Large tsunamis reset growth of massive corals

Kazuhisa Goto,Keitaro Miyazawa,Akifumi Hisamatsu,Masashi Watanabe,Chuki Hongo

doi:10.1186/s40645-019-0265-2

Kazuhisa Goto, Keitaro Miyazawa + Show 3 more

Open Access

https://doi.org/10.1186/s40645-019-0265-2

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Abstract

Corals at Ishigaki Island, Japan, are characterized by their high species diversity. Not only are they struck by storm waves generated annually by typhoons, the corals, especially the massive ones, in the fringing reef were buffeted by huge tsunami waves with a run-up height of ca. 30 m in 1771 Meiwa tsunami and its predecessors at few hundred-year intervals. We present field survey and numerical results demonstrating that such near-field large tsunamis could have reset the growth of massive corals, a phenomenon which large typhoons have not caused. Our field survey revealed that the massive corals in the lagoon are not attached to the bedrock but are instead located on the sandy sea bottom. Therefore, those are movable of sufficiently large wave inundated in the lagoon. Our numerical results further showed that the maximum velocity of the tsunami at the reef edge, calculable as < 21.2 m/s at the study area, is still high in the shallow lagoon, perhaps generating sufficiently strong hydrodynamic force to devastate the massive corals in the shallow lagoon entirely, as well as some presumed damages on tabular and branching corals on the reef crest and reef slope. This numerical result is consistent with the observed fact that even a 9-m long Porites boulder (about 220 t) was cast ashore by the 1771 tsunami. The sizes of the presently living massive corals of Porites spp. are consistent with our hypothesis that they started to grow after the latest 1771 tsunami event. At the coral reefs of high tsunami-risk countries, severe destruction of corals by large tsunami waves should be considered for their growth history because, depending on the bathymetry, coral characteristics, and tsunami hydrodynamic features, tsunamis can radically alter coral habitats.

Highlights

Physical perturbation of corals by extreme waves can generate a remarkably strong influence on marine ecosystems (Madin and Connolly 2006; Puotinen et al 2016)
It is usually considered that local conditions should have affected the growth history of corals and coral reefs
Catastrophic destruction of coral by the tsunami would have been one important factor affecting the coral growth at Ishigaki Island because historical and geological evidence indicates that small and large tsunamis in the past frequently struck eastward of the island at 150 to 400 or 600-year intervals (e.g., Araoka et al 2013; Ando et al 2018)

Summary

Introduction

Physical perturbation of corals by extreme waves can generate a remarkably strong influence on marine ecosystems (Madin and Connolly 2006; Puotinen et al 2016). Coral damage is generally related to the place where the tsunami hydrodynamic forces strengthen (Goto and Imamura 2009; Kawamata et al 2009), the relation is unclear. Coral damage by the 2004IOT at the islands in Thailand was high in straits among the islands and at the northern and southern headlands of the respective islands (PMBC 2005), where the tsunami hydrodynamic forces are stronger (Kawamata et al 2009). Coral damage by the 2004IOT was generally low

Results

Discussion

Conclusion