Christine Pergent-Martini

Gérard Pergent

Briac Monnier

Charles-François Boudouresque

Christophe Mori

Audrey Valette-Sansevin

Audrey Valette

Contribution of Posidonia oceanica meadows in the context of climate change mitigation in the Mediterranean Sea

Keywords: Seagrass, Posidonia oceanica, Carbon Fixation, Carbon Sequestration, Mediterranean Sea, Corsica. Introduction

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 importance of ensuring the integrity of all ecosystems, including oceans'. Major carbon sinks such as wetlands, peatlands, coastal vegetation and phytoplankton were, therefore, for the first time taken into account. The concept of carbon sink covers several very different compartments, which are often erroneously confused in the literature. (i) Long-term sinks correspond to carbon which is not re-mineralized after the death of the primary producer and therefore sequestrated for thousand or million years. This sink corresponds to coal, peat and modern deposits. (ii) Short-term sinks correspond to the amount of carbon which is incorporated into living matter, for a few days to millennia, depending upon the life span of the organisms: days for phytoplankton, months for leaves, centuries to millennia for tree trunks. In fact, this corresponds to the biomass. Part of the so-called carbon sequestration by the European forests is just due the expansion of forests, following agricultural decline, and to the ageing of forests, which accumulate carbon in the wood; this carbon is more or less slowly mineralized after the death of the leaves or of the whole plant. (iii) A number of plants and habitats, such as Posidonia oceanica (Linnaeus) Delile (see below), may fall in between, due to the very low rate of re-mineralization of some tissues (rhizomes and roots), accumulated in the soil, able to persist during several millennia. Despite a very small surface area (less than 0.5% of the oceans), coastal vegetation plays a major role in the fixation and sequestration of Blue Carbon (Nellemann et al., 2009;[START_REF] Fourqurean | Seagrass ecosystems as a globally significant carbon stock[END_REF]. In fact, these ecosystems, such as seagrass beds, kelp forests, mangroves, salt marshes and wetlands, are major carbon sinks [START_REF] Chmura | Global carbon sequestration in tidal, saline wetland soils[END_REF][START_REF] Duarte | Major role of marine vegetation on the oceanic carbon cycle[END_REF][START_REF] Bouillon | Mangrove production and carbon sinks: A revision of global budget estimates[END_REF][START_REF] Reed | The management of natural coastal carbon sinks[END_REF]. Carbon sequestration by seagrass is estimated at 15% of the blue carbon while covering only 17.7 to 61.0 million hectares at the biosphere scale [START_REF] Duarte | Seagrass biomass and production: a reassessment[END_REF][START_REF] Spalding | The distribution and status of seagrasses[END_REF][START_REF] Kennedy | Seagrass Meadows[END_REF]UNEP-WCMC, 2018). Among the sixty-four species of seagrass (Guiry & Guiry, 2020), P. oceanica, a Mediterranean endemic species, appears to be the most effective in carbon storage [START_REF] Pergent-Martini | Primary production of Posidonia oceanica in the Mediterranean basin[END_REF]Mateo et al., 2006;[START_REF] Kennedy | Seagrass Meadows[END_REF]Pergent et al.,2014;[START_REF] Jamaludin | Carbon storage and preservation in seagrass meadows[END_REF] The P. oceanica meadow is the only ecosystem able to 'compete' with peatlands (IPS, 2008) and mangroves [START_REF] Bouillon | Mangrove production and carbon sinks: A revision of global budget estimates[END_REF] because it builds a unique structure: the matte [START_REF] Boudouresque | Protection and conservation of Posidonia oceanica meadows[END_REF]. Made up of rhizomes and roots with sediment that fills the interstices, this little putrescible structure can reach several meters in height, and the organic and inorganic carbon inside can persist for millennia [START_REF] Romero | The belowground organs of the Mediterranean seagrass Posidonia oceanica as a biogeochemical sink[END_REF]Mateo et al., 1997;Lo Iacono et al., 2008;[START_REF] Tomasello | Seagrass meadows at the extreme of environmental tolerance: the case of Posidonia oceanica in a semi-enclosed coastal lagoon[END_REF][START_REF] Boudouresque | The necromass of the Posidonia oceanica seagrass meadow: Fate, role, ecosystem services and vulnerability[END_REF]Monnier et al., in press). The surface area covered by P. oceanica meadows in the Mediterranean Sea has been the subject of several estimates (Pasqualini et al., 1998, UNEP-MAP-RAC/SPA, 2009;[START_REF] Giakoumi | Ecoregion-Based Conservation Planning in the Mediterranean: Dealing with Large-Scale Heterogeneity[END_REF][START_REF] Telesca | Seagrass meadows (Posidonia oceanica) distribution and trajectories of change[END_REF]. The scarcity of data for the eastern basin detracts from the accuracy of these estimates, although recent mapping works of [START_REF] Topouzelis | Seagrass mapping in Greek territorial waters using Landsat-8 satellite images[END_REF] and [START_REF] Traganos | Towards global-scale seagrass mapping and monitoring using Sentinel-2 on Google Earth Engine: The case study of the Aegean and Ionian seas[END_REF], for the Greek coastline (32% of the Mediterranean coastline), Akçali et al. (2019) and [START_REF] Duman | Mapping Posidonia oceanica (Linnaeus) Meadows in the Eastern Aegean Sea Coastal Areas of Turkey: Evaluation of Habitat Maps Produced Using the Acoustic Ground-Discrimination Systems[END_REF] for 3 000 km of Turkish Mediterranean coastline, have made it possible to improve the accuracy of the previous estimates.

