Key species of hermatypic coral for reef formation in the northwest Pacific during Holocene sea-level change

Abstract

Cores from Holocene reefs provided a growth history and species-level identification of corals and demonstrated the most important reef builders during the formation stage. This knowledge is important to determine a principle for reef formation and to provide preservation plans in the near future. A biological and sedimentological study of sediment cores recovered from the Palau Islands and Yoron Island, northwest Pacific, revealed four major facies: corymbose Acropora, arborescent Acropora, massive Porites, and detritus. Species-level observations show that arborescent Acropora (A. muricata and A. intermedia) contributed to reef growth under low- to moderate-energy conditions, whereas corymbose and tabular Acropora (A. digitifera, A. hyacinthus, and A. robusta/A. abrotanoides) and I. palifera were key species for reef formation under high-energy conditions during Holocene sea-level rise and the ensuing period of sea-level stability. Once sea level had stabilized, massive Porites became restricted to areas subjected to low-energy, turbid conditions. These key species are successful corals because the ecological strategy is rapid growth, determinate growth, a high degree of colony integration, strongly resistant to wave action, and rapid local dispersion via fragmentation. Moreover, the western boundary current (Kuroshio) flows along the reefs in the northwest Pacific and it is easy for key species to distribute throughout the region during the period of Holocene sea-level rise and stabilization. These features are a principle for reef formation during sea-level changes. These key species played a significant role in Holocene reef formation in the northwest Pacific; however, coral mortality, caused by climate change, has recently been widely reported. Moreover, the decrease in key species abundance in present-day reefs has been more severe than that in any other species. These geological findings have important implications regarding the appropriate use of coral transplantation and decisions regarding the optimal location and size of marine protected areas.

Introduction

Knowledge of coral reef growth patterns and internal facies during the Holocene has increased greatly in the last 40 years. Reefs from the Indo-Pacific region and the Caribbean have been studied so that it is now possible to determine general and regional patterns of formation, and the relationship between reef growth and corals (e.g., Cabioch et al., 1995, Macintyre and Glynn, 1976, Marshall and Davies, 1982, Montaggioni and Faure, 1997). Previous geological studies have revealed that some genera are important in terms of understanding the above topics. Although more than 200 scleractinian genera are currently found throughout the world (Veron, 2000), few played a part in reef formation during the Quaternary. For example, post-glacial reefs in the Pacific were composed mainly of corymbose and arborescent Acropora (e.g., Cabioch et al., 1995, Hopley and Barnes, 1985, Kayanne et al., 1993, Kennedy and Woodroffe, 2000, Marshall and Davies, 1982, Takahashi et al., 1988). Holocene reefs in the Indian Ocean also comprised mainly corymbose and arborescent Acropora (e.g., Camoin et al., 1997, Gischler et al., 2008, Montaggioni and Faure, 1997). Moreover, the genera Porites and Pocillopora were the main reef builders in the Hawaiian Islands and eastern Pacific (e.g., Cortés et al., 1994, Grossman and Fletcher, 2004, Macintyre et al., 1992). In the Caribbean, reefs were also composed predominantly of Acropora (e.g., Aronson et al., 2005, Blanchon et al., 2002, Gischler and Hudson, 2004, Hubbard et al., 2005, Macintyre, 2007, Macintyre and Glynn, 1976). Other genera (e.g., Favia, Favites, Goniastrea, Platygyra, and Montastrea) were observed in the Indo-Pacific region during the reef-formation stage (Cabioch et al., 1995, Camoin et al., 2004, Marshall and Davies, 1982, Montaggioni and Faure, 1997).

