Spatially Explicit Seagrass Extent Mapping Across the Entire Mediterranean

Kurzfassung

The Posidonia oceanica seagrass is the foundation species of the coastal Mediterranean, whose meadows support significant ecosystem services: food security, coastal protection, biodiversity maintenance, carbon sequestration, amongst others. This endemic in the basin seagrass features the largest carbon storage among seagrasses globally, contributing substantially to global blue carbon stocks.  However, climate change, coastal development, and decreasing water quality all render this slow-growing species at risk of area loss, functional extinction, and, hence, its provided services. This risk is further complicated by the current knowledge gaps in its bioregional extent, necessitating accurate, efficient and spatially explicit mapping and accounting of its distribution and trajectories at a high spatial resolution. Here, we leveraged recent Earth Observation advances—cloud computing, open satellite data, and machine learning—with field data via a cloud-based ecosystem accounting framework to map the spatially-explicit ecosystem extent of P. oceanica seagrass across the whole Mediterranean, at 10m resolution.  Employing 279,186 Sentinel-2 satellite images between 2015-2019, and a human-labelled training dataset of 62,928 pixels, we mapped 19,020 km2 of P. oceanica seagrass area in 22 countries across 56,783 km2 of mapped seabed between 0-25 m of depth. Based on 2,480 independent field-based points, we observe an overall accuracy of 72%. Using a Tier 2 assessment, we estimated the bioregional blue carbon storage of P. oceanica beds to be 722.2 million MgC.  As reference data collections, remote sensing technology and biophysical modelling improve and coalesce, such extent accounts could support physical and monetary accounting of seagrass condition and ecosystem services. We envisage that such holistic seagrass ecosystem accounts could enable effective policy uptake in national climate, biodiversity and protection strategies and necessary financing. This in turn could accelerate transparent natural climate solutions and coastal resilience, beyond the physical location of seagrass beds and the 21th century.