Bleaching hits the southern Great Barrier Reef

By Dr Emma Kennedy

As the global mass bleaching event hits the southern hemisphere, Australia is in the spotlight as the Great Barrier Reef (GBR) experiences the most extreme mass bleaching in its 8000 year history. While northern areas of the GBR are the worst affected (with 95% of reefs showing severe bleaching, and just 4 reefs of 520 showing no bleaching), the southern extent of the bleaching was still being established by aerial surveys this week.

Over 300 marine scientists – from government agencies to universities and volunteer teams – have headed out to the GBR to assess the damage and record the event. Last Thursday, our team from the Australian Rivers Institute headed up to the Capricorn Coast – at the southernmost end of the GBR – to assist the National Bleaching Taskforce in ground-truthing reef sites using scuba surveys as they flew above.

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Feeling a little pale. Branching corals at North Keppel Island on Saturday. The corals on right are losing the golden-brown microalgae from within their tissues, leaving them an paler colour. Photo credit: Kennedy.

Bleaching occurs when environmental stress – often warmer-than-usual sea temperatures – trigger a loss of microscopic algae living symbiotically within coral tissues. As more and more colourful algae are lost, the coral begins to gradually pale, until the bone white skeleton can be seen beneath the coral’s transparent tissue, leaving corals appearing a ghostly white.

While bleaching can look spectacular on mass, it is very troubling for the health of the reef. Without the tiny algae, which photosynthesise to produce sugars that nourish the coral, the coral begins to slowly starve to death.

Alleviation of the stress can bring the algae – and the golden-green colour – back, but sometimes it’s too late for the coral. Up north, over 50% of the bleached corals are starting to die.

There have been 8 mass bleaching events since the 1980s with the world experiencing the very first global mass bleaching event – where coral reefs from the Caribbean to the Pacific were drained of colour – in 1998. Eighty percent of reefs were affected and 16% went on to die. A second event occurred in 2002, but this third global mass bleaching event, coinciding with an El Niño year (which exacerbates warming seawater temperatures), is thought to be more severe than the previous two.

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A cheeky squid investigates the dive team. Photo credit: Kennedy.

The southern area of the reef, including the Keppel Islands, has received little attention as generally it’s thought to be less affected: temperatures haven’t been as warm as the northern areas this summer (partly due to Cyclone Winston). Aerial surveys failed to detect bleaching beyond Mackay, yet the more turbid water of the inshore reefs can make the white colour harder to pick up.

The Keppel Islands, where we surveyed, have been historically been affected by land-based pollution and freshwater inundation when the Fitzroy River floods. These plucky corals bounce back fast from bleaching, but land-based pollution sources (and a severe bleaching event in the region in 2006) have led to reefs around the area becoming gradually degraded over time. Since bleaching had not been reported for this area, our team were hopeful that we wouldn’t see any evidence of bleaching stress.

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Massive coral colonies were the worst affected. Photo credit: Kennedy.

Our Coral Reef Algae Lab team was scrambled to carry out emergency bleaching surveys in our study area in the Keppel Islands, managing to visit 14 reefs in just under two-and-a-half days, covering some of the main reefs – many of them sites for our experiments – in the Keppel Islands group. Using photo and video transects to rapidly assess as much of the area as possible, our scientists were dismayed to find bleached corals at most of the sites we visited.

While these corals are likely to recover as cool weather brings a reprieve, it is clear the extent of the bleaching is more far-reaching than previously believed.

Meanwhile the global mass bleaching event – that begun in Hawaii – continues its merciless sweep around the planet, with West Australia and Indian Ocean reefs now starting to bleach. Sudden dramatic degradation of an entire ecosystem in this manner heralds a new era of rapid global change driven by changes in atmospheric CO2, and should be a wake-up call to governments and individuals alike that humanity is on a very slippery slope to climate chaos.

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