Paulo Bonifácio
email: p.bonifácio@me.com

Solveig Bourgeois

Céline Labrune

Jean-Michel Michel Amouroux

Karine Escoubeyrou

Roselyne Buscail

Alicia Romero-Ramirez

François Lantoine

Gilles Vétion

Sabrina Bichon

Martin Desmalades

Béatrice Rivière

Bruno Deflandre

Antoine Grémare

Spatiotemporal changes in surface sediment characteristics and benthic macrofauna composition off the Rhône River in relation to its hydrological regime

Keywords: Mediterranean Sea, Rhône River, Floods, Temporal variations, Particulate organic matter, Zoobenthos

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Introduction

River-dominated Ocean Margins (RiOMar) are land-ocean margin systems that are impacted by major rivers water, nutrient and particle discharges [START_REF] Mckee | Transport and transformation of dissolved and particulate materials on continental margins influenced by major rivers: benthic boundary layer and seabed processes[END_REF]. As such they include large marine areas. RiOMar provide essential ecosystem services, such as habitat for many plant and animal species, nutrient recycling and fisheries [START_REF] Levin | The function of marine critical transition zones and the importance of sediment biodiversity[END_REF] and are sensitive to a large diversity of natural and anthropogenic disturbances. On continental margins, in front of each river as well as lagoon mouths appears a preferential area of sediment accumulation under the wave storm base [START_REF] Aloïsi | La sédimentation infralittorale. Les prodeltas nordméditerranéens[END_REF]. These deposition areas, commonly named prodeltas, are the subaquaeous extension of aerial deltas in the inner-shelf around 30 m water depth [START_REF] Bourrin | Contribution to the study of coastal rivers and associated prodeltas to sediment supply in Gulf of Lions (NW Mediterranean Sea)[END_REF]. River prodeltas (i.e., the underwater parts of river deltas) are hydrodynamic environments experimenting high nutrient and terrestrial organic matter inputs, which enhance primary production (Cruzado and Velasquez, 1990;[START_REF] Lochet | Transfers between a eutrophic ecosystem, the river Rhône, and an oligotrophic ecosystem, the north-western Mediterranean Sea[END_REF]. Their benthic compartments are affected by several co-occurring sedimentary (e.g. sedimentation/resuspension) and biogeochemical (e.g. bioturbation/mineralization) processes [START_REF] Aller | Mobile deltaic and continental shelf muds as suboxic, fluidized bed reactors[END_REF][START_REF] Lansard | Benthic remineralization at the land-ocean interface: a case study of the Rhône River (NW Mediterranean Sea)[END_REF]Pastor et al., 2011a;Pastor et al., 2011b). Together with shelf sediments, deltaic sediments are the most important area for organic carbon burial [START_REF] Hedges | Sedimentary organic matter preservation: an assessment and speculative synthesis[END_REF] and for the decomposition of both terrestrial and marine particulate organic matter (POM) [START_REF] Aller | Mobile deltaic and continental shelf muds as suboxic, fluidized bed reactors[END_REF].

Deltaic sedimented POM is usually composed of: (1) continental (e.g. vascular land plants debris and soil-derived POM exported by rivers), and (2) marine material (e.g. marine autochthonous primary production and advective inputs) [START_REF] Hedges | Organic matter sources to the water column and surficial sediments of a marine bay[END_REF][START_REF] Goni | A reassessment of the sources and importance of land-derived organic matter in surface sediments from the Gulf of Mexico[END_REF][START_REF] Leithold | Deposition and modification of a flood layer on the northern California shelf: lessons from and about the fate of terrestrial particulate organic carbon[END_REF][START_REF] Gordon | Sources and distribution of terrigenous organic matter delivered by the Atchafalaya River to sediments in the northern Gulf of Mexico[END_REF]. The balance between both sources clearly shifts from the dominance of continental to marine inputs along inshore/offshore gradients [START_REF] Vonk | Molecular and radiocarbon constraints on sources and degradation of terrestrial organic carbon along the Kolyma paleoriver transect, East Siberian Sea[END_REF]. River floods affect temporal changes in this balance. They are important in controlling both the quantity and the quality of continental POM exported to the sea. These 2 parameters vary seasonally depending on water flows [START_REF] Pont | Les débits solides du Rhône à proximité de son embouchure: données récentes (1994-1995)[END_REF] and are also affected by drainage basin compositions. Dry seasons are usually associated with strong contributions of marine production, conversely to wet seasons, which are typically associated with strong contributions of soil-derived POM and plant debris carried by strong flows [START_REF] Yu | Contribution of amino compounds to dissolved organic nitrogen in forest soils[END_REF]. Although many studies have been devoted to the assessment of the relationships between spatiotemporal changes in deltaic sedimentary organics and hydrological regimes [START_REF] Leithold | Deposition and modification of a flood layer on the northern California shelf: lessons from and about the fate of terrestrial particulate organic carbon[END_REF][START_REF] Bianchi | Sources of terrestrially-derived organic carbon in lower Mississippi River and Louisiana shelf sediments: implications for differential sedimentation and transport at the coastal margin[END_REF][START_REF] Yu | Contribution of amino compounds to dissolved organic nitrogen in forest soils[END_REF], no real consensus has been reached yet regarding: (1) the quality (i.e., lability) of continental inputs [START_REF] Leeuw | A review of macromolecular organic compounds that comprise living organisms and their role in kerogen, coal, and petroleum formation[END_REF][START_REF] Wakeham | Degradation and Preservation of Organic Matter in Marine Sediments[END_REF][START_REF] Mayer | Input of nutritionally rich organic matter from the Mississippi River to the Louisiana coastal zone[END_REF][START_REF] Vonk | Molecular and radiocarbon constraints on sources and degradation of terrestrial organic carbon along the Kolyma paleoriver transect, East Siberian Sea[END_REF], and (2) the effects of different types of floods on spatiotemporal changes in sedimentary organics within RiOMar.

The long-term impact of riverine inputs (e.g. sediments, organic matter) in controlling benthic macrofauna composition is well recognized as well. This paradigm was initially established based on the studies of major rivers such as the Amazon [START_REF] Aller | General characteristics of benthic faunas on the Amazon inner continental shelf with comparison to the shelf off the Changjiang River, East China Sea[END_REF][START_REF] Aller | The distribution and seasonal characteristics of benthic communities on the Amazon shelf as indicators of physical processes[END_REF] and the Changjiang [START_REF] Rhoads | Macrobenthos and sedimentary facies on the Changjiang delta platform and adjacent continental shelf, East China Sea[END_REF][START_REF] Aller | General characteristics of benthic faunas on the Amazon inner continental shelf with comparison to the shelf off the Changjiang River, East China Sea[END_REF]. It has then been verified for smaller rivers [START_REF] Occhipinti-Ambrogi | Macrobenthos community structural changes off Cesenatico coast (Emilia Romagna, Northern Adriatic), a sixyear monitoring programme[END_REF][START_REF] Wheatcroft | Time-series measurements of macrobenthos abundance and sediment bioturbation intensity on a flood-dominated shelf[END_REF][START_REF] Akoumianaki | Spatial variability and dynamics of macrobenthos in a Mediterranean delta front area: the role of physical processes[END_REF][START_REF] Harmelin-Vivien | Linking benthic biodiversity to the functioning of coastal ecosystems subjected to river runoff (NW Mediterranean)[END_REF][START_REF] Akoumianaki | Environmental variation and macrofauna response in a coastal area influenced by land runoff[END_REF]. According to the conceptual model proposed by [START_REF] Rhoads | Macrobenthos and sedimentary facies on the Changjiang delta platform and adjacent continental shelf, East China Sea[END_REF] spatial changes in benthic macrofauna composition off (major) rivers result from 2 opposite effects, namely: (1) a reduction of benthic macrofauna at the immediate vicinity of the river mouth due to the inputs of large quantities of sediments resulting in high sedimentation rates and instability, and (2) an increase of benthic macrofauna further offshore resulting from moderate organic enrichment. Conversely, there is no consensus about the occurrence of short-term effects of river inputs on benthic macrofauna, which were reported by [START_REF] Occhipinti-Ambrogi | Macrobenthos community structural changes off Cesenatico coast (Emilia Romagna, Northern Adriatic), a sixyear monitoring programme[END_REF] and by [START_REF] Akoumianaki | Environmental variation and macrofauna response in a coastal area influenced by land runoff[END_REF] but not by [START_REF] Wheatcroft | Time-series measurements of macrobenthos abundance and sediment bioturbation intensity on a flood-dominated shelf[END_REF]. Part of this discrepancy may result from differences between studies in the relative locations of monitored stations relative to inshore/offshore gradients. There is therefore a clear need for new studies combining appropriate spatial and temporal sampling to better describe the effects of changes in riverine inputs on benthic macrofauna composition.

