Coral reefs, which are abundantly distributed on tropical and subtropical coasts, are terrains created by organisms called corals. Although the area is only about 0.1% of the earth's surface, about 90,000 species of organisms live in this ecosystem and it is an important place from the perspective of biodiversity. In addition, the coral reefs serve as good fisheries, providing food that supports the livelihoods of the local people, and also as a tourism resource that attracts many tourists due to its beautiful scenery.

However, coral reefs are currently in danger. A research report, Reefs at Risk Revisited, published by the World Resources Institute in 2011, estimates that 75% of the world's coral reefs are in crisis. The major causes include the discharge of a large amount of sediment from land, depositing on the coral reefs and causing the corals to suffocate, the outbreak of crown-of-thorns starfish that eat the corals, and in recent years, warmer ocean temperatures due to climate change that decline coral activity, resulting in the phenomenon called “coral bleaching”. With regards to the sediment discharge, a large amount of soil that flows out from farmlands and development sites especially after heavy rains is considered as the main source. In the case of the outbreak of crown-of-thorns starfish, nutrients such as nitrogen and phosphorus flow excessively into the sea from land through rivers and groundwater, which increases phytoplankton population, and as a result, survival of crown-of-thorns starfish larvae that feed on the phytoplankton. 

In the 5th Medium- to Long-term Plan that started this month, JIRCAS will target the tropical and subtropical island areas where coral reefs are distributed and we are implementing a project to develop and demonstrate the technologies to control the outflow of soil and excess nutrients from land through appropriate management of forests and farmlands and effective utilization of biological resources such as crop residues. The purpose of this study is to clarify the influences of land use, topography, geology etc. of the basin (the area where rainwater that has fallen on the ground surface collects and flows into the river) on the concentration of sediment and nutrients in river water. We are conducting river water quality surveys on Ishigaki Island, where the JIRCAS Tropical Agriculture Research Front (TARF) is located. The 2021 JIRCAS public mini-lecture Health checkup for rivers: So many countries, so many ‘rivers’?! introduces the state of the river surveys and some of the findings of the surveys so far. Please take a look!
 

References

国立環境研究所, サンゴ礁の過去・現在・未来 ～環境変化との関わりから保全へ～, 環境儀 No. 53 (2014).  https://www.nies.go.jp/kanko/kankyogi/53/02-03.html.（2021年4月13日閲覧）

Burke, L., Reytar, K., Spalding, M., Perry, A. Reefs at Risk Revisited. World Resources Institute (2011). https://www.wri.org/publication/reefs-risk-revisited. Accessed on April 13, 2021
沖縄県, オニヒトデのはなし（第2版）(2004). https://www.pref.okinawa.jp/site/kankyo/shizen/hogo/documents/onihitode….（2021年4月13日閲覧）

Contributor: KIKUCHI Tetsuro (Crop, Livestock and Environment Division)