UAV-based seagrass wrack orthophotos classification for estimating blue carbon

Abstract

Seagrass wrack, the detritus from seagrass meadows that accumulates along the coast, is thought to contribute to carbon sequestration in shallow coastal areas when processed into fixed biochar. Accurate and efficient estimation methods for seagrass wrack area size and carbon content are still being explored, especially across multiple seasons. The carbon content of seagrass wrack along the Futtsu coast of Tokyo Bay, Japan, was evaluated using unmanned aerial vehicle (drone) photogrammetry and in-situ measurements of volume and density. A combination of a low-pass filter in pre-processing and the unsupervised iterative self-organizing data analysis technique algorithm (ISODATA) classification method achieved the best classification accuracy when compared with a majority filter and/or any of the three conventional supervised classifiers (support vector machine, random forest, and maximum likelihood classifier). This approach minimizes the manual labor required for determining the area size of the seagrass wrack. The Futtsu tidal flat seagrass wrack carbon for 2020 was estimated to be 3 tons C/yr, accounting for approximately 2% of the seagrass meadow net primary production. These results can greatly assist blue carbon policy implementation.