The Rhône River (Figure 1) is the major source of freshwater and terrigenous particles to the Gulf of Lions [START_REF] Aloisi | Essai de modélisation de la sédimentation actuelle sur les plateaux continentaux français[END_REF]. Its drainage Basin shows a strong geological heterogeneity and is subjected to highly fluctuating climatic conditions [START_REF] Pont | Les débits solides du Rhône à proximité de son embouchure: données récentes (1994-1995)[END_REF][START_REF] Pont | Medium-term changes in suspended sediment delivery to the ocean: consequences of catchment heterogeneity and river management (Rhône River, France)[END_REF]. Mean annual water and particle flows are 1700 m 3 .s -1 and 7400 10 3 t.y -1 [START_REF] Pont | Medium-term changes in suspended sediment delivery to the ocean: consequences of catchment heterogeneity and river management (Rhône River, France)[END_REF]. Temporal changes in water (up to 11000 m 3 .s - 1 ) and particle flows (up to 22700 10 3 t.y -1 in years with strong floods) are very high, which is a characteristic of Mediterranean Rivers [START_REF] Pont | Medium-term changes in suspended sediment delivery to the ocean: consequences of catchment heterogeneity and river management (Rhône River, France)[END_REF][START_REF] Antonelli | Suspended sediment and 137 Cs fluxes during the exceptional December 2003 flood in the Rhone River, southeast France[END_REF]. Rhône River floods may be classified [START_REF] Pont | Les débits solides du Rhône à proximité de son embouchure: données récentes (1994-1995)[END_REF] as: (1) oceanic when resulting from precipitations in the Northern Basin and characterized by water flows rising slowly and regularly, (2) Cevenol when resulting from intense precipitations in the South-Western Basin and characterized by sudden and violent increase in water flows, (3) extensive Mediterranean when resulting from precipitations affecting the whole Southern Basin and mostly often associated with autumnal western perturbations, and (4) generalized when corresponding to a combination of the 3 first types. These events strongly differ in terms of both the quantity [START_REF] Pont | Les débits solides du Rhône à proximité de son embouchure: données récentes (1994-1995)[END_REF] and the origin [START_REF] Eyrolle | Consequences of hydrological events on the delivery of suspended sediment and associated radionuclides from the Rhône River to the Mediterranean Sea[END_REF] of the particles transferred to the sea. The Rhône River prodelta is characterized by high sedimentation rates (0.40-0.65 cm.y -1 as assessed through 210 Pb measurements), which then decrease offshore (0.20 cm.y -1 ) [START_REF] Zuo | Accumulation rates and sediment deposition in the northwestern Mediterranean[END_REF][START_REF] Radakovitch | 210 Pb and caesium accumulation in the Rhône delta sediments[END_REF][START_REF] Miralles | 210 Pb sedimentation rates from the Northwestern Mediterranean margin[END_REF]. It constitutes a transitional depositional area for terrigenous particles, associated organic matter and contaminants [START_REF] Roussiez | Background levels of heavy metals in surficial sediments of the Gulf of Lions (NW Mediterranean): an approach based on 133 Cs normalization and lead isotope measurements[END_REF], which are later transferred to the deep sea through a succession of resuspension events mostly caused by storms [START_REF] Ulses | Suspended sediment transport in the Gulf of Lions (NW Mediterranean): impact of extreme storms and floods[END_REF]. Overall, strong temporal changes in its hydrological regime together with the heterogeneity of its drainage basin and classical spatial gradients within the deltaic area make the Rhône River an excellent model for the assessment of the effect of a major Mediterranean river on sedimentary organics and benthic macrofauna characteristics.

Spatiotemporal changes in surface sediment characteristics off the Rhône River have already been assessed in several studies [START_REF] Alliot | Biogeochemical impact of a dilution plume (Rhone River) on coastal sediments[END_REF][START_REF] Tesi | Source, transport and fate of terrestrial organic carbon on the western Mediterranean Sea, Gulf of Lions, France[END_REF][START_REF] Lansard | Benthic remineralization at the land-ocean interface: a case study of the Rhône River (NW Mediterranean Sea)[END_REF][START_REF] Cathalot | Temporal variability of carbon recycling in coastal sediments influenced by rivers: assessing the impact of flood inputs in the Rhône River prodelta[END_REF][START_REF] Bourgeois | Distribution and lability of land-derived organic matter in the surface sediments of the Rhône prodelta and the adjacent shelf (Mediterranean Sea, France): a multi proxy study[END_REF]Pastor et al., 2011b;[START_REF] Cathalot | The fate of river organic carbon in coastal areas: a study in the Rhône River delta using multiple isotopic (δ 13 C, Δ 14 C) and organic tracers[END_REF]. Part of these studies [START_REF] Lansard | Benthic remineralization at the land-ocean interface: a case study of the Rhône River (NW Mediterranean Sea)[END_REF][START_REF] Cathalot | Temporal variability of carbon recycling in coastal sediments influenced by rivers: assessing the impact of flood inputs in the Rhône River prodelta[END_REF] have mostly dealt with the assessment of sediment organic matter remineralization and thus have only considered a limited set of bulk biochemical descriptors. Others have included a wider set of biochemical parameters but have been restricted to an assessment of spatial changes [START_REF] Alliot | Biogeochemical impact of a dilution plume (Rhone River) on coastal sediments[END_REF][START_REF] Bourgeois | Distribution and lability of land-derived organic matter in the surface sediments of the Rhône prodelta and the adjacent shelf (Mediterranean Sea, France): a multi proxy study[END_REF]Pastor et al., 2011a;Pastor et al., 2011b;[START_REF] Cathalot | The fate of river organic carbon in coastal areas: a study in the Rhône River delta using multiple isotopic (δ 13 C, Δ 14 C) and organic tracers[END_REF]. To our knowledge, the study by [START_REF] Tesi | Source, transport and fate of terrestrial organic carbon on the western Mediterranean Sea, Gulf of Lions, France[END_REF] is the only one, which combined the study of 2 contrasted situations in terms of Rhône River flows with the sampling of a large number of stations and the assessment of a reasonably large set of sedimentary organics biochemical characteristics. However, it still did not include classical organic matter quality descriptors such as EHAA/THAA and Chl-a/(Chl-a+Phaeo-a).

Several studies have assessed spatiotemporal changes in benthic macrofauna composition off the Rhône River as well [START_REF] Salen-Picard | Responses of a Mediterranean soft bottom community to short-term (1993-1996) hydrological changes in the Rhone river[END_REF][START_REF] Hermand | Macrofaunal density, biomass and composition of estuarine sediments and their relationship to the river plume of the Rhone River (NW Mediterranean)[END_REF][START_REF] Harmelin-Vivien | Linking benthic biodiversity to the functioning of coastal ecosystems subjected to river runoff (NW Mediterranean)[END_REF][START_REF] Labrune | Comparison of ecological quality indices based on benthic macrofauna and sediment profile images: a case study along an organic enrichment gradient off the Rhône River[END_REF]. A few of them have dealt with: (1) the whole benthic macrofauna but were restricted to a single sampling date [START_REF] Hermand | Macrofaunal density, biomass and composition of estuarine sediments and their relationship to the river plume of the Rhone River (NW Mediterranean)[END_REF][START_REF] Harmelin-Vivien | Linking benthic biodiversity to the functioning of coastal ecosystems subjected to river runoff (NW Mediterranean)[END_REF][START_REF] Labrune | Comparison of ecological quality indices based on benthic macrofauna and sediment profile images: a case study along an organic enrichment gradient off the Rhône River[END_REF]. They have shown the occurrence of strong longitudinal gradients in benthic macrofauna composition, which limits their use to ca 10 stations sampled only once in assessing the impact of Rhône River water flow [START_REF] Hermand | Macrofaunal density, biomass and composition of estuarine sediments and their relationship to the river plume of the Rhone River (NW Mediterranean)[END_REF][START_REF] Harmelin-Vivien | Linking benthic biodiversity to the functioning of coastal ecosystems subjected to river runoff (NW Mediterranean)[END_REF]. Another study was conversely restricted to the assessment of polychaete fauna at a single 70 m deep station but focussed on the assessment of the relationship linking Rhône River water flows and fauna compositions based on repeated seasonal sampling [START_REF] Salen-Picard | Responses of a Mediterranean soft bottom community to short-term (1993-1996) hydrological changes in the Rhone river[END_REF]. It has suggested the dependency of benthic macrofauna composition on temporal changes in the Rhône River water flow with the distinction between 2 groups of species: one responding rapidly (i.e., around 3 months) and mostly composed of opportunistic species adapted to organically rich environments, and a second one responding with a 1-2 year time lag and mostly composed of (more) stable species. The question of the extrapolation of these results to other areas off the Rhône River mouth however still remains fully open due to the occurrence of strong spatial gradients and of their possible interactions with temporal changes in Rhône River water flow.

