Radiocarbon isotopic evidence for assimilation of atmospheric CO₂ by the seagrass Zostera marina

Abstract. Submerged aquatic vegetation takes up water-column dissolved inorganic carbon (DIC) as a carbon source across its thin cuticle layer. It is expected that marine macrophytes also use atmospheric CO₂ when exposed to air during low tide, although assimilation of atmospheric CO₂ has never been quantitatively evaluated. Using the radiocarbon isotopic signatures (Δ¹⁴C) of the seagrass Zostera marina, DIC and particulate organic carbon (POC), we show quantitatively that Z. marina takes up and assimilates atmospheric modern CO₂ in a shallow coastal ecosystem. The Δ¹⁴C values of the seagrass (−40 to −10 ‰) were significantly higher than those of aquatic DIC (−46 to −18 ‰), indicating that the seagrass uses a ¹⁴C-rich carbon source (atmospheric CO₂, +17 ‰). A carbon-source mixing model indicated that the seagrass assimilated 0–40 % (mean, 17 %) of its inorganic carbon as atmospheric CO₂. CO₂ exchange between the air and the seagrass might be enhanced by the presence of a very thin film of water over the air-exposed leaves during low tide. Our radiocarbon isotope analysis, showing assimilation of atmospheric modern CO₂ as an inorganic carbon source, improves our understanding of the role of seagrass meadows in coastal carbon dynamics.