[A-CG40] Coastal Ecosystems 2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG40-11] Evaluation of production in Sargassum bed using part-specific measurement by ¹³C labeling method

*Toru Miyata¹, Yu Umezawa¹, Takunari Ida¹, Kenta Watanabe², Atsushi Kajita³, Koji Manabe³, Tomohiro Kuwae² (1.Tokyo University of Agriculture and Technology, 2.Port and Airport Research Institute, 3.SUIKEN RESEARCH)

Keywords:Sargassum bed , 13C labeling method, Annual production, Part-specific measurement

Seaweed beds are representative ecosystems in the coastal zone of Japan and have many important ecosystem functions. Among these functions, the carbon storage function is expected to be used as a measure to reduce atmospheric carbon dioxide. In coastal areas where seaweed beds are distributed, the amount of carbon dioxide absorbed is equal to the gross primary production of the seaweed beds, but it has not been accurately determined. In general, the production of seaweed beds is measured by the stratified clip technique. However, this method cannot measure the amount of DOC released and the amount of loss due to waves and erosion as production. As another method, the ¹³C labeling method can quantify most of the carbon fixed by seaweeds. In the production measurement using a part of the seaweed, such as the ¹³C labeling method, only the growing part of the sample is used, even though the production is expected to be different in each part of the individual. Therefore, in this study, we extrapolated the biomass and photon flux density data and evaluated the annual production of the Sargassum bed by the ¹³C labeling method.

The study site was the Sargassum bed distributed around Heigun Island in Yamaguchi Prefecture. The biomass of the dominant species was measured by the quadrat method in September, October, December 2019, and January 2020. The production of Sargassum horneri, Myagropsis myagroides, and Sargassum macrocarpum was measured by ¹³C labeling method in September and December 2019, January 2020, and September and October 2021.

In this examination, it was found that the production of seaweeds differed by part. If this is not taken into account, there is a possibility of overestimation when scaling up production. The quantification of biomass showed that M. myagroides was the most dominant species in the study site in 2019. Since the growth period of M. myagroides is from September to January, we approximated the annual production of the seagrass beds in the study site as the total production of M. myagroides from September to January. As a result of the estimation, the annual production per unit area of the study site was 1.65×10⁶ mgC/m². This value is close to the gross primary production, and it is possible to accurately evaluate the carbon absorption in coastal areas by accumulating studies using this study method.