The resilience of transplanted seagrass traits encourages detection of restoration success.

Restoration of coastal ecosystems, particularly those dominated by seagrasses, has become a priority to recover the important ecosystem services they provide. However, assessing restoration outcomes as a success or failure remains still difficult, probably due to the unique features of seagrass species and the wide portfolio of practices used on transplanting actions. Here, several traits (maximum leaf length, number of leaves, leaf growth rate per shoot, and leaf elemental carbon and nitrogen contents) of transplanted seagrass Posidonia oceanica were compared to reference meadows in five sites of Western Mediterranean Sea in which restoration were completed in different times. Results have evidenced the resilience of transplanted P. oceanica shoots within a few years since restoration, as traits between treatments changed depending on the elapsed time since settlement. The highlighted stability of the restoration time effect suggests that the recovery of the plants is expected in four years after transplanting.

Operationalizing blue carbon principles in France: Methodological developments for Posidonia oceanica seagrass meadows and institutionalization.

Conservation of ecosystems is an important tool for climate change mitigation. Seagrasses, mangroves, saltmarshes and other marine ecosystems have particularly high capacities to sequester and store organic carbon (blue carbon), and are being impacted by human activities. Calls have been made to mainstream blue carbon into policies, including carbon markets. Building on the scientific literature and the French voluntary carbon standard, the 'Label Bas-Carbone', we develop the first method for the conservation of Posidonia oceanica seagrasses using carbon finance. This methodology assesses the emission reduction potential of projects that reduce physical impacts from boating and anchoring. We show how this methodology was institutionalized thanks to a tiered approach on key parameters including carbon stocks, degradation rates, and decomposition rates. We discuss future needs regarding (i) how to strengthen the robustness of the method, and (ii) the expansion of the method to restoration of seagrasses and to other blue carbon ecosystems.

A multi-approach inventory of the blue carbon stocks of Posidonia oceanica seagrass meadows: Large scale application in Calvi Bay (Corsica, NW Mediterranean).

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 (Corg) and inorganic (Cinorg) carbon stocks in the P. oceanica matte of the Calvi Bay (Corsica) using sub-bottom profiler imagery and biogeochemical analysis of sediment cores. The matte thicknesses map (average ± SD: 2.2 m ± 0.4 m) coupled with marine benthic habitat cartography allows to estimate matte volume at 12 473 352 m3. The cumulative stocks were assessed at 20.2-50.3 kg Corg m-2 and 26.6-58.7 kg Cinorg m-2 within the first meter of depth on matte (3632 ± 486 cal yr BP). The data contributed to estimate the overall carbon stocks at 389 994 t Corg and 615 558 t Cinorg, offering a new insight of the heterogeneity of blue carbon stocks in seagrass meadows. Variability of carbon storage capacity of matte influenced by substrate is discussed.

Quantification of blue carbon stocks associated with Posidonia oceanica seagrass meadows in Corsica (NW Mediterranean).

In the last decades, the increasing necessity to reduce atmospheric carbon dioxide (CO2) concentrations has intensified interest in quantifying the capacity of coastal ecosystems to sequester carbon, referred to commonly as 'Blue Carbon' (BC). Among coastal habitats, seagrass meadows are considered as natural carbon sinks due to their capacity to store large amounts of carbon in their sediments over long periods of time. However, the spatial heterogeneity of carbon stocks in seagrass sediments needs to be better understood to improve the accuracy of BC assessments, particularly where there is high environmental variability. In the Mediterranean, Posidonia oceanica (L.) Delile constitutes extensive meadows considered as long-term carbon sinks due to the development of an exceptional structure known as 'matte', reaching several meters in height, which can be preserved over millennia. In order to specify the role of P. oceanica meadows in climate change mitigation, an estimate of carbon stocks has been conducted along the eastern coast of Corsica (NW Mediterranean). The approach is mainly based on the biogeochemical analysis of 39 sediment cores. Organic carbon (Corg; 327 ± 150 t ha-1, mean ± SE) and inorganic carbon stocks (Cinorg; 245 ± 45 t ha-1) show a high variability related to water depth, matrix (sandy vs rocky substrate) or the depositional environment (coastal vs estuary). The isotopic signature (δ13C) revealed a substantial contribution of allochthonous inputs of organic matter (macroalgae and sestonic sources) mainly in estuarine environment and shallow areas. The carbon stocks in the first 250 cm of matte (average thickness) were estimated at 5.6-14.0 million t Corg (study site) and 14.6-36.9 million t Corg (Corsica), corresponding to 11.6-29.2 and 30.4-76.8 years of CO2 emissions from the population of Corsica.

Sizing the carbon sink associated with Posidonia oceanica seagrass meadows using very high-resolution seismic reflection imaging.

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.

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

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 CO2 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).

Seismic interval velocity in the matte of Posidonia oceanica meadows: Towards a non-destructive approach for large-scale assessment of blue carbon stock.

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.