[A-CG38] Coastal Ecosystems 2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG38-P02] Evaluation of mineral and organic particle captured and retained in seagrass beds of fringing reefs

*Keisuke Umebayashi¹, Yu Umezawa¹, Masaaki Sato³, Toru Miyata¹, Ryoutarou Ueba¹, Kento Suzuki¹, Han Lyu¹, Soh Sugihara¹, Toshihiro Miyajima² (1.Tokyo University of Agriculture and Technology, 2.Atmosphere and Ocean Research Institute University of Tokyo, 3.Japan Fisheries Research and Education Agency)

Keywords:Red soils, Seagrass, Sediment trap, Gypsum ball

In subtropical and tropical regions, adverse effects on coral reef ecosystems have often been reported due to the discharge of terrestrial soils (red soils) into coastal areas. On the other hand, seagrass beds stabilize sediments, which may retain red soils and mitigate the impact on coral reef ecosystems. However, few studies have quantitatively discussed how much red soils are retained by seagrass beds. In this study, mineral particles and organic matter transported from land and/or resuspended within the seagrass bed were collected using sediment traps on the fringing reef of Bise (Okinawa Island), where there is no reported red soil problem, as well as at Shiraho (Ishigaki Island) and Ngiwal (Republic of Palau), where there is a reported red soil problem. The aim was to determine the extent of the differences between seagrass beds and bare sand.
In the previous study conducted in the laboratory, sediment accumulation tended to be higher in the seagrass beds than in the bare sand, but in this study conducted in the field, the opposite trend was also observed. The physical quantity in the vicinity (reflecting flow velocity and turbulence) was determined by gypsum balls. However, no correlation was found between the physical quantity and the amount of collected material in the data including all results. It is suggested that the bottom sediment environment, such as grain size and the number of coral fragments, and seagrass species may have a significant impact on the amount of material collected.