Geographic variability in organic carbon stock and accumulation rate in sediments of East and Southeast Asian seagrass meadows

Organic carbon (OC) stored in the sediments of seagrass meadows has been considered a globally significant OC reservoir. However, the sparsity and regional bias of studies on long-term OC accumulation in coastal sediments have limited reliable estimation of the capacity of seagrass meadows as a global OC sink. We evaluated the amount and accumulation rate of OC in sediment of seagrass meadows and adjacent areas in East and Southeast Asia. In temperate sites, the average OC concentration in the top 30 cm of sediment was higher in seagrass meadows (780-1080 mmol g(-1)) than in sediments without seagrass cover (52-430 mmol g(-1)). The average OC in the top 30 cm of subtropical and tropical seagrass meadow sediments ranged from 140 to 440 mmol g(-1). Carbon isotope mass balancing suggested that the contribution of seagrass-derived carbon to OC stored in sediments was often relatively minor (temperate: 10-40%; subtropical: 35-82%; tropical: 4-34%) and correlated to the habitat type, being particularly low in estuarine habitats. Stock of OC in the top meter of sediment of all the studied meadows ranged from 38 to 120 Mg ha(-1). The sediment accumulation rates were estimated by radiocarbon dating of six selected cores (0.32-1.34 mm yr(-1)). The long-term OC accumulation rates calculated from the sediment accumulation rate and the top 30 cm average OC concentration for the seagrass meadows (24-101 kg ha(-1) yr(-1)) were considerably lower than the OC accumulation rates previously reported for Mediterranean Posidonia oceanica meadows (580 kg ha(-1) yr(-1) on average). Current estimates for the global carbon sink capacity of seagrass meadows, which rely largely on Mediterranean studies, may be considerable overestimations.