Spatial and temporal variations in coral reef recovery amid recurring bleaching events in Okinawa Island, Japan

Abstract

Climate change is altering coral reefs around the world at an unprecedented rate, with increasing frequency and magnitude of mass bleaching events caused by high sea surface temperatures (SSTs). In this study, we examined coral communities through reef transect surveys at two depth zones (1-5m and 7-12m) across 11 sites on Okinawa Island, Japan. This study focused on trends in coral cover and composition in Okinawa Island, particularly following the thermal stress events of 2017 and 2022 and the high SSTs of 2023. We assessed coral community composition and bleaching incidence across coral genera, sites, years and depths. In 2017, the overall bleaching percentage was 33.28%, whereas by 2022, bleaching incidence was 9.47%. Sensitivity to bleaching varied among coral genera, with those typically exhibiting massive to encrusting growth forms, such as Porites, Montipora, Goniastrea, Favites, and Platygyra, being more impacted compared to branching and tabular species. Bleaching incidence also differed between depth zones, with 36% overall bleached in shallow depths and 27% in deeper depths in 2017. Despite recurrent bleaching events, hard coral cover across Okinawa Island increased from 13.85% in 2017 to 28.47% by 2023, demonstrating recovery potential despite recurrent bleaching events. Our findings indicated that recurrent bleaching events could alter the bleaching susceptibility and abundance of key coral genera over spatial and temporal scales. Considering the current state of the Okinawan reefs, developing and implementing effective management and protection strategies remains critical to reduce the impacts from global and local stressors.

Keywords: Coral bleaching; Coral communities; Okinawa island; Reef recovery; Thermal stress.