Long-time monitoring of seagrass beds on the east coast of Hainan Island based on remote sensing images

Seagrass beds have extremely high ecological service value, so their protection and management are very important. The coastal area that stretches from Wenchang to Qionghai in the eastern part of Hainan Island is the largest seagrass bed distribution area in China's South China Sea Bioregion. Understanding the local distribution and changes in seagrass beds is a prerequisite for implementing the protection and management of seagrass bed resources in this area. In this study, Landsat satellite remote sensing images of the study area from 1986 to 2021 were selected; seagrass bed survey data and field investigation data were combined; the Sum Green Index was used to extract and plot seagrass bed distribution information; the spatial distribution changes in seagrass beds in the region over the past thirty years were analyzed; and the driving factors of these changes are discussed. The following conclusions were drawn: (i) The water quality in the study area is clear, the marine sediment is mainly composed of coral debris and sand, and the seagrass plants are relatively tall. The Sum Green Index can effectively extract seagrass bed information and achieve good mapping accuracy. (ii) Between 1986 and 2021, there were significant changes in seagrass bed distribution in the study area, showing a trend of slow growth followed by rapid decline. Overall, over the past thirty years, the total area of seagrass beds in the study area has declined, coverage has decreased, and patches have become fragmented. (iii) The construction of marine and coastal engineering projects, as well as large-scale aquaculture, are important reasons for the decline of seagrass beds in the study area. Other types of mechanical physical damage, such as shipping and shells raking can also have serious impacts on seagrass beds. This study is the first to conduct distribution range mapping and long time series change monitoring based on satellite remote sensing images for the largest seagrass beds in China's South China Sea Bioregion, with the aim of providing a reference for the monitoring and protection of seagrass beds in the South China Sea.