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The seagrass Posidonia oceanica is the main habitat-forming species of the coastal Mediterranean, providing millennial-scale ecosystem services including habitat provisioning, biodiversity maintenance, food security, coastal protection, and carbon sequestration. Meadows of this endemic seagrass species represent the largest carbon storage among seagrasses around the world, largely contributing to global blue carbon stocks. Yet, the slow growth of this temperate species and the extreme projected temperature and sea-level rise due to climate change increase the risk of reduction and loss of these services. Currently, there are knowledge gaps in its basin-wide spatially explicit extent and relevant accounting, therefore accurate and efficient mapping of its distribution and trajectories of change is needed. Here, we leveraged contemporary advances in Earth Observation—cloud computing, open satellite data, and machine learning—with field observations through a cloud-native geoprocessing framework to account the spatially explicit ecosystem extent of P. oceanica seagrass across its full bioregional scale. Employing 279,186 Sentinel-2 satellite images between 2015 and 2019, and a human-labeled training dataset of 62,928 pixels, we mapped 19,020 km2 of P. oceanica meadows up to 25 m of depth in 22 Mediterranean countries, across a total seabed area of 56,783 km2. Using 2,480 independent, field-based points, we observe an overall accuracy of 72%. We include and discuss global and region-specific seagrass blue carbon stocks using our bioregional seagrass extent estimate. As reference data collections, remote sensing technology and biophysical modelling improve and coalesce, such spatial ecosystem extent accounts could further support physical and monetary accounting of seagrass condition and ecosystem services, like blue carbon and coastal biodiversity. We envisage that effective policy uptake of these holistic seagrass accounts in national climate strategies and financing could accelerate transparent natural climate solutions and coastal resilience, far beyond the physical location of seagrass beds.

International audience

7 páginas, 5 figuras, 5 tablas., The pervasive use of ecological indices is increasingly requiring actions of harmonisation. Specifically, within the EU Water Framework Directive, an important effort in methods intercalibration is being done. However, a significant limitation in comparability assessment arises from the datasets used, which have different geographic origins. The purpose of our study was to perform an experimental intercalibration, where data were collected specifically on a set of common sites and following all the requirements of the methods being assessed. Three indices based on the marine angiosperm Posidonia oceanica, the POMI, the BiPo and the PoSte, were applied to sites in three different geographical areas of the western Mediterranean: Catalonia, Corsica and Southern Italy (Ischia), distant between hundreds and more than thousands of kilometers. Two indices, POMI and BiPo, showed not only a very good relationship with human pressures (measured on a common scale for all sites) but also a high comparability, in all aspects investigated. The differences found for the third one (PoSte) are hypothesised as being due to a different rationale used to define reference conditions, the different metrics used in the index, and in particular to a different definition of ecological status in relation to the time scale of the response to anthropogenic pressures. Our study demonstrates that indices with very different approaches can provide fully reliable and comparable results., This study was supported by the POSIDONIA Project, set up within the framework of the Interreg IIIB–MEDOCC Convention. The laboratory of benthic ecology of the Stazione Zoologica di Napoli was supported by a research contract of the Italian Environmental Protection Agency (APAT), for the “Development of methods and analysis of P. oceanica, according to the Water Framework Directive” (APAT, 2005–2006). We are indebted to Mian Vich

International audience; The seagrass Posidonia oceanica is extensively monitored in Mediterranean coastal waters and is an ideal candidate for an eco-regional assessment of the coastal ecosystem. The aim of this study is to evaluate the potential of P. oceanica as eco-regional indicator for its assessment at the scale of Mediterranean basin. For this purpose, regional and national P. oceanica monitoring programmes are identified, and their data and metadata are collected and compared in terms of objectives, strategies, sampling designs and sampling methods. The analysis identifies a number of issues concerning data quality, reliability and comparability. In particular, the adoption of different sampling designs and methods may introduce relevant errors when comparing data. The results of this study stress the necessity of carefully planning monitoring programmes. Moreover, it highlights that the adoption of a number of common tools would facilitate all Mediterranean monitoring activities and allows an optimisation of management efforts at an eco-regional scale.

International audience; The Biotic Index based on Posidonia oceanica (BiPo) is a classification system for evaluation of the ecological status in Mediterranean coastal waters, developed in accordance with the EU Water Framework requirements. The aim of this study is to verify the applicability and reliability of the BiPo index to different geographical areas of the north-western Mediterranean (France, Spain and Italy), to understand whether such a classification system may be applied more extensively, as so far it has only been applied to coastal waters in Corsica. The ecological status determined for sites is verified against pressures revealed from satellite imagery and from trace metal contamination of plants, to identify the sources of pressure that may be responsible for a low ecological status. The results of this study indicate that: (i) the BiPo index responds reliably to pressures, in different areas of the Mediterranean; (ii) sites with an ecological quality ratio (EQR) close to the good/moderate boundary require particular attention to identify and reduce causes of deterioration; (iii) the support of chemical indicators, in this case metal contamination, is relevant to identify potential sources of pressure.

International audience; The aim of this study is to assess the relevance and the potential of Posidonia oceanica as a tracer of past mercury contamination. Shoots were collected on two sites, an impacted site, Rosignano (Tuscany, Italy), and a pristine site, Tonnara (Corsica, France). Lepidochronology was used to measure mercury concentrations in living sheaths and in the corresponding dead sheaths. The results show that there is an overestimation of mercury concentrations in dead sheaths (because of the degradation of this tissue due to its ageing), overestimation which stabilizes itself from the third lepidochronological year onwards (trend significant only for Rosignano). Thus, it is possible to estimate previous mercury concentrations and to date a contamination, by measuring mercury concentration in the dead sheaths of a given lepidochronological year and by taking into account the degradation of the sheaths. Therefore, Posidonia oceanica can be used to reconstruct the evolution of the past mercury contamination

International audience; The aim of this study is to determine metal (Cd, Co, Cr, Hg, Ni, Pb) concentrations in Posidonia oceanica tissues along the Corsican coastline. The results show that except for Cr, all the metals are preferentially accumulated in the blades; this is particularly interesting as it means that future metal analyses may be carried out only on the blades avoiding thus the removal of the shoots. Moreover, they show that metal concentrations may reflect the ''background noise'' of the Mediterranean Sea. Station 15 (Canari) can however be distinguished from the others due to its high Co, Cr and Ni concentrations. This result may be related to the presence of a previous asbestos mine, located near this station. Therefore, this study reinforces the usefulness and the relevance of Posidonia oceanica as a tracer of spatial metal contamination and as an interesting tool for water quality evaluation.

International audience; Little doubt is left that climate change is underway, strongly affecting the Earth's biodiversity. Some of the greatest challenges ahead concern the marine realm, but it is unclear to what extent changes will affect marine ecosystems. The Mediterranean Sea could give us some of the answers. Data recovered from its shores and depths have shown that sea temperatures are steadily increasing, extreme climatic events and related disease outbreaks are becoming more frequent, faunas are shifting, and invasive species are spreading. This miniature ocean can serve as a giant mesocosm of the world's oceans, with various sources of disturbances interacting synergistically and therefore providing an insight into a major unknown: how resilient are marine ecosystems, and how will their current functioning be modified?