Method for Distinguishing Sargassum and Zostera in the Seto Inland Sea Using Sentinel-2 Data

Abstract

Coastal blue carbon ecosystems are crucial to mitigating global warming. To accurately calculate the blue carbon stock, the existing amount of each species in seaweed and seagrass (SWSG) beds must be estimated to calculate the amount of CO2 absorbed by each species. However, there exists no efficient and comprehensive method for separating SWSG species. Remote sensing techniques hold promise in addressing this issue. This study used satellite Sentinel-2 data to differentiate and map the areas in which Sargassum and Zostera flourish in the Seto Inland Sea. A two-step approach was proposed to separate these algae. First, the SWSG bed area was estimated using the bottom index method, which has been commonly used for sediment mapping. Consequently, using spectral characteristics obtained from field surveys, the Sargassum and Zostera distinguishing index was developed to efficiently separate Sargassum and Zostera. This algorithm was applied to Sentinel 2 data to create a distribution map of Sargassum and Zostera in the Seto Inland Sea. When the map was compared with SWSG bed maps, obtained using field survey-based methods, it showed high credibility, meaning that the proposed method can be used to repeatedly and easily understand seasonal changes in SWSG types in this area in the future.