[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-P12] Impacts of the bottom cold water mass on nutrients and the lower trophic ecosystem in Harima-Nada, Seto Inland Sea, Japan

*Siraporn Tong-u-dom¹、Akihiko Morimoto¹、Xinyu Guo¹、Qian Leng¹ (1.Ehime university)

キーワード：Lower trophic ecosystem, Bottom cold water mass, Nutrient release from sediment, Climate changes, Biogeochemical modelling, Harima-Nada, Seto Inlan Sea

A bottom cold water mass (BCWM), also known as a bottom cold dome. It has lower temperature compared to surrounding waters, typically formed in coastal seas during the summer. The presence of BCWM in the Seto Inland Sea has been widely reported through hydrographic observation and numerical modelling. Cold water mass from the previous winter becomes trapped in topographic depressions after the onset of spring, forming the Harima-Nada BCWM (HNBCWM). The intensity of HNBCWM is controlled by vertical and horizontal heat transport processes. The water temperature at the seabed plays a crucial role in the release of nutrients from sediment, which is a viral factor in the lower trophic levels and material cycle in coastal seas. Several studies have suggested that the BCWM prevents water exchange with the surrounding water, potentially leading to increased nutrient concentrations due to decomposition processes inside the BCWM. Furthermore, breakdown of the BCWM in autumn promotes the blooming of phytoplankton. Based on long-term monitoring data in the center of Harima-Nada from 1981 to 2018, obtained from the Broad Comprehensive Water Quality Survey conducted by the Ministry of the Environment, Japan (https://water-pub.env.go.jp/water-pub/mizu-site/mizu/kouiki/dataMap.asp), the data exhibits inter-annual variability of temperature and nutrients. Furthermore, the linear correlation coefficient between nutrients and temperature in the HNBCWM area was 0.61 (n=33) in a negative direction, suggesting a significant role of HNBCWM in concentrating nutrients in the bottom layer. Decomposition processes occur at the bottom layer, where nutrients become trapped inside the BCWM, preventing water exchange with the surrounding water and resulting in an increased nutrient concentration. Additionally, during winter, the linear correlation coefficient was 0.62 in a positive direction, indicating that an elevated water temperature at the bottom layer significantly influences the release of nutrients from sediment. These findings are pivotal in understanding the impact of changes in water temperature, potentially linked to climate changes, on lower trophic ecosystems. Consequently, a comprehensive physic-sediment-ecosystem model is essential to grasp the response of lower ecosystem dynamics to environmental changes. In this study, we performed year-to-year calculations by categorizing strong and weak BCWM in Harima-Nada to elucidate its impact on lower trophic levels and material cycles.