Terrestrial Environmental Factors of Red Soil Runoff Causing Coral Communities Decline and Their Spatial-Seasonal Differences—A Case Study of Ishigaki and Iriomote Islands—

Abstract

In recent years, coral communities have been rapidly declining. 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. This study analyzed the terrestrial environmental factors, such as topography and land use, on a watershed scale 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 as an indicator of red soil pollution to identify the regional and seasonal differences in SPSS values using random forest models. The results showed that important terrestrial environmental factors affecting SPSS values varied spatially and seasonally. Specifically, watershed areas, slope angles, and anthropogenic land use such as rice fields, bare land, sugarcane fields, other agricultural land, and wild grasslands were important. The result indicated that rice and sugarcane fields affected the seasonal changes in SPSS values. Although further studies are necessary to verify the detailed land use conditions and their regional and seasonal variations, the relationship between the terrestrial environmental factors and SPSS values could contribute to the conservation of coral communities.