Jean-François Dechézelles

Tangi Aubert

Fabien Grasset

Stéphane Cordier

Carlos Barthou

Catherine Schwob

Agnès Maître

Renaud A L Vallée

Henri Cramail

Serge Ravaine
email: ravaine@crpp-bordeaux.cnrs.fr.

Fine tuning of emission through the engineering of colloidal crystals

We describe the preparation and characterization of photonic colloidal crystals from silica spheres with incorporated luminescent [Mo 6 Br 14 ] 2-cluster units. These structures exhibit strong angle-dependent luminescent properties. The incorporation of one or several planar defects in the periodic structures gives rise to the creation of a pass band in the stopband. In the energy range of this pass band, an increase of the emission intensity has been found.

Introduction

Photonic crystals (PCs) are a class of artificial structures with a periodic dielectric function in one, two, or three dimensions, in which the propagation of electromagnetic waves within a certain frequency band is forbidden. This forbidden frequency band has been dubbed photonic band gap (PBG). The position, width, depth, and shape of the PBG strongly depend on the periodicity, symmetry properties, dielectric constant contrast, and internal lattice structure of the unit cell. There is a common belief that PCs will perform many functions with light that ordinary crystals do with electrons.

Various important scientific and engineering applications such as control of spontaneous emission, [1][2][3][4][5][6][7][8][9][10][11] zero-threshold lasing, 12 very sharp bending of light, [13][14] lightemitting diodes, [15][16] and so on, have been demonstrated by utilizing the photonic bandgap and/or light-emitters. Especially, a stopband, i.e. an incomplete photonic bandgap, can inhibit light propagation in a certain direction for a given frequency. Many groups have reported that PCs can modify significantly the emission characteristics of emitters (organic dyes, 5,17 semiconductors, 7,18 rare earth doped particles, [19][20] etc.) as the emission wavelength of the optically-active material overlaps the stopband. In most of these studies, the emitters have been incorporated within pre-fabricated PCs owing to a simple infiltration process. As the efficiency of controlled emission is highly dependent on the organization of the PCs, which may get disturbed by the solvent used, and the homogeneous infiltration of the dyes, we chose to incorporate the emitters in the silica particles themselves, which were thus used to fabricate luminescent colloidal crystals following the strategy reported by only a few groups. 10,[21][22] Moreover, more emphasis in the PCs research field is placed on the incorporation of point, channel, and planar defects within the PCs structures for the creation of localized points and pathways for photons. With regard to the planar defects, which can be introduced within PCs by chemical vapor deposition, [23][24] spin-coating, [25][26] or the Langmuir-Blodgett (LB) technique, [27][28][29][30][31] the emergence of a localized state for photons within the stopband, known as a defect mode, has been revealed. The obtained heterostructures present a dip, also called pass band, inside the stop band in the extinction spectrum. As far as we know, there is only one report on the spectral narrowing of emission in self-assembled colloidal photonic superlattices, though no clear effect of the pass band was observed in the fluorescence emission spectrum. 32 This situation motivated us to investigate the influence of the introduction of one or several defect layers inside a 3D colloidal crystal on the emission spectrum of the incorporated emitters.

In this paper, we present a study of stop band / pass band effects on spontaneous emission of [Mo 6 Br 14 ] clusters-doped silica particles organized in 3D colloidal crystals. Let us recall that [(Mo 6 L i 8 )L a 6 ] cluster units (Figure 1a, L = halogen and or chalcogen), constitute the basic building blocks in molybdenum octahedral cluster chemistry. The Mo 6 cluster is face-capped by eight inner ligands (L i ) and is additionally bonded to six apical ligands (L a ). [33][34] The physical properties of Mo 6 solid-state compounds are related to the number of electrons available for metal-metal bonding within the cluster (valence electron count, VEC). Mocentered electrons are located on twelve metal-metal bonding molecular orbitals of the molecular orbital diagram. Their full occupation leads to a closed-shell configuration with a VEC of 24. 35 Using high temperature synthesis, it turns out that halogen ligands favor the formation of solid-state molecular compounds based on [Mo 6 X i 8 X a 6 ] 2-(X = Cl, Br, I) units with VEC values of 24 that can be discrete or poorly interconnected within the solid. The disssolution of inorganic solid-state halide precursors affords nanometric [Mo 6 X 14 ] 2-anionic units that exhibit interesting structural, photophysical, and redox properties that can be used for the formation and organization of supramolecular assemblies as well as hybrid materials. [36][37][38][39] For instance, the large emission region of the [Mo 6 X 14 ] 2-anion in the red and near infrared (580-900 nm) [40][41][42] is particularly interesting for biotechnology applications. In this frame, some of us recently prepared by water in oil microemulsion synthesis luminescent Cs 2 Mo 6 X 14 @SiO 2 and bifunctionnal luminescent magnetic Fe 2 O 3 -Cs 2 Mo 6 X 14 @SiO 2 nanoparticles containing [Mo 6 X 14 ] 2-unit as red inorganic dye. [43][44] Cs 2 Mo 6 Br 14 precursors were encapsulated in silica by a classical sol-gel synthetic procedure and the obtained colloidal spheres were organized by the LB technique to elaborate 3D colloidal crystals.

Heterostructures containing one or three planar defects made of silica spheres with a different size were also fabricated. We clearly observed that the emission of the incorporated light sources was inhibited by the presence of a stop band and was enhanced in the spectral region of a pass band.

Experimental

Cs 2 Mo 6 Br 14 clusters synthesis

The Cs 2 Mo 6 Br 14 ternaries were synthesized from a stoichiometric amount of CsBr and MoBr2 binary halides according to the procedure described in the literature. [45][46] Fabrication of the Cs 2 Mo 6 Br 14 @SiO 2 colloidal particles Cs 2 Mo 6 Br 14 @SiO 2 spheres with a mean diameter of 270 ± 15 nm and 330 ± 15 nm were synthesized following the well-known Stöber-Fink-Bohn method. 47 As an example, for the synthesis of the 330 nm spheres, 100 mg of Cs 2 Mo 6 Br 14 clusters, 195 ml of absolute ethanol (Prolabo), 11 ml of ammonia (29 % in water, J.T. Baker), 29 ml of water were introduced in a 500 mL three-neck round flask equipped with a refrigerating system and stirred at 40°C at 400 rpm to homogenize. 16.5 ml of tetraethoxysilane (TEOS, 99%) (Fluka) were then introduced and the solution was left under stirring during 24 hours. A transmission electron microscopy (TEM) picture of the 270 nm Cs 2 Mo 6 Br 14 @SiO 2 spheres is shown in Figure 1b.

The good monodispersity in size of the particles can be seen. Nevertheless, the Mo 6 Br 14 units cannot be distinguished inside the SiO 2 particles due to the strong absorption of the mineral particles.

The above suspension of 330 nm particles was used as a seed for the growth of silica in order to get particles with a final diameter of 460 ± 25 nm. Typically, 250 mL of the suspension of seeds were introduced in a 500 mL three-neck round flask. A solution of 6.5 mL of TEOS diluted in 25 mL of absolute ethanol was added dropwise at an addition rate of 2 mL.h -1 under stirring at 400 rpm and at room temperature. Synthesis was carried out for 24 hours.

In order to use the Cs 2 Mo 6 Br 14 @SiO 2 colloidal beads as building units to elaborate colloidal crystals by the LB technique, their surface was functionalized with aminopropyltriethoxysilane (99%, Aldrich) as previously described. 48

Fabrication of the colloidal crystals

Multilayer colloidal crystals were deposited by the Langmuir-Blodgett technique onto glass substrates (24 mm x 60 mm). Details of the fabrication have been already published. 48 3D colloidal crystals including one or three defect layers of particles of larger size were elaborated by alternately depositing 5 layers of 330 nm Cs 2 Mo 6 Br 14 @SiO 2 particles (host particles), and one layer of 460 nm Cs 2 Mo 6 Br 14 @SiO 2 spheres (guest spheres).

The 3D heterostructures are stable over years and can be manipulated without any damage if one takes care not to rub them up.

Characterizations

The size of the colloidal particles was determined statistically by analyzing TEM pictures taken with a Hitachi H-600 microscope.

Scanning electron microscopy (SEM) observations of the photonic crystals were performed with a JEOL JSM-6700F scanning electron microscope operating at 10 kV. The specimens were carbon-coated and simply cut into pieces using a glass cutter prior to examination.

