a system with two taxa (whatever the characteristics of the added taxon). Because each taxon can tolerate only a limited range of physicochemical and biotic conditions [START_REF] Tachet | Invertébrés d'eau douce : systématique, biologie, écologie[END_REF], any change in environment (e.g., pollution) beyond these conditions leads to its extinction. Therefore, in the case of environmental changes, the occurrence of three taxa rather than two taxa in the community should provide "insurance" [START_REF] Chapin | Biodiversity and ecosystem processes[END_REF][START_REF] Yachi | Biodiversity and ecosystem productivity in a fluctuating environment: the insurance hypothesis[END_REF] for the functioning of the sediment-water interface. In other words, a higher number of detritivorous taxa should enhance the probability that the river sediment system will provide a consistent level of performance over a given unit of time. This similar effect of two-taxa and three-taxa treatments also suggests that, beyond a given number of taxa, the physical modification of the system was limited to a threshold level of modification of system functioning (sediment reworking, stimulation of bacteria, and O 2 consumption).

Perspectives

Although our results demonstrated the significant impact that bioturbator interactions have on several river sediment processes, our study used a simplified experimental system with average natural density of each taxon and a limited number of taxa. For example, the impact of density of each taxon and assemblages of taxa on ecosystem functioning has not been tested for all treatments (except for tubificid worms; [START_REF] Mermillod-Blondin | Testing the functional redundancy of Limnodrilus and Tubifex (Oligochaeta, Tubificidae) in hyporheic sediments: an experimental study in microcosms[END_REF]) because our additive model supposed that animal density did not strongly affect the impact of asellids or chironomids. The opposite effect of A+C treatment on O 2 concentration in the top 5 cm of sediment in comparison with the A and C treatments supports our assumption because it is very improbable that bioturbation activities of chironomids and asellids (tube building and reworking of fine sediment) leading to O 2 increases would be affected by animal density. However, further investigations using variable densities of each taxon in assemblages should be developed to analyse the possible density-dependent impact on system functioning. Extending the experimental approach developed here to assemblages with much higher species richness is also needed to better understand the links between invertebrate diversity and river sediment functioning.

Fig. 1 .

 1 Fig. 1. Schematic diagram of an experimental unit. DOC, dissolved organic carbon, ETS, electron transport system.

Fig. 2 .

 2 Fig.2. Depth profiles (means ± SD) of luminophores (circles) in the eight treatments and calculated profiles (diamonds) obtained with the advection-diffusion-nonlocal model and the average coefficients of transport. Animal treatments: A, 25 asellids; C, 50 chironomids; T, 100 tubificid worms; A+C, 25 asellids + 50 chironomids; A+T; 25 asellids + 100 tubificid worms; C+T, 50 chironomids + 100 tubificid worms; A+C+T, 25 asellids + 50 chironomids + 100 tubificid worms.

Fig. 3 .

 3 Fig. 3. Coefficients (means ± SD) of (a) biodiffusion and (b) nonlocal transport (expressed as ln(x + 1)) measured in the eight treatments (open bars). Predicted coefficients (shaded bars) are indicated for the two-and three-taxa treatments. Animal treatments: A, 25 asellids; C, 50 chironomids; T, 100 tubificid worms; A+C, 25 asellids + 50 chironomids; A+T; 25 asellids + 100 tubificid worms; C+T, 50 chironomids + 100 tubificid worms; A+C+T, 25 asellids + 50 chironomids + 100 tubificid worms.

Fig. 4 .

 4 Fig. 4. Depth profiles (means ± SD) of O 2 concentrations in the eight treatments. The depth profile of the control treatment (open circles) is reported on all graphs with each animal treatment (solid circles). Animal treatments: A, 25 asellids; C, 50 chironomids; T, 100 tubificid worms; A+C, 25 asellids + 50 chironomids; A+T; 25 asellids + 100 tubificid worms; C+T, 50 chironomids + 100 tubificid worms; A+C+T, 25 asellids + 50 chironomids + 100 tubificid worms.

Fig. 5 .

 5 Fig. 5. Depth profiles (means ± SD) of log 10 (total number of bacteria) in the eight treatments. The depth profile of the control treatment (open bars) is reported on all graphs with each animal treatment (solid bars). Animal treatments: A, 25 asellids; C, 50 chironomids; T, 100 tubificid worms; A+C, 25 asellids + 50 chironomids; A+T; 25 asellids + 100 tubificid worms; C+T, 50 chironomids + 100 tubificid worms; A+C+T, 25 asellids + 50 chironomids + 100 tubificid worms.

Fig. 6 .

 6 Fig.6. Depth profiles (means ± SD) of log 10 (number electron transport system (ETS)-active bacteria) in the eight treatments. The depth profile of the control treatment (open bars) is reported on all graphs with each animal treatment (solid bars). Animal treatments: A, 25 asellids; C, 50 chironomids; T, 100 tubificid worms; A+C, 25 asellids + 50 chironomids; A+T; 25 asellids + 100 tubificid worms; C+T, 50 chironomids + 100 tubificid worms; A+C+T, 25 asellids + 50 chironomids + 100 tubificid worms.

Fig. 7 .

 7 Fig.7. Measured (open circles and bars) and predicted (shaded circles and bars) effects (means ± SD, n = 3) of the two-and threetaxa assemblages on depth profiles of (a) O 2 concentrations, (b) total number of bacteria, and (c) numbers of electron transport system (ETS)-active bacteria. The results of the statistical comparison between observed and predicted values are indicated on the graphs (observed-predicted: **, significant difference; ns, no significant difference). Animal treatments: A+C, 25 asellids + 50 chironomids; A+T; 25 asellids + 100 tubificid worms; C+T, 50 chironomids + 100 tubificid worms; A+C+T, 25 asellids + 50 chironomids + 100 tubificid worms.

Fig. 8 .

 8 Fig.8. Schematic representation of the presumed interactions between invertebrates in the two-taxa treatments. This representation does not take into account a possible modification of the behaviour of the taxa when they were associated.

Table 1 .

 1 Advective, diffusive, and nonlocal transport coefficients are presented as means (SD) (n = 7 for control and n = 3 for animal treatments). Z min and Z max are the respective upper and lower limits of the sediment layer influenced by nonlocal transport. Animal treatments: A, 25 asellids; C, 50 chironomids; T, 100 tubificid worms; A+C, 25 asellids + 50 chironomids; A+T; 25 asellids + 100 tubificid worms; C+T, 50 chironomids + 100 tubificid worms; A+C+T, 25 asellids + 50 chironomids + 100 tubificid worms. Mixing rates of sediment estimated in the eight treatments.

