Assessing the content and origin of organic carbon in Atlantic blue carbon seagrass ecosystems in Brittany

Abstract

Seagrass ecosystems have increasingly attracted academic attention due to their role in climate change mitigation through carbon uptake. They also have been incorporated into the Verified Carbon Standards carbon credit system aiming to offset anthropogenic carbon emissions. However, their carbon uptake potential and mechanisms exhibits substantial interspecific and geographic variability. In order to contribute to understanding of localised and species-specific sequestration dynamics, this research focused on assessing the organic carbon content and origin in Zostera marina seagrass meadows compared to adjacent bare plots. To this end, eight sediment cores of ~ 16 - 30 cm length were collected from Île Callot (northern Brittany) and Pointe d’Arradon (southern Brittany), France. Seagrass tissues were collected at both sites, and salt marsh tissues at Pointe d’Arradon, for organic carbon source identification. Sediment and plant tissue samples were analysed for organic carbon content and stable carbon isotope ratios (δ¹³C). The results indicate that sediments beneath Z. marina patches stored significantly (p < 0.001) more organic carbon (0.97 ± 0.53 %; 0.50 ± 0.24 %, respectively). Statistical analysis also revealed significant (p < 0.001) differences in organic carbon content between both study sites. Furthermore, organic carbon inputs to both bare and seagrass sediments are predominantly derived from allochthonous, non-seagrass primary producers. These findings suggest that the studied Zostera marina meadows enhance organic carbon drawdown by intercepting allochthonous particles from the water column. They further highlight pronounced regional variability in carbon sequestration dynamics, underscoring the need for broader spatial coverage and high-resolution, localised data in seagrass carbon sequestration studies. To better constrain the long-term burial and stability of sequestered carbon, and assess site suitability for carbon credit attribution, further research should investigate burial rates as well as carbon efflux and transport dynamics. It could also sharpen understanding about carbon sources and their relative contributions over time further by applying radioisotope dating techniques in combination with mixing models.