Ultra-violet visible (UV-vis) extinction spectra were obtained by recording the transmitted intensity at normal incidence on a UV4 Spectrometer from Unicam. The extinction spectrum is defined as 1 -T where T is the transmitted intensity spectrum normalized to the incident intensity spectrum on the sample. For specular reflection, samples were illuminated with a fibered and collimated halogen source covering the whole 350-800 nm spectral range. The reflected light was collected by a second optical fiber symmetric to the first one. The fibers were mounted on rotating stages allowing for a precise selection of the incident and collection angles. The light spot on the opal had a size of 4 mm 2 (at 20° incidence) and the distance between the sample and the optical fiber was 10 cm. As for the extinction spectra, specular reflexion spectra are normalized to the incident light spectrum.

The photoluminescence (PL) emission spectra were obtained by exciting the samples by the 337 nm nitrogen laser line (Laser Photonics LN 1000, 1.4 mJ pulse energy, 0.6 ns pulse duration). The emitted light from the sample, collected at 8 cm by an optical fiber on the same side as the excitation, was analyzed with a Jobin-Yvon Spectrometer HR460 and a multichannel CCD detector (2000 pixels). The spectrometer was corrected for wavelength dependence of the transmission of the polychromator and the sensitivity of the detector. This setup has a resolution of 0.3 nm/point for a 30 µm slit.

Simulations

Finite-difference time-domain (FDTD) simulations [START_REF] Taflove | Computational Electrodynamics: The Finite-Difference Time-Domain Method[END_REF] of the extinction, reflection spectra of the various photonic architectures experimentally investigated were performed, integrated in a freely available software package with sub-pixel smoothing for increased accuracy. [START_REF] Farjadpour | [END_REF] The computational cell, in which the incoming wave propagates along the z direction at normal incidence, has been implemented with periodic boundary conditions in x and y directions and perfectly matched layers (PMLs) in the z direction. The resolution of the grid has been refined such that the convergence of the results was ensured. To be in close agreement with the experimental conditions, the opals have been represented by monodiperse spheres of given size (330 nm) and dielectric constant ε = 2.1 arranged along a face-centered cubic (fcc) lattice.

The lattice has been truncated to 25 (for Figure 2c) or 10 (for Figure 3b) planes normal to the direction [111], and the resulting crystal was set on a substrate with a dielectric constant of 2.3, which corresponds to the glass slides that were used, extending to the PMLs in the z direction. In case of a defect layer of larger spheres (with a diameter of 1.5 larger than the host particles) has been introduced between two stacks of 5 layers of host particles on the experimental side, we represented this layer on the simulation side by a bloc of the appropriate thickness with an effective refractive index n e = 1.35.

Results and discussion

The luminescence properties of the Cs 2 Mo 6 Br 14 @SiO 2 particles were first analyzed.

Figure 2a shows the room-temperature PL spectra measured under 337 nm excitation of aqueous solutions of the particles at various concentrations. The spectra show a broad band centered around 705 nm and exhibit a similar shape to those of the starting compounds, 43 evidencing that the emission properties of clusters are preserved when incorporated inside the silica particles.

The photonic crystals fabricated with the Cs 2 Mo 6 Br 14 @SiO 2 particles display a strong iridescence in the visible region of the spectrum, which indicates the presence of a photonic stop band due to the regular ordering of the building spherical units. In fact, the good crystalline quality of a crystal made of 25 layers of 330 nm particles can be seen on the SEM side view shown in Figure 2b. Figure 2c shows the optical extinction / reflection spectra of the crystal (solid lines), taken at various incidence angles (from 0° to 40°). One main peak, which shifts to shorter wavelength when the angle of incidence is increased, is observed according to Bragg-Snell diffraction law:

α λ 2 2 max sin 3 2 2 - = e n D
where λ max is the Bragg diffracted wavelength, α is the angle between the [111] direction and the incident beam, D is the diameter of the colloidal particles and n e is the effective refractive index of the crystal at the wavelength of interest. Spectra were simulated using the FDTD method. Calculated spectra (dashed lines) are plotted along the experimental ones in Figure 2c. Note that the vertical axis is arbitrarily scaled and the spectra are arbitrarily shifted in order to clearly distinguish them, as a function of angle. A good agreement between theoretical and experimental data can be observed.

The successful engineering of the crystal is clearly shown in Figure 2c, as the pseudogap maximum overlaps completely (α = 0, 20, 25, 30°) or partially (α = 35°) the emission band of the Cs 2 Mo 6 Br 14 @SiO 2 particles, represented in the figure as a grey vertical rectangle.

Therefore, one should expect the emission spectrum changes when the detection angle is modified. In order to verify this point, we conducted an angle-resolved emission study during which the excitation wavelength was chosen to be 337 nm, which is outside the stop band of the crystal in order to penetrate well into the sample and interact with the emitters. Figure 2d shows the emission spectra recorded at six different detection angles (solid lines), while the experimental extinction / reflection spectra of the sample are also shown (dashed lines). A profound dip in the emission band is observed. Its position shifts to shorter wavelengths when α increases, coinciding with the spectral position of the stopband. This indicates that an inhibition of the light emission from the Cs 2 Mo 6 Br 14 @SiO 2 particles occurs due to a low density of photonic states in the wavelength range of the stopband. A maximum inhibition of the luminescent emission of ~35% was estimated at α = 0°, whereas the emission band of the Cs 2 Mo 6 Br 14 @SiO 2 particles is unaffected at α = 40° as the stopband is brought completely out of the emission range.

In order to investigate the influence of a pass band on the emission spectrum of the Cs 2 Mo 6 Br 14 @SiO 2 particles, we exploited our capability to build multilayer colloidal crystals in a controlled manner with the Langmuir-Blodgett technique. We engineered a colloidal crystal composed by one layer of 460 nm Cs 2 Mo 6 Br 14 @SiO 2 spheres embedded between two stacks of five layers of 330 nm Cs 2 Mo 6 Br 14 @SiO 2 particles. The crystal's structure can be described as (330) 5 /(460) 1 /(330) 5 by listing the sphere size from bottom to top. Both the defect layer and the good crystalline quality of the sample can be seen on the SEM side view shown in Figure 3a. The insertion of such a defect layer induces the appearance of localized states for photons within the L pseudo-gap of the host particles structure. Accordingly, a pass band is created within the stop band. On the basis of the particle diameters of the guest and host particles used for the preparation of the sample, the pass band position is expected to overlap the emission profile of the colloidal building units. This has been verified experimentally by recording the extinction / reflection spectra of the sandwich-like crystal as a function of angle while tilting the sample over the ΓL direction. Indeed, Figure 3b shows that these spectra (solid lines) exhibit a dip within the pseudogap, which shifts to the blue and passes through the emission range of the Cs 2 Mo 6 Br 14 @SiO 2 particles as α increases. Calculated spectra (dashed lines) are also plotted along the experimental ones in Figure 3b. Here again, the simulations fit the experimental ones accurately in all cases. Figure 3c shows the emission (solid lines) and extinction / reflection (dashed lines) spectra of the sandwich-like crystal recorded at six different detection angles. For 0° ≤ α ≤ 35°, both an inhibition of the emission in the dielectric and air band regions and an exaltation within the pass band are clearly observed, as the result of the redistribution of the photonic local density of states. Only a weak shoulder in the PL spectrum around 650 nm is visible for α = 40°, as a result of the blue shifting of the pass band towards the low wavelength edge of the emission band induced by the tilting of the sample. To quantify the effects of the (330) 5 /(460) 1 /(330) 5 crystal on the emission spectra, we took the ratio of the emission spectra to the spectrum recorded from a reference sample made of 10 layers of 270 nm Cs 2 Mo 6 Br 14 @SiO 2 spheres, which exhibits a photonic band gap at 595 nm (not shown). We thus obtained the relative intensity spectra displayed in Figure 3d. For α = 0°, the data clearly show a 15% attenuation of the emission in the dielectric and air band regions as well as a 8% enhanced emission in the pass band region.

The simultaneous observation of attenuation and enhancement means that the effect of the PC is not a simple filter action.

We further took advantage of the versatility of the Langmuir-Blodgett technique by engineering a photonic heterostructure made by alternated deposition of stacks (A) made of five layers of 330 nm Cs 2 Mo 6 Br 14 @SiO 2 particles with one monolayer (B) of 460 nm Cs 2 Mo 6 Br 14 @SiO 2 particles. Figure 4a shows a SEM side view of a A(BA) 3 heterostructure.