In this context, the aim of this work was to further assess the effect of changes in Rhône River water flows on both: (1) the quantitative and qualitative characteristics of surface sediment, (2) benthic macrofauna composition, and (3) the possible control of the latter by the former.

Materials and Methods

Study area

The Rhône River hydrological basin covers an area of 97800 km² (Figure 1) with a mean daily flow between 602 and 11000 m 3 .s -1 [START_REF] Pont | Medium-term changes in suspended sediment delivery to the ocean: consequences of catchment heterogeneity and river management (Rhône River, France)[END_REF][START_REF] Antonelli | Suspended sediment and 137 Cs fluxes during the exceptional December 2003 flood in the Rhone River, southeast France[END_REF]. Low flows are usually recorded during summer whereas high flows occur during winter and spring [START_REF] Pont | Les débits solides du Rhône à proximité de son embouchure: données récentes (1994-1995)[END_REF]. The mean daily concentration of suspended particulate matter (SPM) is 180 mgDW.l -1 . It can decrease to 26 mgDW.l -1 during dry periods [START_REF] Pont | Medium-term changes in suspended sediment delivery to the ocean: consequences of catchment heterogeneity and river management (Rhône River, France)[END_REF]. The Rhône River accounts for about 80% of total particulate matter riverine inputs to the Gulf of Lions [START_REF] Aloisi | Essai de modélisation de la sédimentation actuelle sur les plateaux continentaux français[END_REF][START_REF] Durrieu De Madron | Particulate matter and organic carbon budgets for the Gulf of Lions (NW Mediterranean)[END_REF]. There are 3 distinct sedimentary units in front of its mouth [START_REF] Aloisi | Sur un modèle de sédimentation deltaïque: contribution à la connaissance des marges passives[END_REF]: (1) the delta front between 5 and 30 m depth, (2) the prodelta between 30 and 60 m depth, and (3) the distal zone between 60 and 100 m.

Rhône River flows

Rhône River water flow (m 3 .s -1 ) and SPM (mgDW.l -1 ) were measured in Arles, (47.5 km upstream of the river mouth, data provided by MOOSE: Mediterranean Ocean Observing System on Environment -http://www.moose-network.fr).

Sampling

The 5 sampled stations were located between 2 and 13 km off the Rhône River mouth along a gradient corresponding to the preferential direction of the river plume (Figure 1, Table 1). Station A was in the delta front; station B in the prodelta and

Sediment characteristics

Sediment cores (9.5 cm internal diameter) were collected using an Oktopus® GmbH MUC 8/100 multicorer in April 2007, May 2008and December 2008;and an Oktopus® GmbH MC 6 multicorer in July 2011. There were 3 cores per sampled station. The upper half centimetre of each core was sliced and homogenized. Each sample was then split in two subsamples (one for granulometry and pigment and one for biochemistry) and frozen at -20°C. The subsamples used for organic carbon, total nitrogen and amino acids were later freeze-dried.

Granulometry

Sediment granulometry was assessed using a Malvern Mastersizer® 2000 laser microgranulometer and expressed as median grain diameter (D 0.5 ). There was no replicate at stations N, C and D in December 2008 and all stations in July 2011.

Organic carbon and total nitrogen

Organic carbon and total nitrogen concentrations (OC and TN, respectively) were measured on homogenized, precisely weighed samples with an automatic CNanalyzer LECO 2000, after acidification with 2M HCl (overnight, at 50 °C) in order to remove carbonates prior to the analyses of organic carbon [START_REF] Cauwet | Contribution of the Rhone River to Organic-Carbon Inputs to the Northwestern Mediterranean-Sea[END_REF].

Precision for OC and TN measurements are about 2 %. C/N ratios were expressed as atomic ratios. There was no replicate for OC in May and December 2008 and for TN in May and December 2008 and July 2011.

Chloropigments

Triplicates (100 to 500 mgFW) samples were extracted overnight (5°C in darkness) in 5 ml of 90% acetone (final concentration). Fluorescence measurements were then performed using a Perkin Elmer® LS55 spectrofluorometer according to [START_REF] Neveux | Spectrofluorometric assay of chlorophylls and phaeopigments using the least-square approximation technique[END_REF]. This allowed for the quantification of chlorophyll-a (Chl-a), chlorophyll-b (Chl-b) and phaeophytin-a (Phaeo-a).

Amino acids

Total hydrolysable amino acids (THAA) and enzymatically hydrolysable amino acids (EHAA) were analysed on triplicates. THAA were extracted by acid hydrolysis.

EHAA were extracted following the biomimetic approach proposed by [START_REF] Mayer | Bioavailable amino acids in sediments: a biomimetic, kinetics-based approach[END_REF]. THAA and EHAA were analysed as isoindole derivatives following reaction with an orthophtaldialdehyde solution [START_REF] Lindroth | High performance liquid chromatographic determination of subpicomole amounts of amino acids by precolumn fluorescence derivatization with o-phthaldialdehyde[END_REF] fluorescent derivatives were separated by reverse-phase high-performance liquid chromatography (Gynkotek-Dionex system) on a C18 HPLC column using non-linear gradient of methanol-acetate buffer, and detected by fluorescence at 450 nm using an excitation wavelength of 335 nm.

Benthic macrofauna

At each station, 5 samples of 0.1 m 2 were collected using a van Veen grab, sieved on a 1 mm mesh and fixed (5% buffered formalin). Macrofauna were then sorted, identified to the lowest tractable taxonomic group and counted.

Sediment Profile Images

SPIs were collected using two similar Ocean Imaging® systems. Ten deployments were carried out at each station, except in May 2008 (4 deployments at station A and 9 at station N due to bad weather conditions). SPIs were analyzed using the SpiArcBase software [START_REF] Romero-Ramirez | Semiautomatic analysis and interpretation of sediment profile images[END_REF].

Data analysis

Sediment characteristics

Non-Metric Multidimensional Scaling (nMDS) and hierarchical clustering (Euclidean distance, group average linking) were performed on normalized sediment characteristics (D 0.5 , OC, TN, THAA, EHAA, Chl-a, Chl-b, Chl-b/Chl-a, Phaeo-a, C/N, EHAA/THAA and Chl-a/(Chl-a+Phaeo-a)). The significance of differences among the groups derived from hierarchical clustering was tested using SIMilarity PROFile (SIMPROF) procedure [START_REF] Clarke | Testing of null hypotheses in exploratory community analyses: similarity profiles and biota-environment linkage[END_REF].

Benthic macrofauna

Total abundance, species richness (SR) and Pielou's evenness (J') were used as bulk descriptors of benthic macrofauna compositions. SR is the number of species present in a sample whereas J' indicates how homogeneous is the individual abundance of each species within a sample. J' is between 0 and 1. This last value indicates that all species are represented by the same number of individuals.

Replicated samples were pooled and abundance-based compositions were also compared through nMDS and hierarchical clustering (square-root transformed data, Bray-Curtis similarity, group average linking). SIMPROF tests (were used together with ANOSIM for the cluster composed by stations N, C and D) to assess the significance of internal structures in identified clusters [START_REF] Clarke | Testing of null hypotheses in exploratory community analyses: similarity profiles and biota-environment linkage[END_REF]. SIMilarity PERcentages analyses [START_REF] Clarke | Non-parametric multivariate analyses of changes in community structure[END_REF] were performed to identify the species contributing most to between-clusters dissimilarity.

Relationships linking sediment characteristics and benthic macrofauna compositions

A BIO-ENV procedure [START_REF] Clarke | A method of linking multivariate community structure to environmental variables[END_REF] was performed to identify the subset of sediment characteristics, which best described spatiotemporal changes in benthic macrofauna composition. The set of tested sediment characteristics included D 0.5 , Chl-a, Chl-b/Chl-a, Phaeo-a, Chl-a/(Chl-a+Phaeo-a), EHAA, THAA, EHAA/THAA, OC and C/N. The correlations of each retained environmental variable with benthic macrofauna compositions were assessed using Mantel tests.

All procedures were completed using the PRIMER 6® software package.

Results

Rhône River flows

Strong temporal changes in daily water and particle flows were observed between 2007 and 2011 (Figure 2). The May 2008 cruise took place during a flood.

The April 2007 and December 2008 cruises took place 42 and 26 days after a flood.

Conversely, the July 2011 cruise took place after an extended (i.e., 191 day long) period of low water flows.

Sediment characteristics

Overall, sediments grains tended to be coarser and more variable in size among dates at station A (between 6.7 in May and 69.2 µm in December 2008; Table 2, Figure 3A). D 0.5 at station B was between 12.9 in July and 23.9 µm in May 2008. Surface sediments tended to be finer and less variable at stations N, C and D.