On the other hand, the increase of the amount of data concerning primary production by P. oceanica at many sites including different locations, depths, meadow structure and human pressures, has made it possible to estimate a probable mean value of the amount of carbon fixed by P. oceanica at the scale of the Mediterranean basin [START_REF] Buia | Primary production and growth dynamics in Posidonia oceanica[END_REF][START_REF] Pergent-Martini | Primary production of Posidonia oceanica in the Mediterranean basin[END_REF]Pergent et al., 1997;[START_REF] Torricelli | Primary leaf production in the Posidonia oceanica meadow of Monterosso al Mare (eastern Ligurian Sea): variations along a depth gradient (in Italian)[END_REF][START_REF] Zupo | A production model for Posidonia oceanica based on temperature[END_REF]Guidetti et al., 2000;Cebrian & Duarte, 2001;[START_REF] Vela | Relevance of Leaf matter loss in the functioning of Posidonia oceanica system[END_REF]. Moreover, the use of lepidochronology associated with the analysis of the elementary composition of tissues allows determination of the part of this carbon sequestered within the matte mainly through rhizomes and dead sheaths [START_REF] Pergent-Martini | Primary production of Posidonia oceanica in the Mediterranean basin[END_REF][START_REF] Pergent-Martini | Primary production of Posidonia oceanica in the Mediterranean basin[END_REF]Cebrian & Duarte, 2001;[START_REF] Gobert | C, N, P concentrations and requirements of flowering Posidonia oceanica shoots[END_REF][START_REF] Vela | Relevance of Leaf matter loss in the functioning of Posidonia oceanica system[END_REF][START_REF] Scartazza | Carbon and nitrogen allocation strategy in Posidonia oceanica is altered by seawater acidification[END_REF]. Based on new data concerning one of the largest P. oceanica meadows in the Mediterranean Sea (100 km of coastline, 20 425 ha; [START_REF] Valette-Sansevin | Continuous mapping of benthic habitats along the coast of Corsica: A tool for the inventory and monitoring of blue carbon ecosystems[END_REF] and the synthesis of available data from the whole of the Mediterranean basin, the aim of this work is (i) to evaluate the amount of carbon fixed each year by P. oceanica and sequestered in the matte, and (ii) to better understand this ecosystem service in relation with the mitigation of the impact of climate change (carbon sink) for each Mediterranean country.

Material and method

The distribution of P. oceanica along the Corsican coast is conventionally subdivided into 14 sectors with similar environmental conditions (Meinesz et al., 1990;Pasqualini et al., 1998;[START_REF] Valette-Sansevin | Continuous mapping of benthic habitats along the coast of Corsica: A tool for the inventory and monitoring of blue carbon ecosystems[END_REF]. The study area is located along the eastern coast of Corsica (Western Mediterranean Sea) characterized by the presence of an extensive P. oceanica meadow, designated as a NATURA 2000 site 'FR9402014 -Grand Herbier de la Côte Orientale' [START_REF] Valette-Sansevin | Continuous mapping of benthic habitats along the coast of Corsica: A tool for the inventory and monitoring of blue carbon ecosystems[END_REF]Fig. 1). Sampling stations are located at 5, 10, 15, 20, 25 and 30 m depth and distributed from north to south. The meadow density (number of shoots per square meter -shoot.m -2 ) was measured for each station (0.16 m 2 quadrat, 20 replicates) while 20 orthotropic shoots of P. oceanica were collected every three months, from July 2015 to July 2016. A phenological analysis [START_REF]Sur une méthode de mesure et de comptage des structures foliaires de Posidonia oceanica (Linnaeus) Delile[END_REF], and a lepidochronological study (Pergent, 1990) were performed. The primary production was estimated according to the crossed lepidochronological method [START_REF] Vela | Fonctionnement et production primaire des herbiers à Posidonia oceanica (L.) Delile en Méditerranée[END_REF]), derived from Pergent & Pergent-Martini (1990). A leaf is formed by a basal non-photosynthetic sheath and a blade. The blade primary production was estimated considering: the average number of annually produced leaves, the average length of the rank 3 adult blades (usually the longest leaf of the shoot), the average width of the rank 1 adult blades, the average density (ratio of leaf area to leaf dry mass) of the rank 1 adult blades (leaf growth completed). The sheath production was estimated considering: the average number of annually produced leaves, the average length, width and density of the rank 1 adult sheaths. Rhizome production was assessed from the segments of the rhizome, with inserted roots, between each pair of dead sheaths of minimum thickness (corresponding to the tissue produced during a one-year period) cut and then dried for 48 h at 70 °C, until constant dry weight was achieved. Hereafter, 'rhizomes' always means 'rhizomes and roots' due to the fact than root tissues are very limited. The portions corresponding to the two most recent years were not considered because their growth is not yet complete [START_REF] Boudouresque | Relation entre la sédimentation et l'allongement des rhizomes orthotropes de Posidonia oceanica dans la baie d'Elbu (Corse)[END_REF]. The carbon content (% C) of adult blades, sheaths and rhizomes, expressed in dry mass (DM) percentage, were determined for each sample using elementary analysis (Elementar Vario MICRO Cube®, Elementar Analysensysteme GmbH, Germany). A 95% confidence interval for each mean was computed. A Shapiro-Wilk test is applied to check the normality of the means; this is one of the most powerful normality tests [START_REF] Ghasemi | Normality Tests for Statistical Analysis: A Guide for Non-Statisticians[END_REF]). Student's t-test is used for comparing the means, while confidence interval (95%) is reported for non-linear regression (Minitab software™). Inter-relationships between variables were investigated by performing Pearson correlation test. The correlation coefficient was calculated together with p-values to determine the significance and strength of each relationship. A significant difference is considered as a p-value<0.05.