The thickness of each stack is controlled at the layer level as the number of deposited layers matches perfectly with the predefined value of transfer cycles. The overall thickness of the sample is uniform despite the presence of defect layers B of larger silica particles which periodically separate the A substructures. The periodic alternation between A and B substructures gives rise to a superlattice, whose periodic parameter corresponds to the sum of the thicknesses of stacks A and B. We have recently shown that the pass band becomes wider as the layering is repeated. 30 The comparison of the extinction spectra of A(BA) 1 and A(BA) 3 heterostructures taken at normal incidence (α = 0°) and shown in Figure 4b confirms this assertion, as the width of the defect state of the latter is larger than for the A(BA) 1 crystal. In agreement with our previous observations, 30 weak oscillations are also visible inside the passband of the A(BA) 3 sample. They can be assimilated to minibands, which result from the coupling of the individual defect modes in the heterostructure. Figure 4b further illustrates the influence of the structure of the colloidal crystals on the emission characteristics of their building units. An enhancement of the emission is observed between 700 nm and 770 nm in the PL spectrum of the A(BA) 3 heterostructure, i.e. in the whole spectral zone of the pass band. One should also note a slight inhibition of the emission at 730 nm and 765 nm, which correspond to the spectral positions of the minibands, which thus act as stopbands.

Conclusions

In summary, we have engineered colloidal crystals from silica spheres with incorporated luminescent [Mo 6 Br 14 ] 2-cluster units. An inhibition of the emission intensity from the light sources was observed in the spectral region of the stop band, which was found to nicely correspond and follow the Bragg-Snell law as the angle between the [111] direction and the incident beam was varied. Heterostructures containing one or three planar defects displayed increased emission in the spectral region of the pass band. The 3D photonic colloidal structures investigated here are very promising for both basic and applied research related to the confinement and exaltation of spontaneous emission in given frequency ranges and thus are good candidates for low-threshold and/or single mode photonic crystal lasers. b)

Figures captionsFigure 1 .

 1 Figures captions

Figure 2 .

 2 Figure 2. a) Concentration dependence of the photoluminescence spectra of the 330 nm

Figure 3 .

 3 Figure 3. a) SEM side view of a colloidal crystal composed by one layer of 460 nm

Figure 4 .

 4 Figure 4. a) SEM side view of a A(BA) 3 heterostructure. b) Experimental extinction (at α =

Figure 1

 1 Figure 1.

  Photoluminescence (a.u.)

Figure 4 .

 4 Figure 4.

Acknowledgements

The authors thank B. Agricole (CRPP) and E. Sellier (CREMEM, Talence) for Langmuir-Blodgett experiments and SEM observations, respectively.


Mel Constant
email: mel.constant@lilo.org

Philippe Kerhervé
email: kerherve@univ-perp.fr.

Morgan Mino-Vercellio-Verollet

Marc Dumontier

Anna Sànchez Vidal

Miquel Canals

Serge Heussner

Constant Mel

Kerhervé Philippe

Mino-Vercellio-Verollet Morgan

Dumontier Marc

Canals Miquel Sanchez-Vidal Anna

Heussner Mino- Vercellio-Verollet Morgan

Beached microplastics in the Northwestern Mediterranean Sea

Keywords: microplastics, Mediterranean Sea, Gulf of Lion, micro-tidal system, beach

Introduction

Numerous reports qualify anthropogenic litter as a pervasive and persistent global threat to natural environments. Indeed, it occurs in terrestrial [START_REF] Zubris | Synthetic fibers as an indicator of land application of sludge[END_REF], subterranean [START_REF] Panno | Microplastic Contamination in Karst Groundwater Systems[END_REF], freshwater [START_REF] Gasperi | Assessment of floating plastic debris in surface water along the Seine River[END_REF] and marine ecosystems [START_REF] Moore | A Comparison of Plastic and Plankton in the North Pacific Central Gyre[END_REF]. Even remote habitats are concerned as shown by the finding of manmade litter in mountain lakes and isolated islands [START_REF] Free | Highlevels of microplastic pollution in a large, remote, mountain lake[END_REF][START_REF] Lavers | Exceptional and rapid accumulation of anthropogenic debris on one of the world's most remote and pristine islands[END_REF]. Oceans and seas are widely considered as a sink for anthropogenic litter, produced either by terrestrial or marine human activities, which triggers a number of environmental and economic issues. Though floating marine litter can provide habitat and shelter to a variety of organisms it is first and above all a threat for marine life mainly through ingestion and entanglement. Marine litter is also undesirable for humans as it affects local and national economies in terms of environmental pollution, tourist choice for beaches and seagoing recreational activities, or by inducing expensive clean-up costs [START_REF] Krelling | Differences in perception and reaction of tourist groups to beach marine debris that can influence a loss of tourism revenue in coastal areas[END_REF].

This pervasive problem should persist in the future even if inputs of new debris would stop now [START_REF] Barnes | Accumulation and fragmentation of plastic debris in global environments[END_REF].

Plastics represent the largest item category described within beached marine litter, which may reach up to 90% of total debris in some beaches [START_REF] Laglbauer | Macrodebris and microplastics from beaches in Slovenia[END_REF]. Furthermore, the number of plastic pieces drastically increases with decreasing size. Particles smaller than 5 mm, called microplastics (MPs hereafter), can represent more than half of all plastic debris [START_REF] Munari | Plastic debris in the Mediterranean Sea: Types, occurrence and distribution along Adriatic shorelines[END_REF]. MPs can be ingested by small invertebrates and potentially transferred to higher trophic levels through the food web [START_REF] Farrell | Trophic level transfer of microplastic: Mytilus edulis (L.) to Carcinus maenas (L.)[END_REF].

Concern has been even formulated on potential impact on human health since MPs have been found in seafood, salt and airborne particles [START_REF] Rochman | Anthropogenic debris in seafood: Plastic debris and fibers from textiles in fish and bivalves sold for human consumption[END_REF][START_REF] Yang | Microplastic pollution in table salts from China[END_REF][START_REF] Dris | Synthetic fibers in atmospheric fallout: A source of microplastics in the environment?[END_REF].

In 2008 the European Union launched the Marine Strategy Framework Directive (MSFD), a policy framework aiming to achieve a good environmental status of marine waters. One of the selected criteria relates to marine litter with a particular attention to small plastic particles, which should be monitored within four compartments: along coastlines (shored MPs), at the sea-surface (floating MPs), in marine sediments (settled MPs), and in the biota (ingested MPs). Protocols regarding the quantitative assessment of macrolitter are well established in Europe (OSPAR 2010;[START_REF] Galgani | Marine litter within the european marine strategy framework directive[END_REF] and the most recent studies comply with them [START_REF] Poeta | The cotton buds beach: Marine litter assessment along the tyrrhenian coast of central italy following the marine strategy framework directive criteria[END_REF][START_REF] Portman | Marine litter from beach-based sources: Case study of an eastern mediterranean coastal town[END_REF]. In contrast, MPs are still studied using a wide range of non-standardized methods and protocols, a situation that does not favor data comparison nor the calculation of budgets.

As a landlocked sea, the Mediterranean Sea is highly exposed to marine litter pollution [START_REF] Cózar | Plastic accumulation in the Mediterranean Sea[END_REF][START_REF] Deudero | Mediterranean marine biodiversity under threat: Reviewing influence of marine litter on species[END_REF][START_REF] Tubau | Marine litter on the floor of deep submarine canyons of the Northwestern Mediterranean Sea: The role of hydrodynamic processes[END_REF]. This is partly explained by high population densities in many of the Mediterranean watersheds, its appeal as one of the most popular tourist destinations in the world, and also by its semi-enclosed nature favoring litter recirculation and limiting an unrestricted transfer to other ocean areas.

Marine litter thus significantly adds to the high anthropogenic pressures threatening the ecosystems of the Mediterranean Sea where at least 134 species are impacted [START_REF] Deudero | Mediterranean marine biodiversity under threat: Reviewing influence of marine litter on species[END_REF].

