Colony of elkhorn corals (Acropora palmata) showing rapid mortality at Sombrero Reef, Fla., on July 20, 2023. Yellow-brown patches on the colony are locations of tissue loss. [Credit: Coral Restoration Foundation]
A NOAA-led study recently published in Science confirmed that a record marine heatwave in 2023 triggered the functional extinction of two coral species in Florida waters—elkhorn (Acropora palmata) and staghorn (Acropora cervicornis) corals.
“Functional extinction means that a species' numbers are so small that it stops playing a significant role in an ecosystem,” said Derek Manzello, coordinator of NOAA’s Coral Reef Watch program. “Elkhorn and staghorn corals are among the most ecologically important and critically endangered reef-building corals on Florida’s Coral Reef. Based on the survey data, these two species are now completely gone from the Middle and Lower Florida Keys, as well as the Dry Tortugas.”
During this survey, Manzello and NOAA’s Coral Reef Watch (CRW)—a satellite-based tool—monitored the marine heat wave in near-real-time, directly communicating with partners on-the-ground about the unprecedented heat stress event as it happened.
“Ocean temperatures as little as two or three degrees Fahrenheit warmer than normal can cause corals to become stressed and experience bleaching,” Manzello said. “The damage that corals experience from marine heatwaves is a function of the magnitude of the temperature anomaly plus the duration of the heat stress.”
Corals can usually recover from bleaching if the heat stress subsides, but lasting consequences include impaired growth and reproduction, as well as increased susceptibility to disease. If the heat stress does not subside quickly enough, the coral will die.
A CRW map of sea-surface temperature anomalies around Florida and the Bahamas on July 13, 2023. [Credit: NOAA]
Manzello and his team found that the 2023 marine heatwave—the ninth mass coral bleaching along Florida’s Coral Reef since 1987—was the reef’s worst on record. “The 2023 heatwave in Florida was unprecedented,” Manzello noted. “It started earlier, lasted longer and was more severe than any previous event in that region.”
Their study found that around the Florida Keys and Dry Tortugas, 98–100% of the elkhorn and staghorn colonies died. Along southeastern Florida where cooler sea temperatures were found, mortality was lower—nearly 38%. “Since the late 1970s, multiple stressors had already reduced the ecological relevance of Acropora corals in Florida, but the 2023 heatwave marked their functional extinction from Florida’s Coral Reef,” Manzello concluded.
While the CRW tool provided critical information for understanding the intensity and scope of the mass bleaching event, it also allowed NOAA to engage in intervention methods to help mitigate the harm to the corals.
CRW was able to pinpoint single-pixel virtual stations at key restoration and nursery sites where coral salvation and relocation efforts were most urgent. Through its Mission: Iconic Reefs program, which focuses on seven critical habitats in the Florida Keys National Marine Sanctuary, NOAA also made significant efforts to offset some of the negative impacts by moving coral nurseries to deeper, cooler waters and salvaging the genetic diversity of the corals by transferring some of them to land-based rescue stations.
Manzello emphasized that monitoring the health of our nation’s coral reefs with Earth-observing satellites is truly a team effort. “The ability of CRW to provide actionable information for our numerous partners in state and federal government, academia, and non-governmental organizations illustrates the unique value NOAA’s Coral Reef Conservation Program provides to the management of the nation’s coral reef resources.”