				Nonlocal transport
			•year -1 )	(g•day -1 )	Z min (cm)	Z max (cm)
	Control	0 (0)	1.06 (0.26)	0.013 (0.011)	0.875 (0.48)	1.875 (0.25)
	A	0 (0)	3.3 (2.3)	0.043 (0.020)	1.27 (0.46)	3.73 (1.27)
	C	0 (0)	8.23 (6.82)	0.068 (0.035)	0.67 (0.29)	2.33 (1.53)
	T	0 (0)	7.8 (5.4)	0.023 (0.002)	0.83 (0.29)	3 (0)
	A+C	0 (0)	4.23 (0.25)	0.047 (0.012)	0.83 (2.9)	3.33 (2.08)
	A+T	0 (0)	2.4 (0.85)	0.040 (0.057)	1 (0)	2 (0)
	C+T	0 (0)	13.13 (14.66)	0.036 (0.029)	1 (0.5)	2.17 (0.76)
	A+C+T	0 (0)	6.0 (2.16)	0.011 (0.001)	1 (0)	2.17 (0.76)

Note:

Table 2 .

 2 ANOVAs for the comparison among treatments (and depth for two-way ANOVA) on coefficients of sediment reworking, O 2 concentrations, and microbial parameters.

	Variable		Source			df	F	p
	One-way ANOVA
	Biodiffusion of surface particles		Treatment		7	4.6	<0.05
			Error			19
	Nonlocal transport of surface particles	Treatment		7	2.4	>0.05
			Error			19
	Two-way ANOVA
	O 2		Treatment		7	19.7	<0.001
			Depth			3	306.3	<0.001
			Treatment × depth	21	3.8	<0.001
			Error			80
	Total number of bacteria		Treatment		7	4	<0.001
			Depth			3	257.1	<0.001
			Treatment × depth	21	4.3	<0.001
			Error			80
	Number of ETS-active bacteria		Treatment		7	12.1	<0.001
			Depth			3	408.2	<0.001
			Treatment × depth	21	3.5	<0.001
			Error			80
		Animal treatment
	Variable	A	C	T	A+C	A+T	C+T	A+C+T
	Biodiffusion	0	+	+	+	0	+	+
	Nonlocal transport	0	+	0	+	0	0	0
	Effect on O 2 concentrations	+	+	-	-	-	-	-
	Effect on total number of bacteria	0	0	+	0	+	0	+
	Effect on ETS-active bacteria	0	-	+	+	+	+	+

Table 3 .

 3 Effects of each animal treatment on coefficients of sediment reworking, O 2 concentrations, and microbial characteristics (comparison with the control treatment).

Corresponding author (e-mail: mermillo@univ-lyon1.fr).

Acknowledgments

This work was a part of the programme entitled "Rôle de la diversité des invertébrés à l'interface sédimentaire eaux de surface -eaux souterraines" supported by the French National Centre for Scientific Research (CNRS) (No. 98N62/0144, "Programme Environnement Vie et Sociétés") within the programme "Biodiversité et Fonctionnement des Ecosystèmes (Programme National Dynamique de la Biodiversité et Environnement)". We thank G. Fauvet for his assistance in microbial analyses; J.-L. Reygrobellet and L. Mauclaire for their assistance in collecting invertebrates; and C.P. Henry, F. Malard, and M.-J. Olivier for reviewing a previous draft of this manuscript.


Pascal Laffaille
email: pascal.lafaille@univ-rennes1.fr

Sébastien Brosse

Eric Feunteun

Aurore Baisez

Jean-Claude Lefeuvre

U M R Ecobio

Cesac

Role of fish communities in particulate organic matter fluxes between salt marshes and coastal marine waters in the Mont Saint-Michel Bay

Keywords: salt marsh, outwelling, organic matter fluxes, fish community, nursery, mullets

Among the 90 fishspecies censused in the Mont Saint-Michel Bay (France), 23 colonise and forage in the salt marshes during flood. Therefore, this environment may play an important trophic and nursery role for these species. This community is largely dominated by mullets (81% of the biomass), Liza ramada and secondarily L. aurata. But gobies (mainly Pomatoschistus minutus and P. lozanoï) and sea bass (Dicentrarchus labrax) are also present; they represent respectively 11% and 4% of the biomass. During the tide cycles, mullets export from salt marshes about 7% of their body weight (FW) containing a mixture of sediment (43%), organic matter (24%) and water (33%). Gobies and sea bass mainly feed on dwelling macro-invertebrates, and they export respectively 4.5% and 10% of their body weight during a tide cycle. Thus, we estimated that 50 tonnes year -1 of particulate organic matter (dry weight POM) are exported from the 4000 ha of salt marshes to the marine coastal waters. These fish communities appear to be POM transporters and could play a significant role in the global energy budgets of coastal environments such as Mont Saint-Michel Bay. According to the seasons and the years, the energy exported by fish communities is assumed to range between 0 and 10% of the total POM output.

Introduction

Salt marshes are complex ecotones between terrestrial and marine systems, regularly flooded by tides. Fluxes of particulate organic matter (POM), dissolved organic matter (DOM) and inorganic nutrients occur between salt marshes and coastal environments. Research on these fluxes and the global functioning of these systems were undertaken by [START_REF] Odum | Energy flow in ecosystems: a historical review[END_REF] in the development of outwelling hypothesis. According to this concept, up to 45% of organic matter produced in salt marshes may be exported to the coastal ocean that benefits from this fertilisation to support secondary production [START_REF] Teal | Energy flow in the salt march ecosystem of Georgia[END_REF]. However, several studies have shown that this concept should not be applied to all salt marshes, and that nutrient and or-ganic matter fluxes between adjacent systems ought to be systematically measured (i.e. [START_REF] Pomeroy | The Ecology of a Salt Marsh[END_REF]Lefeuvre & Dame, 1994). Many authors thus have quantified these fluxes by abiotic vectors such the tidal currents and the drainage, between various salt marshes and adjacent marine coastal waters (e.i., Teal, loc. cit.;[START_REF] Valiela | The nitrogen budget of a salt marsh ecosystem[END_REF][START_REF] Nixon | Beetwen coastal marshes and coastal waters: A review of 20 y of speculation and research on the role of salt marshes in estuarine productivity and water chemistry[END_REF][START_REF] Dame | The outwelling hypothesis and North Inlet Estuary, South Carolina[END_REF][START_REF] Chambers | Ammonium and phosphate dynamics in a Virginia salt marsh[END_REF], and more specific to this study in the Mont Saint-Michel Bay (e.i., Lefeuvre et al., 1994;[START_REF] Troccaz | Methodology for studying exchanges between salt marshes and coastal marine waters[END_REF][START_REF] Bouchard | Production et devenir de la matière organique des halophytes dans un marais salé européen en système microtidal (Baie du Mont Saint-Michel)[END_REF][START_REF] Troccaz | Evolution de la dynamique d'un marais salé: processus fonctinnels internes et relations avec le milieu côtier (la Baie du Mont Saint-Michel[END_REF][START_REF] Troccaz | A nutrient budget methodology in the Mont Saint-Michel Bay salt marsh, France[END_REF].

