An improved estimation of the poleward expansion of coral habitats based on the inter-annual variation of sea surface temperatures

Coral Reefs (2015) 34:1125–1137 DOI 10.1007/s00338-015-1347-2 REPORT An improved estimation of the poleward expansion of coral habitats based on the inter-annual variation of sea surface temperatures S. Takao1 • H. Yamano2 • K. Sugihara2 • N. H. Kumagai2 • M. Fujii1 Y. Yamanaka1 • Received: 12 December 2014 / Accepted: 3 September 2015 / Published online: 11 September 2015 Ó The Author(s) 2015. This article is published with open access at Springerlink.com Abstract The poleward expansion of coral habitats has been observed along the Japanese coast since the 1930s. Previous modeling studies have projected a poleward expansion using decadal-mean sea surface temperatures (SSTs) in the coldest months. However, this poleward expansion could be affected by the inter-annual variation of SST in the coldest months, which has not been considered before. In this study, the simulated pattern of poleward expansion was compared between cases where coral mortality was considered based on the inter-annual variation of SST and the decadal-mean SST in the coldest months. Modeled monthly mean SSTs for historical and future global warming simulations from the most recent climate projection model (MIROC4h) were used. The poleward expansion of corals simulated by considering mortality based on the inter-annual variation of SST in the coldest months better reproduced the observed poleward expansion speed compared to the simulations without such a consideration. Our results show the importance of considering coral mortality based on the inter-annual variation of seawater temperature to produce a more realistic poleward expansion of coral habitats. Communicated by Biology Editor Prof. Brian Helmuth & S. Takao 1 Faculty of Environmental Earth Science, Hokkaido University, North 10 West 5, Kita-ku, Sapporo, Hokkaido 060-0810, Japan 2 Center for Environmental Biology and Ecosystem Studies, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan Keywords Poleward expansion  Coral reefs  Future projection  Global warming  Inter-annual variation  Sea surface temperature Introduction Zooxanthellate scleractinian corals (hereafter corals) provide essential habitats and energy for thousands of associated organisms and are also valuable resources for tourism (e.g., Moberg and Folke 1999). Therefore, any shift in their distribution due to climate change could have major implications for biodiversity, ecological function, biogeochemical cycling, and human society. The distributional limits of marine species are closely related to their limits of thermal tolerance (Sunday et al. 2012), and both extremely high and low seawater temperatures could affect the distribution of suitable habitats for corals (Saxby et al. 2003; Hoegh-Guldberg et al. 2005). Rising seawater temperatures due to global warming have enhanced the poleward expansion of coral habitats by lessening lowtemperature stress in winter in northern coral communities. Recently, Yamano et al. (2011) reported the first largescale evidence of a poleward expansion of corals, based on 80 years of national records from the temperate areas of Japan. The rate of this expansion has reached up to 14 km yr-1, although the sporadic cooling of seawater in winter, which may cause coral bleaching and mortality, could limit the poleward expansion of corals (Saxby et al. 2003; Hoegh-Guldberg and Fine 2004; Hoegh-Guldberg et al. 2005; Nomura 2009; Yamano and Namizaki 2009). Previous field studies reported that coral bleaching with low-temperature stress could happen within several days in the southern Great Barrier Reef (Hoegh-Guldberg et al. 2005) to one month around Japan (Yamano and Namizaki 123 1126 2009), while laboratory experiments showed that coral bleaching occurred after brief (12–18 h) exposure at low temperatures (Saxby et al. 2003). These studies indicated that low-temperature exposure for several days to 1 month would lead to coral bleaching, and this would constrain new coral settlement beyond the distributional limits. Japan is uniquely suited for assessing the poleward expansion of corals due to climate change for several reasons. First, Japan extends across a wide latitudinal range, covering subtropical to temperate areas (Fig. 1), and the latitudinal limits of coral reefs and coral distributions have been reported around the Japanese islands (Veron and Minchin 1992; Yamano et al. 2012). Second, the Japanese islands form an almost continuous chain, with subtropical and temperate corals able to inhabit the extent of the latitudinal gradient, due to the warm Kuroshio Current and its branches (Fig. 1; Veron and Minchin 1992; Sugihara et al. 2009). Third, seawater temperatures have been rising more rapidly around Japan than in most of the other oceans in the world (Japan Meteorological Agency 2013). The average SSTs around Japan have risen by ?1.08 °C per century from 1891 to 2012, which is twice the corresponding value for the world’s oceans (?0.51 °C per century). Finally, corals around Japan have been monitored and documented periodically since the early twentieth century (Yamano et al. 2011), providing a much-needed baseline against Fig. 1 Location of the study sites around Japan where on-site monitoring of the poleward expansion of coral habitats was conducted (eight sites, dots) or has subsequently been examined (two sites, stars). Gray arrows indicate the paths of the Kuroshio and Tsushima Currents 123 Coral Reefs (2015) 34:1125–1137 which to assess the impacts of changes in seawater temperatures on their distribution. Many studies have projected the future impacts of rising seawater temperatures on coral habitats using high- and/or low-temperature stress indices (Donner et al. 2007; Donner 2009; Teneva et al. 2012; van Hooidonk et al. 2013; Yara et al. 2009, 2011, 2012, 2014). Most studies have used a high-temperature stress index such as degree heating months, which is calculated from positive anomalies relative to mean water temperatures in the warmest months, to project the area and frequency of coral bleaching in several regions (Donner et al. 2007; Donner 2009; Teneva et al. 2012; van Hooidonk et al. 2013; Yara et al. 2009, 2014). Alternatively, some studies (including this one) have used low-temperature indices, which are defined from absolute water temperature values in the coldest months, based on the current distributional limits of coral reefs and corals, and have projected a poleward expansion of coral habitats (Yara et al. 2009, 2011, 2012). Yara et al. (2011) investigated the potential future poleward expansion of three coral communities in response to rising sea surface temperatures (SSTs) along the coasts of Japan, based on decadal-mean SSTs in the coldest months obtained from multiple climate models and simplified indices of poleward expansion. However, the estimated decadal-mean speeds of coral expansion were one order smaller than those reported by Yamano et al. (2 (...truncated)