Necessity of bottom topography measurements in coral reef regions

Abstract

High water temperatures resulted in damage to coral in Japan's Okinawa prefecture during the world wide coral bleaching event of 1998. However Sekisei lagoon, which has a radius of 20 km and is 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? Maintenance of water temperatures that were nonstressful to the corals in Sekisei lagoon is probably due to the physical environment's effects on the flow regime. The flow regime in coral reef regions is an important environmental factor affecting the health, growth, distribution, and structure of the corals. The northeastward flowing Kuroshiro current passes offshore of Japan's coral reef region and thus influences water exchange and the transport of coral eggs and larvae. However, many patch reefs are scattered throughout the Sekisei lagoon. And, the existence of patch reef which uncharted has complicated the bottom topography of this lagoon. Therefore, the health, growth, distribution, and structure of the corals including the circulation in Sekisei lagoon is influenced by the small-scale complex bottom topography. Bottom topography data and continuous physical environmental data, such as water flow and water temperature, are currently being measured by JAMSTEC in order to understand the physico-chemical characteristics of the environment from the deep sea (Kuroshima knoll) to shallow water (Sekisei lagoon) in coral reef regions. Recently, water temperature increases over short time periods have occurred, and it is worried what effects they will have on the coral reef ecosystem in the Sekisei lagoon. In this study, we discuss the necessity of bottom topography measurements, using SEABAT8125, in the Sekisei lagoon, including physical environment data.