Functioning of salt marshes of Mont Saint-Michel Bay have been studied for 10 years (Lefeuvre et al., 1994). The salt marshes provide high primary productivity. Some vascular plants (e.g. Atriplex portula-coides and Elytrigia aetherica) produce up to 3600 g DW m 2 y -1 (Lefeuvre et al., loc. cit.), among which less than 1% is exported to the sea as macrodetritus during spring tide [START_REF] Bouchard | Production et devenir de la matière organique des halophytes dans un marais salé européen en système microtidal (Baie du Mont Saint-Michel)[END_REF]. The remaining is decayed rapidly (Bouchard, loc. cit.) and exported as dissolved organic matter to the open sea by abiotic vectors (tidal currents and especially drainage) [START_REF] Troccaz | Evolution de la dynamique d'un marais salé: processus fonctinnels internes et relations avec le milieu côtier (la Baie du Mont Saint-Michel[END_REF][START_REF] Troccaz | A nutrient budget methodology in the Mont Saint-Michel Bay salt marsh, France[END_REF]. This contributes to sustain a high biodiversity in Mont Saint-Michel Bay, especially in fish and mollusks [START_REF] Legendre | La pêche artisanale sur le domaine intertidal de la Baie du Mont Saint-Michel[END_REF][START_REF] Feunteun | Les peuplements piscicoles de la Baie du Mont Saint-Michel[END_REF]. On the other hand, salt marshes globally trap sediment and particulate organic matter imported by marine waters (Troccaz, loc. cit.; Troccaz & Giraud, loc. cit.). Although significant fluxes of N and C occur between salt marshes, marine waters and atmosphere by rain and evapotranspiration (Hemond & Fifield, 1981), they had not been measured in Mont Saint-Michel Bay.

Beside these biogeochemical and physical processes, the role played by fauna, as biotic vector of organic matter fluxes, in the global functioning of estuarine systems has received scant attention [START_REF] Bildstein | The relative importance of biotic and abiotic vectors in nutrient transport[END_REF]. For instance, animals are able to export organic matter from salt marshes, where they forage, to other ecosystems where they rest or breed (i.e. 6% of phosphorus inputs were due to birds in a lake of western France [START_REF] Marion | The importance of avian-contributed nitrogen (N) and phosphorus (P) to Lake Grand-Lieu, France[END_REF][START_REF] Feunteun | Assessment of Grey Heron predation on fish communities: the case of the largest European colony[END_REF]).

This role of animals in energy fluxes had not been studied in Mont Saint-Michel Bay either. However, the high primary productivity of Mont Saint-Michel Bay contributes to an exceptional invertebrate and vertebrate biodiversity. In particular, its fish community is very rich [START_REF] Legendre | La pêche artisanale sur le domaine intertidal de la Baie du Mont Saint-Michel[END_REF]Feunteun & Laffaille, in press). Moreover, the Bay's mud flats represent one of the main nurseries of the French coast of the English Channel for many species such as sea bass, fishes and clupeids [START_REF] Beillois | Nurseries littorales en Baie du Mont Saint-Michel et du Cotentin Est[END_REF]Legendre, loc. cit.). Many adult fishes also utilise the marshes and sustain a traditional fishery (Legendre, loc. cit.).

The objective of this study was thus assess the role of fishes in transferring particulate organic matter (POM) between salt marshes and marine waters in the Mont Saint-Michel Bay. Fish communities were studied in order to determine which species are most involved in these fluxes of organic matter, and to quantify and compare these fluxes with those due to abiotic vectors.

Study site

The Mont Saint-Michel Bay (France) is a wide littoral zone situated in the Normano-Breton Gulf (lat. 48 • 40 ′ N, long 1 • 40 ′ W) and which extends over 500 km 2 (Figure 1). The tidal marshes cover 220 km 2 , including 180 km 2 of mud flats and 40 km 2 of salt marshes. This Bay is characterised by strong tidal ranges which reach up to 16 m. Salt marshes are incised by a more or less dense creek network where sea water comes in during a number of tidal cycles.

The study site is located in accreting salt marshes (30-50 ha per year) west of Mont Saint-Michel (Lefeuvre et al., 1994), dominated by Halimione portulacoides [START_REF] Bouchard | Production et devenir de la matière organique des halophytes dans un marais salé européen en système microtidal (Baie du Mont Saint-Michel)[END_REF]. It is situated at 2.5 km from the coastline on a 10 m wide creek which drains a 5.7 ha watershed. The water only reaches this canal in 43% of the tides when the water level is > 11.25 m, [START_REF] Troccaz | Methodology for studying exchanges between salt marshes and coastal marine waters[END_REF].

Materials and methods

Sampling fish community of the salt marsh

Fishes were caught in the creek between November 1995 and September 1996 during a total of 12 spring tides (Table 1). The samplings were made during ebb or ebb and flood tide in order to compare gut contents before and after residence in the salt marsh. Sampling selectivity was reduced by combining fyke net (4 mm mesh size, 5 m deep, 1.80 m high, 20 m long) to catch small fishes and trammel net (30 to 70 mm mesh size, 2 m high and 30 m long) to capture bigger fishes, especially mugilids, that could escape by avoiding the fyke net. Both devices were set in the canal to catch every fish that accessed the salt marsh during flood or left it during ebb. Sampling was conducted in order to quantify the fish which colonised the marsh and to analyse the variation of population and community structure during the tidal cycle. Every 15 min, a fyke and two trammel nets were set simultaneously. After five minutes, the fyke net was emptied. The trammel nets were taken up after 10 min. Thus, seven to 10 samples were collected depending upon the duration of the tidal cycle. Large fish were sampled during 66% of the time (10 min of fishing every 15 min) with trammel nets and fyke net. Small individuals such as larvae and juvenile, but also small species (i.e. gobies, gasterosteids, etc.), were caught 33% of the time (5 min of fishing every 15 min) with the fyke net. All samples were stored in a refrigerated box and deep frozen at the laboratory (-18 • C) until analysis. Later, they were identified to species using various guides [START_REF] Wheeler | The fish of the British Isles and North West Europe[END_REF][START_REF] Russel | The Eggs and Planktonic Stages of British Marine Fishes[END_REF][START_REF] Farrugio | Clés commentées pour la détermination des adultes et des alevins de Mugilidae de Tunisie[END_REF][START_REF] Reay | Identification of grey mullets (Teleosteï, Mugilidae) juveniles from British waters[END_REF][START_REF] Serventi | The use of pigmentation and morphological characters to identify Italian mullet fry[END_REF]. Individual fish was measured to the nearest 1 mm (fork length, FL) and weighed to the nearest 0.01 g (individual fresh body weight, BW). They were then weighed to the nearest 0.1 g for species (total fresh weight, TFW) to establish the biomass and numeric composition of the community during each sampling. The sampling design enabled us to assess the total numbers and biomass of fish that colonized the salt marsh at each tide studied by extrapolation with moving average of the samples: (i) relative number and biomass frequency of species at each tide and (ii) number and biomass of fish caught per min (CPUE). According to the sampling design, two methods were based on the calculation of estimating the composition (number and biomass abundance of species caught, %NA and %BA) of the fish community during this study: When the sampling was only conducted during ebb,

N j = N i t i + N i + 1 t i + 1 2 xtj, (1)
CPUE = N j + N i / t j + t i , (2)
with: t i = sampling time (mn) for each sample and each device, t j = time between sample i and i + 1; N i = quantity of fish (numbers or g) caught per sample and per species; N j = estimated quantity of fish (numbers or g) which entered in the canal between sample i and i + 1.

