Late Holocene reef ecosystem baseline: Field evidence from the raised reef terraces of Kodakara and Kikai Islands, Ryukyu Islands, Japan

Abstract

To better understand and evaluate how present-day and future reef ecosystems are being impacted by natural climate change and human activity, it is important to establish reef ecosystem baselines from paleoecological records. Here we present paleoecological data, including coral cover and genus and species composition, for two raised reef islands (Kodakara and Kikai islands) located in the Ryukyu Islands (Japan) covering the last 2.4 and 4.1 kyr, respectively. Evidence of a mean cover of 20–40% coral on pristine reefs at the study sites was found for a millennial-scale period of ocean environment stability, in terms of sea level, solar radiation, and sea surface temperature. This coral cover has been maintained on the modern reef at Kikai Island since at least 4.1 kyr ago. The coral community was a reef crest–upper reef slope community, with Acropora and Goniastrea as the dominant genera and Acropora digitifera and Goniastrea retiformis as the dominant species from the late Holocene to present. Millennial-scale persistence of A. digitifera and G. retiformis may have been influenced by a strong Kuroshio Current, increased genetic diversity, and an increased potential for adaptation to environmental conditions. Our coral cover and species composition results provide important information for the development of effective reef restoration plans in light of the likely future degradation of reefs.