Characteristics of physical environment of Japanese coral reef region in Sekisei lagoon (Okinawa Prefecture, Japan)

High water temperatures resulted in damage to the coral in Japan's Okinawa prefecture during the worldwide coral bleaching event of 1998. However, Sekisei lagoon, with a radius of 20 km and Japan's largest coral reef area (between Ishigaki Island and Iriomote Island in Okinawa Prefecture), suffered damage to only about 30% of its corals. Why was the influence of coral bleaching relatively weak in Sekisei lagoon? The maintenance of water temperatures that were non-stressful to the corals in Sekisei lagoon is probably due to the physical environment's effects on the flow regime. The flow regime of a coral reef region is an important environmental factor affecting health, growth distribution and structure of the corals. The northeastward flowing Kuroshio Current passes offshore of Japan's coral reef region and thus influences water exchange and the transport of coral eggs and larvae. The circulation in Sekisei lagoon is determined by the complex bottom topography and water exchange is accomplished through channels in the lagoon. Because of the effect of bottom topography on the water circulation we classified the lagoon into open areas, semi-closed areas, and closed areas. In this paper, based upon the results of short-term observations, e.g. current directions and velocities, we show the relation between fluctuations in the Kuroshio Current and both water circulation and coral distribution in Sekisei lagoon. There was good correspondence between each classified area and coral distribution. This study also suggested that the water in Sekisei lagoon may be warming as an effect of the offshore Kuroshio Current and that the flow regime makes it almost impossible for coral eggs and larvae to be transported from Sekisei lagoon to offshore.