[A-HW22] Material transportation and cycling in watershed ecosystems; from headwaters to coastal areas

[AHW22-10] Reconstruction of long-term change in external nitrogen loading and its effect on coastal sediment of Osaka Bay, western Japan

*Mitsuyo Saito¹, Shin-ichi Onodera², Kunyang Wang³ (1.Graduate School of Environmental and Life Science, Okayama University, 2.Graduate School of Advanced Science and Engineering, Hiroshima University, 3.Graduate School of Integrated Arts and Sciences, Hiroshima University)

Keywords:long-term nitrogen loading, coastal sediment, Osaka Bay watershed

Evaluation of long-term nutrient loading from the watershed to the coastal area is important for understanding the current environmental status and taking measures for environmental management. Especially for the coastal area faced with megacities, it is important to estimate how much the nutrient loading during significant economic growth. However, there are few studies to reconstruct the "net" nutrient loading during the past high-loading period and its effect on nitrogen accumulation in coastal sediment. We aimed to reconstruct the long-term change in external nitrogen loading and its effect on nitrogen accumulation in coastal sediment of Osaka Bay, western Japan.

Soil & Water Assessment Tool (SWAT) was applied to Yamato River catchment inflowing to Osaka Bay to reconstruct the long-term change in nutrient loading. The results indicate that total nitrogen (TN) loading was at a peak from 1960 to 1970s due to the increase of domestic and industrial pollution loading and caused by economic growth, then it has been decreased due to the comprehensive water quality control in the watersheds of Seto Inland Sea (e.g. development of sewage treatment facilities). However, based on the previous studies, TN accumulation in the sediment of Osaka Bay shows an increasing trend from the 1990s to the present. This mismatch between the peak in external TN loading and TN accumulation trend in the coastal sediment suggests that the other mechanism should be considered about the nutrient cycle in the coastal area.


*This work is supported by the research grant of “Asia-Pacific Network for Global Change Research, CRRP2019-09MY-Onodera, 2019-2022, PI: Shin-ichi ONODERA”.