When the sample was conducted during ebb and flood:

CPUE = N j + N i + N k t i + t j (3)
with: N k = Quantity of fish (numbers or g) caught per species and per sampling device during the flood.

Gut contents analysis

After thawing, measuring and weighing fishes, the entire content of the digestive tract was removed from every thin-lipped mullet (Liza ramada, FL > 200 mm), 213 sand gobies (Pomatoschistus minutus, FL > 29 mm) caught in March 1996 and every sea bass (Dicentrarchus labrax, FL > 59 mm) caught in November. The percentage of empty stomachs was noted (vacuity index, V). Then, the gut contents were weighed to the nearest 0.01 g (gut content fresh weight, FW). To assess the relationship between FW and DW, for each studied species, we also measured the dry weights of gut contents to nearest 0.01 g (gut content dry weight, DW) after desiccation at 50 • C in a drying oven.

In L. ramada, the percentage of particulate organic matter (POM) in the gut of 30 specimens caught during ebb was determined by loss on ignition at 480 • C. In order to remove mineral carbon, 10 samples of gut contents of thin-lipped mullets caught during the ebb tide were decarbonated by HCl (1 N, pH 2.0), rinsed with distilled water and lyophyllised. The proportion of total organic carbon (%Ctot) and nitrogen (%Ntot) and the molar C/N ratio was determined with a CHN analyser (Carlo-Erba 1500). Diatoms were also identified in the stomach contents of 15 L. ramada specimens. These were separated from sediment by centrifugation and density gradient in a 40% silica gel: Ludox HS solution [START_REF] Blanchard | Méthode simplifiée pour l'extraction du microphytobenthos des sédiments marins par le gel de silice Ludox[END_REF]. Diatom identifications were made according to [START_REF] Peragallo | Diatomées marines de France et des districts maritimes voisins[END_REF], Hustedt (1927Hustedt ( -1933;;[START_REF] Hustedt | Die Kieselagen Deutschlands, Österreichs und der Schweiz mit Berücksichtigung der übrigen Länder Europas sowie der angrenzenden Meeresgebiete[END_REF][START_REF] Hustedt | Die Kieselagen Deutschlands, Österreichs und der Schweiz mit Berücksichtigung der übrigen Länder Europas sowie der angrenzenden Meeresgebiete[END_REF], [START_REF] Hendey | An introductory account of the smaller algae of British coastal waters. Part 5: Bacillariophyceae (Diatoms)[END_REF], [START_REF] Patrick | The diatoms of the United States[END_REF], [START_REF] Germain | Flore des diatomées. Diatomophycées. Eaux douces et eaux saumâtres du Massif Armoricain et des contrées voisines d'europe occidentale[END_REF] and [START_REF] Ricard | Atlas du phytoplancton marin[END_REF]. The frequency of occurrence (%FO) was estimated and the diatoms classified into three categories (I); accidental: FO < 0.10, secondary: 0.10 < FO < 0.50 and preferential: FO > 0.50 [START_REF] Albertini-Berhault | Biologie des stades juvéniles de téleostéens mugilidae Mugil aurata Risso, 1810, Mugil Capito Cuvier, 1829 et Mugil Saliens Risso, 1810. 1. Régime alimentaire[END_REF].

In sand gobies and sea bass, the food items of 100 and 26 stomach contents were identified to lowest possible taxonomic level and counted. The frequency of occurrence (%FO) and numerical frequency (%N) of macroinvertebrates were estimated according to [START_REF] Hureau | Biologie comparée de quelques poissons antarctiques (Nototheneiidae)[END_REF].

Estimation of POM exported by fish community

The amount of particulate organic matter exported by a fish community is mainly related to three factors: quantity of POM exported by species, weather conditions (seasonal fluctuation of composition of fish community) and duration of submersion at each tide. These variables were measured automatically. During each tide and for each species, we assessed the abundance and composition of fishes that grazed in the salt marshes, and, thus we estimated the POM they carried. For each species studied, we calculated the instantaneous ration (Ir), ratio between gut contents weight (FW) and body weight (BW). The difference of Ir between ebb and flood tide was assumed to represent the POM budget of each species during a tide cycle. Assuming that the quantity of fish which exploited the salt marsh is proportional to the total duration of the submersion, it is possible to assess the quantity of POM transferred per tide cycle and hence per month (Q(POM) m ), where:

Q(POM) m = CPUE m × t m × δ (Ir) (4)
where: CPUE m = average CPUE per species (g/mn -1 ) during the month m, t m = total duration (mn) of the submersion during the month m, and δ (Ir) = total dif- ference of Ir between flood and ebb during month m.

Results

Composition of the fish community

We caught 23 species belonging to 12 families (Table 2). Among these, eight occurred frequently and could be regarded as permanent species, including three species of mullets (L. aurata, L. ramada and Chelon labrosus (Risso, 1826)), two gobies (P. minutus and P. lozanoï), sea bass, sprat (Sprattus. sprattus) and three-spined stickleback (Gasterosteus aculeatus). Thus, the community's biomass was highly dominated by mullets (81.1%), especially by thin lipped grey mullet (78.9%) and secondarily by Pomatoschistus (10.5%) and sea bass (3.9%). The community was numerically dominated by gobies (73.2%), especially by sand gobies (60.3%), flatfish larvae (12.6%) but only 3.5% are mugilids. Glass eels (Anguilla anguilla) and flatfish larvae were not quantified because the fyke net was not considered an efficient sampler of these small fishes. Except for gobies, mullets, sea bass and three-spined stickleback, the fishes were mainly found as larvae and juveniles and they only occurred for short period, in particular during early spring. During these early stages, most fishes feed on microplankton [START_REF] Russel | The Eggs and Planktonic Stages of British Marine Fishes[END_REF] which is transported the water during the tide; therefore, we assumed that they were not significantly involved in POM fluxes.

The salt marsh was colonised by fish every time the tide reached the creek. Nevertheless, abundances were variable: CPUE varied between 2.8 g min -1 (March 1996) and 400 g min -1 (September 1996) (Figure 2a). The first abundance peak occurred in November 1995; it was mainly due to gobies (100 g.min -1 ) (Figure 2b). No data was collected during winter but, considering the low water temperatures, we assume that abundances dropped and remained low in accordance with other sites [START_REF] Feunteun | Le peuplement piscicole du marais littoral endigué de Bourgneuf-Machecoul (France, Loire-Atlantique). Approche méthodologique pour une analyse quantitative de la distribution spatiale du peuplement piscicole et de la dynamique de certaines de ses populations[END_REF] and in this creek in winter 1997 (Laffaille, unpublished data). The CPUE began to increase again at the beginning of March (2.8 g.min -1 ) and in August (150 g.min -1 ), when catches were generally dominated by mullets between April and September.

