Biological response of coral reefs to sea surface temperature variation: evidence from the raised Holocene reefs of Kikai-jima (Ryukyu Islands, Japan)

Abstract.

The island of Kikai-jima (central Ryukyu Islands, Japan) provides a rare opportunity to define both environmental and biological variations within a reef ecosystem over a geological time frame. This paper documents the palaeo-environmental records archived in the δ¹⁸O composition of four Holocene Porites cores collected from the raised reef terraces of Kikai-jima. These coral samples record mean sea surface temperatures of 23.5 °C at 4,220 years B.P., falling to 22.2 °C at 3,790 years B.P. and to a minimum of 21.4 °C at 3,400 years B.P. After this time mean sea surface temperatures rose to 23.5 °C at 1,860 years B.P. before reaching modern-day conditions of 24.9 °C. During the cool water period between 3,790 and 3,400 years B.P., the coral isotope data indicate that sea surface temperatures were below the currently accepted 18 °C minimum temperature for reef development approximately 14% of the time. Biological changes (total coral abundance, colony size, individual coral coverage, genera diversity and evenness) preserved in the raised reef terraces of Kikai-jima can be explained by these variations in Holocene sea surface temperature. The biological data indicate that the coral community at Kikai-jima was ecologically stressed during the cool water period from 3,790 to 3,400 years B.P.; however, in the gross morphological sense the coral community did still constitute a coral reef.