A number of studies have recently addressed floating MPs in the Mediterranean Sea, and specifically in the Gulf of Lion in its northwestern part [START_REF] Collignon | Neustonic microplastic and zooplankton in the North Western Mediterranean Sea[END_REF][START_REF] Collignon | Annual variation in neustonic micro-and meso-plastic particles and zooplankton in the Bay of Calvi (Mediterranean-Corsica)[END_REF][START_REF] Faure | An evaluation of surface micro-and mesoplastic pollution in pelagic ecosystems of the Western Mediterranean Sea[END_REF] 5 / 31 [START_REF] Alathea | Captioner: Numbers figures and creates simple captions[END_REF][START_REF] Pedrotti | Changes in the Floating Plastic Pollution of the Mediterranean Sea in Relation to the Distance to Land[END_REF][START_REF] Schmidt | Occurrence of microplastics in surface waters of the Gulf of Lion (NW Mediterranean Sea)[END_REF][START_REF] Constant | Floating Microplastics in the Northwestern Mediterranean Sea: Temporal and Spatial Heterogeneities[END_REF]. However, only a few investigations have been conducted on MPs stranded on beaches [START_REF] Kaberi | Microplastics along the shoreline of a greek island (kea isl., aegean sea)-types and densities in relation to beach orientation, characteristics and proximity to sources[END_REF][START_REF] Laglbauer | Macrodebris and microplastics from beaches in Slovenia[END_REF][START_REF] Lots | A large-scale investigation of microplastic contamination: Abundance and characteristics of microplastics in European beach sediment[END_REF][START_REF] Munari | Plastic debris in the Mediterranean Sea: Types, occurrence and distribution along Adriatic shorelines[END_REF][START_REF] Abidli | Microplastics in sediments from the littoral zone of the north Tunisian coast (Mediterranean Sea)[END_REF]. The MSFD guidance document [START_REF] Galgani | Marine litter within the european marine strategy framework directive[END_REF] for sampling beached MPs focuses on intertidal sediments and high tide lines. However, in the Mediterranean Sea, which is a microtidal environment, alongshore depositional line marks are not due to tides but to hydrodynamic conditions such as surges and storms. Thus, the first objective of our study was to sample nearby Mediterranean beaches representative of different settings and depositional conditions at high spatial resolution in search for patterns in the quantitative (density) and qualitative (size, shape and polymer composition) distribution, temporal variation and properties of MPs in the 5 mm -63µm size range. The second objective was to examine the potential role of external forcing such as precipitation, flooding, surging, wind regimes and specific events on the distribution and temporal variation of beached MPs.

Materials and Methods

Study area

The Gulf of Lion is a passive margin covering 11,000 km 2 from Cap Croisette in the northeast to Cap Creus in the southwest (Fig. 1). This river-dominated continental margin is a micro-tidal environment characterized by large supplies of freshwater, sediments, nutrients and contaminants (Durrieu de [START_REF] Durrieu De Madron | Particulate matter and organic carbon budgets for the Gulf of Lions (NW Mediterranean)[END_REF][START_REF] Durrieu De Madron | Shelf-slope nutrients and carbon fluxes in the mediterranean Sea. In "Carbon and nutrient fluxes in continental margins. A global synthesis[END_REF][START_REF] Ludwig | River discharges of water and nutrients to the Mediterranean and Black Sea: Major drivers for ecosystem changes during past and future decades?[END_REF][START_REF] Bouloubassi | Sources, dispersal pathways and mass budget of sedimentary polycyclic aromatic hydrocarbons (PAH) in the NW Mediterranean margin, Gulf of Lions[END_REF]. The most important river flowing into the Gulf of Lion is the Rhône River that at present is also the highest freshwater input to the Mediterranean Sea with a mean annual flow rate of 1700 m 3 s -1 and yearly floods higher than 5000 m 3 s -1 [START_REF] Maillet | Morphological changes and sedimentary processes induced by the December 2003 flood event at the present mouth of the Grand Rhône River (southern France)[END_REF], Ludwig et al., 2009). Inputs of particulate matter from the Rhône River represents on average 80 to 95% of the total inputs to the gulf [START_REF] Bourrin | Contribution to the study of coastal rivers and associated prodeltas to sediment supply in the Gulf of Lions (NW Mediterranean Sea)[END_REF][START_REF] Sadaoui | Controls, budgets and variability of riverine sediment fluxes to the gulf of lions (nw mediterranean sea)[END_REF]. Its mouth is located 180 km northeast from our sampling area in the southwestern Gulf of Lion. Suspended sediments at sea are mainly carried by the Northern Current and generally follow a south-southwest direction as far as the southwestern end of the gulf [START_REF] Arnau | River Mouth Plume Events and Their Dispersal in the Northwestern Mediterranean Sea[END_REF].

Several small coastal rivers, as the Têt River, discharge also freshwater, sediments and contaminants along the central and western part of the gulf [START_REF] Sadaoui | Controls, budgets and variability of riverine sediment fluxes to the gulf of lions (nw mediterranean sea)[END_REF] but with a lower contribution than the Rhône River. These coastal rivers are characterized by small watersheds (< 5,000 km 2 ) and a highly variable discharge regime governed by episodic flood events. Indeed, their contribution to sediment inputs into the Gulf of Lion reaches a long-term average of about 5% of the total amount of riverine sediments, except in some particular years with frequent flash-floods (up to 90% and 27% of the monthly and yearly sediment discharge respectively). The 116 km long Têt River, a typical Mediterranean coastal river with its source in the Pyrénées mountain range and a mean slope of 12.1°, dominates continental inputs to the study area (Fig. 1). With a long-term annual average discharge of 10 m 3 s -1 this river, as most Mediterranean coastal rivers, shows long periods of low-water stages (<3 m 3 s -1 ) interrupted by yearly flash-flood events (>100 m 3 s -1 ) peaking at 1.800 m 3 s -1 [START_REF] Serrat | Spatial and temporal variability of sediment fluxes from a coastal Mediterranean river: the Têt (France)[END_REF]. The lower course of the Têt River flows across a 600 km 2 agricultural alluvial plain. Perpignan is the main town there with around 150,000 inhabitants. The urban wastewater treatment plant of Perpignan has a direct effect on the chemistry of the Têt River, by increasing nutrients and pollutants [START_REF] Garcia-Esteves | Predicting the impact of land use on the major element and nutrient fluxes in coastal Mediterranean rivers: The case of the Têt River (Southern France)[END_REF][START_REF] Reoyo-Prats | Multicontamination phenomena occur more often than expected in Mediterranean coastal watercourses: Study case of the Têt River (France)[END_REF].

Two markedly different beaches, both in terms of setting and sedimentary conditions, have been selected (Table 1). Both beaches are located 25 km (straight) and 35 km (alongshore) apart. These beaches are "La Crouste" Beach, to the north, (hereafter also called "northern site") nearby the Têt River outlet, and "Fourat Beach" (hereafter "southern site") (Fig. 1).

The northern site is bordered at its southern end by the Têt River mouth and at its northern end by a man-made jetty (Fig. 2A). The combined effects of the jetty and a northward alongshore current lead to preferential sediment accumulation at the northern part of the beach. At its innermost, upper part, the beach is bounded by dune vegetation. The southern site backshore is bounded by natural boulders and a concrete wall (Figs. 2E andF). Onshore winds and associated waves induce a southward alongshore current [START_REF] Brunel | 20th century sediment budget trends on the Western Gulf of Lions shoreface (France): An application of an integrated method for the study of sediment coastal reservoirs[END_REF], and sediment accumulates mainly at the southern end of the beach. This site may receive sediments and plastic materials used in the surrounding terraced vineyards through a small, 0.6 km 2 dry creek during rainfall events. Both sites are affected by "Tramontane", a strong, persistent and dry NNW wind with gusts often exceeding 100 km.h -1 . They are also affected by eastern sea storms associated to humid winds and precipitation [START_REF] Sanchez-Vidal | Impacts on the Deep-Sea Ecosystem by a Severe Coastal Storm[END_REF]. As for most Mediterranean beaches, they are both popular leisure spots in summer. No activities related to plastic production or treatment have been identified around these study sites.

Sample collection

Given the microtidal regime of most of the Mediterranean Sea, deposit lines on beaches are mainly dependent on surges due to atmospheric conditions (pressure and wind) and on wave height. Three debris deposit lines at different heights can be distinguished most of the time on the shores of the Gulf of Lion (Figs. 2B andF). The first deposit line, at the lower beach, is freshly produced and generally wet. The second one, close to the beach mid-height, is composed of dried up materials stranded during gales. The third one, at the upper beach, corresponds to materials accumulated during stormy conditions and subsequently trapped in the backshore. At the northern site, trapping is due to the dune vegetation, while at the southern site it is favored by the concrete wall and beach-end boulders. At the northern site, the mid-beach line was associated to a berm crest (Fig. 2D). Sampling was performed along the 3 deposit lines named from here onwards "lower", "mid" and "upper", at the crossing of four cross-beach transects termed "north", "mid-north", "mid-south" and "south" (Figs. 2C, D, G, andH).