Thus, only five species were able to export signiforganic matter produced in the salt marshes or on the mud flats: two species of mullets (Liza ramada and L. aurata), two species of gobies (Pomatoschistus minutus, P. lozanoï) and the sea bass (Dicentrarchus labrax). Their abundances decreased in winter and started increasing again in April, reaching a peak in summer (Figure 2). This could be due to the fact that, in the northern hemisphere, mugilids graze more actively in spring and in summer (i.e, [START_REF] Hickling | A contribution a natural history of the English grey mullets (Pisces, Mugilidae)[END_REF][START_REF] Odum | Utilisation of the direct grazing and plant detritus food chains by the stripped mullet Mugil cephalus[END_REF][START_REF] De Silva | Studies on the biology of young grey mullet, Mugil cephalus L[END_REF][START_REF] Albertini-Berhault | Biologie des stades juvéniles de Mugilidae dans la région marseillaise[END_REF]. The scarceness of adult during winter could be related to the low temperatures, but also to a fasting period during gonad maturation between October and April [START_REF] Cassifour | Contribution à l'étude des Mugilidés des réservoirs à poissons du bassin d'Arcachon[END_REF]. Young mullets exclusively feed on zooplankton, but their diet focuses on phytobenthos with increasing age (i.e, Hickling, loc. cit.;Odum, loc. cit.;[START_REF] Blaber | The feeding ecology of juvenile mullet (Mugilidae) in south-east African estuaries[END_REF]Albertini-Berhault, loc. cit.). Therefore, we assume that only large individuals contribute significantly to organic matter fluxes. We found that the diet of large mullets was mainly composed of sediment, phytobenthos, zooplankton and various detritus. Most of the food items were diatoms. Among the 32 identified species, 11 dominant species were determined from the literature to be benthic forms (Table 3): Nitzschia sigma, Coscinodiscus excentricus, Scoliopleura tumida, Podosira stelliger, Surirella ovata, Paralia sulcata, Actinoptychus undulatus, Biddulphia rhombus, Surirella gemma, Raphoneis amphiceros and Navicula phyllepta (Peragallo & Peragallo, 1887;Hustdet, 1927Hustdet, -1933;;[START_REF] Hustedt | Die Kieselagen Deutschlands, Österreichs und der Schweiz mit Berücksichtigung der übrigen Länder Europas sowie der angrenzenden Meeresgebiete[END_REF][START_REF] Hustedt | Die Kieselagen Deutschlands, Österreichs und der Schweiz mit Berücksichtigung der übrigen Länder Europas sowie der angrenzenden Meeresgebiete[END_REF][START_REF] Hendey | An introductory account of the smaller algae of British coastal waters. Part 5: Bacillariophyceae (Diatoms)[END_REF][START_REF] Patrick | The diatoms of the United States[END_REF]; Germain, 1981; [START_REF] Ricard | Atlas du phytoplancton marin[END_REF]. At the same period, these species were also present in the creeks sediment [START_REF] Brosse | Intersystem exchanges, role of epipelic algae in the retention and transfer of matter between a salt marsh and coastal sea water[END_REF]. This suggests that L. ramada grazed the superficial sediment layer and confirms several former studies (i.e. Hickling, loc. cit.;Odum, loc. cit.;[START_REF] Fagade | The food and feeding interrelationship of the fishes in the Lagos lagoon[END_REF][START_REF] Brusle | Food and feeding in grey mullet[END_REF][START_REF] Almeida | The feeding strategies of Liza ramada (Risso, 1826) in fresh and brackish water in the river Tagus, Portugal[END_REF]. We found a confirmation of these observations in the occurrence of grazing marks in the silt of the channels after the passage of mullets. None of the sampled mullets had empty guts (V = 0) during ebb or during flood (Table 4). This shows that the grazing started on the mud flats adjacent to the marsh during flood. However, the average repletion index increased from 5.9%, when the mullets arrived from the sea with the flood, to 12.7% when they left the marsh at ebb. Thus, on average, mullets swallowed about 6.9% of their body weight while they grazed on the salt marsh at high tide.

A significant regression correlation (r = 0.96) was found between the dry weight and fresh weight of the gut contents of 30 mullets. On average, DW is 66.9% of FW (sd = 7.44). However, no significant relation (r = 0.26) was found between POM biomass and total gut contents (FW). The proportion of POM was comprised between 24.2% and 64.3% (average = 36.3%, sd = 13.1). Therefore, it was not possible to predict POM biomass from total ingested food biomass. This result was not surprising, because the proportion of organic matter depends on factors such as feeding behaviour, sediment composition, site, etc. For instance, [START_REF] Farrugio | Les muges (Poissons Téléostéens) de Tunisie. Répartition et pêche. Contribution à leur étude systèmatique et biologique[END_REF] assessed that one third of the gut contents was composed of sediment, and [START_REF] Jacot | Age, growth and scale characters of Mugil cephalus and Mugil curena[END_REF] showed that 40% of Mugil spp. stomach content were 'non digestible matter '. Tung (1971[START_REF] Brusle | Food and feeding in grey mullet[END_REF] showed that the proportion of organic matter varied from 65% of sediment when mullets fed on diatoms to only 35% when they fed on copepods. Moreover, the proportion of POM varies considerably according to seasons and to trophic areas [START_REF] Almeida | The feeding strategies of Liza ramada (Risso, 1826) in fresh and brackish water in the river Tagus, Portugal[END_REF]. The proportion of total organic carbon and nitrogen contained in the digestive tract of ten mullets (Table 5) averaged 5.7% (sd = 1.34) and 0.8% (sd = 0.21). These proportions were low, certainly because sediment was important in the gut contents. The average C/N ratio is 8.7% (sd = 0.89). Thus, inter-individual differences appeared to be relatively low.

Gobies

Five species of Gobiidae were sampled in the creek. But, because of their commonness, only Pomatoschistus minutus and P. lozanoï were significantly involved in organic matter fluxes. Gobies were present in the salt marsh during the whole study period except at the end of May (Figure 2 b). At the beginning of May, only 7.1% of males were present whereas 39.4% were mature females full of ovocytes. At the end of May, no gobies were caught and after this date high concentrations of larvae occurred (up to 125 gobies min -1 ), but the biomass was low (3.6 g min -1 ) which suggests that gobies did not participate significantly to organic matter flux at that period of year. A high abundance of adults occurred in autumn, when CPUE ranged between 60 and 100 g mn -1 .

