365 Days of Climate Awareness 197: Decline of the Great Barrier Reef: Main Threats

The largest dangers to the Great Barrier Reef are the same as faced by coral reefs worldwide:

• Warming water
• Increasing acidity (dropping pH)
• Increasing storm activity
• Pollution
• Overfishing
• Predatory outbreaks (esp. crown-of-thorns)
• Shipping

A number of monitoring and protection regimes have been enacted to protect parts or all of the reef, notably 2015’s “Reef 2050 Plan” between both the Australian national and Queensland governments.

When coral polyps are under stress, especially from heat, they expel the algae--zooxanthellae—from their tissues, a process known as “bleaching”. These algae help nourish the polyps and while the coral can survive while bleached, they are weaker and more susceptible to other threats. (Bleaching can also occur during anomalously cold water temperatures but this is much more rare.) 

As in many cases worldwide, it is not one threat but a combination which proves deadly. The process is reversible: when the source of stress--high water temperatures, usually--is removed, the corals will accept the zooxanthellae back into their bodies and resume normal living. Bleaching is a stress response but not a 100% predictor of coral death.

Thetford Reef (near Cairns) 2016, pre-bleaching.

A survey conducted in 2016 found that the Great Barrier Reef could be divided roughly into three zones based on extent of bleaching. The most widespread bleaching was in the north, in the tropical region. Moving southward the bleaching became progressively less severe, but still did occur.

Thetford Reef 2017, post-bleaching.

Tomorrow: warming waters in the Great Barrier Reef.

Be brave, and be well.