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Marine macrophytes help protect coastal habitats by modifying the energy of their environment, attenuating wave action, and stabilizing the sediment. The aim of this research was to examine the effect of the above ground parts of different seagrass and seaweed species on the deposition, retention, physical (dry bulk density and grain size), and chemical properties (organic carbon content and organic carbon density) of sediment. Six seagrass and seaweed communities with different above ground morphologies were studied between the dry (April 2015) and wet seasons (August 2015). The results suggested that the highest canopy complexity index was in the green alga, Halimeda macroloba Decaisne patches, which had the highest canopy score. High canopy complexity patches were likely to retain more sediment and store more organic carbon than lower canopy complexity patches and bare sand, respectively. The sediment retention and percent of organic carbon content were also affected by seasons. The wave action in the wet season affected the sediment through higher sediment deposition and movement, which caused increased sediment instability and led to changes in sediment properties. In addition, shoot density in the dry season was lower than that in the wet season, most likely due to damage from long exposure during low tide. Our results indicate that the canopy complexity of marine macrophytes and seasons influenced both sediment retention and properties of the sediment.

Seagrass meadows have one of the highest carbon sequestration and storage capacities than any other ecosystems. Carbon that is stored in the ecosystem is accumulated in the deposited sediment as well as in the living

Coastal areas have high potential to store carbon from both terrestrial and marine materials due to the interconnectivity among the habitats. We investigated the contributions of the various sources to the sedimentary organic carbon by examining the δ13C, δ15N, %TOC (total organic carbon) and %TN (total nitrogen) of several marine macrophytes, which included three seagrass species: Cymodocea rotundata Ascherson & Schweinfurth, Cymodocea serrulata (R. Brown) Ascherson & Magnus, and Halophila ovalis (R. Brown) J.D. Hooker; three seaweed species: Halimeda macroloba Decaisne, Halimeda borneensis W.R. Taylor, and Halimeda opuntia (Linnaeus) J.V. Lamouroux; and one coral species, Porites sp., over a vertical gradient at Tangkhen Bay, Thailand. The distribution of organisms differed significantly between distances from the shore and seasons (p < .001), with seagrass occupying the area of 150–350 m from the shore, while Halimeda was most abundant from 150 to 400 m from the shore. The %TOC and %TN in the sediment were significantly different along the shore (p < .001), with the highest values at 300 m from the shore where the highest organism variability was recoded. Seagrass and Halimeda played major roles in sediment organic matter and carbon contributions, contributing up to 42% of the sedimentary organic carbon, while allochthonous sources such as ocean and terrestrial sources had lower contributions. This study elucidates the importance of macrophytes as organic carbon contributors to sedimentary carbon storage, especially in small semi‐enclosed bays.

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