The coral reef environment has turned worse on a global scale and reefs of the Okinawa islands are no exception. Although it is generally accepted that the main cause of coral degradation is related to rise and fluctuation of sea surface temperatures driven by the global warming, a coral reef ecosystem could also be disturbed by a variety of extremely local ecological and/or anthropogenic influences, resulting in coral bleaching and drastic population decrease. The aim of this study is to make clear the process of changes in the shallow-water coral reef environment with regard to land use alterations in the area of Shiraho on Ishigaki Island located in Okinawa, south west of Japan.
Okinawa, a former Japanese territory under the U.S. administrative authority following the World War II, was returned to Japan in 1972. Thereafter, Okinawa was incorporated in the Japanese economy under the three Okinawa Development Plans in order to rapidly accomplish the social transformation. In Okinawa, several land improvement projects were performed under these development programs, followed by large-scale topographic changes accompanying extensive deforestation and sudden changes of land use practices. As a result, red soil outflow, overloaded runoffs from the farmlands, eutrophication, environmental perturbations of shallow-water reef ecosystems and quasi-extinction of hermatypic corals occurred. In this study, the impacts of the development process on the island ecosystem were assessed by examining the changes in the land use patterns on the island after 1972. Furthermore, using seven series of aerial photographs taken after 1972. I investigated the relevance of the changes on land areas to the deterioration of Shiraho coral reefs.
The ratio of the sea grass bed colonizing the coral reef moat in 1972 was only 1.2%. After about 30 years (2004), the sea grass bed spread over 7.5% of the same area. The sea grass bed spread most extensively in the place adjacent to land improvement projects where large accumulation of red soil and nutrients took place. The combination of the impact of land improvement projects, the excrement of artificial manure and the runoff flow to the lagoon from the beef cattle breeding caused the spatial extension of the sea grass bed. Because the growth of sea grass is faster than that of hermatypic corals, the hermatypic corals lose their habitat as seaweeds spread.
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