Temporal changes in OC, TN and THAA were the lowest at stations N, C and D. They were the highest at station A for OC (Figure 3B) and TN, and at station B for THAA (Table 2). All 3 descriptors showed decreasing values offshore during April Temporal changes in EHAA/THAA were slightly higher at station A than at the 4 other stations (Table 2, Figure 3E). EHAA/THAA always tended to be similar at all stations with the exception of station A during July 2011. EHAA/THAA also tended to be higher during July 2011 at all stations. Temporal changes in Chl-a/(Chl-a+Phaeoa) were higher at station A than at the 4 other stations (Table 2, Figure 3F). There were always strong decreasing inshore/offshore gradients.

The nMDS (Figure 4A) confirmed that the temporal variability of surface

Benthic macrofauna

4558 specimens belonging to 142 taxa were identified. Benthic macrofauna was mainly composed of polychaetes (80% of total overall abundance) followed by crustaceans and molluscs (7% each) and minor groups, including echinoderms, sipunculans, echiurans, cnidarians, hemichordates, nemerteans, platyhelminthes and phoronideans (for a total of 6%). Sternaspis scutata (Polychaeta) accounted for 36% of total abundance, whereas all others species accounted for less than 5% Temporal changes in SR were the highest at station A (Table 2, Figure 5B). The distinction between the 4 types of floods of the Rhône River is often based on SPM, with oceanic floods associated with the lowest concentrations (i.e., <500 mgDW.l -1 ) [START_REF] Pont | Les débits solides du Rhône à proximité de son embouchure: données récentes (1994-1995)[END_REF]. It can also rely on the activities of 238 U, 232 Th, 137 Cs and (239+240) Pu [START_REF] Eyrolle | Consequences of hydrological events on the delivery of suspended sediment and associated radionuclides from the Rhône River to the Mediterranean Sea[END_REF]. This last approach has been used to classify Rhône River floods between 2000 and 2012 [START_REF] Zebracki | Tracing the origin of suspended sediment in a large Mediterranean river by combining continuous river monitoring and measurement of artificial and natural radionuclides[END_REF]. The March 2007, May 2008 and November 2008 floods were respectively classified as: oceanic, generalized and Cevenol. Sampling was thus associated with 3 different types of floods. Floods can also be of different intensities [START_REF] Pont | Les débits solides du Rhône à proximité de son embouchure: données récentes (1994-1995)[END_REF]; Figure 2A). In the last 20 years, several studies have reported water flows and suspended particulate discharges for different time periods [START_REF] Baldock | Aspects of the chemical structure of soil organic materials as revealed by solid-state 13 C NMR spectroscopy[END_REF]: -1995: , 2003: and 2006: -2008[START_REF] Pont | Medium-term changes in suspended sediment delivery to the ocean: consequences of catchment heterogeneity and river management (Rhône River, France)[END_REF][START_REF] Antonelli | Suspended sediment and 137 Cs fluxes during the exceptional December 2003 flood in the Rhone River, southeast France[END_REF][START_REF] Eyrolle | Consequences of hydrological events on the delivery of suspended sediment and associated radionuclides from the Rhône River to the Mediterranean Sea[END_REF]. Maximal water flow was reported by [START_REF] Antonelli | Suspended sediment and 137 Cs fluxes during the exceptional December 2003 flood in the Rhone River, southeast France[END_REF] in December 2003 with an extreme flow of 11000 m 3 .s -1 for a total particulate discharge of 5400 10 3 t.flood -1 . Maximal suspended particulate discharge was of 12624 10 3 t.flood -1 in the 1992-1995 time period. The May 2008 flood, with a water flow of 4156 m 3 .s -1 and a particulate discharge of 4670 10 3 t.flood - 1 [START_REF] Eyrolle | Consequences of hydrological events on the delivery of suspended sediment and associated radionuclides from the Rhône River to the Mediterranean Sea[END_REF], can thus be considered as reasonably strong regarding

Sedimentary organics

Quantitative changes

Previous studies have shown the occurrence of decreasing inshore/offshore gradient in sediment grain size [START_REF] Cathalot | Temporal variability of carbon recycling in coastal sediments influenced by rivers: assessing the impact of flood inputs in the Rhône River prodelta[END_REF][START_REF] Bourgeois | Distribution and lability of land-derived organic matter in the surface sediments of the Rhône prodelta and the adjacent shelf (Mediterranean Sea, France): a multi proxy study[END_REF]Pastor et al., 2011b) and sedimentary organics concentrations [START_REF] Alliot | Biogeochemical impact of a dilution plume (Rhone River) on coastal sediments[END_REF][START_REF] Hermand | Macrofaunal density, biomass and composition of estuarine sediments and their relationship to the river plume of the Rhone River (NW Mediterranean)[END_REF][START_REF] Lansard | Benthic remineralization at the land-ocean interface: a case study of the Rhône River (NW Mediterranean Sea)[END_REF][START_REF] Cathalot | Temporal variability of carbon recycling in coastal sediments influenced by rivers: assessing the impact of flood inputs in the Rhône River prodelta[END_REF][START_REF] Bourgeois | Distribution and lability of land-derived organic matter in the surface sediments of the Rhône prodelta and the adjacent shelf (Mediterranean Sea, France): a multi proxy study[END_REF]. The resulting positive correlation between sediment grain size and sedimentary organics concentrations contradicts the general negative correlation linking these 2 parameters [START_REF] Mayer | Surface area control of organic carbon accumulation in continental shelf sediments[END_REF]. This mostly results from the fact that the Rhône River constitutes the major source of organic inputs in the studied area (Pastor et al., 2011b). During the present study, decreasing inshore/offshore gradients in both sediment grain size and sedimentary organic contents were observed during April 2007 and July 2011. Spatial distributions differed during May and December 2008 with: (1) much finer and much coarser sediments in the delta front, respectively; and

(2) higher sedimentary organics concentrations in the prodelta than in the delta front at both dates. Differences in sediment grain size in the delta front can be related with: (1) the type of the last flood, and (2) the time lag between this flood and sampling. The occurrence of finer sediment in May 2008 probably results from the fact that the May 2008 generalized flood mostly affected the Durance River, which required the opening of the Serre-Ponçon dam. This resulted in the liberation of large amounts of fine particles, which later sedimented in the delta front as indicated by the flood layer observed on 29 th May 2008 (Figure 7). Such a pattern was not observed at station B, which was sampled on 28 th May 2008 (i.e., just at the beginning of the flood). However, [START_REF] Cathalot | Temporal variability of carbon recycling in coastal sediments influenced by rivers: assessing the impact of flood inputs in the Rhône River prodelta[END_REF] later observed a 30 cm thick flood layer with low OC on 6 th June 2008 near this station. Moreover, based on the analysis of 7 Be and 210 Pb xs activities on SPM collected in Arles, [START_REF] Eyrolle | Consequences of hydrological events on the delivery of suspended sediment and associated radionuclides from the Rhône River to the Mediterranean Sea[END_REF] suggested that SPM during the May 2008 flood mostly originated from the reworking of old degraded soils initially trapped in the Serre-Ponçon dam, which is known to trap organically poor fine clay particles originating from intense erosion acting on a soil lacking vegetation [START_REF] Pont | Medium-term changes in suspended sediment delivery to the ocean: consequences of catchment heterogeneity and river management (Rhône River, France)[END_REF]. Overall, the occurrence of finer surface sediment grain size in the delta front than in the prodelta during May 2008 largely results from the fact that station B was sampled before being affected by the deposition of the flood layer. Accordingly, the occurrence of lower sedimentary organic contents in the delta front probably results from the fact that the OC of the sediment constitutive of the flood layer was low [START_REF] Cathalot | Temporal variability of carbon recycling in coastal sediments influenced by rivers: assessing the impact of flood inputs in the Rhône River prodelta[END_REF].

The occurrence of sandy sediments in the delta front in December 2008 probably results from 2 distinct processes. First, the November 2008 flood classified as Cevenol. Such events are sudden and result in high water and particle flows. [START_REF] Antonelli | Suspended sediment and 137 Cs fluxes during the exceptional December 2003 flood in the Rhone River, southeast France[END_REF] found a positive correlation between SPM grain size in Arles and particle flow. Since, sandy particles settle quicker than fine particles, this may account for the occurrence of higher concentrations of sand following the flood.

