Reef coral bleaching to be assessed by new classification system

Coral bleaching in the Great Barrier Reef will be categorised by severity under a new system.

Coral bleaching in the Great Barrier Reef will be categorised by severity under a new system. Photo: Getty

Coral bleaching will be categorised by a system similar to the one used for tropical cyclones under a new framework for the Great Barrier Reef.

“This now really helps us to clearly articulate the level of bleaching event that’s occurring on the Great Barrier Reef at any given time or over a particular season,” the Great Barrier Reef Marine Park Authority’s Fred Nucifora said.

Coral bleaching is a stress response due to changes to the environment, such as increased water temperatures or freshwater flooding.

Coral has single-celled algae called zooxanthellae living inside it which gives it colour and food.

But when the coral gets stressed it expels the algae and turns white.

A bleaching event does not necessarily mean the coral is dead.

The new categorisation system, much like tropical cyclones, is ranked category one through to five with the latter indicating widespread bleaching devastation on the reef.

A key difference between the two systems is that cyclones are gauged by wind intensity while bleaching is measured by sea temperature and a four-tier assessment framework.

The framework looks at the exposure and duration of heat stress on the reef.

It also assesses the response of coral colonies to the heat stress including how many are bleached, the prevalence across habitats and depths, and also how much of the reef is affected.

In a worst-case scenario, if the reef is exposed to above-average sea surface temperatures for more than 16 degree
“heating weeks”, full bleaching to all species of coral and mortality for most species of coral on a widespread scale would occur.

(A heating week is a special measurement that includes intensity and duration, where 1 degree Celsius above the maximum over one week would be referred to as one-degree heating week.)

However, a single category may not be applied across the reef but rather be designated to different areas of the structure.

Unlike cyclones where the Bureau of Meteorology can forecast the severity, the category of coral bleaching will be released after the summer period.

“A tropical cyclone might happen over 24 hours as it crosses the Great Barrier Reef on our coastline but the bleaching event might unfold over several months,” the Great Barrier Reef Marine Park Authority’s Dr Mark Read said.

He said the new framework will help reef caretakers prepare for a bleaching event and will provide a more detailed assessment of the impacts after the fact.

Read said the team will also be applying the categorisation framework retrospectively to previous summers to show broader trends.

This summer the reef has so far been hit by two tropical cyclones and runoff caused by flooding – two key impacts on reef health.

Read said some reefs near Cairns and Townsville have had some physical damage from the cyclones barrelling over and causing waves.

However, other parts of the reef are in good condition.

Freshwater flooding and subsequent bleaching have also impacted some of the offshore reefs near Cairns this summer.

A full assessment of which category of bleaching the reef has experienced this summer is expected to be released in a few months.


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