Results

Meadow density decreases regularly with depth, from 550.5 shoots.m -2 at -5 m to 106.1 shoots.m -2 at -30 m (Fig. 2). The number of leaves produced each year also exhibits a declining trend from shallow water to deep water (from 8.55 to 6.35 leaves.yr -1 ; Fig. 2). Whatever the depth, the adult blade of rank 1 (external leaf) is on average shorter than the adult blade of rank 3 (t-test, p-value<0.0001) due to the higher percentage of leaves broken by herbivore grazing and water movement. The mean density of the blades of rank 1 and rank 3 is not significantly different (3.68 ± 0.44 and 3.85 ± 0.38 mg DM.cm -2 , respectively; t- test, p-value=0.579). Conversely, the density of blades is always significantly smaller than sheaths (9.11 ± 1.27 mg DM.cm -2 ; t-test, p-value<0.0001). Blade and sheath density decrease significantly with depth (respectively R 2 = 0.98, p-value<0.0001 and R 2 = 0.89, p-value<0.005; Fig. 3). The mean carbon content in the leaves shows no significant difference between blades (41.3 ± 0,4 %) and sheaths (41.1 ± 0.7 %) (t-test, p-value=0.663; Table I). However, the mean carbon content in rhizomes is higher (42.2 ± 0.7 %) and exhibits a significant difference with blades (t-test, p-value=0.022) and sheaths (t-test, p-value=0.024). Blade and sheath production can be estimated for each depth (Table II). The entire leaf production varies between 1 844.3 mg DM.shoot -1 .yr -1 at -5 m and 736.1 mg DM.shoot -1 .yr -1 at -30 m (752.2 to 301.4 mg C.shoot -1 .yr -1 ). Rhizome production varies between 59.2 and 34.8 mg DM.shoot -1 .yr -1 (24.9 to 14.7 mg C.shoot -1 .year -1 ). Total primary production (blades, sheaths and rhizomes) decreases with depth. The mean production, in carbon per shoot, corresponds to 79 % for blades, 17 % for sheaths and 4 % for rhizomes. The mean total carbon fixation (blades, sheaths and rhizomes) varies between 426.6 and 33.5 g C.m -2 with a clear decreasing trend according to depth (Fig. 4).

Discussion

Carbon fixation and sequestration by the P. oceanica meadow, measured in sector 2 (Fig. 1), corresponds to the sector with the largest extension of this habitat (more than 38% of the areas covered by P. oceanica along the Corsican coast; [START_REF] Valette-Sansevin | Continuous mapping of benthic habitats along the coast of Corsica: A tool for the inventory and monitoring of blue carbon ecosystems[END_REF]. Moreover, recorded values are similar to those measured sporadically in the Bay of Calvi, on the western coast of Corsica (Bay, 1984;[START_REF] Pergent-Martini | Primary production of Posidonia oceanica in the Mediterranean basin[END_REF][START_REF] Vela | Relevance of Leaf matter loss in the functioning of Posidonia oceanica system[END_REF].

Based on the values measured in sector 2, it is therefore possible to extrapolate the carbon fixation and sequestration, by P. oceanica meadows, for the entire coast of Corsica (Fig. 4).

