Coral Zonation of fringing reefs of Ishigaki-jima, Ryukyu Islands
Kaoru Sugihara1), Yasufumi Iryu2) and Toru Nakamori2)
1) Department of Earth System Science, Fukuoka University, 814-0180 Fukuoka, Japan
2) Institute of Geology and Paleontology, Tohoku University, 980-8578 Sendai, Japan
The Ryukyu Islands (Ryukyus) are located at relatively high latitudes (24ﾟ20'N-30ﾟ44'N). However, well-developed fringing reefs are distributed over the islands, where species diversity of hermatypic corals is high. This study aims to describe the distribution patterns of coral communities in Ishigaki-jima (Ishigaki Island) and to present a typical coral zonation on fringing reefs in the Ryukyus based on the comparison of two sites with different reef scale.
A quantitative survey using the quadrat method was conducted to describe topographic and coral zonation on the two sites (Urasaki and Shiraho) in Ishigaki-jima. A topographic profile was drawn covering areas from shore to shallow reef slope (down to 5 m) perpendicular to the shoreline to identify the topographic zonation. The species composition and coral cover were documented at one-square-meter quadrats at an interval of 10 m along the transect. Multidimensional scaling ordination based on these species composition and coverage was used to display the relationships between reef topography and coral distribution patterns as well as the interrelationships. Cluster analysis based on presence/absence of coral species common at both sites was conducted to define typical coral communities in Ishigaki-jima.
Topographic and coral zonation were clearly discernible parallel to the shoreline in both sites. Reef flat was subdivided into 5 topographic subareas (shallow lagoon, inner reef flat, reef crest, outer reef flat, and reef edge). The number of topographic zones was the same at the two site although the reef flat at the Urasaki site is about two times as wide as that at the Shiraho site. Ninety species in 30 genera were found within 171 quadrats at the Urasaki site, while 68 species in 25 genera within 86 quadrats at the Shiraho site. Fourty-five species in 16 genera within 194 quadrats and were found in the same topographic subareas of both sites. Ordinations of topographic subarea and coral species show that distribution patterns of corals correlate well with the topographic zones. The cluster analysis also gives results similar to those by the ordination. Coral species occurred within the quadrats are divided into 2 coral communities (community in wave-exposed habitat and community in sheltered habitat) against the reef crest. The former community is subdivided into 2 subcommunities; an Acropora pulchra com. on the outer reef flat and a tabular Acropora com. on the reef edge to shallow reef slope. The latter one is subdivided into 4 subcommunities; a Goniastrea aspera com., a Montipora digitata com., a Porites cylindrica com., and an Acropora formosa com. The scale of the fringing reef represented by the width may not affect the distribution patterns of the community in wave-exposed habitat. However, significant differences are recognized between two sites in the distribution of coral species in the sheltered habitat (especially in the shallow lagoon). Thus depth and the distance from shore are regarded as the most critical factor controlling the distribution in such environment.
Our study will support a scientific synthesis of monitoring, assessment, and management of coral reef environments in the Ryukyus and is useful to compare the different scale of reefs such as fringing reefs, atoll and barrier reefs in the Indo-Pacific regions.