In total, 12 samples were collected on each site using a 0.5 x 0.5 m wood quadrat. The top first centimeter of sand was sampled with a steel trowel and sieved through an 8 mm steel sieve placed on top of a metallic funnel to avoid obstruction of the funnel by plant residues and/or gravels. The <8 mm fraction was funneled into glass bottles. This sampling was repeated twice at each site: on March 2 and March 30, 2016 for the northern site, and on February 22 and March 30, 2016 for the southern site.

Sample treatment

Contamination of samples was minimized by wearing laboratory cotton coats and by using glass or steel equipment and tools, rinsed with MilliQ® water and wrapped in aluminium foils. Back in the laboratory, samples were dried at 50°C during one week and weighed.

Samples were sieved through a column of sieves (5, 2.5, 1 mm; 500, 315, 63 µm) placed in a mechanical shaker. Parts <63 µm were discarded, as the available equipment did not allow accurate measurements of such small particles. The protocol for MP density-based extraction from sediments provided by [START_REF] Thompson | Lost at Sea: Where Is All the Plastic?[END_REF] was adapted to our samples: 550 mL of concentrated saline solution (density 1.2 g mL -1 ) were added to 500 g of sorted sand in a boiling flask and hand-stirred for two minutes. After 30 minutes the supernatant was filtered on GF/F Whatman filters (47mm diameter; 0.7µm porosity). This extraction procedure (stirring and decantation) was repeated until all floating particles were recovered. At least 3 repetitions were performed, and four repetitions were generally sufficient to reach full extraction. Filters were dried at 50ºC overnight and then examined under a Wild Heerbrugg dissecting stereo-microscope (6 x, 12 x, 25 x and 50 x magnification). Several contamination control filters were simultaneously performed as blanks with the same concentrated saline solution. Particles were separated into 5 shape categories: fibers (including filaments and fishing line fragments; Figs. 3A andB), fragments (items with a three-dimensional shape; Fig. 3C), micro-beads (solid spheres; Fig. 3C), films (items with a two-dimensional shape; Fig. 3C), and foams (pieces or spheres with a spongy, soft structure; Fig. 3C). These categories have been used in previous studies [START_REF] Kaberi | Microplastics along the shoreline of a greek island (kea isl., aegean sea)-types and densities in relation to beach orientation, characteristics and proximity to sources[END_REF][START_REF] Faure | An evaluation of surface micro-and mesoplastic pollution in pelagic ecosystems of the Western Mediterranean Sea[END_REF][START_REF] Abidli | Microplastics in sediments from the littoral zone of the north Tunisian coast (Mediterranean Sea)[END_REF].

Polymer analysis with FTIR spectroscopy

Five items for each size fraction (including the >5 mm fraction) and shape category (except beads) were randomly selected at each site and analyzed to determine their plastic nature and where appropriate their polymeric composition [START_REF] Löder | Methodology used for the detection and identification of microplastics-A critical appraisal[END_REF]. A total of 40 fibers, 60 fragments, 60 films, 60 foams and 4 beads were examined (fibers were only present inside four size classes). MPs were analyzed with a Perkin Elmer Frontier FTIR spectrometer, and fibers, due to their small volumes, were analyzed with a microscope IR-Plan Spectra Tech coupled to a Bomem ABB FTLA FTIR spectrometer.

The identification of each polymer was performed by comparison with a self-collected spectrum database including polyethylene (PE), polypropylene (PP), polyamide (PA), polyvinyl chloride (PVC), polystyrene (PS), poly(ethylene-vinyl acetate) (PEVA), poly(ethylene terephthalate) (PET), polyester (PES), polyurethane (PUT), acrylic (A) and some non plastic materials such as cotton, wool, wood, paper, calcium carbonate, talcum powder and plant fibers. Only polymers reaching at least 70% of similarity with reference spectra were accepted, according to the suggestion of [START_REF] Thompson | Lost at Sea: Where Is All the Plastic?[END_REF]. The rejected items were counted in the unidentified category.

Data analysis

FTIR analysis was used to correct the initial sorting and accurately determine true MPs in our samples. For each shape category, an identified plastic ratio was accordingly applied.

This ratio was calculated by dividing the number of true particles, with a "plastic spectra", by the total number of sorted items and visually described as "potential plastics".

Results

MPs spatial distribution

A total of 15,664 particles have been sorted from the 48 samples collected on the two beaches and at the two periods (Table 2). After FTIR analysis, it appeared that only 45% of them were really plastics. Mean concentration of MPs was 166 ± 205 items kg -1 for the northern site (range 33 to 798 items kg -1 ) and 58 ± 53 items kg -1 for the southern site (range 12 to 187 items kg -1 ).

Both sites significantly differ regarding MP concentrations (p-value <0.01; Fig. 4A). Though concentrations of MPs changed by a factor up to three for some limited cross-beach transects between the first and the second period no statistically significant differences were found between the two sampling periods at each site (northern site: p-value = 0.79; southern site: p-value = 0.47; Fig. 4A). The data were therefore consequently pooled to gain readability (Table 3; Figs 5 and6).

The range of MP concentrations measured in the northern site spanned over an order of magnitude, from a minimum of 33 to a maximum of 798 items kg -1 (Table 2).

Concentrations at the two extreme cross-beach transects (North and South) were quite heterogeneous, while middle cross-beach transects (Mid-North and Mid-South) showed low and stable values. Highest mean concentrations (>300 items kg -1 ), significantly different from the other "baseline" samples (Fig. 4D), were located at the medium and high alongshore transects along the "North" and "South" cross-beach transects (Fig. 5). The 3 major impacted zones ("High Concentration Zones"), represented half (52%) of the total number of MPs found in the 24 samples collected on that beach. Finally, MP concentrations did not significantly differ between alongshore transects (p-value = 0.16; Fig. 4B), crossbeach transects (p-value = 0.06; Fig. 4C) or the interactions among cross-beach transects and alongshore transects (p-value = 0.65).

MP concentrations measured at the southern site ranged from a low of 12 to a high of 187 items kg -1 (Table 2). They were more stable than in the northern site. The major, obvious outcome of the survey there was the strong difference in concentrations between alongshore transects (p-value <0.01; Fig. 4B). The highest concentrations were all located in the high deposition line (Fig. 6). With an overall mean concentration of 115 items kg -1 , compared to the overall means of 30 items kg -1 for the other two alongshore transects, the high line concentrated more than half (57%) of the total MPs in that area (Fig. 6). Though differences between cross-beach transects were not significant (p-value = 0.15; Table 3), it seems that an increase of MP concentrations occurred along the beach from North to South, i.e. in the direction of the alongshore current, especially in the high alongshore transect.

Again, the interaction among cross-beach transects and alongshore transects was not significant (p-value = 0.97). MPs distribution throughout the beach sampled at the end of February exhibited a very similar spatial pattern than the one observed at the end of March (data not shown).

Shape and polymers

Due to the high number of shape categories considered, data from the different samples were pooled within each study site. Fibers were the most abundant shape found at both beaches (northern site: 59% ; southern site : 77%; Fig. 7), followed by fragments (northern site: 25%; southern site : 17%) and foams (northern site: 12%; southern site : 5%). Films were less represented (<5%) and micro-beads were negligible (<1%). Some changes in the shape distributions occurred between the two sampling periods. In the northern site, fragments, films, foams and beads were mainly present during the first sampling period, whereas fibers were mainly present during the second sampling period. In the southern site, beads were only present during the first sampling period, fibers and films were equally present in both sampling periods, and fragments and foams were mainly present during the second sampling period.