The areas covered in each sector as well as their average bathymetric extension provide a basis for determining the carbon fixation and sequestration rate (Table III). Total fixation is estimated at 69 275 t C.yr -1 , an average of 1.29 t C.ha -1 .yr -1 , while sequestration rate in the matte (dead sheaths and rhizomes) is estimated at 14 250 t C.yr -1 , an average of 0.27 t C.ha - 1 .yr -1 (Table III). Although the P. oceanica meadow in sector 2 covers a very large area, it only allows an average fixation rate of 1.08 t C.ha -1 .yr -1 and a sequestration rate of 0.22 t C.ha -1 .yr -1 , the lowest values recorded along the coast of Corsica, because the average bathymetric range of this meadow is the deepest one (-21 m; Table III). This is explained by its deep upper limit due to significant hydrodynamic forces (exposed coast) and of areas unsuitable for P. oceanica, near the mouth of coastal rivers (Fig. 1; Pasqualini et al., 1998;[START_REF] Boudouresque | Protection and conservation of Posidonia oceanica meadows[END_REF]. Conversely, for the sectors where the average bathymetric extension of the meadow is shallower, the carbon fixation rate reaches 1.70 t C.ha -1 .yr -1 and the sequestration rate 0.35 t C.ha -1 .yr -1 (sector 4 = East coast of the Natural Reserve of the Strait of Bonifacio; sector 6 = Gulf of Valinco). The primary production of the P. oceanica meadow has been the subject of numerous studies, but they mainly concern the North-Western Mediterranean Sea (Bay, 1984;[START_REF] Buia | Primary production and growth dynamics in Posidonia oceanica[END_REF][START_REF] Pergent-Martini | Primary production of Posidonia oceanica in the Mediterranean basin[END_REF]Cebrian et al., 1997;[START_REF] Torricelli | Primary leaf production in the Posidonia oceanica meadow of Monterosso al Mare (eastern Ligurian Sea): variations along a depth gradient (in Italian)[END_REF][START_REF] Guidetti | The use of lepidochronology to assess the impact of terrigenous discharges on the primary leaf production of the Mediterranean seagrass Posidonia oceanica[END_REF][START_REF] Romero | Las praderas de Fanerógamas marinas. La producción primaria y su destino. Caractérísticas de los restos de la planta[END_REF][START_REF] Vela | Relevance of Leaf matter loss in the functioning of Posidonia oceanica system[END_REF], even if some studies are available for other areas of the Mediterranean Sea; Algeria [START_REF] Boumaza | Evaluation of past and present primary production in Posidonia oceanica beds from the cove of Kouali (Tipasa, Algeria) using the lepidochronological method[END_REF], Croatia (Bakran-Petricioli & Schultz, 2010), Sicily (La [START_REF] Loggia | Influence of hydrodynamic conditions on the production and fate of Posidonia DFMR Republic of Cyprus[END_REF], Turkey [START_REF] Pergent-Martini | Primary production of Posidonia oceanica in the Mediterranean basin[END_REF]. These data confirm that, for a continuous meadow, primary production (mg DM.shoot -1 ) decreases with depth (Fig. 6). However, for a given depth, the values can be highly variable from one site to another (e.g. between 310 and 2 250 g DM.shoot -1 at -2 m; [START_REF] Boumaza | Evaluation of past and present primary production in Posidonia oceanica beds from the cove of Kouali (Tipasa, Algeria) using the lepidochronological method[END_REF], Vela et al., 2006). The synthesis of a hundred measurements made throughout the Mediterranean Sea and at depths between 0.5 and 32.0 m provides a basis for estimation of the average primary leaf production (in g DM.m -2 ) according to the depth (Fig. 7). For a depth of 15 m, which corresponds to the average depth of P. oceanica meadows in the Mediterranean Sea (mean extension between -2 and -28 m, in UNEP/MAP-RAC/SPA, 2015), annual primary leaf production (blades and sheaths) is estimated at 343 g DM.m -2 . Linéaire (Port-Cros) Linéaire (Corsica) Linéaire (Ischia) Linéaire (Rukavac)

