Record Water Temperatures Testing Resilient Coral Reefs

Coral reefs are some of the most diverse and beautiful ecosystems on the planet. Roughly 25% of the ocean’s fish and over half a billion people depend on these underwater habitats, according to the National Oceanic and Atmospheric Administration (NOAA). But, like most other ecosystems, coral reefs are being threatened by the impacts of global climate change. This summer, record water temperatures stemming from a strong El Niño weather pattern are causing coral bleaching and death events in reefs throughout the world.

Lisa Rodrigues, PhD, professor of environmental science, is an expert in coral reef ecosystems and ocean environments. She has been following the reports of coral bleaching when they began surfacing in July.

“Coral bleaching often coincides with El Niño years,” she said. “Since water takes much longer to heat than air, July is very early in the year for water temperature to be so high and that is one of the main causes for concern.”

Coral bleaching is a stress reaction emanating from high water temperatures, among other causes. When stressed, corals expel the microscopic algae inside of their tissues. The absence of these algae reveals corals’ white skeletons.

“In a healthy coral system there is a symbiotic relationship between the coral host (an animal) and the endosymbiotic algae (a plant),” Rodrigues said. “Corals prefer to live in a fairly narrow range of temperature, which is typically when the symbiotic relationship can be successful. During bleaching, the symbiotic relationship is broken down and the two partners no longer live together.”

If corals’ energy stores are low, or if the relationship is compromised for an extended period, corals can die.

“This can have long-lasting and negative impacts on ecosystems, as a healthy structure provides a habitat for fish and other organisms,” she said. “For humans that live near the coast, loss of reefs means lost coastal protection, lost economic revenue from fishing and tourism and a lost piece of their natural history.”

Reports of coral bleaching and death have stretched across the Americas and the NOAA is issuing bleaching warnings in southeast Asia. Water temperatures off the coast of Florida this summer have been measured at over 100 degrees in some areas, and temperatures could continue to rise into September.

Despite dismal conditions, especially in the Gulf and Caribbean waters, there is still hope for these reefs.

“Death doesn’t always happen following bleaching and we also know that recovery from bleaching can occur,” Rodrigues said. “We have learned a lot from past bleaching events. Over the longer term following a bleaching event corals and coral reefs can recover, but the process is slow and dependent on the stressor(s) that caused the event in the first place.”

Rodrigues notes there is a species of coral in Hawai’i that is able to sustain itself during bleaching events and can even reproduce, proving these underwater marvels won’t go down without a fight.

“It’s unknown how many species are able to do this, but there is evidence that corals have a wide array of susceptibility and resilience to bleaching associated with high temperatures.”