The carbon, nitrogen and the stable carbon isotopic composition, δ13C, of organic matter has been used to elucidate the source(s) of this material to coastal sediments. Sediments were collected at 15 coastal locations in the Philippines and Vietnam, which broadly represented different depositional environments ranging from seagrass meadows, through seagrass meadows located near mangroves and to mangrove stands. In addition, short-term sediment traps were deployed at 12 of the sites where seagrass was present. Mean sediment organic carbon concentration and C/N increased and δ13C of organic matter decreased from seagrass to mangrove dominated settings. The organic carbon flux measured by the sediment trap deployments was very variable (32 ± 3 to ∼ 700 mmol m-2 d-1) and represented only a small fraction of the total particle flux. The importance of seagrass as a source of organic matter to the sediments was assessed by using a simple mixing model and the average δ 13C values for seagrass, their epiphytic community and surface water particulate matter (seston). A positive correlation between seagrass leaf biomass and sediment δ13C in seagrass dominated settings suggests that these macrophytes do significantly influence the composition of sedimentary organic matter. Seagrass was however rarely found to be the dominant source of organic matter to the underlying sediments. Both sediment trap and sediment data suggest that material of planktonic origin was the dominant source of sedimentary organic matter in these settings. At the sites dominated by mangroves the concentration of organic matter (∼ 1-13 × 103 mmol g-1) in the sediment is generally higher than at seagrass dominated sites due to the outwelling of organic matter from the mangrove stands. Mangrove organic matter often dominates the sedimentary input but other sources of organic matter must contribute to cause the observed range in sediment δ13C. © 2004 Elsevier Ltd. All rights reserved.

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