Award: OCE-2347805

Project Outcome Report for NSF award 2347805 - RAPID: A comparison of acute heat stress and fish abundance influencing coral survival by Dr. Michael Childress, Principal Investigator. Coral reefs are one of the most endangered communities on the planet due to multiple stressors such as rising sea surface temperatures, nutrient pollution, overfishing, and the accumulation of marine debris. But what is often overlooked are features of the community that confer resilience to these stressors and potentially give us reason for hope. This study examined the fate of the coral reef community of the middle Keys in the Florida Keys National Marine Sanctuary after the devastating marine heat wave of 2023. Extreme temperatures, often exceeding 95 degrees F at the bottom, caused widespread hard coral bleaching and significant mortality in soft corals. However, much to our surprise, over 95% of the hard corals that bleached survived this extreme temperature event. Our analysis of how the community responded before, during, and after this event gives us insight into the key features of resiliency that seem to be at work. First, despite the death of soft corals (sea fans, sea rods, sea whips, etc.) fish communities remained relatively unchanged. The only decrease in reef fish abundance we observed were the abundance of reef predators inshore, likely due to their movement toward deeper and cooler waters. After the heat wave passed, these top predators returned to nearshore reefs at the levels they were before the heat wave. Important reef fishes, such as herbivorous parrotfishes, remained at high levels throughout the heat wave. Second, small cryptic invertebrates that make up the base of the reef food chain actually increased their biomass during the heat wave while shifting slightly toward more worm taxa. Third, while soft coral cover decreased due to mortality, they were replaced by increases in turf algae rather than fleshy macroalgae. Fleshy macroalgae, known to stress and compete with hard corals, actually decreased during the marine heat wave. Taken together this study reveals that coral reef communities have multiple mechanisms of resiliency that resist changes caused by extreme heat events. Hard corals that are slowest to grow and take the longest to replace, were largely unaffected by this extreme heat wave because most sensitive individuals have perished in previous heat waves and disease outbreaks. The soft corals lost are known to recruit quickly and have the potential to repopulate within a few years as they did after the disturbance of Hurricane Irma in 2017. The stability of the cryptic invertebrates and reef fish communities indicates that despite the extreme nature of this heat wave on corals, that the ecosystem processes including primary productivity, primary consumption, and secondary consumption remained relatively undisturbed. How long will this last given the slow decline of hard coral cover is hard to know, but so far resiliency is maintaining a functional coral reef ecosystem in the face of a rapidly changing environment.

Last Modified: 03/02/2026
Modified by: Michael J Childress