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In Mediterranean, Posidonia oceanica develops a belowground complex structure ('matte') able to store large amounts of carbon over thousands of years. The inventory of blue carbon stocks requires the coupling of mapping techniques and in situ sediment sampling to assess the size and the variability of these stocks. This study aims to quantify the organic (C

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The anchoring of large recreational craft constitutes one of the main threats in shallow marine habitats. In the Mediterranean, this activity has seen constant development during the last decades, causing major physical disturbances in Posidonia oceanica meadows and associated ecosystem services. In this context, the main aims of the present study are to estimate the impact of this anchoring on P. oceanica meadows surface areas and carbon fixation and sequestration capacities, in a particularly highly-frequented area (Sant'Amanza bay, Corsica Island). Accurate benthic marine habitats mapping was performed yearly (2019–2021) using a drone coupled with ground-truthing data. The evaluation of carbon fixation and sequestration by the plant was measured at 12 stations within the bay (−5 m to −30 m). The maps of the marine habitats reveal an extensive regression of P. oceanica meadows (7.5 ha) between 2019 and 2020. This destruction represents a 9% decline in the total carbon fixation and sequestration performed each year by P. oceanica meadows within the bay. The related ecosystem services loss is estimated at 4.72 million € yr−1. Although an overall decline of boat anchoring in the seagrass meadow has been observed (e.g. recent enforcement of anchoring regulations), other solutions should be experimented to manage this major carbon sink.

Among blue carbon ecosystems, seagrass meadows have been highlighted for their contribution to the ocean carbon cycle and climate change mitigation derived from their capacity to store large amounts of carbon over long periods of time in their sediments. Most of the available estimates of carbon stocks beneath seagrass meadows are based on the analysis of short sediment cores in very limited numbers. In this study, high-resolution seismic reflection techniques were applied to obtain an accurate estimate of the potential size of the organic deposit underlying the meadows of the Mediterranean seagrass Posidonia oceanica (known as ‘matte’). Seismic profiles were collected over 1380 km of the eastern continental shelf of Corsica (France, Mediterranean Sea) to perform a large-scale inventory of the carbon stock stored in sediments. The seismic data were ground-truthed by sampling sediment cores and using calibrated seismo-acoustic surveys. The data interpolation map highlighted a strong spatial heterogeneity of the matte thickness. The height of the matte at the site was estimated at 251.9 cm, being maximum in shallow waters (10–20 m depth), near river mouths and lagoon outlets, where the thickness reached up to 867 cm. Radiocarbon dates revealed the presence of seagrass meadows since the mid-Holocene (7000–9000 cal yr BP). Through the top meter of soil, the matte age was estimated at 1656 ± 528 cal yr BP. The accretion rate showed a high variability resulting from the interplay of multiple factors. Based on the surface area occupied by the meadows, the average matte thickness underneath them and the carbon content, the matte volume and total Corg stock were estimated at 403.5 ± 49.4 million m3 and 15.6 ± 2.2 million t Corg, respectively. These results confirm the need for the application of large-scale methods to estimate the size of the carbon sink associated with seagrass meadows worldwide., This work would not have been possible without the participation of the oceanographic research vessel L'Europe (IFREMER) and its crew (GENAVIR), provided by the Flotte Océanographique Française for the CoralCorse, PosidCorse and Carbonsink surveys. This research was financially supported by the Office Français de la Biodiversité (AAMP/15/065 and UCPP 2510-AFB/2018/274), the Collectivité de Corse (PADDUC-CHANGE program; 17-DESR-SR-87), the Office de l’Environnement de la Corse (UCPP 2019-156) and the Direction Régionale de l’Environnement, de l’Aménagement et du Logement de Corse (2015073–0001). This research was part of the Interreg Italy-France Marittimo 2014–2020 cooperation program - GIREPAM project (E76J16001050007). The authors would like to express their gratitude to PhD researcher C. Luzzu (Biosurvey Company), and researchers from the University of Palermo for their contributions to the seismic acquisition during Sismat survey as well as the University Grant program of the Information Handling Services company (IHS Inc. www.ihs.com) by providing the access to the Kingdom PAKaged Suite + software.

Este artículo contiene 15 páginas, 8 figuras, 4 tablas., Along most Mediterranean coasts, the endemic seagrass species Posidonia oceanica builds extensive meadows and complex peat-like bioconstruction known as ‘mattes’. These belowground deposits are recognized as a valuable long-term archive allowing the reconstruction and the study of palaeo-climatic and palaeo-ecological changes in the coastal environment over the Holocene period. One of the P. oceanica matte cores sampled during a coring survey along the eastern continental shelf of Corsica Island (France, NW Mediterranean) revealed the unprecedented finding of a dead bank of the scleractinian coral Cladocora caespitosa embedded in the matte. Measurement of the morphological and biometrical features of corallite fragments coupled to biogeosedimentological analysis and radiocarbon dating contributed to provide a basis for the reconstruction of the stratigraphic sequence since the mid-Holocene (last 4750 years). The study of the sediment core enabled identification of three major phases: (i) the settlement of the C. caespitosa colonies (~4750-3930 cal yr BP), (ii) the coexistence of the C. caespitosa bank and the P. oceanica meadow (~3930-1410 cal yr BP), followed by (iii) the death of the coral bank and the development of only the P. oceanica meadow (~1410 cal yr BP-present). The sclerochronological analysis completed on the well-preserved corallite fragments revealed that the mean annual growth rate of the coral ranged between 1.9 and 3.1 mm yr− 1 with a mean value estimated at 2.3 ± 0.8 mm yr− 1 . Trend analysis showed semi-millennial to millennial oscillations in annual growth rates which are probably related to environmental climatic changes since the Cold Phase of the Subatlantic period (2925-2200 cal yr BP). During the Roman Warm Period (2200-1500 cal yr BP), the decline and the death of the bank (~1410 cal yr BP) was probably due to the combined effect of a prolonged increase in summer temperatures and an increase in the competition with the P. oceanica meadow., This research was financially supported by the Office Français de la Biodiversit´e (UCPP 2510-AFB/2018/274), the Collectivit´e de Corse (PADDUC-CHANGE program; 17-DESR-SR-87) and the Office de l’Environnement de la Corse (UCPP2019-156). This research was part of the Interreg Italy-France Marittimo 2014–2020 cooperation program - GIREPAM project (E76J16001050007).

