[A-CG41] Coastal Ecosystems -2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG41-P02] Diversity of coral distributions and the difference in Sakiyamawan-Amitoriwan nature conservation area, Iriomote Island, Japan

*Shinya Shimokawa¹, Tomokazu Murakami¹, Hiroyoshi Kohno², Akira Mizutani² (1.National Research Institute for Earth Science and Disaster Resilience, 2.Okinawa Reagional Research Center, Tokai University)

Keywords:Reef building coral, Diversity index, Intermediate disturbance hypothesis

The Sakiyamawan-Amitoriwan nature conservation area, Iriomote Island, is the only oceanic nature conservation area in Japan. The area has no access roads; therefore, it conserves a natural environment with very little human impact and various coastal marine ecosystems such as those containing reef-building corals and Enhalus acoroides (a kind of seagrass; its northern distribution limit is the ocean area surrounding the Yaeyama Islands, Japan). Sakiyama and Amitori bays are adjacent, but the coral distributions in both bays are largely different because of the difference of the geographical environmental gradients such as water depth, bay length and bay shape.
In this study, the difference of coral distributions between both bays are investigated from a view of diversity index (MacArthur and MacArthur, 1961, Clark & Warwick, 2001, McCune and Grace, 2002) and intermediate disturbance hypothesis (Connell, 1978) based on the results of coral distribution field survey, physical environmental observation such as wave height and soil particle number, and numerical simulations using the observation data that we have conducted since 2011 (e.g., Shimokawa et al., 2014, 2015, 2016, 2020).
In Amitori Bay, the diversity index (here, based on colony shapes) was at the maximum in the middle part of the bay, and intermediate disturbance hypothesis was realized by considering wave height and soil particle as external disturbance. Also, Enhalus acoroides is scarce and there is no large community. On the other hands, in Sakiyama Bay, the diversity index was at the maximum in the edge of coral reef and intermediate disturbance hypothesis was not realized. Also, Enhalus acoroides is distributed broadly from the inner part of the bay to the middle part of the bay. In the presentation, the reason of the difference between both bays is also stated from a view of the difference of the effect of soil particle inflow due to geographical environmental gradients between both bays.

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