Pomatoschistus minutus is considered as a generalist predator [START_REF] Hamerlynck | The food of Pomatoschistus minutus (Pisces, Gobiidae) in Belgian coastal waters, and a comparison with the food of its potential competitor P. lozanoi[END_REF]. But, with regard to the food items in the gut (Table 6), Pomatoschistus sp. can also be considered as an opportunist. They can either feed on microinvertebrates, but also on macroinvertebrates, and cases of cannibalism are occasionally observed. Orchestia gammarellus were the preferential food items in the canal (%OF = 0.600 and %N = 0.710). This amphipod is a resident species at Mont Saint-Michel salt marshes [START_REF] Fouillet | Evolution des peuplements d'Arthropodes des schorres de la Baie du Mont Saint-Michel[END_REF].

In Pomatoschistus minutus, V was low (V = 1.5%) during flood tides, but it dropped to 0% during ebb a few minutes later (Table 7). Meanwhile, Ir increased from 9.7% (flood tide) to 14.4% (ebb tide), indicating that these fishes fed actively while they colonised the salt marshes and that they exported on average 4.7% of their body weight during a tidal cycle. A significant correlation was found between dry weight and fresh weight of guts contents (r = 0.96). On average, DW represents 59% (sd = 8.3) of gut contents.

Sea bass

Dicentrarchus labrax were present as juveniles stages, 0+ and 1+ (Table 2). They were absent in winter, but started arriving in the middle of spring (Figure 2). Un- til the beginning of summer, their abundances were generally low (0 to 5.1 g min -1 ), and peaked at the end of summer and especially in autumn (7 to 13 g min -1 ).

The diet was exclusively composed of macroinvertebrates (Table 6), mainly the amphipod Orchestia gammarellus, which occurred in 92% of the stomach, and secondarily the isopod Sphaeroma rugicauda, found in 54% of the stomach contents. These preferential food items (%N = 0.910) are resident species in the Mont Saint-Michel salt marshes [START_REF] Fouillet | Evolution des peuplements d'Arthropodes des schorres de la Baie du Mont Saint-Michel[END_REF].

During flood, vacuity (Table 8) averaged 61.8% and dropped to 0% when the bass left the marsh at ebb. Ir (Table 8) increased from 0.96% at flood to 11.04% at ebb tide. The sea bass ingested about 10.08% of their body weight while they were in the salt marshes. This organic matter was then transferred to the marine coastal waters. There was a significant correlation between DW and FW of the gut contents (r = 0.97). Thus, it was possible to predict the DW from the FW as DW represented 62% (sd = 6.6) of the FW.

Discussion -Conclusion

At least 90 fish species are present in the Bay of Mont Saint-Michel [START_REF] Beillois | Nurseries littorales en Baie du Mont Saint-Michel et du Cotentin Est[END_REF][START_REF] Legendre | La pêche artisanale sur le domaine intertidal de la Baie du Mont Saint-Michel[END_REF][START_REF] Feunteun | Les peuplements piscicoles de la Baie du Mont Saint-Michel[END_REF]. Of these, 23 species colonise the salt marsh mainly during their early life stages. Other species are also known to frequent salt marshes, for instance shad Alosa alosa (Linnaeus, 1758) and salmon Salmo salar (Linnaeus, 1758) are occasionally caught in traditional fisheries (Lefeuvre, unpublished data), and juveniles of Belone belone (Linnaeus, 1758) and Lepadogaster lepadogaster (Bonnaterre, 1788) have also been collected during recent samplings (Laffaille, unpublished data). Therefore, the productivity of this environment, that is known to be exploited by birds [START_REF] Schricke | Distribution spatio-temporelle des populations d'Anatidés en transit et en hivernage en Baie du Mont Saint-Michel en relation avec les activité humaines[END_REF]Lefeuvre et al., 1994), is also actively foraged by fish communities. The Bay of Mont Saint-Michel (Beillois et al., loc. cit.) as with comparable shallow marine coastal waters such as estuaries and lagoons (i.e. [START_REF] Costa | The Tagus and Mira estuaries (Portugal) and their role as spawning and nursery areas[END_REF][START_REF] Elie | The inshore brackish water domain of the French Atlantic coast: ecological functions for the exploited species-impact of physical development[END_REF], are known to play a nursery role for many fish species. This function is usually described as restricted to the mud flats of the midlittoral zones in Mont Saint-Michel Bay.

The present study shows that the salt marshes of the supralittoral zone also play an important nursery role (i.e. [START_REF] Boesch | Dependence of fishery species on salt marshes: the role of food and refuge[END_REF]. Salt marshes are believed to serve as nurseries for many fishes as a result of the high production of vascular plant detritus and diatoms. The refuge role generally described in shallow bays may not be as significant in Mont Saint-Michel Bay; despite scarce predators (a few seals, piscivorous birds and large fish), salt marshes are submerged for too short period (max. 2 h) and the currents are too strong (megatidal system) to represent a signifiant refuge role for young fish. We found that a minimum of five fish species feed actively in the salt marshes of Mont Saint-Michel Bay during tidal floods: two species of mullets, two species of gobies and juvenile sea bass. These five seem to colonise marshes to exploit their high productivity. Mullets exploit the primary productivity (mainly diatoms), whereas gobies and sea bass exploit the secondary productivity (especially Orchestia gammarellus). These resident species feed on diatoms and vascular plant detritus [START_REF] Creach | Origines et transferts de la matière organique dans un marais littoral: utilisation des compositions isotopiques naturelles du carbone et de l'azote[END_REF]. Moreover, these fishes play a dominant role in the food web and in the fisheries of the Mont Saint-Michel Bay either as prey of other fishes and birds (gobies, and young mullets) or as valuable commercial species (sea bass) [START_REF] Feunteun | Les peuplements piscicoles de la Baie du Mont Saint-Michel[END_REF]. Therefore, the primary productivity of salt marshes appears to play a fundamental role in the global functioning of Mont Saint-Michel Bay.

The magnitude of organic matter fluxes mainly depends on the biomass of fish that forage on the salt marsh during the tidal cycle. To summarise, during the tidal cycles, mullets export a mixture of sediment and organic matter from the salt marshes to the ma-rine coastal waters of the Bay. During a given tide cycle, each fish exports about 6.9% of its fresh weight, comprising 42.6% of sediment, 24.3% of particulate organic matter and 33.1% of water. The dry gut contents POM averages 5.7% organic C and 0.8% organic N. Sand gobies and sea bass mainly feed and export respectively 4.5% and 10% of their body weight during a tide cycle. The difference of Ir takes into account excretion and gastric evacuation in the creek. Retention time of the ingested material is highly variable. For example, it ranges from 2 to 6 h for mullets [START_REF] Brusle | Food and feeding in grey mullet[END_REF], which is much longer that the tidal submersion period of the creeks and the salt marsh. Therefore we assume that POM is low in the salt marsh and creeks during the tide cycle.

