Earth Observation for Seagrass Blue Carbon Assessment in East Africa

The plan to mitigate the rise in global temperature includes nature-based solutions such as the blue carbon stocks of coastal ecosystems. Seagrass meadows rank among the most efficient organic carbon sinks on Earth. However, due to the high costs of the required equipment and enormous environmental challenges, field data collection of underwater habitats are expensive. In order to fill the knowledge gap, we utilized openly available Earth Observation data and algorithms which enabled us to remotely assess the blue carbon stocks here. In this study, we opted for the Sentinel-2 optical satellite images to map the national extent of seagrass meadows across the East African coastline. We developed and applied our mapping framework in the geospatial cloud platform Google Earth Engine of which processors are capable of processing 16,453 images available from 2018-2020 at 10-m spatial resolution. We estimated a regional extent of 4,316 km^2 of shallow water seagrass meadows along the coastlines of Kenya, Tanzania, Mozambique, and Madagascar with the overall accuracy of 84.3%, up to 23 m of depth. By pairing the country-specific in situ soil carbon data, we estimated regional seagrass blue carbon stocks between 11.2-40.2 million MgC in East Africa. Our regional seagrass map and its resulting blue carbon stock estimates are useful to highlight the importance of seagrass restoration and conservation for coastal biodiversity and blue carbon ecosystem services worldwide