Boosting blue carbon via seagrass restoration: Decadal insights from Nanozostera noltei and Ruppia cirrhosa

The restoration of seagrass meadows is increasingly promoted as a nature-based solution for climate change mitigation, due to their ability to sequester and store carbon. This study assessed sedimentary carbon accumulation and standing biomass carbon sequestration in restored aquatic angiosperm meadows of Nanozostera noltei and Ruppia cirrhosa in transitional coastal environments over a 10-year period. Sediments colonized by N. noltei and R. cirrhosa accumulated an average of 24 ± 13 g organic carbon (Corg) m-2 y-1 in the top 5 cm, roughly half (12 ± 5.6 g m-2 y-1) deriving from the seagrasses themselves. When analysed separately, N. noltei showed a higher Corg accumulation rate (34 ± 7.8 g m-2 y-1; 14 ± 2.1 g m-2 y-1 from seagrass origin) compared to R. cirrhosa (20 ± 13 g m-2 y-1; 10 ± 6.7 g m-2 y-1 from plant-derived carbon). Restoration actions led to a sustained recolonization of 33.5 km2 over a 10-year period. Corg standing stocks at 10 years retained in plant biomass from 1,740 to 3,740 tonnes across both investigated species, with a mean of 2,740 tonnes (equivalent to 10,000 CO2 tonnes). Stable isotope analysis indicated that approximately 50 % of the sediment Corg originated from seagrass biomass, while the other half was likely contributed by autochthonous algal production and allochthonous sources, such as saltmarshes and riverine inputs. In contrast, inorganic carbon, mainly composed of allochthonous dolomite, showed no net accumulation by weight over the monitoring period, suggesting negligible contribution to active carbon sequestration. Overall, our findings highlight the role of small-sized aquatic angiosperms in enhancing Corg accumulation and stabilizing sediments in choked lagoon systems, supporting their suitability for large-scale restoration initiatives.