Tracing Sources and Spatial Distribution of Seagrass Sediments, Yao Yai Island, Thailand

Abstract

Coastal vegetated ecosystems are recognised for their long-term carbon sequestration and high capacity carbon storage that can mitigate associated impacts of global climate change. However, due to the catchment-coast connectivity, coastal ecosystems are often at the receiving end of terrestrial-derived pollution. As sedimentation is a major threat to coastal ecosystems, tracing may allow managers to identify point sources which can be targeted for mitigation strategies. The significance of considering ecosystem connectivity is demonstrated by using d13C and d15N isotope tracers and the SIAR mixing model to map the composition and distribution of deposited sediment in a seagrass bay of Yao Yai Island, Thailand. Through mixing polygon diagrams, weak acidification (acid fuming) on organic matter adsorbed on sediments, to remove inorganic carbon, was found to provide the most suitable source signatures and seagrass sediment signatures for the sediment mixing model.

Kriging interpolation showed that 50-60% of sediments in close proximity to river mouths were terrestrial- and mangrove-derived. Rivers enhance connectivity from catchments to the coastal bay, implying that mangroves may not be effective buffers for seagrass ecosystems when major flow pathways directly link terrestrial and coastal areas. Landscape composition and configuration of the catchment was identified as an important factor contributing to the extension of the channel network which improves hydrologic connectivity of the system and delivery of sediments to the coast. Thus, a wider catchment-coastal system perspective and approach must be adopted when managing sedimentation problems in coastal ecosystems. Mitigation measures should not focus on adaptation response at the coast, but concentrate on selecting suitable targeted solutions for land use management which addresses erosion and hydrological/landscape connectivity.