R 2 =0.78 R 2 =0.94 R 2 =0.99 R 2 =0.85
Figure 7: Leaf annual primary production of P. oceanica meadow (g DM.m -2 ) according to depth in the Mediterranean Sea (data from: [START_REF] Wittmann | Effects of cropping on growth in the Mediterranean seagrass Posidonia oceanica (L.) Delile[END_REF]Bedhomme et al., 1983;Bay, 1984;[START_REF] Thelin | Production de feuilles dans un herbier superficiel à Posidonia oceanica évaluée par une méthode dérivée de la méthode de ZIEMAN[END_REF][START_REF] Wittmann | Temporal and morphological variations of growth in a natural stand of Posidonia oceanica[END_REF][START_REF] Esteban | Dinamica, ciclo de hojas y produccion foliar en praderas de Posidonia oceanica del litoral de la comunidad valenciana (Mediterraneo occidental)[END_REF][START_REF] Romero | Primary production of Posidonia oceanica beds in the Medas Islands (Girona, NE Spain). International Workshop on Posidonia beds[END_REF][START_REF] Esteban | Leaf biomass and production of Posidonia oceanica at Spanish Eastern Coast[END_REF][START_REF] Buia | Primary production and growth dynamics in Posidonia oceanica[END_REF][START_REF] Pergent-Martini | Primary production of Posidonia oceanica in the Mediterranean basin[END_REF]Pergent et al., 1997;[START_REF] Torricelli | Primary leaf production in the Posidonia oceanica meadow of Monterosso al Mare (eastern Ligurian Sea): variations along a depth gradient (in Italian)[END_REF][START_REF] Zupo | A production model for Posidonia oceanica based on temperature[END_REF][START_REF] Boumaza | Evaluation of past and present primary production in Posidonia oceanica beds from the cove of Kouali (Tipasa, Algeria) using the lepidochronological method[END_REF][START_REF] Guidetti | Leaf primary production in Posidonia oceanica: two reconstructive aging techniques give similar results[END_REF]Guidetti et al., 2000;[START_REF] Guidetti | The use of lepidochronology to assess the impact of terrigenous discharges on the primary leaf production of the Mediterranean seagrass Posidonia oceanica[END_REF][START_REF] Loggia | Influence of hydrodynamic conditions on the production and fate of Posidonia DFMR Republic of Cyprus[END_REF][START_REF] Vela | Fonctionnement et production primaire des herbiers à Posidonia oceanica (L.) Delile en Méditerranée[END_REF][START_REF] Vela | Relevance of Leaf matter loss in the functioning of Posidonia oceanica system[END_REF]Bakran-Petricioli & Schultz, 2010;this study). Trend curve: y = -227.2 x ln(depth) + 958.4 (R 2 = 0.43). Carbon concentrations in the leaf tissue of P. oceanica show great variability depending on the site (Table IV); thus, they are on average higher in Corsica (38%, [START_REF] Gobert | Variations temporelles de la phénologie et de la composition biochimique de la phanérogame marine Posidonia oceanica en baie de Calvi[END_REF]41.2%, this study), low at Alicante (27.9%, Romero et al., 1998) and intermediate at Ischia (30.6 %, Romero et al., 1992;31.3 %, Scartazza et al., 2017;34.9 %, Garrard, 2013). However, the carbon concentration in the tissues does not seem to be significantly impacted by the depth or the season [START_REF] Garrard | The effect of ocean acidification on plant animal interactions in a Posidonia oceanica meadow[END_REF]; Table I) although Alcovero et al. (2000) highlights a slight variation between winter and spring, probably related to the age of the leaf tissue. The average of the values available in the Mediterranean Sea, for all sites and depths combined, is estimated at 37.8 ± 1.6% for leaf tissue and 42.0 ± 1.1% for rhizomes (Table IV). These values are lower than those measured for terrestrial Magnoliophyta: 47.5% as a mean [START_REF] Dupouey | Carbon stocks and flux in French forests; Stocks et flux de carbone dans les forets francaises[END_REF] and 44% in the case of mangrove trees [START_REF] Bouillon | Mangrove production and carbon sinks: A revision of global budget estimates[END_REF]. The average annual carbon fixation and sequestration rate throughout the Mediterranean basin can be estimated from (i) the annual primary leaf production (blades and sheaths) for the integrating depth of -15 m (343 g DM.m -2 ), (ii) the average proportion of the different tissues produced estimated in this study (blades 79%, sheaths 17%, rhizomes 4%), and (iii) the average carbon concentration of these tissues (37.8 ± 1.6% for leaf tissue and 42.0 ± 1.1% for rhizomes). Thus, the fixation rate of the blades is estimated at 102.4 g C.m -2 .yr -1 , that of the sheaths at 22.0 g C m -2 .yr -1 and that of the rhizomes at 5.8 g C m -2 .yr -1 ; i.e. a total fixation rate of 130.2 g C m -2 .yr -1 and sequestration rate (dead sheaths and rhizomes) of 27.8 g C m -2 .yr -1 . Based on the available meadow areas for each country, to date (UNEP-MAP-SPA/RAC, 2019), it is possible to make a first evaluation of the amount of carbon fixed and sequestered by the P. oceanica meadow each year (Table V). The total amount of carbon fixed is estimated at 3 638 909 tons per year and the amount sequestered at 776 971 tons per year, equivalent respectively to 13 343 878 and 2 849 154 tons per year of CO2 emission/release equivalent. 14 The large Mediterranean islands deserve special attention because the carbon fixation by P. oceanica is higher given the lower population density and the large areas of seagrass beds they shelter (Table VII). The annual carbon fixation is on average 3.1% of CO2 emissions/releases but can reach almost 14.4% for Corsica. Moreover, the percentage of carbon fixation in comparison with carbon release/emission in the large Mediterranean islands appears higher in the Western basin. et al., 2019), 'C Fixation' (total island carbon fixation by P. oceanica in t C.yr -1 ), . 'C Sequestration' (total island carbon sequestration by P. oceanica in t C.yr -1 ), '% CO2 Fixed' (% CO2 fixed by P. oceanica/CO2 released in t CO2.yr -1 ) and 'CO2 Release Inhab' (total island CO2 released per inhabitant in t CO2.yr -1 .inhab -1 according to http://www.globalcarbonatlas.org/en/CO2-emissions). The areas covered by the P. oceanica meadows between 0 and -40 m are significantly greater for the islands located to the north (r = 0.941; p-value<0.01) and to the west (r = 0.904; p-value<0.05) of the Mediterranean Sea (i.e. Sardinia, Corsica, Balearic Islands). Conversely, CO2 emissions per capita/inhabitant are negatively correlated with latitude (r = 0.863; p-value<0.05) reflecting higher CO2 emissions for the inhabitants of the islands in the south of the Mediterranean basin (i.e. Cyprus, Crete). Thus, the northwestern Mediterranean islands, characterized by lower CO2 emissions per capita/inhabitant and greater coverage of P. oceanica meadows within the bathymetric range from 0 to -40 m, appear as sectors where the ratio CO2 fixed / CO2 released is the highest (r = 0.880; p-value<0.05) Finally, the carbon sequestered by P. oceanica meadows in the matte is significantly correlated with the fixation of carbon produced by the plant (r = 0.979; p-value<0.001). [START_REF] Fao | State of Mediterranean Forests 2018.Food and Agriculture Organization of theUnited Nations, Rome and Plan Bleu[END_REF] and more than 20% for the six large Mediterranean islands [START_REF] Fao | State of Mediterranean Forests 2018.Food and Agriculture Organization of theUnited Nations, Rome and Plan Bleu[END_REF]. Morever, the major advantage of the P. oceanica meadow lies in its capacity to store carbon in soil from centuries to millennia. The average carbon stock found in the matte, resulting from the accumulation of 21% of annual primary production, varies between 40 and 237 kg C.m -2 [START_REF] Romero | The belowground organs of the Mediterranean seagrass Posidonia oceanica as a biogeochemical sink[END_REF], Mateo et al. 1997, Serrano et al., 2012;[START_REF] Serrano | Influence of water depth on the carbon sequestration capacity of seagrass[END_REF]. These values are similar or higher than those measured in several terrestrial ecosystems considered to be efficient in carbon storage such as peatlands (120 kg C.m -2 ; [START_REF] Warner | Implications of peat accumulation at Point Escuminac, New Brunswick[END_REF]

Figure 1 :

 1 Figure 1: Left: Corsica island, with sectors 1 through 14. Middle: The studied sector (sector 2). Right: Location of the study sites and sampling stations of P. oceanica shoots.

Figure 2 :

 2 Figure 2: Meadow density (mean number of shoots.m -2 and confidence level) and number of leaves produced annually (mean number per shoot and confidence level) according to depth. Density trend curve: y = -226.1 x ln(depth) + 914.8 (R 2 = 0.91); leaf number trend curve: y = -1.045 x ln(depth) + 10.093 (R 2 = 0.73).

