Sentinel-2 satellite images show Great Barrier Reef bleaching

[Australia] Scientists observed the bleaching of Australia’s Great Barrier Reef early this year using satellite images. While capturing these events from space has been difficult in the past, Sentinel-2’s frequent revisits and its resolution makes it possible.

An article published on the website of the European Space Agency (ESA) shows satellite imagery of coral bleaching along the Great Barrier Reef.

As previously reported, the Great Barrier Reef has suffered back-two-back bleaching events over the past two years, thought to be caused by the warming effect of a prolonged El Niño event and the impact of global warming.

Coral Bleaching occurs when symbiotic algae known as zooxanthellae – responsible for the coral’s nutrition and colour – are expelled due to higher-than-usual surrounding water temperature.

The coral will recover if temperatures return to normal. However, if the algae are not recovered quickly enough, the coral polyps will die, causing secondary effects to the reef ecosystem.

Satellite images showing the whitening of coral reef between visits (Copernicus Sentinel data (2016–17), processed by J. Hedley).

Satellite imagery is not generally seen as a reliable indicator of bleaching events. The apparent whitening of the coral may be caused by other factors, as explained by Dr John Hedley, scientific leader of the Sen2Coral monitoring project.

‘In general, interpreting changes is ambiguous. You can’t just jump to the conclusion brightening is bleaching because the brightness of any spot on a reef varies from image to image for many reasons due to both the water and bottom changes,’ said Dr Hedley

The subsequent darkening of the area may indicate that the coral has either recovered or has already died and has been overgrown by an algal bloom that – unlike the zooxanthellae – is not beneficial to the coral. Only a closer examination of the coral beds can distinguish between the two.

The frequency of Sentinel-2’s passing over the GBR, however, allowed scientists to monitor the colouration of the reef between January and April 2017. Images captured in February suggested that a bleaching effect was in progress, lasting at least 10 days.

Dr Chris Roelfsema of the University of Queensland’s Remote Sensing Research Centre, and lead of the Great Barrier Reef Habitat Mapping Project, conducted field studies in the area, and his observations confirmed the detrimental effects of the bleaching event.

The area of concern for the reef is highlighted in the diagram above (conceptual model by C. Roelfsema).

‘Sadly, in the areas where bleaching can be seen, the abundant coral cover we observed in January was in April mostly overgrown with turf algae, with only some individual coral species surviving. The imagery and field data suggest this area has been hit badly,’ he said.

Proper monitoring of the health of coral reefs requires regular inspection, usually carried out by low-level flight or scuba diving. Variations in the water and other local environmental changes make satellite imagery unreliable, however as the report suggests, the frequent revisits of Sentinel-2 allow these variations to be and help the detection of bleaching events before coral recovery or algal overgrowth turns the area dark again.

‘Sentinel 2 is a game-changer for coral reef remote sensing; the combination of frequent revisit and spatial resolution is enabling us to see genuinely new things,’ said Dr Hedley.

‘We now know bleaching can be visible in satellite imagery, but the challenge is to produce reliable software that can map or quantify that.’

“It has to account for all sources of temporal variation and, importantly, the uncertainty in the methods. The methods have to be open for scientists to be able to interpret the outputs.”

Towards this aim, a set of software specifically for coral reef applications is being created by ESA’s Sen2Coral project and is expected to be available by the end of the year.


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