Adaptation Advantage on Coral Reefs

"It's one enormous natural selection event."
"Despite the fact that Year 1 was hotter, we saw less bleaching."
"[A shorter lapse of time between bleaching events has importance] because it means we no longer have the luxury of studying these bleaching events as if they were rare with a lot of time between them for a full recovery."
"We have to study them as not just stand-alone events, but sequences of events that are interacting with each other."
Terry Hughes, coral reefs expert, James Cook University, Australia

"Baselines are shifting and changes occurring so rapidly that it may be difficult to predict if this pattern will continue."
Kuulei S. Rodgers, researcher, University of Hawaii, Manoa

Researchers found that the response of corals to heat stress during the second of two unprecedended back-to-back bleaching events on the Great Barrier Reef was markedly different from the first. Credit: Tane Sinclair-Taylor

In 1998 and in 2002 the Great Barrier Reef experienced mass bleaching. According to scientists, bleaching was seen to have occurred for the first time, two years in a row, between the years 2016 and 2017. Coral reefs support an estimated quarter of the world's marine species, providing 17 percent of the animal protein eaten by humans, according to the United Nations.

Coral reefs are constructed as colonies with tiny animals known as coral polyps secreting layers of calcium carbonate under their bodies as they build the structure they live upon. A symbiotic relationship is formed between the coral polyps building the reef and microscopic algae called zooxanthelae. The limestone structure that results protects the coral polyps. The bright reds and purples that distinguish the reefs result from algae thriving in their cells providing oxygen to allow the corals to grow.

A researcher accesses minor damage at Day Reef on the Great Barrier Reef following the March 2016 mass coral bleaching event.  Credit: ARC Centre of Excellence for Coral Reef Studies/ Gergely Torda

In 2016 a heat wave destroyed many of the most heat-sensitive corals, leaving the corals that proved capable of withstanding higher ocean temperatures. Warming oceans threaten the survival of coral reefs; as they become harmed by the change of water temperature the vibrant colours become bleached out. 

Researchers discovered after a surge in ocean temperature around the Great Barrier Reef in 2016 that surviving corals became more resistant to a follow-up period of extreme warming the year that followed. Their study appeared last month in the journal Nature Climate Change.

Terry Hughes and a team of researchers undertook that study. They found that when parts of the reef were exposed to four to eight degrees of warmer temperature than normal for the ocean, approximately fifty percent of the coral bleached within four to five weeks in 2016. However, the coral had to be exposed to those same warm temperatures for eight to nine weeks in 2017 before it succumbed to the same level of bleaching.

According to Dr. Hughs's findings, ecological memory was in play; the theory behind a past experience of a biological community influencing its ecological response following the event to the present, or into the future. Too-cold temperatures of ocean water and the corals will respond negatively just as too-warm temperatures also threaten the integrity of the corals by having the algae and corals separate, to leave the corals stripped of their colour.

In turn, bleached corals become more susceptible to infection and death. Yet the good news is that not all corals are at risk. It takes about ten years for a reef to fully recover from an incident of bleaching. The study concludes that there is hope the coral reefs may be capable of surviving as oceans continue to warm, given the identified pattern of harm and recovery.

ThoughtCo