The Great Barrier Reef (GBR), the world’s largest tropical reef ecosystem (as seen below), has been in the spotlight this week for its’ battle with a warming planet. The attention was due to a trending article that gives a bleak outlook of the reef and proclaims it to be dead after a productive 25 million years. Cause of death: ‘a long term sickness’ – we can only infer to mean humanity. Although its’ death was quickly refuted by scientists that went to the tabloids in uproar, the overwhelming sense of tragedy this instilled rippled through the web and got people talking – can the reef return from a critical state? Are we watching an ecosystem in meltdown?

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Great Barrier Reef, Queensland, Australia. Source.

 

The Current State

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Coral bleaching. Images adapted from  XL catlin seaview survey. Source.

These questions come to light after the GBR experienced the largest coral bleaching event ever recorded during the 2015 to 2016 El Niño event – a phenomenon that results in warm water spreading through the Pacific. 93% of reef-forming corals were reported by scientists to be ‘bleached’ in the Northern sections of the reef, due to increased sea-surface temperatures of 1-2.6°C above average temperatures (1961-1990) for this time of year. A bleached coral looks a ghostly white colour. Although alarming the coral is still alive. However, the coral has expelled its’ colourful symbiotic-algae called ‘zooxanthellae’, that are vital to them through providing energy via photosynthesis.

The reason for corals to expel their food provider is considered to be a response mechanism in times of stress. Corals do not cope well under stress. Their survival depends upon an extremely narrow range of environmental conditions, including particular light levels and ocean temperatures. As temperature increases the zooxanthellae can go into ‘hyperdrive’ and the coral will expel them, perhaps in the hope of taking up another symbiont that is better equipped to cope. Once conditions return to normal the coral will reabsorb its’ symbiont. The problem is conditions could be prolonged and other zooxanthellae adapted to the altered conditions may not exist. This will leave them in a far weakened state and susceptible to disease, predation, or overrun with seaweeds.

The opportunity to bounce back from a bleached state is possible once conditions return, as observed in lower regions of the GBR. However, within the Northern Quarters of the GBR scientists saw almost instant mortality in many species. Over a few short weeks in March temperatures reached critical levels and boiled many corals. The extent of mortalities has not yet been fully assessed, but initial surveys have reported more than 50% mortality within some of the Northern sections of the reef (seen in the figure below), including 200-300 year-old corals. Fortunately, this sight was not widespread, the mid to lower regions of the reef are reported to be unscathed by this disaster. But is this mass bleaching event a taste of what lies ahead?

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Current observed coral mortality (as of November 2016) from this years bleaching event along the Great barrier Reef. Image produced by Australian Institute of Marine. Source

The future

The major driver that is at the core of bleaching events is global warming. The double edged blade of global warming simultaneously increases sea temperatures and ocean acidity. Global warming is due to increased carbon dioxide in the environment, 30% of which is absorbed into the ocean. This decreases the levels of carbonate ions in the water and increases the acidity – a process known as ocean acidification. In the last 100 years we have seen a drop in the pH of the ocean of 0.1. This may sound low but these levels of acidity are outside of those seen in the last million years, and could exceed anything the planet has seen in 40 million years if global warming continues at the predicted rate.

To corals global warming is an attack on many fronts. The increased acidity of future oceans may make corals struggle to build up their carbonate bodies (studies suggest some corals may compensate to changing acidity– but at what cost?). Rising temperatures cause them to expel their zooxanthellae leading to weakening and mortality. Furthermore, global warming is predicted to increase the severity and frequency of storms and El Niño events, that as we have seen can decimate reefs over extremely short time periods.

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Bleached coral has died and become rapidly taken over by algae. Images captured by the Catlin Seaview Survey at Lizard Isalnd in March/ May 2016. Source.

A loss of reef-forming corals will alter the GBR ecosystem, and its future looks uncertain in the face of these challenges. Major regime shifts in tropical-reef ecosystems have been observed after bleaching events, transforming a coral-reef to a less diverse seaweed-dominated ecosystem. This shift will cause a cascading loss of fish and other animals that are dependent upon coral structures. However, the chances of a shift are decreased for reefs in deeper waters, those with more complex structures, and areas with more juveniles that allow the reef to build up again. This could provide patches of refuge to the GBR and enable future recolonisation to damaged areas with enough time. There is also evidence of novel reef ecosystems emerging that, although different from the previous pristine systems, could still support a diversity of life.

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Life on the reef. Source.

With the damage observed this year the future health of the GBR may never be what it was 100 or even 40 years ago within our lifetime, but it is still very much alive, and it is the time to fight for its’ future. This is exactly what scientists are doing. Current studies are looking to create ‘supercorals’, whereby they are attempting to speed up the rate of evolution so corals can take up zooxanthellae better adapted to cope in the face of global warming. There is research focussing on understanding why some tropical coral species and regions show  more resilience to bleaching. Increasingly effective management strategies to tackle ocean acidification. Tools that reduce the combined effects of other stressors that can weaken corals, such as sedimentation and predation (check out this starfish-killing robot), making it harder for them to fight the battle against climate change.

The current mass bleaching events on the Great Barrier Reef leave us on the brink of an extreme ecological disaster. If a global change is not rapid to mitigate the effects of a warming planet we may see a meltdown of the Great Barrier Reef. But humans are undoubtedly one of the most adaptable species on the planet  – change can occur.

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