2. Carbon Sequestration in Sediment as an Ecosystem Function of Seagrass Meadows

Seagrass meadows have the potential to sequester large amounts of organic carbon (OC) in underlying sediments as detrital OC. This ecosystem function of seagrass meadows depends not only on the high primary productivity of seagrasses and associated algae, but also on the physical and hydrological properties of seagrass meadows that accumulate fine-grained mineral sediment and allochthonous organic matter and stabilize the sediment. The burial efficiency of OC in marine sediment is constrained principally by (i) structural recalcitrance of accumulated organic matter, (ii) enhancement of OC preservation through organic–mineral interactions, and (iii) the length of the oxygen exposure time in the reactive layer of sediment. We discuss how these mechanisms contribute to and constrain OC sequestration in seagrass sediments. From the sediment source–sink perspective, long-term carbon sequestration capacity can be viewed as an emergent property of coastal vegetated ecosystems such as seagrass meadows. Sequestration of OC also depends on the nature of the OC supplied, which is largely determined by the origin of the OC. The major sources of OC to seagrass meadow sediments are reviewed and classified in terms of their mode of transport and degradability in the early diagenetic process. The available methods of evaluating various OC sources, including recently developed environmental DNA techniques, are also critically reviewed. Finally, available data are compiled on the concentration and burial rate of OC in seagrass meadow sediments and a tentative global estimate of the long-term (~1000 years) OC sequestration rate in seagrass meadows as 1.6–9.4 Tg C year⁻¹ is provided. This is lower than previous estimates of the global OC burial rate in seagrass meadows.