Acute heat stress and the extirpation of a threatened coral species from a remote, subtropical reef system

The ecological significance of the reef-building elkhorn coral, Acropora palmata, is threatened by heat-stress-induced mortality. The intensity and duration of the ocean heatwave affecting Dry Tortugas National Park in the summer of 2023 was historically unprecedented in its early timing and maximum temperatures reached and resulted in 100% A. palmata mortality. To understand the lethality of this event, we examined temperature data measured by in situ data loggers and estimated from satellites (National Oceanic and Atmospheric Administration’s Coral Reef Watch) to investigate the timing of peak ocean temperatures and coral mortality. The in situ dataset revealed warmer water temperatures during the heatwave than those reported from the satellites, and the difference between the datasets was significantly pronounced during the summer. Our results support that, at least for this subtropical population, A. palmata may have an upper-threshold temperature that caused lethality faster than expected from the gradual accumulation of heat stress.