[H-GG02] Dialogues on natural resources and environment between earth and social sciences

[HGG02-06] Terrestrial environmental factors of red soil runoff causing coral reef decline and its regional differences
-A case study of Ishigaki and Iriomote Islands-

*Yamamoto Ayuka¹, Hitoshi SAITO¹ (1.Nagoya University)

Keywords:red soil runoff, coral reef, environmental problem, watershed

In recent years, coral reefs haves been rapidly declining. The seas surrounding Okinawa Prefecture, Japan, are characterized by rich coral reefs. However, red soil runoff caused damage to coral reefs. In Okinawa Prefecture, natural conditions such as topography, soil, and heavy rainfall combined with, human activities such as agriculture and land development cause red soil runoff. Although several studies analyzed red soil runoff and the factors, they have not yet comprehensively addressed both the factors contributing to red soil runoff and their regional and seasonal differences. This study analyzed the terrestrial environmental factors such as topography and land use in a watershed to explore the links between these factors and red soil runoff. We statistically analyzed the relationship between these factors and SPSS (Suspended Particles in Sea Sediment) values at river mouths to identify the regional and seasonal differences in SPSS values. The study areas are Ishigaki Island and Iriomote Island in Okinawa Prefecture, Japan. We analyzed topography and land use at a watershed scale using QGIS. Then the important terrestrial environmental factors were examined using correlation analysis and random forest methods. The results showed the seasonal and regional differences in the important terrestrial environmental factors for SPSS values. Especially in regions with extensive human-induced land use such as Ishigaki Island, SPSS values tended to correspond to the types of land use. The results also indicated the importance of rice paddies, pastures and meadows, and wild grasslands for red soil runoff which have not been addressed in the past. Further studies are necessary to analyze the detailed terrestrial environmental factors and their regional and seasonal variations. Temporal-spatial analysis of the terrestrial environmental factors and corals may contribute to the conservation of coral reef ecosystems.