When corals are stressed by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic algae living in their tissues, causing them to turn completely white.
In a huge bleaching event this year, over one third one-third of the coral reefs of the central and northern regions of the Great Barrier Reef have died, Queensland researchers said.
Corals north of Cairns which cover a last portion of the Great Barrier Reef , north of Australia had a record highest mortality rate of about 35 % rising to about 50 % in some other areas.
Area affected by Coral Bleaching
Professor Terry Hughes, director of the ARC Center of Excellence for Coral Reef Studies at James Cook University, said he was “gobsmacked” by the scale of the coral bleaching which far exceeded the two previous events in 1998 and 2002.
Bleached corals in the Great Barrier Reef
It is fair to say we were all caught by surprise,” Professor Hughes said. “It’s a huge wake up call because we all thought that coral bleaching was something that happened in the Pacific or the Caribbean which are closer to the epicenter of El Nino events.”
The El Nino of 2015-16 was among the three strongest on record . Reefs in many regions, such as Fiji and the Maldives, have also been hit hard.
Abnormal conditions such as warm seas cause corals to expel tiny photosynthetic algae, called Zooxanthellae. Corals turn white without these and therefore cannot perform photosynthesis and therefore die if they are not recolonized by these photosynthetic algae.