Blue carbon dynamics following widespread seagrass loss across tropical coastal sediments in Thailand

Seagrass ecosystems are major sinks of sedimentary organic carbon (C_org), but the temporal changes of the C_org following the seagrass loss remains limited, particularly across different gradients of disturbance severity. In this study, the temporal changes of the sedimentary C_org across nine intertidal seagrass meadows along Andaman coast and Gulf of Thailand were estimated, by combining historical data sets (2015-2021) with the field re-sapling in 2025 following the same protocols. Sediment properties and C_org stocks were analyzed for the surface sediments (0-20 cm) and whole sediment cores using mixed effect models.

Sites that were affected by the long-term or complete seagrass loss had substantial decline in sedimentary C_org stocks, with annual losses up to 17 Mg C ha⁻¹ yr⁻¹ and associated potential CO₂ emissions over 60 Mg CO₂ ha⁻¹ yr⁻¹. These Corg losses are accompanied by decreases in dry bulk density and C_org content, indicating sediment softening and destabilization and reduced organic inputs. On the other hand, sites with partial loss and intact seagrass meadows showed different trajectories: some meadows retained long term C_org stocks with some surface losses, while others exhibited net declines in both surface and long term C_org stocks despite low changes of C_org content. This indicates that C_org enrichment does not ensure long-term carbon retention where physical sediment reorganization and lateral redistribution dominate.

Our results demonstrate that seagrass loss severity and sediment physical dynamics jointly regulate sedimentary carbon stability and CO₂ release. Distinguishing between surface reworking and whole-core carbon loss is therefore essential for accurately assessing blue carbon vulnerability and for integrating seagrass degradation into coastal carbon budgets, greenhouse-gas inventories, and climate mitigation strategies.