[A-CG51] Coastal Ecosystems-2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG51-P06] Changes in coral distribution and diversity before and after the 2016 large-scale coral bleaching in Amitori Bay, Iriomote Island, Japan

*Uji Yosuke¹, Tomokazu Murakami², Shinya Shimokawa² (1.University of Tsukuba, 2.National Research Institute for Earth Science and Disaster Resilience)

Keywords:Iriomote Island, Amitori Bay, Coral bleaching, Coral distribution, Coral diversity

Coral reefs are the richest source of biodiversity in the oceans and provide mankind with rich marine resources and economic benefits as tourist destinations. In recent years, however, coral reefs have been threatened by coral bleaching worldwide caused by high sea temperatures due to global warming. It is important to investigate the changes in coral distribution and diversity before and after bleaching in order to conserve coral reefs. Amitori Bay in Iriomote Island is a place rich in nature with a variety of corals due to the large physical environmental gradient and low human impact. Large-scale bleaching occurred in Amitori Bay in 2016. Therefore, in this study, changes in coral distribution were clarified by analysing coral coverage based on codrait photographs taken before and after this bleaching. The Shannon-Wiener’s diversity index H' (Ref. 1) was then calculated based on the coral coverage to reveal changes in diversity. Furthermore, data on the planar distribution of significant wave heights and number of soil particles in Amitori Bay calculated by observation and numerical simulation (Ref. 2) were organised to obtain the average significant wave heights and number of soil particles at the sites where the cover analysis was conducted, and the relationship between coral cover and diversity and significant wave heights and number of soil particles was analysed. The following results were obtained. (1) Branching and tabular corals were dominant before the bleaching, but after the bleaching, branching and tabular coral coverage decreased by more than 80% and were no longer dominant(Fig.1). (2) Massive corals showed the smallest rate of decline at around 10%(Fig.1). (3) Among branching corals, Acropora had the greatest coverage before the bleaching, but after the bleaching, the coverage of Acropora decreased significantly, and the coverage of Pocillopora became the maximum. In addition, the coverage of Pocillopora increased compared to that before the bleaching(Fig.2). (4) More sites had increased diversity index after the bleaching. (5) Tabular corals were dominant at sites with the greatest significant wave height. (6) Massive and encrusting coral coverages were greater at sites with the highest number of soil particles. (7) The intermediate disturbance hypothesis was not valid for the relationship between the diversity index and the physical environment. The reasons for (1) and (2) are considered to be that branching and tabular corals grow faster but are less tolerant of bleaching, while massive corals grow slower but are more tolerant of bleaching. (3) suggests that Acropora was greatly reduced by the bleaching, allowing Pocillopora to move into the space created by the bleaching due to its spawning cycle. (4) suggests that diversity may become richer when the dominant life forms and genera are reduced by bleaching. (5) and (6) are consistent with previous studies, while (7) is not consistent with previous studies. The latter may be due to the small number of survey sites. In this presentation, we will also show the results of increasing the number of sites for the coverage analysis.

References:
1.Clarke K. R, Gorley R. N, Somerfield P. J, Warwick R. M(2014)CHANGE IN MARINE COMMUNITIES: An Approach to Statistical Analysis and Interpretation 3rd edition. PRIMER-E Ltd, Plymouth, 262p.
2.Shimokawa S., Murakami T., Ukai A., Kohno H., Mizutani A., Nakase K.(2014a)Relationship between coral distributions and physical variables in Amitori Bay, Iriomote Island, Japan. Journal of Geophysical Research : Oceans, 119, 8336-8356.