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Bleaching events can leave corals weaker in the face of pollution and other stresses. AAP Image/University of Queensland/Ove Hoegh-Guldberg

Great Barrier Reef bleaching event: what happens next?

Monitoring the scale and intensity with which coral bleaching manifests in real time is not a job we wish for, but in reality it provides a powerful tool to enable better management against future events.

It is crucial we learn from El Niño events, which we can treat as natural experiments, to show just how much bleaching occurs as conditions change. It can show us which species are most affected and to what extent the patterns and timing of bleaching reflect the reef’s weakened state from other stresses such as pollution.

To facilitate this, the Great Barrier Reef Marine Park Authority (GBRMPA) has a Coral Bleaching Response plan that allows researchers and managers to gauge the impacts. The widespread bleaching seen in the Great Barrier Reef’s north has now seen the “response level” raised to level 3, indicating the most severe situation catered for by the plan.

Threat and response

Two key criteria are used to determine the response level: bleaching severity and spatial extent. Being at response level 3 basically means that the coral species that are normally most sensitive to bleaching, such as the colourful branching and table corals, have already moved from paling (and therefore some chance of recovery) to death.

Differing levels of coral bleaching severity in the GBRMPA bleaching response plan. GBRMPA

Furthermore, at level 3 the more bleaching-tolerant boulder corals have begun to dramatically pale at more than 10 different sites in several parts of the reef. To put it in simple, stark terms, multiple areas of the reef are now dead and dying.

This is not just linked to the current El Niño, which is now showing signs of waning. Response level 3 calls for continued monitoring and stronger management measures such as the tightened pollution caps discussed by federal environment minister Greg Hunt in the wake of the bleaching seen over the past few days.

This process will last for months beyond the current bleaching outbreak, as researchers monitor the reef for signs of post-heat disease outbreaks among immune-compromised corals. They will also check for any apparent signs of potential recovery.

Our ability to have enough “eyes on the ground” is critical to be effective in classifying the appropriate response level. Ever-improving remote sensing of sea temperatures over the reef has transformed our ability to assess bleaching risk and mobilise rapidly.

Reef researchers and managers around the world rely on a network of “virtual bleaching stations” operated by the US National Oceanic and Atmospheric Administration. This helps to determine the probability of a bleaching event by detecting where (and by how much) temperatures are warmer than the seasonal average. This can help reef managers to work preemptively to spot areas that are in danger, and showing where and when researchers need to be in the field to monitor the severity and scale of bleaching at first hand.

Ongoing monitoring

What management options exist now that the Great Barrier Reef is at response level 3? While we understand that such mass bleaching events are driven by sea temperature anomalies, they are made much worse in places where coral are stressed by the presence of pollution or extra organic matter in the water.

Response level 3 gives a clear impetus to improve water quality immediately and to encourage responsible use of the Great Barrier Reef Marine Park and its surroundings. But this is exactly the same for response level 2, so in effect all the recent upgrade adds to the picture is a sense of increased urgency. Emergency funding announced by Hunt in response to the move to level 3 will support unprecedented surveying of the reef’s condition over the coming months, to truly understand the scale of impact.

However, intensive and immediate management of local stresses is the bottom line under response level 3. Corals remain severely weakened by heat stress because of their compromised immune systems and depleted energy reserves. Heat stress fuels the activity of disease-causing microbes in the water, so corals that may have survived the heat stress event might later succumb to disease.

A stressful event such as the recent high temperatures can leave corals even more vulnerable to further stresses, even among those corals that have so far appeared healthy and resisted the current round of bleaching. To choose an apt if unfortunate metaphor, the reef is not out of hot water yet.

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