Spatially Explicit Seagrass Extent Mapping Across the Entire Mediterranean

Abstract

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.

Bahamian seagrass extent and blue carbon accounting using Earth Observation

“…On the other hand, both remaining non-spatially-explicit estimates by UNEP-WCMC and Short (2018) and McKenzie et al (2020) feature wide underestimating trends-2,227 and 2,250 km 2 , respectively-approximately one order of magnitude less than our mapped minimum seagrass extent of 39,210 km 2 . This observation corroborates our recent findings in the Mediterranean and East Africa Traganos et al, 2022a;Traganos et al, 2022b) reflecting the potential biases introduced by the utilization of nondata-driven and non-nation-specific mapping designs in blue carbon and, more broadly, coastal ecosystem accounting at both the physical and monetary level in and beyond The Bahamas (Bertram et al, 2021). We expand more into this latter issue in Section 4.4, namely the downstream implication regarding the uptake of such accounts in policy and financial crediting frameworks.…”

“…Utilizing region-specific seagrass carbon data, this spatially-explicit extent translates to a Tier 2 blue carbon assessment up to 771.4 million Mg C and an annual blue carbon sequestration rate of 123 Mt CO 2 (Table 1). The second most important finding is that our seagrass ecosystem accounting efforts enrich our previously published research in 27 different tropical and temperate countries across the Mediterranean, East Africa and the Western Indian Ocean (Lee et al, 2022;Traganos et al, 2022a;Traganos et al, 2022b). The common denominator of all 28 national seagrass ecosystem accounting endeavors is the utilization of a standardized technological Earth Observation framework which supports an effective qualitative and quantitative comparison of its yielded physical accounts by minimizing both technological and environmental differences.…”

“…The synergy of Earth Observation technology and Ecosystem Accounting has more recently showcased promising results towards quantifying the nationwide natural climate solutions of blue carbon ecosystems 4 (Traganos et al, 2022a;Traganos et al, 2022b).…”

“…Our Bahamian seagrass ecosystem accounting increases the total mapped seagrass extent of DLR's Global Seagrass Watch project to around 76,000 km 2 across more than 306,000 km 2 of mapped seabed, 74,000 km of coastline in 28 temperate and tropical countries, and three seagrass bioregions (Lee et al, 2022;Traganos et al, 2022a;Traganos et al, 2022b). All of these high-resolution spatially explicit seagrass ecosystem extent accounts are produced through an ever-advancing standardized Earth Observation framework.…”

“…The lack of recognition and protection arises from insufficient knowledge of spatially explicit seagrass distribution and related carbon accounts, especially in remote and/or data-poor areas (McKenzie et al, 2020;Macreadie et al, 2021). Recent advances in Earth Observation utilize high temporal and spatial resolution satellite data archives, cloud computing, and machine learning to minimize mapping uncertainties (Lyons et al, 2020;Traganos et al, 2022a;Traganos et al, 2022b). This provides an alternative to traditionally expensive and time-consuming surveying methods (Hossain et al, 2015;Veettil et al, 2020).…”