Assessment of a contemporary Earth System Science approach and Essential Biodiversity Variables for subtidal seagrasses in Mozambique

Kurzfassung

The interest in the blue carbon sequestration provided by seagrasses, mangroves and tidal flats has been increasing in the past few years due to the potential of these nature-based solutions in climate change mitigation within a conservation and restoration context. Despite only covering less than 0.2% of the global seafloor, seagrass meadows store 10% of the world’s atmospheric carbon deep in the soils. The extent of seagrass meadows has been declining at an alarming rate of 1.5%/year on average. Field data collection of the underwater habitats can be costly due to the environmental challenges. Our goal in the Biodiversity of the Coastal Ocean: Monitoring with Earth Observation (BiCOME) project is to develop analysis-ready products that can be used for Essential Biodiversity Variable (EBV) quantification and downstream impact. This study highlights our efforts in mapping the seagrass extent and stored blue carbon across the entire Mozambican coastlines. We processed 9,089 100x100km2 Sentinel-2 image tiles across 34,193km2 of the coastal area in Mozambique acquired between 14 December 2018 and 20 April 2020 with our multitemporal processor to produce an image synthesis with minimized effects from the environmental noises such as waves, sunglint, cloud cover, and turbidity. By using in-situ bathymetry data, and both in-situ and self-annotated habitat data, we mapped the full extent of bathymetry and seagrass habitats in the optically shallow water areas of Mozambique. We mapped the bathymetry using the Clustering-based Lyzenga method, which helps detect the depth at which the seagrass meadows are located. The seagrass extent was mapped by employing the Random Forest machine learning image classification, resulting in 1,779.3 km2 of seagrass meadows, which covers 2.7% of the study area, located between depths of 1.6 and 9.2 m. We estimated the seagrass blue carbon stocks by multiplying the mapped nationwide seagrass extent with their corresponding Tier 1 and Tier 2 carbon stock estimates. Based on our Tier 1 carbon stock assessment, the national seagrass blue carbon stock was estimated at 1.78-147 million Mg and our Tier 2 carbon stock assessment results in 1.64-4.35 million Mg, which highlights a large overestimation of the Tier 1 globally averaged assessment. These analysis-ready products we developed with multidisciplinary methodologies (extent, bathymetry, and carbon stock processors) can be further used to design and develop relevant EBVs for subtidal seagrass meadows. Moreover, the estimated seagrass habitat map can be useful to highlight the importance of seagrass restoration and conservation which can promote more cost-effective and targeted action for coastal biodiversity and blue carbon ecosystem services of seagrasses in tropical regions worldwide.