Radiocarbon isotopic evidence for assimilation of atmospheric CO₂ by the seagrass <i>Zostera marina</i>

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.