Another possible cause of the discrepancies in sediment grain size within the delta front between May and December 2008 is linked to the fact that station A was sampled during the flood in May and 26 days after the flood in December. Prodeltaic systems of the Gulf of Lions are areas of transitory deposits for continental inputs, which are later resuspended and transferred offshore [START_REF] Buscail | Seasonal variability of the organic matter in a sedimentary coastal environment: sources, degradation and accumulation (continental shelf of the Gulf of Lions -Northwestern Mediterranean Sea)[END_REF][START_REF] Durrieu De Madron | Particulate matter and organic carbon budgets for the Gulf of Lions (NW Mediterranean)[END_REF][START_REF] Ulses | Suspended sediment transport in the Gulf of Lions (NW Mediterranean): impact of extreme storms and floods[END_REF]. For instance, 2 kilometres off the Rhône River mouth, [START_REF] Marion | In situ record of sedimentary processes near the Rhône River mouth during winter events (Gulf of Lions, Mediterranean Sea)[END_REF] observed the erosion of a 5 cm deposits within 20 days after the November 2006 flood. Resuspension preferentially affects fines and mostly occurs during autumn and winter [START_REF] Ulses | Suspended sediment transport in the Gulf of Lions (NW Mediterranean): impact of extreme storms and floods[END_REF]. It may thus have contributed to clear surface sediments at station A of their finest components between their deposition (i.e., during the November 2008 flood) and December 2008 sampling. The occurrence of higher sedimentary organic concentrations in the prodelta during December 2008 may result from the granulometry effect [START_REF] Mayer | Surface area control of organic carbon accumulation in continental shelf sediments[END_REF].

Qualitative changes C/N and Chl-b/Chl-a ratios classically show decreasing inshore/offshore gradients off the Rhône River mouth [START_REF] Lansard | Benthic remineralization at the land-ocean interface: a case study of the Rhône River (NW Mediterranean Sea)[END_REF][START_REF] Cathalot | Temporal variability of carbon recycling in coastal sediments influenced by rivers: assessing the impact of flood inputs in the Rhône River prodelta[END_REF][START_REF] Bourgeois | Distribution and lability of land-derived organic matter in the surface sediments of the Rhône prodelta and the adjacent shelf (Mediterranean Sea, France): a multi proxy study[END_REF]. For both descriptors, results from the present study show no strong temporal changes except in December 2008 in the delta front and to a lesser extent in the prodelta where high Chl-b/Chl-a supported the contribution of continental plant detritus to sedimentary organics [START_REF] Jeffrey | A report of green algal pigments in the central North Pacific Ocean[END_REF][START_REF] Meyers | Preservation of elemental and isotopic source identification of sedimentary organic matter[END_REF][START_REF] Hedges | What happens to terrestrial organic matter in the ocean?[END_REF][START_REF] Tesi | Source, transport and fate of terrestrial organic carbon on the western Mediterranean Sea, Gulf of Lions, France[END_REF][START_REF] Cathalot | The fate of river organic carbon in coastal areas: a study in the Rhône River delta using multiple isotopic (δ 13 C, Δ 14 C) and organic tracers[END_REF]. This may be related to the November 2008 Cevenol flood since these events mostly originate from the Cevennes area, which is densely forested. Besides changes in continental inputs, higher Chl-b/Chl-a close to the Rhône River mouth may also result from subsequent physical sorting [START_REF] Tesi | Source, transport and fate of terrestrial organic carbon on the western Mediterranean Sea, Gulf of Lions, France[END_REF], which tends to export fine particles with lower contents of vascular plant detritus offshore [START_REF] Goni | A reassessment of the sources and importance of land-derived organic matter in surface sediments from the Gulf of Mexico[END_REF][START_REF] Leithold | Deposition and modification of a flood layer on the northern California shelf: lessons from and about the fate of terrestrial particulate organic carbon[END_REF]. This pattern is supported by spatiotemporal changes in C/N. The C/N of soil organic matter is typically between 7 and 15 [START_REF] Baldock | Aspects of the chemical structure of soil organic materials as revealed by solid-state 13 C NMR spectroscopy[END_REF]. Values recorded during April 2007, May 2008 and July 2011 were in the upper range (11-15) with no clear inshore/offshore gradient. Values recorded in December 2008 were higher with a clear decreasing inshore/offshore gradient, which is coherent with a higher contribution of continental vascular plant material (C/N>20; [START_REF] Baldock | Aspects of the chemical structure of soil organic materials as revealed by solid-state 13 C NMR spectroscopy[END_REF]) to sedimentary organics, and/or a reduction of this contribution due to dilution or preferential degradation during the transport offshore [START_REF] Baldock | Aspects of the chemical structure of soil organic materials as revealed by solid-state 13 C NMR spectroscopy[END_REF]. The first of these 2 hypotheses is probably the most valid because C/N ratios did not decrease offshore during the 3 other cruises and EHAA/THAA never decreased offshore. ratios correlated negatively with Chl-b/Chl-a ratios, which is coherent with the lower lability classically attributed to continental than to marine plant material [START_REF] Wakeham | Molecular indicators of diagenetic status in marine organic matter[END_REF]. The decreasing trend and the lack of temporal changes at higher depth result from the fact that: (1) Rhône River inputs are the major source of plant material to the sediment/water interface of all stations, and (2) irrespective of slight differences in the original freshness of bulk plant materials, its most labile components are quickly degraded during their transfer offshore. EHAA/THAA ratios are indicative of a different, much larger and overall less labile component of sedimentary organics [START_REF] Wakeham | Molecular indicators of diagenetic status in marine organic matter[END_REF]. The lack of offshore gradient probably results from the fact that the degradation taking place during the offshore transfer affects a minor fraction of the nitrogenous fraction of sedimentary organics.

Spatiotemporal changes in

Conversely, the occurrence of significant temporal changes in EHAA/THAA may reflect differences in the lability of the nitrogenous component of sedimentary

Benthic macrofauna

Although still significant in the distal zone (according to the results of the ANOSIM test at least), temporal changes in macrofauna composition were clearly much more marked in the delta front and to a lesser extent in the prodelta, which is similar to what was observed for surface sediment characteristics.

In the Rhône River delta front, sedimentation rates can reach up to 0.65 cm.y - 1 [START_REF] Zuo | Accumulation rates and sediment deposition in the northwestern Mediterranean[END_REF][START_REF] Miralles | 210 Pb sedimentation rates from the Northwestern Mediterranean margin[END_REF] which, according to the model proposed by [START_REF] Rhoads | Macrobenthos and sedimentary facies on the Changjiang delta platform and adjacent continental shelf, East China Sea[END_REF], can account for minimal SR and abundance of benthic macrofauna in the delta front during April 2007, May and December 2008. The fact that benthic macrofauna SR (April 2007, May andDecember 2008) and abundances (April 2007) tended to be higher in the prodelta, which is located further offshore supports this model. Conversely, benthic macrofauna abundances in the delta front and the prodelta were close in May and December 2008 (i.e., following the generalized and Cevenol floods, respectively), which may be indicative of the extension of the negatively affected area during major floods and therefore also tends to support the [START_REF] Rhoads | Macrobenthos and sedimentary facies on the Changjiang delta platform and adjacent continental shelf, East China Sea[END_REF] model.

The nMDS and the analysis of benthic macrofauna characteristics at stations

A and B show that July 2011 clearly differed from the 3 other sampling dates with:

(1) much higher abundances and SR, and (2) higher abundances of Sternaspis scutata (Polychaeta), Laonice cirrata (Polychaeta), Lumbrineris latreilli (Polychaeta, station A only) and Thyasira flexuosa (Mollusca). The extended period of reduced water flows before July 2011 clearly enhanced the establishment of a more mature benthic macrofauna community in the delta front and the prodelta. This community included both tube-dwelling and deep-burrowing macrofauna as indicated by sediment profile imagery in the prodelta (Figure 7). More generally, July 2011 corresponded to the inshore enlargement of the spatial distributions of several species including Abra nitida (Mollusca), Abyssoninoe hibernica (Polychaeta), Alpheus glaber (Crustacea), Ampharete grubei (Polychaeta), Apseudes spp.

