陸域に接するサンゴ礁浅海域の脆弱性に関する考察 : 石垣島白保サンゴ礁を例に

Many coral reefs around the Ryukyu Islands are “fringing” reefs which grow adjacent to the land. This makes them highly susceptible to impacts from the land. In recent years, the regional coral reefs have been decreasing because of bleaching caused by elevated water temperatures. In addition, red soil runoff from nearby rivers and groundwater nutrients continue to degrade the coral reef environment. This study examines the vulnerabilities that fringing coral reefs are exposed to, using the Shiraho Coral Reef of Ishigaki Island as a case study.<n>The Shiraho Coral Reef has a wide shallow lagoon 800 m in width from the shore and a well-developed reef crest running parallel to the shore from north to south. The 3-meter-deep reef extends between the beach and the ocean, whereas the ocean-facing ridge divides the ocean from the shallow lagoon. The ridge dries out when the water level drops below mid-tide and acts as a barrier to the ocean waves. This creates a calm and closed-off area where the reef’s shallow lagoon is unaffected by the waves. Large colonies of blue coral reside in the reef at Shiraho. However, the area has low coral species diversity because the location makes it difficult for many coral species to survive. It is believed that the blue coral’s resistance to high water temperatures enables it to survive in this environment. Although various coral species inhabit the northern side of the reef, the number of species is now significantly decreasing across the entire Shiraho reef. The Todoroki River to the north and the Miyara River to the south-west meet the coral reefs as they enter the ocean. The basins of both the rivers are home to large expanses of pasture for cows and agricultural fields for sugar cane. In monsoon, red soil flows from the rivers to the reef. This river runoff has continued for many years.<n>Furthermore, as coral typically survives in oligotrophic environments, the eutrophication of the ocean habitat also contributes to its reduction.<n>As such, in addition to the red soil, the nutrient water resulting from eutrophication from river runoff is causing a decline in the corals. Moreover, the land adjacent to the reefs is calcareous, which gives it excellent water permeability. It is now clear that human activities add nutrient water to the ground water, which runs off into the ocean. As described above, the Shiraho Coral Reef, which is adjacent to a land, is vulnerable to impact from river and underground runoff of red soil and nutrient water. In addition, there has been a recent increase in the number of tourists visiting Okinawa. When residents offer snorkeling tours of the reefs and other sightseeing activities, there are concerns that the coral can be damaged by the tourists entering the water and that the substances contained in sunblock are affecting the coral ecosystem.<n>The land-adjacent coral reefs of Shiraho are susceptible to a wide variety of impacts from the land. Although coral reef environments face further declines because of global warming, it will be necessary to explicitly elucidate the negative pressures on coral reefs in various regions and to devise conservation measures tailored to each area.