Figure 3 :

 3 Figure 3: Blades and sheaths density (mean and confidence level 95%) according to depth. The high values of the confidence interval are related to seasonal variations in leaf tissue density.

Figure 4 :

 4 Figure 4: Annual mean carbon fixation (dotted line) and sequestration (solid line) by P. oceanica meadow (g C.m -2 ) according to depth; confidence interval (95%) is also reported. Fixation trend curve: y = -202.5 x ln(depth) + 724.6 (R 2 = 0.95); sequestration trend curve: y = -40.5 x ln(depth) + 145.5 (R 2 = 0.98).

Figure 5 :

 5 Figure 5: Annual carbon fixation (A) and sequestration (B) by P. oceanica meadow (g C.m -2 ) according to depth.

Figure 6 :

 6 Figure6: Annual primary production of P. oceanica (mg DM.shoot -1 ) according to depth and site, R 2 = determination coefficient is reported (from[START_REF] Pergent-Martini | Primary production of Posidonia oceanica in the Mediterranean basin[END_REF] Bakran-Petricioli & Schultz, 2010 and this study).

  , wetlands (13-73 kg C.m -2 ; Laffoley & Grimsditch 2009) and the boreal forests (9-34 kg C.m -2 ; Serrano et al. 2014) and Mediterranean forest (5.7 kg C.m -2 ; FAO & Plan Bleu, 2018). Acknowledgments This work would not have been possible without the financial support of the Agence Française pour la Biodiversité, the Collectivité de Corse (PADDUC-CHANGE programme), the Uffiziu di Ambiente di a Corsica. A special thanks to Maria Cristina Buia (Stazione Zoologica Anton Dohrn, Ischia -Italy) and Sylvie Gobert (University of Liège) for providing raw data and Michael Paul for proof-reading the manuscript. References Akçalı B., Kaboğlu G., Guçlusoy H., 2019. A review on Posidonia oceanica (Linnaeus) Delile coverage along the Turkish coasts until 2019. J. Black Sea/Mediterranean Environment, 25(1): 115-124. Alcoverro T., Manzanera M., Romero J., 2000. Nutrient mass balance of the seagrass Posidonia oceanica: the importance of nutrient retranslocation. Marine Ecology Progress Serie, 194: 13-21. Bakran-Petricioli T., Schultz S., 2010. Biometry and leaf lepidochronology of the seagrass Posidonia oceanica in the Croatian Adriatic. Proceedings of the 4 th Mediterranean Symposium on marine vegetation (Yasmine-Hammamet, Tunisia), 2-4 December 2010, El Asmi S., Langar H., Belkacem W (eds.), RAC/SPA publ., Tunis: 28-31. Bay D., 1984. A field study of the growth dynamics and productivity of Posidonia oceanica (L.) Delile in Calvi Bay, Corsica. Aquatic Botany, 20: 43-64. Bédhomme A.L., Thelin I., Boudouresque C.F., 1983. Mesure de la production primaire des feuilles de Posidonia oceanica : modification de la méthode de Zieman. Botanica

Table I :

 I

					Blade R3	Sheaths R1
		16,0
		14,0
		12,0
	Foliar density (mg.cm-2)	6,0 8,0 10,0
		4,0
		2,0
		0,0
		0	5	10	15	20	25	30	35
						Depth (m)
			-5 m			-10 m	-15 m
			Blades	Sheaths		Blades	Sheaths	Blades	Sheaths
		July	41.2 ± 1.9	43.0 ± 2.5	41.9 ± 1.2	43.0 ± 2.2	42.2 ± 0.6	42.6 ± 2.8
	October	41.3 ± 1.1	40.2 ± 1.2	42.9 ± 0.3	41.7 ± 2.3	40.4 ± 1.1	41.0 ± 0.3
	January	40.7 ± 0.2	37.7 ± 3.1	40.6 ± 0.4	38.2 ± 3.7	40.1 ± 0.2	37.6 ± 2.7
		April	40.5 ± 4.2	40.0 ± 3.1	41.7 ± 2.6	40.4 ± 3.9	42.4 ± 4.8	41.6 ± 1.5
			-20 m		-25 m	-30 m
			Blades	Sheaths		Blades	Sheaths	Blades	Sheaths
		July	42.2 ± 0.9	43.2 ± 2.4	41.7 ± 1.1	41.7 ± 3.0	41.5 ± 1.6	39.8 ± 1.6
	October	41.3 ± 0.5	41.8 ± 0.4	41.5 ± 0.8	41.3 ± 0.2	40.4 ± 8.0	45.3 ± 7.3
	January	40.4 ± 0.2	39.0 ± 1.8	39.4 ± 0.2	38.6 ± 5.8	40.5 ± 0.3	40.2 ± 2.7
		April	41.5 ± 1.2	40.0 ± 2.5	41.6 ± 2.7	40.1 ± 5.5	41.0 ± 1.4	40.6 ± 1.3

Carbon content (in %; mean and confidence level 95%) in P. oceanica leaves according to depth and sampling period (July 2015 to July 2016).

Table II :

 II Leaf primary production according to depth at the studied site.

