Great Barrier Reef Coral Bleaching

Great Barrier Reef off the east coast of Australia where currents swirl in the water around corals. Image acquired by Landsat-8 on 23 August 2013. Image Courtesy of USGS/ESA.

Coral bleaching on the Great Barrier Reef in Australia was worse than expected last year, and a further decline is expected in 2017 according to the Great Barrier Reef Marine Park Authority. In a document issued this week they noted that, along with reefs across the world, the Great Barrier Reef has had widespread coral decline and habitat loss over the last two years.

We’ve written about coral bleaching before, as it’s a real barometer of climate change. To put the importance of the Great Barrier Reef into context:

  • It’s 2300 km long and covers an area of around 70 million football pitches;
  • Consists of 3000 coral reefs, which are made up from 650 different types of hard and soft coral; and
  • Is home to over 1500 types of fish and more than 100 varieties of sharks and rays.

Coral bleaching occurs when water stress causes coral to expel the photosynthetic algae, which give coral their colours, exposing the skeleton and turning them white. The stress is mostly due to higher seawater temperatures; although cold water stresses, run-off, pollution and high solar irradiance can also cause bleaching. Whilst bleaching does not kill coral immediately, it does put them at a greater risk of mortality from storms, poor water quality, disease and the crown-of-thorns starfish.

Last year the Great Barrier Reef suffered its worst bleaching on record, aerial and in-water surveys identified that 29% of shallow water coral reefs died in 2016; up from the original estimation of 22%. The most severe mortality was in an area to the north of Port Douglas where 70% of the shallow water corals died. This is hugely sad news to Sam and I, as we explored this area of the Great Barrier Reef ourselves about fifteen years ago.

Whilst hugely concerning, there is also a little hope! There was a strong recovery of coral in the south of the Great Barrier Reef, as bleaching and other impacts were less.

Images from the Copernicus Sentinel-2A satellite captured on 8 June 2016 and 23 February 2017 show coral turning bright white for Adelaide Reef, Central Great Barrier Reef. Data courtesy of Copernicus/ESA, and contains modified Copernicus Sentinel data (2016–17), processed by J. Hedley; conceptual model by C. Roelfsema

The coral bleaching event this year has also been captured by Sentinel-2. Scientists from ESA’s Sen2Coral project have used change detection techniques to determine bleaching. Images between January and April showed areas of coral turning bright white and then darkening, although it was unclear whether the darkening was due to coral recovery or dead coral being overgrown with algae. In-water surveys were undertaken, which confirmed the majority of the darkened areas were algal overgrowth.

This work has proved that coral bleaching can be seen from space, although it needs to be supported by in-situ work. ESA intends to develop a coral reef tool, which will be part of the open-source Sentinel Application Platform (SNAP) toolkit. This will enable anyone to monitor the health of coral reefs worldwide and hopefully, help protect these natural wonders.