International audience; Coastal marine vegetation has been recently highlighted for its highly efficient carbon storage capacity. Among the sixty-four species of seagrass, Posidonia oceanica, a Mediterranean endemic species, appears to be the most effective in carbon fixation and storage. Based on new data from the study of one of the largest P. oceanica meadows in the Mediterranean Sea (100 km of coastline, 20 425 ha), and a synthesis of available data from the whole of the Mediterranean basin, the aim of this work is to evaluate the amount of carbon fixed each year by P. oceanica and sequestered in the matte, in relation with the mitigation of the impact of climate change (carbon sink). The mean total carbon fixation (blades, sheaths and rhizomes) per year varies between 33.5 and 426.6 g C.m − 2 and the mean carbon sequestration (long-term sink in the matte), corresponding to the sheath and rhizome tissues, varies between 7.7 and 84.4 g C.m − 2 , with a clear decreasing trend according to depth because of the meadow density decrease. 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 estimating the average annual carbon fixation and sequestration rate throughout the Mediterranean basin. The fixation of the blades is estimated at 1 024 t C.ha − 1 .yr − 1 , that of the sheaths at 220 t C ha − 1 .yr − 1 and that of the rhizomes at 58 t C ha − 1 .yr − 1 ; i.e. a total fixation rate of 1 302 t C ha − 1 .yr − 1 and sequestration rate (dead sheaths and rhizomes) of 278 t C ha − 1 .yr − 1. This annual carbon fixation represents only 0.61% on average of CO 2 emissions/releases for all Mediterranean countries but in the large Mediterranean islands this fixation is on average 3.1% and can reach almost 14.4% for Corsica. Moreover, the major advantage of the P. oceanica meadow lies in its capacity to store carbon from annual carbon sequestration for centuries to millennia and can be compared to several terrestrial ecosystems considered to be efficient in carbon storage (peatlands).

Over the past few years, the mapping of Mediterranean marine habitats has become a priority for scientists, environment managers and stakeholders, in particular in order to comply with European directives (Water Framework Directive and Marine Strategy Framework Directive) and to implement legislation to ensure their conservation. Side-scan sonar (SSS) is recognised as one of the most effective tool for underwater mapping. However, interpretation of acoustic data (sonograms) requires extensive field calibration and the ground-truthing process remains essential. Several techniques are commonly used, with sampling methods involving grabs, scuba diving observations or Remotely Operated Vehicle (ROV) underwater video recordings. All these techniques are time consuming, expensive and only provide sporadic informations. In the present study, the possibility of coupling a camera with a SSS and acquiring underwater videos in a continuous way has been tested. During the ‘PosidCorse’ oceanographic survey carried out along the eastern coast of Corsica, optical and acoustic data were respectively obtained using a GoPro™ camera and a Klein 3000™ SSS. Thereby, five profiles were performed between 10 and 50 m depth, corresponding to more than 20 km of data acquisition. The vertical images recorded with the camera fixed under the SSS and positioned facing downwards provided photo mosaics of very good quality corresponding to the entire sonograms's blind band. From the photo mosaics, 94% of the different bottom types and main habitats have been identified; specific structures linked to hydrodynamics conditions, anthropic and biological activities have also been observed as well as the substrate on which the Posidonia oceanica meadow grows. The association between acoustic data and underwater videos has proved to be a non-destructive and cost-effective method for ground-truthing in marine habitats mapping. Nevertheless, in order to optimize the results over the next surveys, certain limitations will need to be remedied.

High-resolution seismic reflection data have been used over the last decades to estimate the thickness of the long-term Blue Carbon sink associated to the below-ground sediment deposit (matte) of the Posidonia oceanica meadows. Time-to-depth conversion of these geophysical datasets was usually performed assuming a sound velocity in this structure, but appropriate seismic interval velocity measurements is necessary to achieve accurate calibration. This study describes the first methodology to estimate the seismic interval velocity in the matte. This approach performed on the eastern continental shelf of Corsica island (France, NW Mediterranean) is based on measurements of the vertical matte profile from high-resolution seismic reflection profiles (s TWTT) and from seafloor morpho-bathymetric DTM (multibeam echosounders - MBES and Light Detection and Ranging - LiDAR surveys) calibrated with ground-truthing data. A biogeosedimentological analysis of horizontal cores sampled in vertical matte escarpments has been undertaken to identify the potential relationship of sediment and environmental parameters with sound velocity. The cross-comparison and the data intercalibration show significant correlation of MBES (R2 = 0.872) and LiDAR datasets (R2 = 0.883) with direct underwater measurements. Seismic interval velocities (n = 367) have been found to range between 1631.9 and 1696.8 m s−1 (95% confidence interval) and are estimated on average at 1664.4 m s−1, which is similar to the literature for unconsolidated marine sediments. The prediction map provided by the ordinary kriging method emphasized, however, a high variability of sound velocity within the study area. The results showed that changes in sound velocity in the matte are positively and strongly correlated with sand and gravel content and environmental factors such as distance to coastal river mouths and coastline. However, it was found that a negative relationship linked sound velocity with total and coarse organic content of matte deposits.