What is coral bleaching?
Coral colonies are of made up of many individual animals, called polyps. Most corals have small algae living in their tissue, called zooxanthellae. The algae provide the coral with nutrients, while the corals provide the algae with shelter and carbon dioxide. The tiny algae have a big job and can provide up to 90% of the nutrients corals need to grow. The pigment in algal cells also provides coral with their vivid colours. This relationship exists in a fine balance.
If the coral polyp becomes stressed by changing environmental conditions (increased temperature, excess solar radiation(UV), nutrient pollution, etc.), then it expels the algae in a process known as coral bleaching. Bleached corals are still alive, but can only exist in this state for a period of time and will ultimately perish. Generally, if the stressor is removed within a few weeks, the coral can recover the algae and survive (albeit it a weakened state). After a bleaching event, the coral may need years to fully recover; experiencing reduced reproduction and growth, as well as increased vulnerability to disease.
- GBRMPA Coral bleaching factsheet
- GBRMPA Coral Bleaching on the Great Barrier Reef video
- GBRMPA Coral bleaching basics: causes and past events
- Learn more about mass bleaching from Australian Institute of Marine Science
- A very thorough video on the process of coral bleaching from HHMI BioInteractive
- Ocean Ark Alliance resources
- Information about GBR health from Great Barrier Reef Foundation
Mass coral bleaching in 2016 & 17
Global coral bleaching on the Great Barrier Reef in 2016 and 2017 has led to widespread declines in coral cover. Impacts were variable across the Reef, with the most extreme impacts in the northern GBR (2016) and central sections of the GBR (2017), where an estimated average two-thirds of corals died (based on assessments of 60 reefs in the region). That is the largest coral loss ever recorded for the GBR from a discrete event. Prior to this event, the northern area had been considered one of the most pristine and healthy areas of the reef. Generally, there were lower levels of bleaching impacts recorded moving southward along the huge 3,200 kilometre span of the Reef. Many reef areas in the southern section of the Reef reported little to no loss of corals from coral bleaching.
In February 2017, reports indicated that varying levels of coral bleaching were being seen on parts of the Great Barrier Reef and in Sydney. Water temperatures remained warm over the winter, prolonging the stress and currently sea temperatures are approximately 2 degrees celcius warmer than average.
- Current bleaching update from Great Barrier Reef Marine Park Authority
- Observe global oceans through Coral Reef Watch Satellite Monitoring
Collecting citizen science data for coral bleaching
Around the world, Reef Check teams are tracking bleaching impacts on their reefs. The globally-standardised protocols of Reef Check offer the platform to understand this event at a global scale.
- Eye on the Reef: Report your Sightings and photos of what you have seen out on the reef through the Eye on the Reef app. To take your reporting to the next level, participate in the online training course to undertake Rapid Monitoring Surveys to document key indicator species and crucial reef health observations.
- CoralWatch: Learn more about coral bleaching with the Coral Reefs and Climate Change book and online resources. Then, grab a Do It Yourself Kit to document the colour of corals on reefs you visit.
- REEFSearch: Share your photos and findings via the REEFSearch education and observation program.
- Join a survey team: Help collect reef monitoring data by joining as a trained volunteer to undertake globally-standardised reef health surveys.