Catastrophic bleaching in protected reefs of the Southern Great Barrier Reef

The iconic Great Barrier Reef (GBR) experienced mass coral bleaching in early 2024. In the southern GBR, heat stress triggered severe and widespread bleaching to levels not previously recorded and impacted a diverse range of coral genera at One Tree Reef (OTR). Over 161 d, we tracked the health of 462 coral colonies from heatwave peak to autumn and winter cooling. In February and April, 66% and 80% of the colonies were bleached, respectively. By May, 44% of the bleached colonies were dead and 53% in July. In July, 31% of colonies were still bleached and 16% recovered. Goniopora developed black band disease contributing to high mortality. Colony collapse occurred in Acropora (95% mortality) with accumulation of algal-fouled fragments. In-water tracking of individual colonies showed rapid bleaching, disease onset and mortality. The protected status and offshore location did not protect OTR from heat stress bleaching and mortality.