In order to assess the relative importance of POM fluxes from the salt marsh due to fish, we assume that respectively Liza ramada and L. aurata, and Pomatoschitus minutus and P. lozanoï export, on average, an equal quantity and assumable quality of organic matter. Since sea bass and gobies' diets were largely dominated by Orchestia gammarellus, we used the proportion of Ctot and Ntot assessed by [START_REF] Creach | Origines et transferts de la matière organique dans un marais littoral: utilisation des compositions isotopiques naturelles du carbone et de l'azote[END_REF] in the Mont Saint-Michel Bay: Ctot = 50.9 ± 4.13% (n = 20), Ntot = 8.7 ± 0.97 (n = 20) and C/N = 9.7 ± 0.9 (n = 20). On this basis, we estimated the organic matter fluxes due to these fish per tide cycle and per month (Figure 3). During the study, a total of 50 g ha -1 to 1700 g ha -1 of organic matter (DW) was exported monthly towards the marine coastal waters. On average, 67% of this organic matter is exported by mullets, 23% by gobies and 10% by sea bass. But during late autumn, gobies export more than 70% of the organic matter transported by fish. The study site has a central location among the salt marshes of the Bay of Mont Saint-Michel. Therefore, assuming such a geographical representativity, the extrapolation of our data set permits to assess that the fish communities export about 50 tons DW of organic matter per year from the salt marshes towards the sea, comprising 10.3 tons of organic C and 1.7 tons of N. Of course, part of the POM exported by fish is decayed and can be imported later by tidal currents as POM, DOM or dissolved nutrients. However, we assume that the POM output by fish was very low compared to other sources during the study period. More, recent samplings have shown that the biomass of mullets present in the creek may be at least 15 times higher (unpublished data) which suggests that important interannual variations of fish contribution to organic matter fluxes between salt marshes and marine coastal waters are expected. Even if the total input of OM to the Bay of Mont has been estimated in the past years [START_REF] Troccaz | Evolution de la dynamique d'un marais salé: processus fonctinnels internes et relations avec le milieu côtier (la Baie du Mont Saint-Michel[END_REF], we still do not know the proportion of this organic matter which is exported by fish. In order to improve the understanding of the coastal biogeochemical cycles, it is interesting to compare the POM budgets due to fish communities with those due to abiotic vectors (tidal currents, interstitial water, etc.). On a Bay-wide scale, the salt marsh traps POM during the tide cycles: for instance in 1991, the input was 2.9 kg m -2 [START_REF] Troccaz | Evolution de la dynamique d'un marais salé: processus fonctinnels internes et relations avec le milieu côtier (la Baie du Mont Saint-Michel[END_REF][START_REF] Troccaz | A nutrient budget methodology in the Mont Saint-Michel Bay salt marsh, France[END_REF]. In comparison, during the ebb tides between 1991and 1994, Troccaz et al. (1994), Troccaz & Giraud, (loc. cit.) and Troccaz (loc. cit.) estimated that between 0.7 g m -2 y -1 and 5.8 g m -2 y -1 of total N, and 83.3 g m -2 y -1 to 114.2 g m -2 y -1 of total organic C was exported from these salt marshes. Of course, such comparison is difficult because of several uncertainties. Firstly, we noted an important temporal heterogeneity of the POM budgets due to fish transport and also to abiotic vectors. Therefore, a simultaneous survey of both POM fluxes vectors ought to be conducted a longer period. Secondly, the assumption that the study site is reprentative of the whole salt marsh will have to be verified because its productivity depends on several factors. For instance, in the parts of the marsh which are grazed by sheep, the original vegetation cover, dominated by Atriplex portulacoides, is replaced by Puccinellia maritima. Thirdly, the 4000 ha of salt marshes have variable ages, the oldest have had about 60 y of existence, whereas the youngest are only a few years old: 30 ha of pioneer mud flat zones are colonised every year, especially by Spartina spp., and Salicorna spp. [START_REF] Levasseur | Transferts entre systèmes écologiques terrestre et marin en Baie du Mont Saint-Michel: rôle des marais salés[END_REF]. It has been shown that salt marshes have a tendancy to export more OM as they get older (Dame & Lefeuvre, 1994). Therefore, these variables add another variability source which has to be investigated in order to make accurate comparisons between the OM bugets due to fish communities and abiotic vectors. Nevertheless, as a preliminary approach, we assume that the fish community is responsible for part of the output of nitrogen and carbon from the salt marsh, which varies strongly according to years, and to characteristics of the salt marsh. We conclude that fish communities play an important role in the global functioning of the Bay and more generally of coastal environments, at least in terms of organic matter fluxes.

Figure 1 .

 1 Figure 1. Location of the Mont Saint Michel Bay and study site.

Figure 2 .

 2 Figure 2. Seasonial fluctuations of the fish abundance. a. biomass (g) of fish caught per min (CPUE). b. biomass frequency (%).

Figure 3 .

 3 Figure 3. Monthly transfers of particulate organic matter by fishes between the studied salt marshes and marine coastal waters in the bay of Mont Saint Michel, between November 1995 and August 1996.

Table 1 .

 1 Characteristics of the 12 tides sampled for fish in Mont Saint-Michel Bay, November 1995 to September 1996.

	Dates	Tide	Duration of	Sampling during
		amplitude	submersions	flood ebb
		(m)	(min)	tide	tide
	07/11/95	11.60	100	Yes	Yes
	23/11/95	12.45	155	Yes	Yes
	07/03/96	12.00	125	No	Yes
	18/03/96	12.25	150	Yes	Yes
	05/04/96	12.30	130	No	Yes
	17/04/96	12.20	120	No	Yes
	03/05/96	12.15	125	No	Yes
	17/05/96	11.80	100	Yes	Yes
	02/06/96	12.05	120	No	Yes
	01/07/96	12.55	140	No	Yes
	31/07/96	12.65	160	No	Yes
	01/09/96	12.60	155	No	Yes

Table 2 .

 2 Composition of fish community caught in the canal between November 1995 and September 1996 during 12 tides. %Occ: percentage of samples in which the species was present,%BA: biomass abundance (%),%NA: numeric abundance (%), FL: fork lenght (mm), n: number of measured fish, * : no quantification of A. anguilla.

