Marine Heatwaves and Coral Bleaching Potential in the Java Sea During the 2016 Extreme Thermal Event

Abstract

The intense thermal event of 2016 in the Java Sea was analysed to assess the link between marine heatwave (MHW) and coral bleaching potential. We used daily sea surface temperature data from 1982 to 2023 to detect marine heatwaves using the 90^th percentile threshold and generate coral bleaching heat stress in the form HotSpot and Degree Heating Weeks (DHW). The analysis was carried out in the region of the Java Sea and three reef-associated sites, namely Kepulauan Seribu, Karimunjawa, and Kangean. The results demonstrate that the 2016 event was a global marine heatwave, but that bleaching-level heat stress was geographically and temporally selective. The greatest area exposed to DHW ≥4°C-weeks was 26.3% of the entire Java Sea area, or around 182,000 km². Alert Level 2 was not observed, as no area achieved DHW ≥8°C-weeks. The Kangean site saw the highest bleaching potential among the three reef locations, with a maximum DHW of 4.87°C-weeks and a duration of 53 days. Kepulauan Seribu has minimal accumulated heat stress, and Karimunjawa was slightly below Alert Level 1. These results show that MHW incidence alone is not enough to represent coral bleaching potential, HotSpot and DHW give a better basis for reef monitoring and management.