	Depth	Mean leaf	Mean blade	Blade production	Carbon	Blade production
	(m)	number.yr -1	mass (mg DM)	(mg DM.shoot -1 )	(%)	(mg C.shoot -1 )
	5	8.6	177.7	1 519.3	40.9	621.5
	10	7.5	139.5	1 046.3	41.7	436.8
	15	6.6	164.7	1 093.6	41.3	451.4
	20	7.0	121.6	851.2	41.4	352.0
	25	7.3	124.1	909.7	41.0	373.2
	30	6.4	93.8	595.6	40.8	243.1
	Depth	Mean leaf	Mean sheath	Sheath production	Carbon	Sheath production
	(m)	number.yr -1	mass (mg DM)	(mg DM.shoot -1 )	(%)	(mg C.shoot -1 )
	5	8.6	38.0	324.9	40.2	130.7
	10	7.5	33.1	248.6	40.8	101.5
	15	6.6	32.7	217.3	40.7	88.4
	20	7.0	25.2	176.3	41.0	72.2
	25	7.3	26.1	191.6	40.4	77.4
	30	6.4	22.1	140.4	41.5	58.3

Table III :

 III P. oceanica meadow surface area, mean depth (from Valette et al., 2019), carbon fixation (-202.5 x ln(depth) + 724.56 and carbon sequestration (-40.46 x ln(depth) + 145.53) in the different sectors. In grey, sector 2, corresponding to the studied site, *= updated in this study; C = carbon.

		P. oceanica surface area (ha)	Mean depth (m)	Mean C fixation (t C.ha -1 .yr -1 )	Carbon fixation (t C.yr -1 )	Mean C sequestration (t C.ha -1 .yr -1 )	Carbon sequestration (t C.yr -1 )
	Sector 1	4 137	20	1.21	5 005	0.25	1 032
	Sector 2	20 425	21*	1.08	22 068	0.22	4 565
	Sector 3	3 078	19	1.34	4 116	0.27	846
	Sector 4	6 179	16	1.68	10 395	0.34	2 124
	Sector 5	3 534	18	1.42	5 001	0.29	1 026
	Sector 6	1 430	16	1.70	2 424	0.35	495
	Sector 7	2 765	18	1.38	3 804	0.28	781
	Sector 8	2 981	18	1.40	4 168	0.29	856
	Sector 9	763	20	1.21	927	0.25	191
	Sector 10	719	18	1.44	1 038	0.30	213
	Sector 11	1 505	18	1.35	2 029	0.28	417
	Sector 12	2 940	20	1.21	3 572	0.25	736
	Sector 13	1 784	17	1.55	2 766	0.32	566
	Sector 14	1 497	19	1.31	1 961	0.27	403

Table IV :

 IV Carbon concentration (%) in leaf tissue and in the rhizomes of P. oceanica in the

	2500 France / Corsica	10.0		32.1			Gobert et al., 2005
	France / Corsica	11.0		35.6			Leoni et al., 2007
	France / Provence	2.0		38.2			Vela, 2006
	500 1000 1500 2000 France / Corsica Italy / Sardinia Tunisia / Cap Bon Tunisia / South Italy / Ischia Italy / Ischia France / Corsica Primary production (g DM.m-2.an-1) France / Corsica	2.0 2.0 2.0 2.0 3.0 3.0 5.0 -30.0 8.0 -37.0		38.7 38.0 37.7 37.4 33.5 -35.8 31.3 37.6 -43.2 35.1 -42.4	38.3 38.0 -45.8	Vela, 2006 Vela, 2006 Vela, 2006 Vela, 2006 Garrard, 2013 Scartazza et al., 2017 This study Gobert unpublished
	0
	0	5	10	15	20	25	30	35
				Depth (m)
	Mediterranean Sea.
	Country / Location	Depth (m)		% C in leaves	% C in rhizome	References
	Spain / Catalonia	5.0 -13.0		34.1 -35.3	35.6		Romero, 1989
	Italy / Ischia	20.0		30.6	35.7		Romero et al., 1992
	Spain	0.5 -15		38.1			Enriquez et al., 1995
	France / Corsica	10.0		30.5 -45.5			Gobert et al., 1995
	Spain / Alicante	5.0 -20.0		26.9 -29.0	38.1 -38.6	Romero et al., 1998
	Spain / Catalonia	5.0		30.5 -35.0	39.0		Alcovero et al., 2000
	Spain / Murcia	7.5		40.0			Ruiz, 2000

Table V :

 V Annual carbon fixation and sequestration by P. oceanica meadows in different Mediterranean countries. * UNEP-MAP-SPA/RAC, 2019 ; ** Telesca et al., 2015 ; *** Duman et al., 2019 ; **** Pergent-Martini et al., 2013. Fixation and sequestration of carbon in P. oceanica meadows can present a significant part 323 of the CO2 emissions/releases of the various Mediterranean countries (Table VI). Indeed, 324 even if this fixation represents only 0.61% on average for all Mediterranean countries, it 325 must be borne in mind that (i) seven countries, some of which have a large population (e.g. 326 Egypt, Morocco), have not identified any P. oceanica meadow, and (ii) that the length of the 327 Mediterranean coast is sometimes very limited compared to the surface area of the country 328 (e.g. Slovenia, Turkey, Morocco). However, this fixation may be high for certain countries 329 (e.g. 24.6% for Tunisia, 5.6% for Croatia). 330 331