Our FTIR results confirmed the necessity of performing plastic polymer analysis for correcting the data. Indeed, we found that less than half of binocular-sorted items (45%)

were plastic. The overall percentage was only 37% for fibers, which accounted for 63% of all the plastic items sampled during this study. Such a low percentage of plastic likely relates to the difficulty in discriminating plastic fibers from cotton and plant fibers with a binocular or a dissecting stereo-microscope. Among the plastic fibers, polyester, acrylic and polyacrylamide were the three most abundant polymers. Fragments were also characterized by a low percentage of plastic. Only half (50%) of the sorted fragments were really plastic polymers, mostly composed of polypropylene (17%), polyethylene (15%) and polystyrene (9%). The presence of calcium carbonate and cellulose identified by FTIR spectra suggests that they have been probably mistaken for shells and plant debris. The identified plastic rate reaches 67% for films with the same major polymers than those in fragments: polypropylene (37%), polyethylene (18%) and polystyrene (10%). A low error rate was measured for foams, probably due to their larger size and particular texture, as 81% of foam items were really plastics, predominantly and logically made of polystyrene (74%). Sorted beads were only plastic items made of polyethylene (100%).

Size

Regarding MP distributions within the sand size classes obtained by sieving, no major differences were observed between the northern and southern sites (Fig. 8). The former had higher MP concentrations within each size class. MPs within the smallest sand fraction (<0.5 mm) were mainly fibers, fragments and films, whereas in the largest ones (>2.5 mm) foams predominated and fibers prevailed in the other fractions.

Meteorological and hydrological context

Two meteo-hydrological periods can be distinguished: a first period from October 2015 to the end of February 2016 (the first sampling date of the southern site) and a second period until the last sampling date at the end of March 2016 (Fig. 9). The first period was mainly characterized by a winter storm associated to a short but intense rainfall event (36 mm of rain fell within a few hours; Fig. 9A) at the beginning of November 2016 that resulted in a peak of the Têt River flow (daily mean peak of 150 m 3 s -1 ; Fig. 9D), just below the flood threshold (170 m 3 s -1 , biennial return period). Simultaneously, south to southeasterly winds at around 13 m s -1 (Fig. 9C) generated the highest swells (up to 5 m height; Fig. 9B) and a consequent storm surge on the shoreline. From this seasonal event to the first sampling date (February 22, 2016) no particular meteorological or hydrological events happened at the local scale. The Têt River was characterized by a long period of low water stage (< 5 m 3 s -1 ) and swells and winds did not exceed 3 m and 16 m s -1 , respectively. The continental, dry and intense wind ("Tramontane") blew often during this interval. The second period was characterized by an important rainfall event (up to 62 mm) generating only a small Têt River flow peak (12 m 3 s -1 ) that occurred between the two first sampling dates, at the southern (February 22, 2016) and northern (March 2, 2016) sites. This second period was further characterized by strong SSE winds (up to 13 m s -1 ) and high swells (up to 4 m).

Then, the wind turned mainly northwest and no significant rainfalls or storms were recorded. The Rhône River did not experience any flood (>3,500 m 3 s -1 ; Fig. 9D) during this 6-month period. Its flow rate peaked three times in 2016 over 3,200 m 3 s -1 (peak at 3,850 m 3 s -1 February 2016), but they were far from reaching the flood biennial return frequency of 5,700 m 3 s -1 .

Discussion

Levels of MP pollution on NW Mediterranean beaches

MP concentrations found along two French Mediterranean beaches varied within the range of MP concentrations measured so far in various locations along the Mediterranean coast (Table 4). [START_REF] Kaberi | Microplastics along the shoreline of a greek island (kea isl., aegean sea)-types and densities in relation to beach orientation, characteristics and proximity to sources[END_REF] and [START_REF] Munari | Plastic debris in the Mediterranean Sea: Types, occurrence and distribution along Adriatic shorelines[END_REF] found low concentrations that may be explained, at least for the former, by their focus on only larger MPs (1-4mm) and thus the lack of significant amounts of fibers. Fibers accounted, on average, for 63% of all MP items collected in our study. They predominated also on Slovenian and Tunisian beaches [START_REF] Laglbauer | Macrodebris and microplastics from beaches in Slovenia[END_REF][START_REF] Abidli | Microplastics in sediments from the littoral zone of the north Tunisian coast (Mediterranean Sea)[END_REF] and reached up to 99% of MPs on various Mediterranean beaches [START_REF] Lots | A large-scale investigation of microplastic contamination: Abundance and characteristics of microplastics in European beach sediment[END_REF]. Synthetic fibers in oceans and beaches can originate from clothes [START_REF] Napper | Release of synthetic microplastic plastic fibres from domestic washing machines: Effects of fabric type and washing conditions[END_REF] but also from fishing gears [START_REF] Claessens | Occurrence and distribution of microplastics in marine sediments along the belgian coast[END_REF][START_REF] Lots | A large-scale investigation of microplastic contamination: Abundance and characteristics of microplastics in European beach sediment[END_REF]. Fiber textiles can break due to pilling in laundry washing machine and reach wastewater treatment plants (WWTP). [START_REF] Napper | Release of synthetic microplastic plastic fibres from domestic washing machines: Effects of fabric type and washing conditions[END_REF] estimated at up to 700,000 the number of fibers produced during a single wash. Although WWTP strongly reduce MPs in wastewaters (up to 98%, [START_REF] Murphy | Wastewater Treatment Works (WwTW) as a Source of Microplastics in the Aquatic Environment[END_REF], the treated effluent flowing into rivers or seas still contain significant amounts of MPs. Moreover, synthetic fibers have been also found in sewage sludges used for agricultural fertilization [START_REF] Zubris | Synthetic fibers as an indicator of land application of sludge[END_REF], representing thus a supplementary source to agricultural watersheds such as the Têt River. [START_REF] Lots | A large-scale investigation of microplastic contamination: Abundance and characteristics of microplastics in European beach sediment[END_REF] also described the so far highest Mediterranean hotspot of stranded MPs with concentrations up to 1,512 items kg -1 , a value about twice the highest MP concentration observed in our study (798 items kg -1 ). According to the authors this hotspot may be explained by the geographical position of the site, situated between the mouths of two rivers. Considering that sample collection and treatment, especially visual sorting and FTIR validation, potentially create noticeable differences between studies, even such concentrations should be considered, in a first approximation, as similar. Our highest MP concentrations found in the northern site thus probably lie close to the upper limit of the contamination range observed so far on Mediterranean beaches. The Mediterranean Sea has been identified as a region of particularly high floating plastic concentrations in comparison to the range measured for the global ocean [START_REF] Cózar | Plastic accumulation in the Mediterranean Sea[END_REF]. However, our results (Table 2) and those obtained by previous studies (Table 4) indicate that this situation is not as dramatic for MPs stranded on beaches elsewhere. They are indeed much lower than the 250,000 items m -2 found in a

Hong-Kong beach [START_REF] Fok | Hong Kong at the Pearl River Estuary: A hotspot of microplastic pollution[END_REF] or 24,000 items m -2 found in Hawaï [START_REF] Mcdermid | Quantitative analysis of small-plastic debris on beaches in the Hawaiian archipelago[END_REF]. More studies are needed to determine if such high concentrations exist in the Mediterranean Sea, and particularly along the south Mediterranean coast where recent models of plastic distribution predict high beaching [START_REF] Mansui | Modelling the transport and accumulation of floating marine debris in the mediterranean basin[END_REF].

Small scale spatial variability in the distribution of MPs

The two study sites have been selected to represent markedly different environmental settings, within a limited geographical area of the Northwestern Mediterranean Sea: a sandy beach bordered by a river mouth and thus directly affected by river litter inputs and a sandy beach located on a rocky coast supposed to be less affected by direct anthropogenic pressures (rivers and anthropized watershed). Despite some similarities in both beaches, such as the presence of high concentration zones (HCZ), the low and quite constant MP concentrations in the lower alongshore transects, or the predominance of fibers (overall mean of 63% for both beaches), the distribution of the concentration of MP at a small scale was quite different between the northern and the southern sites.

The northern site shows the highest MP concentrations (mean value up to 798 items kg -1 and maximum of 4,654 items m -2 ), about three times higher than those found in the southern site. This quantitative difference is likely due to the close proximity of the Têt River mouth at the southern end of the site. This small and typical coastal Mediterranean river drains a small watershed, mainly wild in its upper part but highly anthropized downstream. The riverine MP content ranges between 1 and 2 items m -3 (Constant, unpublished results), a concentration 10 times higher than that in surficial coastal waters a few kilometers off the river mouth [START_REF] Constant | Floating Microplastics in the Northwestern Mediterranean Sea: Temporal and Spatial Heterogeneities[END_REF]. These values suggest that MPs delivered by the Têt River and transported by the northward alongshore drift can be an important if not the major direct source of plastics to this beach. Such a direct connexion has been already described by [START_REF] Munari | Plastic debris in the Mediterranean Sea: Types, occurrence and distribution along Adriatic shorelines[END_REF] for a beach located next to the Po River.

