Posidonia oceanica meadows are important Mediterranean blue carbon ecosystems capable of storing substantial quantities of carbon within long-lived sedimentary matte structures. However, the environmental drivers controlling organic carbon (Corg) and calcium carbonate (CaCO3) storage remain poorly understood at local scales. This study quantified and spatially modelled sediment Corg and CaCO3 stocks within P. oceanica meadows in Baix Empordà, north-eastern Spain, using sediment core analyses, Earth Observation-derived environmental data, and Boosted Regression Tree modelling. Sedimentological variables were stronger predictors of carbon storage than broader environmental conditions. Sediment grain size was the dominant driver of both carbon pools, with fine sediments associated with higher Corg storage and coarse sediments linked to greater CaCO3 accumulation. The final models explained 34.8% and 60.9% of the cross-validated deviance for Corg and CaCO3, respectively. Across the 99 ha mapped meadow extent, total stocks within the upper 30 cm were estimated at 1,980 ± 64 Mg Corg and 19,357 ± 1,241 Mg CaCO3. Carbonate-corrected estimates indicated that the meadows function overall as a net CO2 sink, storing 1,894 ± 416 Mg CO2e in sediments. These findings provide a baseline for seagrass conservation, restoration, carbon accounting, and emerging blue carbon initiatives in the Mediterranean region.
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