The Sources of Organic Matter in Seagrass Sediments and Their Contribution to Carbon Stocks in the Spermonde Islands, Indonesia

Seagrass ecosystems have a potential role in climate change mitigation due to their ability to store high amount of carbon, particularly in the sediment. Studying the factors and mechanisms responsible for this storing capacity is essential to understand seagrass carbon sink function. Therefore, in this study, we identified the sources of organic carbon (C-org) in seagrass sediments and the implication to C-org stocks from four islands in the Spermonde Islands that located at different zones. We used the Bayesian stable isotope mixing model to estimate the proportional contribution of different sources to sediment carbon. Seagrass meadows that located in adjacent to high anthropogenic activities (deforestation and aquacultures) with direct exposure to wave actions, such as on the Bauluang Island, accumulated organic carbon that derived from multiple sources, where phytoplankton contributed the highest, while on the other three islands that are relatively protected from wave actions, the highest contribution (similar to 75%) was from autochthonous production (seagrass-derived). Sediment C-org stocks vary spatially, ranging from 11.9 to 32.1 Mg C ha(-1) (based on the obtained depth of 20-55 cm), or 40.5 to 83.5 Mg C ha(-1) if extrapolated to 1 m depth. The variability of sediment properties and C-org stocks in this study is not solely determined by the geographical differences (inshore, nearshore and offshore islands), but also influenced by other local factors such as hydrodynamics that control the distribution of carbon sources, anthropogenic pressures and species composition. These factors should be taken into account when developing coastal management strategies, as efforts are being undertaken to include coastal ecosystems (including seagrass ecosystems) on the National Green House Gasses Reduction Strategy.