Mean MP concentrations in the northern site were similar between the two sampling periods (182 and 150 items kg -1 ) but the HCZ moved from a position close to the river

External forcing of MP distribution on NW Mediterranean beaches

Local parameters such as swell height, wind, precipitation and river flow affect marine litter transport and may explain MP distributions and the presence of HCZs on beaches [START_REF] Galgani | Global distribution, composition and abundance of marine litter[END_REF]. We hypothesize that most of MPs stranded on the southern site and sampled in February are MPs washed away from the urbanized watershed and discharged by local rivers such as the Têt River in November, during a flood associated with intense southeastern winds and surge waves. This November storm and the related surge accounted also for the location of HCZs always within the high deposit line of the southern site. In addition, the MP accumulation towards the southern part of this beach is explained by the alongshore current prevailing in this area (Fig. 2). Another important feature of the southern site is the relative similarity of MP distributions throughout the beach between the two sampling periods (40 days). The remote location of this beach from the urbanized plain and its rivers, as well as the lack of extreme meteorological events during this interval may explain this small scale stability.

MP inputs to the northern site are more difficult to untangle. Besides the November storm, one must also consider the intense rainfall of February 2016 (60 mm), a few days before sampling. Precipitation was probably sufficient for washing away all plastic litter from local watersheds and concentrating them into streams and rivers. As the rainfall was spread over the day, the daily flow rate of the Têt River did not exceed 10 m 3 s -1 , but the related southeastern winds and swell heights were probably sufficient to ground large amounts of litter and MPs at the proximity of the Têt mouth and at the upper beach. The second sampling date (end of March 2016) was mainly preceded by a period of northwestern winds, low water stages and swell heights. Such a continental wind can blow the stranded litter offshore [START_REF] Galgani | Litter on the sea floor along european coasts[END_REF] and thus erode the HCZ previously shaped close to the Têt river mouth. It is interesting to note that the HCZs on the northern site have been fully modified in three weeks. The new HCZ observed at its northern part may have been formed by the northward alongshore current. The lack of stormy sea during these 3 weeks explains the MP deposition at mid-beach level.

The contribution of the Rhône River to MP inputs into the Gulf of Lion and onto these local beaches cannot be ruled out as the southern general circulation combined with eastern winds and waves may beach some floating MPs. The Rhône River is currently the largest Mediterranean river in terms of freshwater and sediment discharges. [START_REF] Sadaoui | Controls, budgets and variability of riverine sediment fluxes to the gulf of lions (nw mediterranean sea)[END_REF] found that coastal rivers contribute on average only to slightly more than 5% of long-term sediment inputs to the Gulf of Lion and the Rhône River for 95%. The Rhône River did not experience any flood during the 6 months preceding the first sampling. Moreover, the periods of outflow peaks coincided with periods of low swell heights that cannot explain the HCZs along the upper part of both beaches. Some changes of shape assemblages due to external forcing have been noticed by [START_REF] Browne | Spatial Patterns of Plastic Debris along Estuarine Shorelines[END_REF], who observed denser MPs in UK down-wind beaches when compared with upwind beaches, and by [START_REF] Fazey | Debris size and buoyancy influence the dispersal distance of stranded litter[END_REF] who observed along the South African coast an increasing shift in the shape distribution with increasing distance from a potential terrestrial source. No obvious relation was found in our study. Shape assemblages were quite similar between the two sampling sites and the two sampling periods. The major change observed was between the HCZs of the two sampling periods at the northern site. At the first sampling period, fragments, foams and fibers were present with similar proportions in the HCZs, while at the second sampling period fibers prevailed. This difference may be due to different transport pathways of different sources of MPs. The combination of precipitation and stormy sea before the first sampling period could have brought high amounts of fragments and foams close to the Têt mouth, whereas deposition of MPs with a majority of fibers at the HCZ of the second sampling period could have resulted from atmospheric fallout [START_REF] Dris | Synthetic fibers in atmospheric fallout: A source of microplastics in the environment?[END_REF], input of wastewater treatment plant effluent in the river [START_REF] Mason | Microplastic pollution is widely detected in us municipal wastewater treatment plant effluent[END_REF] or marine activities [START_REF] Claessens | Occurrence and distribution of microplastics in marine sediments along the belgian coast[END_REF].

Conclusion

This study on beached MPs innovates in performing a high-resolution spatial survey on the micro-tidal Mediterranean Sea, moreover with high methodological concerns. A number of conclusions may be drawn from this study on the spatial distribution of MPs on two markedly different beaches from the Gulf of Lion (NW Mediterranean Sea). Some pertain to the quality assessment of MPs and some concern the level of contamination of these beaches and the role of external forcing, such as rainfall, river flow, wind and swell height on MP distribution.

(1) Including fibers among the MP shapes considered is crucial for any study dealing with beached MPs. Though sorting fibers is a time-consuming operation that may be furthermore affected by contamination during sample treatment, it appears as being excessively important since fibers represented an overall 63% of our 15,664 sorted items.

(2) The analytical confirmation (and identification) of plastic polymers is critical to accurately quantify and qualify MPs. Analyses by FTIR spectrometers of more than 200 suspected MPs revealed that only 45% of them were really plastic polymers. This analytical step is particularly relevant when samples are mainly composed of fibers, which were minorly (37%) made of plastic in this study.

(3) Both NW Mediterranean beaches were affected by MP contamination but their concentrations (means: 166 ± 205 items kg -1 and 58 ± 53 items kg -1 ) varied within the range of concentrations observed so far along the Mediterranean coast. Highest concentrations (up to 800 items kg -1 or 4,650 items m -2 ) are close to the highest Mediterranean beach concentrations (1,512 items kg -1 ; [START_REF] Lots | A large-scale investigation of microplastic contamination: Abundance and characteristics of microplastics in European beach sediment[END_REF] but far from the world most polluted beaches (250,000 items m -2 ; [START_REF] Fok | Hong Kong at the Pearl River Estuary: A hotspot of microplastic pollution[END_REF].

(4) Our high-resolution study highlights the complexity of MP distribution on beaches.

The spatial distribution can be strongly heterogeneous at small scale and can change within a 1-month interval. MP concentrations quickly dropped around HCZ over a distance of only a few tens of meters. Therefore, stranded MP concentrations depend on the sampling strategy, especially along the Mediterranean micro-tidal coast, where the upper deposition line seems to concentrate and to retain MPs.

(5) The geographical context of the beach and the proximity of potential MP sources (e.g. river, urban area) seem to influence the level of contamination by MPs, whereas the morphological setting and external forcing may affect both the quantity and the type of MPs at a small spatial scale. concentration zones (HCZ) and the other sampling quadrats (Baseline) for both study sites. Alongshore transects: "Upper" (U), "Mid" (M) and "Lower" (L). Cross-beach transects: "North" (N), "Mid-North" (MN), "Mid-South" (MS) and "South" (S). See details in Figure 2). Bars represent the standard deviation from the means. ns: non significant difference (p-value >0.05). * : significant (p-value <0.05). ** : Highly significant (p-value <0.01). *** : very highly significant (p-value <0.001). See details about statistical tests in section 2.5. Figure 5. Mean MP concentrations ±1 SD (items kg -1 ) at the northern site calculated for two sampling periods (March 2, 2016;March 30, 2016). Alongshore transects: "Upper" (U), "Mid" (M) and "Lower" (L). Cross-beach transects: "North" (N), "Mid-North" (MN), "Mid-South" (MS) and "South" (S). See details in Figure 2. Figure 6. Mean MP concentrations ±1 SD (items kg -1 ) at the southern site calculated for the two sampling periods (February 22, 2016;March 30, 2016). Alongshore transects: "Upper" (U), "Mid" (M) and "Lower" (L). Cross-beach transects: "North" (N), "Mid-North" (MN), "Mid-South" (MS) and "South" (S). See details in Figure 2.