(Crustacea), Chaetozone spp. (Polychaeta), Goniada maculata (Polychaeta), L. cirrata (Polychaeta), Malmgrenia lilianae (Polychaeta), T. flexuosa (Mollusca) and Thysanocardia procera (Sipunculida), which were found up to the delta front. Such a positive effect of low flow on benthic macrofauna abundance and SR has already been reported for several major rivers [START_REF] Aller | The distribution and seasonal characteristics of benthic communities on the Amazon shelf as indicators of physical processes[END_REF][START_REF] Occhipinti-Ambrogi | Macrobenthos community structural changes off Cesenatico coast (Emilia Romagna, Northern Adriatic), a sixyear monitoring programme[END_REF]. This supports previous observations (on an annual basis and without any time lag in the response of benthic polychaete fauna to changes in Rhône River differs from what was observed in the delta front and prodelta (see above). The balance between the 2 opposite effects constitutive of the [START_REF] Rhoads | Macrobenthos and sedimentary facies on the Changjiang delta platform and adjacent continental shelf, East China Sea[END_REF] model thus clearly differs in the delta front and in the distal zone. In the former, negative effects are most pronounced due to high sedimentation [START_REF] Zuo | Accumulation rates and sediment deposition in the northwestern Mediterranean[END_REF][START_REF] Miralles | 210 Pb sedimentation rates from the Northwestern Mediterranean margin[END_REF] and direct inputs of POM are occasionally so high that they negatively affect benthic macrofauna [START_REF] Pearson | Macrobenthic sucession in relation to organic enrichment and pollution of the marine environment[END_REF][START_REF] Labrune | Comparison of ecological quality indices based on benthic macrofauna and sediment profile images: a case study along an organic enrichment gradient off the Rhône River[END_REF]. Conversely, disturbances resulting from sedimentation and direct POM inputs are much smaller in the distal zone, which tend to switch the overall balance of the effects of floods towards positive values. Such discrepancies between the delta front and the prodelta, and the distal zone probably account for differences in the nature and the intensity of the response of benthic macrofauna composition to changes in the hydrological regime of the Rhône River. They are as well likely to account for differences in the time lag associated with these responses since the negative effect of sedimentation is almost immediate [START_REF] Wheatcroft | Time-series measurements of macrobenthos abundance and sediment bioturbation intensity on a flood-dominated shelf[END_REF]), whereas, for some feeding types at least, the response to changes in POM availability is much longer [START_REF] Salen-Picard | Responses of a Mediterranean soft bottom community to short-term (1993-1996) hydrological changes in the Rhone river[END_REF].

Relationship between sediment characteristics and benthic macrofauna

Sediments characteristics, in general, and organic matter availability, in particular, are known to largely control both the spatial and temporal patterns of benthic macrofauna composition [START_REF] Pearson | Macrobenthic sucession in relation to organic enrichment and pollution of the marine environment[END_REF][START_REF] Grémare | Relationships between sedimentary organics and benthic meiofauna on the continental shelf and the upper slope of the Gulf of Lions (NW Mediterranean)[END_REF][START_REF] Labrune | Comparison of ecological quality indices based on benthic macrofauna and sediment profile images: a case study along an organic enrichment gradient off the Rhône River[END_REF]. The other way around, benthic macrofauna may also alter both the physical and biogeochemical properties of marine sediments through nutrition and/or bioturbation [START_REF] Meysman | Bioturbation: a fresh look ar Darwin's las idea[END_REF][START_REF] Bernard | Experimental assessment of particle mixing fingerprints in the deposit-feeding bivalve Abra alba (Wood)[END_REF]. Both of these interactions may contribute to the correlation between sediment characteristics and benthic macrofauna composition. Based on the sampling of 16 stations during April 2007, [START_REF] Labrune | Comparison of ecological quality indices based on benthic macrofauna and sediment profile images: a case study along an organic enrichment gradient off the Rhône River[END_REF] reported that spatial changes in benthic macrofauna composition off the Rhône River mouth correlated with the OC contents of surface sediments. Pastor et al. (2011b) reported a similar correlation between OC and benthic oxygen consumption, with no significant effect of the quality/lability of sedimentary organics, which they attributed to the strong dominance of the organic matter source constituted by the inputs from the Rhône River. Results from the present study show that spatiotemporal changes in benthic macrofauna composition are best described when combining 4 factors including D 0.5 , EHAA, Chla/(Chl-a+Phaeo-a) and Chl-b/Chl-a. Besides sediment granulometry (D 0.5 ), these include a quantitative descriptor of a labile component of sedimentary organics (EHAA) and a qualitative descriptor of sedimentary organics (Chl-a/(Chl-a+Phaeoa)). Results from the present study thus support the better correlation found between meiofauna abundance and quantitative descriptors of labile (i.e., EHAA and lipids) rather than bulk (i.e., OC, TN and THAA) components of sedimentary organics already found in the open Gulf of Lions [START_REF] Grémare | Relationships between sedimentary organics and benthic meiofauna on the continental shelf and the upper slope of the Gulf of Lions (NW Mediterranean)[END_REF]. Moreover, they suggest that temporal changes in the quality of sedimentary organics in relation with changes in the hydrological regime of the Rhône River contribute to control temporal changes in the composition of benthic macrofauna.

There are however several lines of evidences suggesting that these parameters are not the only ones accounting for differences between the compositions of benthic macrofauna during July 2011 and the 3 other dates. First, OC and EHAA contents in the delta front and the prodelta were almost equivalent during April 2007 and July 2011, whereas the characteristics and the compositions of benthic macrofauna conversely strongly differed between these 2 dates. Second, the co-variation between abundance, SR and J' did not match the classical [START_REF] Pearson | Macrobenthic sucession in relation to organic enrichment and pollution of the marine environment[END_REF] model since low evenness were associated with high abundances but also with high SR during July 2011. It is therefore likely that spatiotemporal changes in benthic macrofauna composition were also affected by other factors than the Rhône River water flow and resulting organic inputs. High abundances of small individuals of the dominant polychaete Sternaspis scutata (Polychaeta) during July 2011 may for instance result from a recent recruitment. This hypothesis is consistent with the reported preferential recruitment of this species during summertime in the Chinhae Bay [START_REF] Lim | Distribution and growth pattern of Sternaspis scutata (Polychaeta: Sternaspidae) in Chinhae Bay, Korea[END_REF]. However, [START_REF] Hermand | Macrofaunal density, biomass and composition of estuarine sediments and their relationship to the river plume of the Rhone River (NW Mediterranean)[END_REF] reported the preferential recruitment of S. scutata (Polychaeta) during wintertime (i.e., December and January) off the Rhône River, which suggest that high abundances of S. scutata (Polychaeta) in the delta front and the prodelta in July 2011 are indeed associated with low water fluxes and do not result from an interaction with its lifecycle. This is further supported by the fact that low water flow periods are known to enhance the colonization of more inshore zones of the Amazon continental shelf by juveniles of benthic macrofauna [START_REF] Aller | The distribution and seasonal characteristics of benthic communities on the Amazon shelf as indicators of physical processes[END_REF]. Letters refer to stations and symbols to cruises.

  stations N, C and D in the distal zone. Sampling took place in April 2007, May and December 2008 and July 2011 for: (1) sediments characteristics and (2) benthic macrofauna. Sediment Profile Images (SPIs) were also collected in April 2007, May 2008 and July 2011.

  . During July 2011, THAA and EHAA were quantified by fluorescence measurements (excitation wavelength of 340 nm and emission wavelength of 453 nm) using a Perkin Elmer® LS55 fluorescence spectrometer. During April 2007, May 2008 and December 2008,

  2007 and July 2011. Conversely, May and December 2008 were characterized by low values of OC, TN and THAA at station A and maximal ones at station B. This pattern was especially marked for THAA. Overall, EHAA showed the same pattern for the 3 bulk descriptors of sedimentary organics with: (1) high temporal changes at station A, (2) decreasing inshore/offshore gradients during April 2007 and July 2011, and (3) low concentrations at station A and the highest concentrations at station B during May and December 2008 (Table 2, Figure 3C). Temporal changes in EHAA at station B were however low. Temporal changes in Chl-a (Figure 3D) and Chl-b were high at station A, intermediate at station B and low at stations N, C and D (Table 2). Decreasing inshore/offshore gradients were marked during April 2007 and July 2011. Concentrations at stations A and B were much higher during April 2007 than July 2011. Chl-a and Chl-b concentrations were maximal at station B during May and December 2008. This pattern was especially marked for Chl-b during December 2008. Temporal changes in C/N were highest at station A (Table 2). It was especially high at station A in December 2008 and tended to be highest at this particular date than during the 3 other cruises at all other 4 stations. Temporal changes in Chlb/Chl-a were the highest at station A, intermediate at station B, and low at stations N, C and D (Table 2). During April 2007, May 2008 and December 2008, there were clear inshore/offshore gradients with the highest values at stations A and B during December 2008. During July 2011, the highest Chl-b/Chl-a was recorded at station B.

  sediment characteristics was higher at station A, intermediate at station B and lower at stations N, C and D. Station A during May 2008, and station B during July 2011, showed the characteristics of sedimentary organics the closest to those of stations N, C and D. The hierarchical clustering (Figure 4B) confirmed this pattern with the identification of 4 significantly differing clusters: (I) station A in April 2007 and July 2011, and station B in April 2007, May and December 2008, (II) stations N, C and D in April 2007 and May 2008, (III) stations N and C in December 2008, and (IV) stations C and D in July 2011. Station A in May and December 2008, station D in December 2008 and stations B and N in July 2011 were not included in these main clusters.

  each. S. scutata (Polychaeta) was the top rank species at all stations except station A in April 2007 (Thyasira flexuosa, Mollusca), May 2008 (Heteromastus filiformis, Polychaeta) and December 2008 (Polycirrus sp., Polychaeta); station B during December 2008 (Lumbrineris latreilli, Polychaeta); and station N in May 2008 (Poecilochaetus serpens, Polychaeta). Four taxa (S. scutata, H. filiformis, Nephtys kersivalensis (all Polychaeta) and nemerteans) were always present at station A versus 13, 12, 11 and 13 taxa at stations B, N, C and D, respectively. Temporal changes in abundances were higher at stations A and B, intermediate at station N lower at stations C and D (Table 2, Figure 5A). Abundances at station A were between 144 and 1522 ind.m -2 during April 2007 and July 2011. Abundances at station B were between 310 and 1700 ind.m -2 during May 2008 and July 2011. Temporal patterns were similar at these 2 stations except for higher abundances at station B during April 2007. Abundances at station N were between 238 and 622 ind.m -2 during May 2008 and December 2008. They were intermediate during April 2007 (358 ind.m -2 ) and July 2011 (426 ind.m -2 ). Abundances at station C and D were higher during April 2007 (436 and 294 ind.m -2 , respectively) and July 2011 (442 and 284 ind.m -2 , respectively)) and lower during May (258 and 202 ind.m -2 for stations C and D) and December 2008 (246 and 210 ind.m -2 for stations C and D).