The genus Acropora is an especially important scleractinian taxa on reefs throughout much of the Pacific Ocean and the Caribbean. Although the genera Pocillopora and Porites are represented by fewer than 10 species and approximately 80 species, respectively, Acropora is notable as the most diverse coral, with over 200 species (Veron, 2000). In the Caribbean, the present day and the Holocene reefs were dominated by Acropora palmata and Acropora cervicornis (e.g., Aronson et al., 2005, Blanchon et al., 2002, Gischler and Hudson, 2004, Hubbard et al., 2005). In the Indo-Pacific region, the distribution of Acropora species in the present ocean has been intensely studied (e.g., Veron, 1986, Veron, 2000, Wallace, 1999), but reconstructions of reef growth history are usually based on data derived from growth forms, genera, and combinations of certain species. For example, the Holocene reef at One Tree Reef, Great Barrier Reef, was composed of corymbose and arborescent Acropora (Marshall and Davies, 1982), but species-level identification remains to be performed. In Vanuatu, reefs were dominated by several Acropora species, among other corals (Porites, Astreopora, and Montipora), during the past 23,000 years (Cabioch, 2003), although the relationship between species patterns and reef growth remains unknown. Such results have been reported throughout the Indo-Pacific region (e.g., Cabioch et al., 1995, Camoin et al., 1997, Kan and Kawana, 2006); consequently, the distribution patterns of species during past reef formation remains poorly understood.

Johnson et al. (2008) proposed that species diversity was not an important control on reef growth in the Caribbean. Hongo and Kayanne (2010b) also showed that species diversity is not a prerequisite in terms of Holocene reef growth in the Indo-Pacific and the Caribbean and this study indicated that few species are important for reef growth. For example, at Ishigaki Island in the Ryukyu Islands, the reef consisted mainly of several Acropora species (A. digitifera and A. hyacinthus) and other species during the period of Holocene sea-level change (Hongo and Kayanne, 2009). At Kikai Island, the Holocene reef also comprised corymbose and tabular Acropora (A. digitifera, A. hyacinthus, and A. gemmifera) and Isopora palifera (Ota et al., 2000, Webster et al., 1998). In the Caribbean reefs, reefs were composed of robust and arborescent Acropora (A. palmata and A. cervicornis) during the Holocene (Blanchon et al., 2002, Gischler and Hudson, 2004, Hubbard et al., 2005).

Moreover, the species-level records from these fossil corals suggest that the above corals are likely to be key species for reef formation in the near future. These records are valuable in terms of coral preservation, including the designation of marine protected areas (MPAs) and the selection of appropriate species for transplantation (Dizon and Yap, 2006, Edwards and Clark, 1998, Lindahl, 2003, Soong and Chen, 2003) to address the increasing decline in coral reefs caused by climate change and human impacts (Bellwood et al., 2004, Gardner et al., 2003, Hughes et al., 2003). For example, Acropora has been severely affected by coral bleaching events (Loya et al., 2001) and this genus shows a high degree of endemism; consequently, the loss of Acropora species would cause a decline in reef development in the Indo-Pacific and the Caribbean.

In reefs of the northwest Pacific, bio-lithological descriptions and radiometric dates have been obtained from numerous drilling cores recovered since the 1970s and observations of submarine-trench walls have revealed the detailed internal structure of reefs (Table 1). Data on reefs of the northwest Pacific are available from the Palau Islands (Kayanne et al., 2002), Ishigaki Island (Hongo and Kayanne, 2009, Yamano et al., 2001, Yamano et al., 2003), Kikai Island (Konishi et al., 1978, Konishi et al., 1983, Ota et al., 2000, Webster et al., 1998), Yoron Island (Yonekura et al., 1994), and other reefs (e.g., Hamanaka et al., 2008, Kan et al., 1991, Kan et al., 1995, Kan et al., 1997, Takahashi et al., 1988). However, little is known of the reef builder in other reefs during the Holocene because these data are restricted to genera-level observations, with the exception of Kikai Island (28°20′N, 130°00′E) and Ishigaki Island (24°25′N, 124°10′E). The purpose of the present study is to identify the Holocene species-level record from the Palau Islands (7°24′N, 134°21′E) and Yoron Island (27°02′N, 128°26′E) in the Ryukyu Islands, northwest Pacific and to identify the key species for reef formation and for preservation plans in the near future.