The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

1392 / 2024-09-26 15:24:58

El Nino and Positive IOD Trigger Massive Seagrass Habitat Loss on Thailand’s Eastern Andaman Coast

Seagrass ecosystems,ENSO,IOD,sea level depression,3D numerical simulation

During 2019 and 2023, when El Niño and a positive Indian Ocean Dipole (IOD) coincided, Thailand experienced significant seagrass bed degradation in Trat Province, located in the southwestern part of the eastern Andaman Sea. This unprecedented loss, primarily affecting eelgrass (Zostera marina), was observed around Koh Libong, Koh Mook, and Had Chao Mai National Park, once home to Thailand’s healthiest seagrass habitats. These seagrass beds transformed into sandy flats, forcing the region's largest dugong population to migrate in search of new feeding grounds, and leading to a decline in local ecological services and artisanal fisheries. This study aims to investigate the hydrographic changes driving these losses. Data from in-situ water level measurements, remote sensing, and regional hydrodynamic models indicate that during 2019 and 2023, water temperatures and mean sea levels significantly deviated from long-term averages. Multi-level downscaled, three-dimensional hydrodynamic model simulations provided detailed insights into the most affected areas, incorporating accurate bathymetry data from Satellite Derived Bathymetry (SDB) for shallow and intertidal zones. The simulations revealed water temperatures reaching up to 36°C in some areas, coupled with lower sea levels. Prolonged exposure of seagrass beds during low tides doubled desiccation times from 40 to 80 minutes in daytime per day. This extended exposure and high water temperature likely to contributed to the decline of eelgrass beds, as their long leaves require deeper water, explaining the massive die-off observed.