  All MP contents and relative contents of shapes given in the following text, figures and tables are based on identified MPs. Mean values are provided ± 1 standard error. Finally, concentrations of MPs were estimated by dividing the corrected numbers of identified plastic items by the size of the surface sampled on the beach (items per m -2 ) or by the weight of the dry sand sampled (items per kg -1 ). Statistical analyses were performed using R software (R Core Team, 2018; caption package "captioner": Alathea, 2015; manuscript package "rmarkdown": Allaire et al., 2018; data manipulation package "dplyr": Wickham et al., 2017; graphical package "ggpubr": Kassambara, 2017; graphical package "scales": Wickham, 2017). As normality of distributions was never observed (Shapiro-Wilk test), three non-parametric tests were used: the Wilcoxon-Man-Whitney test to compare means of two groups; the Scheirer-Ray-Hare test to compare variances with two factors and the interaction among factors; and the post hoc Dunn's test to compare differences of all possible pairs and pinpoint which specific means are significantly different from the others.

  Fibers and foams showed skewed unimodal distributions towards larger size classes. Most of the fibers appeared within the median sand size classes (0.5-2.5 mm) and only a few fibers were found within the >2.5 mm size classes. Foams belonged to the largest sand size classes (1-5 mm) with high variations between samples from the northern site. Fragments and films were more evenly distributed in the different classes with higher concentrations within the lowest sand size, especially in the northern site where the lowest sand size class (0.063-0.315 mm) had the highest fragment and film concentrations. Beads only belonged to the upper size classes (> 1 mm).

  mouth at the upper deposition line to the mid-deposition line of the opposite and northernmost cross-beach transect within 1 month. The limited spatial extension of the HCZs is also noteworthy as the high concentrations quickly dropped at a distance of a few tens of meters around. The northern HCZ observed during the second sampling period was located within a beach accretion area generated by a jetty. The medium deposition line of this northernmost cross-beach transect was more impacted by MPs than the high and low alongshore transects likely because the presence of a slight depression on the bern at this line. The high deposition line is probably not a final accumulation zone because previous extensive storm surges washed this flat place and moved the debris towards the vegetated area bordering the beach.The southern site is not directly affected by close river inputs or urbanized areas.Consequently it receives less MPs than the northern site. MP concentrations there are highly dependent on the sampled deposit line, and the high deposit line may be qualified as an HCZ. The presence of a concrete wall and boulders along the upper part of the beach obviously prevents the migration of litter beyond that physical barrier during surge periods and concentrates all types and sizes of floating debris there. MPs found on this site are probably originating from floating MPs drifting with the general circulation. The degradation and fragmentation processes of large plastic debris deposited on the beach can be another source, but probably limited since sporadic cleaning operations are performed during the summer season, limiting the residence time of large debris to less than a year. Moreover,[START_REF] Hinata | An estimation of the average residence times and onshore-offshore diffusivities of beached microplastics based on the population decay of tagged meso-and macrolitter[END_REF] estimated that MPs have shorter residence times on beaches than large plastic litter. They hypothesized that macrolitter is pushed towards the backshore while micro-and mesolitter are backwashed to the sea by waves. Our study therefore points out that in the micro-tidal Mediterranean Sea the small scale spatial distribution of MPs is highly heterogeneous and strongly dependent on the morphological and environmental settings of the studied beaches. Among the few studies carried out at a high spatial resolution,[START_REF] Dekiff | Occurrence and spatial distribution of microplastics in sediments from Norderney[END_REF] found that MPs were rather homogeneously distributed within a beach of Norderney Island, a meso-tidal region of the North Sea, whereas[START_REF] Turra | Three-dimensional distribution of plastic pellets in sandy beaches: Shifting paradigms[END_REF] showed a high spatial variability in the MP distribution within Brazilian beaches.

Figure 1 .

 1 Figure 1. Location maps and main features of the study area. A) General map of the Northwestern

Figure 2 .

 2 Figure 2. Aerial (A, E), on site pictures (B, F) and sketches (C, D, G, H) showing the sampling strategy for

Figure 3 .

 3 Figure 3. Examples of the five microplastic shape categories considered in the present study. A and B)

Figure 4 .

 4 Figure 4. Mean MP concentrations ±1 SD per kg of dry sand (items kg -1 ) and statistical significance of

Figure 7 .

 7 Figure 7. Shape distribution of MPs found on both sites (northern and southern data are pooled; Fig. 1)

Figure 8 .

 8 Figure 8. Mean MP concentrations ±1 SD (items kg -1 ) of the different shape categories (fibers,

Figure 9 .

 9 Figure 9. Meteorological and hydrological conditions prevailing before and during the sampling periods

Table 1 .

 1 Main features of the two sampled beaches in the southwestern part of the Gulf of Lion (NW Mediterranean Sea). Northern and southern sites are located 25 km apart (35 km alongshore).

		Northern site	Southern site
	GPS coordinates 42.716758 N, 3.039624 E	42.499864 N, 3.128756 E
	Beach size	700 m long x 70 m wide (49,000 m 2 )	200 m long x 15 m wide (3,000 m 2 )
	Backshore boundary	Vegetation	Concrete wall and boulders
	Touristic activity High (mainly in summer)	High (mainly in summer)
	River mouth	Têt	None
	Shore type	Flat and sandy	Rocky with sand and gravel
	Alongshore current direction	Northward	Southward

Table 2 .

 2 Mean and range of MP concentrations (items kg -1 and items m -2 ±1 SD) collected within the two sites at both periods.

		Northern site	Southern site
		Sampling 1 Sampling 2 Sampling 1 Sampling 2	Unit
	Mean ± SD	182 ± 233	150 ± 180	60 ± 47	56 ± 61 Items kg -1
		1,100 ± 1,629	731 ± 528	254 ± 141	290 ± 292 Items m -2
	Min	33	33	16	12 Items kg -1
		145	298	92	117 Items m-2
	Max	798	707	154	187 Items kg -1
		4,653	2,258	567	1,152 Items m -2
	Total number
	of counted
	particles	6,366	5,370	1,928	2,000 Items
	Identified
	plastic ratio	52	41	40	44 %
	Total number
	of identified
	plastic items	3,301	2,192	763	871 items

Table 3 .

 3 Factor analyses performed on all MP concentrations from both sites and sampling periods. *: significant (p-value <0.05). **: Highly significant (p-value <0.01). ***: very highly significant (p-value <0.001). See details about statistical tests in section 2.5.

	Factors

Table 4 .

 4 Beached microplastics concentrations and polymeric composition observed along the Mediterranean coastline. Concentration values in brackets are maximum values. Decimal values are rounded to the nearest integer. ND: no data.

	Country Size range Polymer analysis	% Fibers Concentration Reference
	Items kg -1
	Slovenia 0.25-5 mm No	75%	178 (444)	Laglbauer et al., (2014)
	Italia				122
	Spain				152
	France				124
	Italia Turkey	0.3-5 mm	70%PES, 20%PP, 10%PE (n=10)	99%	1,512 248	Lots et al., (2017)
	Greece				232
	Israel				168
	Bosnia				76
			38%PE, 35%PP,
	Italia	ND	12%Nylon, 9%PS, 4%PET, 2%PVC, <1%PUT	10%	12 (22)	Munari et al., (2017)
			(n=80)
			Fragments : 86%PE,
	Tunisia	0.1-5 mm	14%PP; Fibers: 100%PP; Films & pellet: 100%PE;	71-99%	316	Abidli et al., (2018)
			Foams: 100%PS (n=17)
	France	0.063-5 mm	21%PS, 20%PES, 20%A, 12%PE, 10A%, 10%PP, <5% other plastics (n=114)	59% 77%	166 (798) 58 (187)	This study: northern site This study: southern site
	Items m -2
	Greece	1-2 mm 2-4 mm	PE, PP, PET, PS	8% 0%	ND (977) Kaberi et al., (2013) ND (1,218)
	France	0.063-5 mm	<5% other plastics 21%PS, 20%PES, 20%A, 12%PE, 10A%, 10%PP,	59%	915 (4,653)	This study: northern site
			(n=114)	77%	272 (1,152) This study:

Acknowledgement

We acknowledge the "Ecole Doctorale ED305" from the University of Perpignan Via Domitia for funding the PhD grant of Mel Constant and the "Parc naturel marin du golfe du Lion" for the Master grant of Morgan Mino-Vercellio-Verollet. We thank Núria Ferrer from the "Centres Científics i Tecnològics" of the University of Barcelona for her technical support during FTIR analyses. We are also grateful to the "Centre d'études et d'expertise sur les risques, l'environnement, la mobilité et l'aménagement" (CEREMA) for providing us swell data, "Météo-France" for the access to weather data and the Banque Hydro hosted by the "French Ministère de l'Environnement et du Développement durable" for hydrological data. This research has also been supported by a Catalan Government Grups de Recerca Consolidats grant (2014 SGR 1068).