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

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 scale. 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 site and landscape scales. We sampled within and across six intertidal seagrass meadows representing three geomorphic settings, including reef-associated settings dominated by Cymodocea spp., estuaries Halophila spp. and lagoons dominated by Thalassia spp., across Singapore. Within and across these sites, we measured soil organic carbon (C org ) stocks and greenhouse gas fluxes using the static chamber method. Tropical intertidal seagrass meadows stored 27.5±19.9 Mg C org ha −1 (mean ± SD) in the top 0–25 cm of soil, which varied significantly within sites (min–max: 5.1–78.9 Mg C org ha −1 , n =42). Plot-level mean fluxes averaged 27.5±25.3 mgm-2h-1 of CO 2 (min–max: −18.6 to 82.4, n =56) and 0.5±18.2 µgm-2h-1 of CH 4 (min–max: −66.4 to 57.5, n =56). Mixed-effects models indicated that geomorphic setting did not explain variation in 0–25 cm stocks or in CO 2 and CH 4 fluxes. Sensitivity analyses for 0–100 cm stocks showed a marginal geomorphology effect for measured-only totals ( p =0.074) that was not present after extrapolation to 100 cm ( p =0.816). In multivariate driver models, CO 2 flux was negatively associated with water temperature ( p =0.011) and showed an overall geomorphology effect ( p =0.027), while CH 4 drivers were not resolved (all p ≥0.158. Plot-level modelling further showed that CO 2 decreased with increasing 0–25 cm C org stock ( p =0.044), whereas CH 4 showed no association with C org stocks ( p =0.469). Overall, these results highlight strong spatial heterogeneity in tropical intertidal seagrass carbon dynamics and underscore the need for spatially and temporally explicit sampling and modelling frameworks that reflect nested spatial structure.