	Families	Species	%Occ	%BA %NA FL min FL max FL mean
						(mm)	(mm)	(n)
	Mugilidae	Mugil sp.	100	0.5	3.1	19	60	25 (454)
		Liza aurata (Risso, 1810)	33	1.7	0.1	95	200	136 (16)
		Liza ramada (Risso, 1826)	50	78.9	0.3	285	494	408 (50)
	Gobiidae	Aphia minuta (Risso, 1810)	8	0.0	0.0	21	55	45 (9)
		Crystallogobius linearis (Von Düben, 1845)	8	0.0	0.0	33	38	36 (6)
		Pomatoschistus microps (Kroyer, 1838)	8	0.1	0.7	42	53	49 (7)
		Pomatoschistus minutus (Pallas, 1770)	83	8.1	60.3	19	58	40 (610)
		Pomatoschistus lozanoï (de Buen, 1923)	75	2.4	12.2	19	70	34 (117)
	Serranidae	Dicentrarchus labrax (Linnaeus, 1758)	67	3.9	0.8	60	260	85 (200)
	Atherinidae	Atherina presbyter (Cuvier, 1829)	17	1.8	1.4	61	85	72 (118)
	Pleuronectidae	Limanda limanda (Linnaeus, 1758)	25	0.0	2.3	9	15	12 (150)
		Platichthys flesus (Linnaeus, 1758)	17	0.9	0.0	195	210	202 (4)
		P. flesus (larvae)	33	0.0	3.1	8	21	14 (200)
		Pleuronectes platessa (Linnaeus, 1758)	17	0.7	0.0	222	232	227 (2)
		P. platessa (larvae)	50	0.0	7.0	8	20	13 (200)
	Clupeidae	Clupea harengus (Linnaeus, 1758)	25	0.2	1.3	23	110	37 (165)
		Sprattus sprattus (Linnaeus, 1758)	75	0.8	6.3	26	86	39 (515)
	Gasterosteidae	Gasterosteus aculeatus (Linnaeus, 1758)	67	0.1	0.3	21	68	39 (45)
	Ammodytidae	Ammodytes tobianus (Linnaeus, 1758)	8	0.0	0.4	14	20	17 (62)
	Soleidae	Solea vulgaris (Quensel, 1806)	33	0.0	0.2	9	27	15 (30)
	Syngnathidae	Hippocampus hippocampus (Linnaeus, 1758)	8	0.0	0.0	/	/	120 (1)
		Syngnathus abaster (Risso, 1826)	8	0.0	0.0	/	/	82 (1)
		Syngnathus rostellatus (Nilson, 1855)	25	0.0	0.1	65	99	81 (18)
	Trachinidae	Echiichtys vipera (Cuvier, 1829)	8	0.0	0.0	/	/	11 (1)
	Anguillidae	Anguilla anguilla (Linnaeus, 1758)	50	*	*	60	79	69 (35)

Table 3 .

 3 Diatoms identified in the gut contents of L. ramada caught during ebb tide. %FO: frequency of occurrence, I: food items importance (P: preferential, S: secondary, A: accidental).

	Food items		I	Benthic	Planktonic
	Nitzschia sigma (Kützing) Wm. Smith	1.00	P *
	Coscinodiscus excentricus Ehrenberg	1.00	P *
	Scoliopleura tumida de Brébisson	1.00	P *
	Podosira stelliger (Bailey) Mann	0.93	P *	*
	Surirella ovata Kützing	0.93	P *
	Paralia sulcata (Ehrenberg) Kützing	0.93	P *
	Actinoptychus undulatus (Bailey) Ralfs	0.87	P *	*
	Biddulphia rhombus (Ehrenberg) Wm. Smith	0.87	P *
	Surirella gemma Ehrenberg	0.87	P *	*
	Raphoneis amphiceros Ehrenberg	0.87	P *
	Navicula phyllepta Kützing	0.73	P *	*
	Auliscus sculptus (Wm. Smith) Ralfs	0.47	S *
	Navicula palpebralis de Brélisson ex Wm. Smith	0.47	S *
	Biddulphia mobiliensis Bailey	0.27	S *	*
	Navicula cincta (Ehrenberg) Van Heurck	0.27	S *
	Melosira westii Wm. Smith	0.27	S *	*
	Gyrosigma obtusatum A. Boyer	0.20	S *
	Navicula praetexta Ehrenberg	0.20	S *
	Coscinodiscus rothii var. subsalsa (Juhl-Dannf) Hustedt	0.13	S	*
	Diploneis crabro Ehrenberg	0.13	S *
	Navicula digito-radiata (Gregory) Ralfs	0.13	S

*

Contribution to organic matter budget

Mullets

Mullets colonised the salt marshes at different life stages during the whole year. Large mullets (L. ramada and L. aurata) mainly occurred in November.

Table 4 .

 4 Comparison

		, between ebb tide and flow
	tide, of average instantaneous ration (%Ir) and
	vacuity index (%V) of L. ramada. sd: standard de-
	viation, n: number of digestive contents analysed.
		% Ir	%V n
		average	sd
	Flood tide	5.86	1.43 0	11
	Ebb tide	12.71	3.15 0	39
	Difference	6,.5
	T test	p < 0,01%

Table 5 .

 5 Proportion

			of total organic car-
	bon (%Ctot) and nitrogen (%Ntot) and the
	molar C/N ratio determined in mullet's di-
	gestive contents. sd: standard deviation, n:
	number of digestive contents analysed.
		%Ctot	%Ntot	C/N
	Average	5.74	0.78
	sd	1.34	0.21	0.89
	n	10	10	10

Table 6 .

 6 Food items identified in the stomach contents of D. labrax (n = 26) and P. minutus (n = 100) caught during ebb tide. %FO: frequency of occurrence,%N: numeric frequency, n: number of stomach contents analysed.

	Food items	Sea bass		Sand gobies
		(n 26)		(n = 100)
		%FO	%N	%FO	%N
	Orchestia gammarellus (Pallas)	0.923	0.666	0.600	0.710
	Sphaeroma rugicauda (Leach)	0.538	0.244	0	0
	Eurydice pulchra (Leach)	0.308	0.014	0.050	0.025
	Corophium volutator (Pallas)	0.346	0.020	0	0
	Bathyporeia sp.	0.192	0.028	0	0
	Neris diversicolor (O.F. Müller)	0.269	0.011	0.180	0.076
	Mysidacea n.i.	0	0	0.160	0.143
	Crangon crangon (Linnaeus)	0.077	0.003	0	0
	Carcinus maneas (Linnaeus)	0.115	0.003	0	0
	Tellinidae sp.	0.038	0.001	0	0
	Insect	0.115	0.005	0	0
	Fish	0	0	0.050	0.025
	Other	0.077	0.005	0.050	0.021

Table 7 .

 7 Comparison, between ebb tide and flow tide, of average instantaneous ration (%Ir) and vacuity index (%V) of P. minutus caught the 18 th March. sd: standard deviation, n: number of digestive contents analysed.

		% Ir		%V	n
		average	sd
	Flood tide	9.71	3.84	1.45	72
	Ebb tide	14.4	4.61	0	141
	Difference	4.69
	T Test	p < 0,01%

Table 8 .

 8 Comparison, between ebb tide and flow tide, of average instantaneous ration (%Ir) and vacuity index (%V) of D. labrax caught in November 1995. sd: standard deviation, n: number of digestive contents analysed.

		% Ir		%V	n
		average	sd
	Flood tide	0.96	1.84	61.8	99
	Ebb tide	11.04	2.24	0	68
	Difference	10.08
	T Test	p < 0,01%

Acknowledgements

We are grateful to Dr M. T. Schricke and B. Savouré for identification of the macro-invertebrates and diatom species. We thank other members of Laboratoire d'Evolution des Systèmes Naturels et Modifiés for their laboratory and field assistance. We are indebted V. Bouchard and two anonymous referees for their remarks and corrections of the manuscript.