	Syria, Lebanon,
	Palestinian territories,	Not present
	Israel
	Egypt	No data		0
	Libya	1 235	11 %	11 227	14 618	3 121
	Tunisia	1 332 815	81 %	1 645 451	2 142 377	457 435
	Algeria	4 085	16 %	25 531	33 241	7 098
	Morocco	Not present	100 %	0
	Total	2 216 116		2 794 861	3 638 909	776 974
	322
		Available P.	Surface	Estimated P.	Estimated C	Estimated
		oceanica	mapped	oceanica	fixation	sequestratio
		surface area	(%)**	surface (ha)	(t C.yr -1 )	n
		(ha)*				(t C.yr -1 )
	Spain	115 904	100 %	115 904	150 907	32 221
	France	87 680	100 %	87 680	114 159	24 379
	Monaco	14	100 %	14	18	4
	Italy	337 611	100 %	337 611	439 570	93 856
	Slovenia	1	100 %	1	1	0
	Croatia	31 437	14 %	224 550	292 364	62 425
	Bosnia Herzegovina	Not present	100 %*	0
	Montenegro	51	10 %*	512	667	142
	Albania	4 803	100 %	4 803	6 254	1 335
	Malta	5 860	100 %	5 860	7 630	1 629
	Greece	251 010	100 %*	251 010	326 815	69 781
	Turkey	39 983	58 %***	68 937	89 756	19 164
	Cyprus	3 627	23 %****	15 770	20 533	4 384

Table VI :

 VI Carbon fixation in the different Mediterranean countries. Meadow surface area is 332 estimated from the UNEP-MAP-SPA/RAC (2019), 2018 population and national CO2 emissions from 333 the site http://www.globalcarbonatlas.org/en/CO2-emissions.

	334
		Estimated	Estimated CO2	Population	Estimated CO2	CO2 fixation
		C fixation	release*	(inhabitant)	release	by P. oceanica
		(t C)	(Mt yr -1 )		(t CO2.inhab -1 )	(%)
	Spain	150 907	268.0	46 692 858	5.74	0.21
	France and Monaco	114 178	338.0	64 990 511	5.20	0.12
	Italy	439 570	338.0	60 627 291	5.58	0.48
	Slovenia	1.3	14.0	2 077 837	6.74	0.00
	Croatia	292 364	19.0	4 156 405	4.57	5.64
	Bosnia Herzegovina	0	22.0	3 323 925	6.62	0.00
	Montenegro	667	2.0	627 809	3.19	0.12
	Albania	6 254	4.6	2 882 740	1.60	0.50
	Malte	7 630	1.6	439 248	3.64	1.75
	Greece	326 815	74.0	10 522 246	7.03	1.62
	Turkey	89 756	428.0	82 340 088	5.20	0.08
	Cyprus	20 533	7.5	1 189 265	6.31	1.00
	Syria	0	28.0	16 945 057	1.65	0.00
	Lebanon	0	24.0	6 859 408	3.50	0.00
	Ter. Palestinians	0	3.2	4 862 979	0.66	0.00
	Israel	0	64.0	8 381 516	7.64	0.00
	Egypt	0	239.0	98 423 598	2.43	0.00
	Libya	14 618	54.0	6 678 559	8.09	0.10
	Tunisia	2 142 377	32.0	11 565 201	2.77	24.55
	Algeria	33 241	156.0	42 228 408	3.69	0.08
	Morocco		66.0	36 029 093	1.83	0.00
	Total (mean)	3 638 909	2 182.9	511 844 042	4.26	0.61

Table VII :

 VII Carbon fixation in the large Mediterranean islands. Meadow surfaces are estimated from the report of the UNEP-MAP-SPA/RAC (2019) and from Valette et al. (2019); national CO2 emissions from the site http://www.globalcarbonatlas.org/en/CO2-emissions; * : This study. Inter-relationships between variables were investigated by performing a Pearson correlation test (Table VIII); variables correspond to 'Po cover 0-40' (% P. oceanica surface covered/total surface available between 0 and -40 m in Valette

		Surface P.	Carbon fixation	Release	Fixation by P.
		oceanica (ha)	(t C.yr -1 )	(t CO2.yr -1 )	oceanica (%)
	Balearic Islands	63 316	82 437	6 818 687	4.4
	Corsica	53 736	69 275	1 763 983	14.4
	Sardinia	153 382	199 703	9 143 077	8.0
	Sicily	76 000	98 952	27 875 235	1.3
	Cyprus	15 770	20 533	7 500 000	1.0
	Crete	14 500	18 879	4 465 284	1.6
	Total	396 927	516 799	59 166 267	3.1

Table VIII :

 VIII Pearson's correlation matrix between geographical, environmental and carbon variables. P. oceanica is high with an average of 1 302 t C ha -1 .yr -1 . Even if the areas covered by P. oceanica meadows are smaller than those of other terrestrial ecosystems, this ecosystem represents around 4% of the areas covered by forest ecosystems in all Mediterranean countries

	Latitude	*		0.941	0.544	0.547	0.880	-0.863
	Po cover 0-40	*	**		0.473	0.511	0.794	-0.681
	C Fixation	NS	NS	NS		0.979	0.316	-0.594
	C Sequestration	NS	NS	NS	***		0.312	-0.523
	% CO2 Fixed	NS	*	NS	NS	NS		-0.673
	CO2 Release Inhab	NS	*	NS	NS	NS	NS
	Carbon fixation by
	Level of significance: *p≤0.05, **p≤0.01, ***p≤0.001; NS, p≥0.05; Significant correlations in bold (r
	value).
	Variables	Longitude Latitude	Po cover 0-40	C Fixation	C Sequestration	% CO2 Fixed	CO2 Release Inhab
	Longitude		-0.854	-0.904	-0.574	-0.621	-0.529	0.720

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