Soil Carbon Stocks in Fijian Seagrass: Environmental Drivers, National Inventories, and Implications for Blue Carbon Accounting in Small Island Settings

Pacific Island seagrass systems remain critically underrepresented in global blue carbon assessments despite their potential significance for climate mitigation. We present the first national-scale assessment of seagrass soil carbon stocks for a Pacific Island nation, based on 135 sediment cores from 47 sites across Fiji. Mean organic carbon (OC) stocks were 5.6 ± 2.5 Mg C ha⁻¹ (upper 15cm) and 11.7 ± 9.2 Mg C ha⁻¹ (wholecore), representing values 2–12 times lower than global averages. National Tier 2 OC stocks were 114 Gg C (15cm) and 237 Gg C (wholecore), based on satellite-derived seagrass area of 203 km². Application of Tier 1 defaults would overestimate these stocks by an order of magnitude. Variance partitioning showed that spatial heterogeneity at sub-island scales dominated OC variability, whereas biological factors (species and coverage) explained <10% variance. Statistical analyses using map-derived geomorphic predictors showed that OC was controlled primarily by hydrodynamic setting. The dominant controls on wholecore stocks were export_distance (proximity to deep channels, lagoons, or reef slopes) and wind-wave exposure, with threshold effects at ~700 m export_distance and ~20% exposure. Near-surface (15cm) stocks were additionally influenced by seagrass cover and local seabed slope. These findings suggest that Fijian seagrass meadows function more as carbon outwelling systems than local sedimentary sinks, potentially transferring carbon to adjacent deep-ocean basins. This study demonstrates a transferable framework for Tier 2 carbon estimates. However, our results emphasize that island-reef geomorphology must be considered when interpreting the magnitude and climate relevance of sediment carbon pools in small-island settings.