  Both the lowest (15 taxa.0.5m -2 during May 2008) and the highest SR (57 taxa.0.5m - 2 in July 2011) were recorded at this station. High SR were as well recorded at station B during April 2007 (39 taxa.0.5m -2 ), station N during December 2008 (37 taxa.0.5m -2 ) and station C during May 2008 (36 taxa.0.5m -2 ). The only clear inshore/offshore gradient was recorded during July 2011 with SR ranging from 57 to 39 taxa.0.5m -2 at stations A and D, respectively. J' were between 0.55 (station C in April 2007) and 0.91 (station N in May 2008). They did not show any clear temporal or spatial patterns (Table 2, Figure 5C). At stations A and B, the lowest values were recorded during July 2011, when both total abundances and SR were the highest. The nMDS (Figure 6A) showed that temporal changes in macrofauna composition were highest at station A, intermediate at station B and lowest at stations N, C and D. The hierarchical clustering (Figure 6B) confirmed this pattern with the identification of 4 clusters: (I) station A in April 2007, May and December 2008, (II) stations A and B in July 2011, (III) station B in April 2007, May and December 2008, and (IV) stations N, C and D during all cruises. There was no internal structure within this last cluster (SIMPROF tests, p>0.05). Conversely, the ANOSIM test (=0.87, p=0.001) showed significant temporal changes at stations N, C and D. Average dissimilarity between groups I and II was 76.6% with Sternaspis scutata (12.3%, Polychaeta), Laonice cirrata (5.2%, Polychaeta), Lumbrineris latreilli (4.6%, Polychaeta) and Thyasira flexuosa (4.2%, Mollusca) contributing most. Average dissimilarity between groups I and III was 70.7% with Labidoplax digitata (5.2%, Echinodermata), S. scutata (5.2%, Polychaeta), L. latreilli (5.1%, Polychaeta), Nephtys hystricis (4.62%, Polychaeta) and Goniada maculata (4.3%, Polychaeta) contributing most. Average dissimilarity between groups II and III was 54.3% with S. scutata (11.1%, Polychaeta), L. cirrata (6.5%, Polychaeta) and T. flexuosa (5.0%, Mollusca) contributing most. Average dissimilarity between subgroups IVa and IVb was 47.5% with S. scutata (7.7%, Polychaeta), Athanas nitescens (3.9%, Crustacea), Harpinia dellavallei (3.3%, Crustacea), Chaetozone spp. (3.1%, Polychaeta) and Abra nitida (3.0%, Mollusca) contributing most. Average dissimilarity between subgroups IVa and IVc was 50.9% with S. scutata (4.5%, Polychaeta), Ampharete grubei (4.3%, Polychaeta), L. cirrata (3.4%, Polychaeta) and L. latreilli (3.0%, Polychaeta) contributing most. Average dissimilarity between subgroups IVb and IVc was 49.4% with A. grubei (5.0%, Polychaeta), L. cirrata (4.4%, Polychaeta) and S. scutata (3.8%, Polychaeta) contributing most. 3.4 SPIs Temporal changes in SPIs characteristics (Figure 7) were the strongest at station A with: (1) a very thin apparent Redox Potential Discontinuity (aRPD) layer and almost no biogenic structures in April 2007, (2) a thick flood layer with a few large biogenic structures in May 2008, and (3) an average thickness of the aRPD of 38 mm with numerous biogenic structures including tubes in July 2011. The flood layer observed in May 2008 was thickest at station A and then tended to decrease offshore (data not shown). The aRPD at stations B, N, C and D tended to be thicker in May 2008 than in April 2007 and in July 2011. 3.5 Relationship between sediment characteristics and benthic macrofauna compositions The best correlation (=0.795, p=0.01) between the similarity matrices of benthic macrofauna composition and sediment characteristics was found using D 0.5 , Chl-b/Chl-a, Chl-a/(Chl-a+Phaeo-a) and EHAA. Each of these variables correlated positively with benthic macrofauna composition (Mantel tests, p=0.001 in all cases). 4 Discussion 4.1 Sampling in relation with the hydrological regime of the Rhône River

  particle flows. Conversely, the March 2007 and the November 2008 floods (3269 m 3 .s -1 and 4806 m 3 .s -1 , respectively) can be considered as of low and intermediate intensity based on water flows. Another source of heterogeneity is the time lag between the last floods and samplings. These were 42 and 26 days in April 2007 and December 2008, whereas the May 2008 cruise took place during the flood. The July 2011 cruise took place after an extended (i.e., 191 day long) period of low water flows. Sampling thus proved representative of a large variety of hydrological regimes and therefore allows for the assessment of the responses of both surface sediment characteristics and benthic macrofauna composition, provided that the temporal dynamics of such responses are properly taken into account.

  Chl-a/(Chl-a+Phaeo-a) and EHAA/THAA ratios clearly differed. Chl-a/(Chl-a+Phaeo-a) ratios always decreased offshore with, except to some extent in the delta front, no marked temporal change. Conversely, EHAA/THAA ratios showed no offshore gradient during either cruise but did show temporal changes with high values in July 2011, intermediate values in April 2007 and low values in May and December 2008. In the delta front Chl-a/(Chl-a+Phaeo-a)

  organics depending on seasons or hydrologic conditions of the Rhône River. Results from the present study suggest that such changes are mainly related with changes in the intensity of continental inputs with higher values during April 2007 (intermediate particle flow, oceanic flood) and July 2011 (low particle flow, dry period) and lower ones during May and December 2008 (high particle flows, generalized and Cevenol floods).

  flow) by[START_REF] Harmelin-Vivien | Linking benthic biodiversity to the functioning of coastal ecosystems subjected to river runoff (NW Mediterranean)[END_REF] and is also in good agreement with the[START_REF] Rhoads | Macrobenthos and sedimentary facies on the Changjiang delta platform and adjacent continental shelf, East China Sea[END_REF] model. SPI data suggest an almost immediate impact of the May 2008 flood in the delta front with the onset of the deposition of a flood layer only 1 day after the beginning of the flood event (Figure7). Together with subsequent observations by Cathalot et al. (2010) (see above) they suggest an almost immediate effect of the hydrological regime of the Rhône River on the composition of benthic macrofauna within its delta front and prodelta. Moreover, the analysis of temporal changes in benthic macrofauna compositions in the delta front and the prodelta shows that May 2008 (during a generalized flood) was intermediate between December 2008 (26 days after a Cevenol flood) and April 2007 (42 days after an oceanic flood). This underlines the importance of the type of floods relative to the time lag between last flood and sampling in affecting benthic macrofauna composition within the delta front. Spatiotemporal changes in benthic macrofauna composition were much more reduced in the distal zone. Salen-Picard et al. (2003) reported a high interannual variability at a 70 m deep station which was attributed to interannual changes in the hydrological regime of the Rhône River. The results of the ANOSIM test support the existence of such variability, even though the overall weakness of differences in benthic macrofauna composition associated with 4 contrasted hydrological conditions clearly complicates the assessment of a causal relationship between water flows and benthic fauna compositions. The analysis of between-cruises similarities in benthic macrofauna compositions at stations in the distal zone shows that July 2011 was closest to December 2008 and farthest from April 2007, which

Figure 1 :

 1 Figure 1: Delimitations of the different Rhône River basins (A) (from Antonelli et al., 2008) and

Figure 2 :

 2 Figure 2: Temporal changes in Rhône River mean daily water flow (A) and suspended particulate

Figure 3 :

 3 Figure 3: Spatiotemporal changes in surface sediment characteristics: D 0.5 : median grain size (A),

Figure 4 :

