For the last three years, coral bleaching due to ocean warming associated with climate change has impacted coral reefs worldwide. Mass coral bleaching events occur during extended periods of elevated sea surface temperatures and have the potential to result in significant and widespread loss of coral (GBRMPA, 2017).

In 2016, the worst coral bleaching on record impacted the Great Barrier Reef Marine Park. A second consecutive bleaching event occurred in early 2017, the impacts of which are still being quantified.

What is coral bleaching?

Corals are able to survive in a limited temperature range (between 23°C to 29°C, although some species can tolerate up to 40°C for short periods) and when the temperature exceeds this limit, they experience heat stress. When under stress, they expel their symbiotic, microscopic algae (called zooxanthellae) that give coral much of their colour and food. Without their zooxanthellae, coral tissues become transparent, exposing their bright white skeleton underneath. If the stress is prolonged, bleached corals begin to starve without their zooxanthellae to feed them. After they have died, coral skeletons are colonised by larger algae.

Healthy, bleached, and dead coral.

Picture credit: GBRMPA © Copyright Commonwealth of Australia 1996 – 2007

The 2016 coral bleaching event

The Great Barrier Reef Marine Park Authority and Queensland Parks and Wildlife Service undertook in-water surveys during March – June 2016 and then September – November 2016 to assess the severity of the bleaching event and the survivorship of the same reefs six months later. Reefs in the Whitsundays were monitored as part of this process. No in-water surveys were undertaken in the Mackay or Isaac areas.

The first round of surveys documented widespread but patchy bleaching with varying levels of severity throughout the marine park in 2016 (GBRMPA). Reefs in the Whitsunday area sustained minor bleaching.

 

Map showing Great Barrier Reef coral bleaching severity.

Reef-wide pattern of bleaching severity impacts on the Great Barrier Reef in 2016 on 62 reefs surveyed using the Reef Health Impact Survey method. Each circle represents a survey reef and colours indicate severity category (© Copyright Commonwealth of Australia 1996 – 2007 GBRMPA, 2017).

In areas that experienced the most severe coral bleaching subsequent coral mortality increased substantially during 2016, particularly in the north of the Marine Park. The June 2016 surveys showed reefs in the Whitsundays sustained low to no mortality.

Map showing coral mortality.

Reef-wide pattern of observed recent coral mortality due to coral bleaching as at June 2016 on the Great Barrier Reef. Each triangle represents a reef, and colours indicate the percentage of coral cover that died (mortality level)© Copyright Commonwealth of Australia 1996 – 2007 GBRMPA, 2017.

In the southern half of the Marine Park, including in the Whitsundays, the second round of coral surveys (November 2016) found that coral cover remained at similar levels to immediately prior to the mass-bleaching event. On the majority of surveyed reefs, coral bleaching was patchy within transects and restricted to a few individual colonies. Increases in coral cover at the majority of these sites demonstrate the very limited impact of the bleaching event at that time. Only one middle shelf reef surveyed in the Whitsundays sustained medium levels of coral loss.

Map showing coral loss by November 2016.

Reef-wide pattern of estimated coral loss (proxy for coral mortality) as at November 2016 on the Great Barrier Reef. Each triangle represents a reef, and colours indicate the percentage of coral cover that died (mortality level). © Copyright Commonwealth of Australia 1996 – 2007 GBRMPA, 2017.

2017 coral bleaching event

Aerial and in-water surveys undertaken by the ARC Centre of Excellence for Coral Reef Studies as part of the coral bleaching taskforce, revealed that some middle shelf reefs surveyed offshore of Mackay suffered bleaching (refer figure below). Read more here.

Coral bleaching map for 2016 and 2017.

Results of aerial surveys undertaken by the ARC Centre of Excellence in response to the 2016/17 bleaching event. © Copyright ARC CoE

This page will be updated when the impact of the 2017 bleaching event has been further quantified for the Mackay-Whitsunday-Isaac Region.

More information is available on GBRMPA’s website as well as the website of the ARC Centre of Excellence for Coral Reef Studies.