Corals at Scott Reef.
Credit: N. Thake
Coral reefs may be more independent and resilient than previously thought.
New research shows that an isolated reef off the northwest coast of Australia that was severely damaged by a period of warming in 1998 has regenerated in a very short time to become nearly as healthy as it was before. What surprises scientists, though, is that the reef regenerated by itself, found a study published today (April 4) in the journal Science.
Until now, scientists have thought that damaged reefs depend on new recruits from nearby reefs to quickly heal themselves, said study author James Gilmour, a researcher at the Australian Institute of Marine Science. But this study found that may not always be the case ― at least with reefs like this one, which has good water quality and isn't heavily impacted by humans, Gilmour told OurAmazingPlanet in an email.
In 1998, unusually warm weather heated up waters off the northwest coast of Australia by about 4 degrees Fahrenheit (2 degrees Celsius) above average. These temperatures persisted for several weeks.
The heat led to the bleaching of the corals, in which corals kick out the tiny symbiotic algae housed within them that provide corals food. If the water's temperature quickly returns to normal, the coral can recover. But often, it dies, becoming a white skeleton of its former self.
The 1998 event killed 70 percent to 90 percent of corals in various parts of the reef, and the number of coral embryos collected by researchers monitoring the reef dropped to almost zero. Gilmour said this shows that the remaining corals weren't reproducing and that there weren't any coral embryos washing in from surrounding reefs. Recovery was expected to take many decades, Gilmour said. [Stressed Coral: Photos of Great Barrier Reef]
At first, the reef grew slowly, mostly through the enlargement of existing coral colonies. But to really recover, the coral needs to sexually reproduce, creating sperm and egg that form embryos that then land on the ocean floor and grow into adult corals ― if all goes well. These larvae can survive for hundreds of miles, swept along by ocean currents, and colonize new areas under the right circumstances.
Larvae floating in from other reefs could have helped the reef, had it not been so isolated.
But amazingly, after about six years, the surviving corals matured and began to reproduce, creating even more new colonies than before the bleaching. "They recovered, and the larvae they produced settled and survived, at much higher rates than is often reported," Gilmour said. By 2012, the reef was basically back to its old self.
The study suggests that, when it comes to reefs, being isolated from human activity may trump being connected to other reefs. Why? Human activities can hurt reefs in a number of ways. Overfishing, for example, removes fish that keep algae from choking out and outcompeting corals, and sediment and pathogens in runoff water can lead to coral diseases and death.
But the results also mean that local decisions about fishing and other issues can help preserve reefs, which are threatened by global warming. "Managing local conditions is a tangible way to maximize the resilience of coral reefs while the more difficult problem of addressing the causes of climate change are resolved," Gilmour said.