Carbon Sequestration Processes in a Shallow Seagrass Meadow: Isotopic Approaches

Seagrass meadows are one of the most productive ecosystems on Earth. Seagrass meadows play an important role as carbon sinks in the ocean, storing large amount of carbon in the sediments. However, there are little empirical studies dealing with how seagrass meadows contribute to the carbon sequestration processes. In this study, we evaluated the contribution of the seagrass Zostera marina to the processes of both carbon storage in sediments and atmospheric CO₂ uptake using isotopic approaches. Stable isotopic and elemental signatures (δ¹³C, δ¹⁵N and C/N ratio) were analyzed statistically to investigate the origin of both particulate organic carbon (OC) and sedimentary OC in a brackish lagoon. Phytoplankton-derived OC dominated the particulate OC (65–95%) in the lagoon; however, it was minor in the sediments (3–29%). In contrast, terrestrial and seagrass-derived OC were relatively minor contributors in the water column but were major contributors in the sediments (49–78% and 19–36%, respectively), indicating that terrestrial and seagrass-derived POC were selectively stored in the sediments. We also estimated the contribution of atmospheric CO₂ to assimilated seagrass carbon by a carbon-source mixing model using radiocarbon concentrations (Δ¹⁴C). The model indicated that the seagrass assimilated 0–40% (mean, 17%) of its inorganic carbon as atmospheric CO₂. Our findings demonstrate that the shallow seagrass meadows efficiently capture atmospheric CO₂ and sequester OC in the sediments, mitigating atmospheric CO₂ increase.