 4 Figure 4: Non-metric Multidimensional Scaling (nMDS) (A) and hierarchical clustering (Euclidean

Figure 5 :

 5 Figure 5: Spatiotemporal changes in benthic macrofauna main characteristics: abundance (A),

Figure 6 :

 6 Figure 6: Non-metric Multidimensional Scaling (nMDS) (A) and hierarchical clustering (Bray-Curtis

Figure 7 :

 7 Figure 7: Examples of sediment profile images collected at 3 stations along the main

  Figure 1

Figure 5 Figure 6

 56 Figure 5

Figure 7

 7 Figure 7

Table 1 :

 1 Latitude, longitude (WGS84, degrees and decimal minutes), depth and 1 distance from the Rhône River mouth of the 5 sampled stations. 2

	Station	Latitude (N)	Longitude (W)	Depth (m)	Distance from the Rhône River mouth (km)
	A	43°18.690' 04°51.042'	24	1.9
	B	43°18.013' 04°50.068'	54	3.0
	N	43°17.626' 04°47.896'	67	5.5
	C	43°16.343' 04°46.565'	76	8.6
	D	43°14.917' 04°43.613'	74	13.0

Table 2 :

 2 Mean values of surface sediment and benthic macrofauna characteristics. Sta: Stations, D 0.5 : median grain size, OC: organic carbon, TN: total nitrogen, C/N: ratio between organic carbon and total nitrogen, Chl-a: chlorophyll-a, Chl-b: chlorophyll-b, Chl-b/Chl-a: ratio between chlorophyll-b and chlorophyll-a, Phaeo-a: phaeophytin-a, Chl-a/(Chl-a+Phaeo-a): ratio between chlorophyll-a and the sum of chlorophyll-a and phaeophytin-a, EHAA: enzymatically hydrolysable amino acid, THAA: total hydrolysable amino acid, EHAA/THAA: ratio between enzymatically and total hydrolysable amino acids, SR: species richness and J': Pielou's evenness. ± standard deviation (n=3).

	Cruise	Sta.	Sampling date	D0.5 (0-0.5mm) (µm)	OC (%DW)	TN (%DW)	C/N (atomic ratio)	Chl-a (µg.g -1 DW)	Chl-b (µg.g -1 DW)	Chl-b/Chl-a (%)	Phaeo-a (µg.g -1 DW)	Chl-a/(Chl-a+Phaeo-a) (%)	EHAA (mg.g -1 DW)	THAA (mg.g -1 DW)	EHAA/THAA (%)	Abundance (ind.m -2 )	SR (taxa.0.5m -2 )	J'
	April 2007	A	4/20/2007	37.40±3.05	1.83±0.18 0.17±0.004	12.90±1.42	9.78±0.82	01.38±0.06	14.20±0.82	20.51±1.49	32.26±0.52	1.17±0.19	3.77±0.30	28.21±1.07	144	20	0.88
		B	4/20/2007	14.83±0.55	1.53±0.08 0.15±0.003	12.10±0.45	4.99±0.49	0.51±0.06	10.17±0.24	21.76±1.24	18.62±0.88	0.86±0.05	3.14±0.03	27.49±1.76	642	39	0.69
		N	4/24/2007	14.01±2.42	1.19±0.21 0.10±0.002	13.46±2.11	2.57±0.60	0.18±0.13	6.29±4.33	14.31±0.18	15.13±2.95	0.68±0.02	2.82±0.35	24.52±2.67	358	26	0.68
		C	4/23/2007	11.38±1.12	1.20±0.05 0.11±0.003	12.78±0.63	1.64±0.12	0.06±0.05	3.79±3.63	12.19±1.19	11.90±0.48	0.69±0.03	2.38±0.07	28.91±1.99	436	34	0.55
		D	4/23/2007	10.45±0.22	1.02±0.03 0.11±0.001	11.23±0.38	1.56±0.29	0.00	0.00	13.76±2.83	10.21±0.43	0.72±0.04	2.54±0.09	28.17±1.85	294	34	0.68
	May 2008	A	5/29/2008	6.74±0.63	1.13	0.09	14.81	3.07±0.43	0.29±0.07	9.47±2.84	4.58±0.57	40.16±0.77	0.43±0.04	2.18±0.13	19.68±0.80	178	15	0.73
		B	5/28/2008	23.87±6.67	1.75	0.17	12.3	3.80±0.63	0.31±0.09	8.02±1.65	17.70±1.11	17.60±1.52	0.93±0.12	5.62±0.63	16.57±1.47	310	28	0.82
		N	5/30/2008	10.65±1.90	1.00	0.10	12.03	1.62±0.59	0.07±0.06	3.48±3.41	12.31±6.59	14.14±8.60	0.49±0.05	2.57±0.20	19.05±2.38	238	28	0.91
		C	5/30/2008	14.51±1.39	1.16	0.10	13.01	1.48±0.62	0.05±0.06	2.83±2.64	8.37±0.21	14.82±5.36	0.52±0.01	2.69±0.11	19.48±1.03	258	36	0.87
		D	06/08/2008	11.37±1.27	1.00	0.10	12.03	0.09±0.13	0.00	0.00	5.47±2.24	1.30±1.84	0.51±0.14	2.33±0.50	21.73±1.36	202	24	0.80
	December 2008	A	12/04/2008	69.18±19.22	1.22	0.05	28.15	2.14±1.06	0.56±0.31	25.71±5.52	6.49±3.43	24.83±1.86	0.41±0.20	2.57±1.37	16.27±0.70	260	21	0.75
		B	12/03/2008	21.58±3.85	1.96	0.13	18.09	5.01±0.80	1.05±0.12	21.05±0.99	17.45±0.98	22.20±1.87	0.83±0.04	5.48±0.59	15.25±1.07	344	32	0.83
		N	12/08/2008	13.58	1.36	0.10	16.28	1.47±0.21	0.05±0.02	3.53±0.74	13.17±1.04	10.03±1.41	0.61±0.06	3.06±0.11	19.87±2.65	622	37	0.64
		C	12/04/2008	17.11	1.49	0.10	18.01	0.98±0.20	0.00	0.00	13.28±0.72	6.85±1.00	0.55±0.05	3.05±0.27	18.07±0.95	246	25	0.78
		D	12/08/2008	15.57	1.16	0.08	16.08	0.18±0.03	0.00	0.00	10.77±1.31	1.66±0.28	0.45±0.06	2.53±0.13	18.01±2.54	210	28	0.78
	July 2011	A	7/26/2011	26.08	1.80±0.14	0.15	14.03	5.94±0.91	0.59±0.21	9.86±7.14	7.14±0.07	45.22±3.92	1.13±0.02	2.96±0.21	38.44±2.25	1522	57	0.61
		B	7/21/2011	12.92	1.54±0.14	0.14	12.98	1.04±0.54	0.17±0.11	15.85±6.52	6.52±1.90	13.24±2.62	0.81±0.07	2.71±0.20	30.04±2.12	1700	45	0.57
		N	7/30/2011	11.07	1.25±0.05	0.11	12.91	0.80±0.29	0.06±0.02	06.93±4.13	4.13±0.52	15.89±3.21	0.66±0.05	2.02±0.12	32.58±0.97	426	41	0.85
		C	7/25/2011	10.32	1.21±0.10	0.09	15.7	0.39±0.17	0.00	0.00	3.25±0.21	10.72±4.90	0.65±0.00	2.03±0.06	32.14±1.12	442	41	0.69
		D	7/28/2011	9.88	1.08±0.03	0.08	15.36	0.43±0.02	0.00	0.00	3.78±0.10	10.27±0.67	0.62±0.07	1.88±0.09	32.62±2.18	284	39	0.83

Acknowledgements

We would like to thank C. Rabouille who coordinated the CHACCRA program.

We thank the captains and crew members of the RV Tethys II and Côte de la Manche (CNRS-INSU) for technical assistance during sampling. Special thanks to L. Rigouin, D. Cabanes, M. Richard for their technical assistance in laboratory during this work. In particular, the authors thank F. Charles for his help at sea and A. Pruski for her contribution to amino acids data. The authors acknowledge the AERMC and the SOERE-MOOSE sponsored by INSU and Alliance ALLENVI for supplying the data of the Rhône River (water and SPM flows). This work was partly funded by the French national ANR program CHACCRA (Contract n° ANR-VULN-06-001-01), the French EC2CO-PNEC program RiOMar and the French EC2CO-PNEC and LEFE-CYBER program BIOMIN. It is part of the PhD Thesis of P. Bonifácio who was funded by IFREMER (convention 09/3211321) and the Agence de l'Eau Corse-Méditerranée (convention n°2010 0871). S. Bourgeois was supported by a grant from the French Ministry of Research.