Landscape-scale spatial variability of blue carbon stocks and fluxes in tropical seagrass meadows

Abstract. Seagrass meadows are emerging natural climate solutions for climate change mitigation through their high potential for organic carbon sequestration and storage, also known as blue carbon. However, the variability in current blue carbon stock and flux estimates is high, particularly at landscape scales. This knowledge gap highlights the need for evaluating blue carbon at spatial scales that are both locally robust and globally relevant. We quantified the magnitude of variability in blue carbon stocks and fluxes in tropical intertidal seagrass meadows at the landscape scale. We sampled six intertidal seagrass meadows representing three geomorphic settings, including reef-associated settings, estuaries and lagoons, across Singapore. Across these sites, we measured soil organic carbon (C_org) stocks and greenhouse gas fluxes using the static chamber method. We found that tropical intertidal seagrass meadows stored 132 ± 78 Mg C_org ha⁻¹ (mean ± SD) in the top 100 cm of soil, which varied significantly within sites and geomorphic settings (min–max: 19–303 Mg C_org ha⁻¹), and were positively associated with salinity. Seagrass fluxes averaged 660 ± 695 mg m^-2 d^-1 of CO₂ and 12 ± 484 µg m^-2 d^-1 of CH4, which, unlike stocks, did not appear to vary significantly across geomorphic settings. However, we identified redox (positive) and bulk density (negative) as independent drivers of CO_2, and C_org as an independent, strong predictor of CH₄ after accounting for spatial hierarchy and geomorphic setting. Spatially explicit stock assessments and inclusion of greenhouse gas fluxes are important to inform robust coastal carbon budgeting and support the inclusion of seagrass in national climate mitigation frameworks.