The Great Barrier Reef Library

A comprehensive guide to The Great Barrier Reef



By on November 10, 2017


The Great Barrier Reef is currently facing many threats, including those listed below. Unfortunately, it is a very complex issue as a number of these threats are connected. Many scientists predict that The Great Barrier Reef could be gone in the next 100 years, with some believing only 15 years! Between 1985 and 2012, The Great Barrier Reef lost half of its coral. 48% of this loss was due to tropical storms and cyclones, 42% was due to Crown of Thorns Starfish (Acanthaster planci) and 10% was due to Coral Bleaching.21 Despite this, there is still hope. Many projects are underway to help protect our reefs – click here for more information.

Increasing Temperatures

Global temperatures have risen by 0.8 degrees Celsius since the 1880’s, which has had a number of impacts on The Great Barrier Reef as the oceans are believed to absorb up to 93% of the additional heat.12,13 Temperature is an important indicator for many organisms to trigger events such as spawning, meaning that even a change of as little as 1 degree can have a massive effect.  The Intergovernmental Panel for Climate Change has predicted that the average sea surface temperature of The Great Barrier Reef may be at least 2.5 degrees warmer by the year 2100. This is not to say that the reef will all be the same temperature, more that the number, size and duration of “hot spots” will increase.10,11 This will lead to further stress from increased frequency of Coral Bleaching and increased frequency and severity of extreme weather events.

Coral Bleaching

Coral is made of three things. It has a Calcium Carbonate, or Limestone skeleton, an animal part, called a Polyp, which is like an upside down jellyfish and then there is an algae in its skin called Zooxanthellae (Symbiodinium). This algae gives corals their colour. The polyps are very small and join together to form colonies, filter feeding through the water. But, since tropical water is relatively nutrient-poor, the polyps can only get roughly 10% of their energy needs by filter feeding. The other 90% comes from the Zooxanthellae, which makes energy from the sun, like all plants and shares its energy with The Coral Polyp.

Causes of Bleaching

Bleaching can be caused by any number of factors, including changes in UV rays, salinity, visibility, pH and temperature – which is the one that is of major concern today. When temperatures increase, the Zooxanthellae become over productive and make too much energy, resulting in the production of additional toxic molecules that damage the coral polyps. This leaves the polyp in a bit of a dilemma. Remove the Zooxanthellae and lose 90% of its food source, or maintain it, along with its toxic molecules. The lesser of two evils is to remove the algae and so the damaged cells are removed by either exocytosis, or digestion.

With no Zooxanthellae, the corals appear ghostly white, what we call bleached coral, but these are not dead, yet. We are then looking at the polyp, without the algae and it is still making 10% of its own energy.  It’s starving, but it is still alive and some corals are believed to be able to survive in this state for over two months, in good conditions, although most only survive two or three weeks. If conditions do not return to normal, the corals cannot produce enough energy to survive and they suffer rapid tissue necrosis and eventually die.

History of Bleaching

Coral bleaching is not a new phenomenon, as it has been recorded in core samples from thousands of years ago, however, what is new, is the frequency and scale of the bleaching events – see our timeline for more information on bleaching events. So bleaching is not really the problem in itself, it is more the frequency of bleaching events, leading to less recovery time for the coral. As the world continues to warm, predictions state that bleaching events may eventually become a seasonal expectation, rather than an anomaly. This would be extremely bad for coral reefs worldwide. 2016 gave us the most severe bleaching event on record, which was followed up by a second bleaching event in 2017 (this is the first time on record that the reef has bleached two years in a row). Coral can adapt to changing conditions – for example it is found in both shallow and deep water, warm and cold water – but the question is, can it adapt quick enough to match the speed of the changing environment.11,12,13,14,15,16,17,18

There are currently many studies going on to help combat the effects of coral bleaching. For more on this, please visit our conservation page.

Extreme weather events

As the temperature on Earth continues to rise, extreme weather events, such as storms and cyclones, are becoming more severe and more frequent. We have had six major cyclones on The Great Barrier Reef, since 2005. They can be extremely destructive, in some cases ripping up entire reefs structures through wave action. This is particularly a problem for the shallow reef flats.

Linked effects

The associated flooding also stresses the inshore reefs as salinity and visibility reduces quickly (which can cause Coral Bleaching). There is also an associated increase of nutrients and agricultural runoff. These nutrients and chemicals can directly kill corals and sea grasses if exposure is prolonged, but they can also lead to algal blooms. The additional algae, or phytoplankton, can take most of the nutrients from the water, thus starving corals. They are also a favourite food of Crown of Thorns Starfish (COTs) larvae. Therefore, if timed with the spawning of COTs, it can aid their survival rate. Extreme weather events can also have a negative impact on iconic species, such as Turtles and Dugongs. These animals rely on sea grass beds for food, which may be ripped up during storms.11,18,22

Recent Storms

The most recent storm was Cyclone Debbie that affected the Whitsundays area on 28th March 2017. Some of the affected sites suffered up to 97% coral loss due to this storm. This shows that recovery from storm damage is slow, as very few corals survive to repopulate the area. There was also lots of flooding associated with this storm, causing flood plumes, which reduced the water quality and linked to secondary issues, such as bleaching.11,18,22

Cyclone Yasi hit The Great Barrier Reef near Cardwell in 2011 and was one of the biggest storms on record, with some parts of South East Queensland experiencing 400% more rainfall than usual.22

Water Quality

Good water quality is vital for the health of The GBR, as reefs grow best in water that contains naturally low concentrations of nutrients, such as phosphorus and nitrogen. This is one of the reasons that The Great Barrier Reef has been able to create such a diverse ecosystem. Unfortunately, many of the habitats of The Great Barrier Reef Marine Park are now in decline. This is partly due to the excess use of nutrients, pesticides and fertilizers and sediment run-off, which all greatly reduce the quality of the water reaching our reefs. This has been identified as the second most significant pressure on The Great Barrier Reef.

The land and the sea are connected through the run-off from 35 basins, draining approximately 424,000km2 of coastal Queensland. The associated rivers provide the biggest influx of nutrients to the inshore areas of The Great Barrier Reef Marine Park. This run-off has been steadily increasing over the past 150 years.

The increased sedimentation and flow of nutrients causes reduced light, which may smother corals. It also increases pollutants and algal blooms in the water. These algal blooms can compete better for nutrients and may prevent the growth of new corals.

The Reef 2050 Water Quality Improvement Plan aims to tackle this issue and improve our water quality to help maintain a healthy reef system. This initiative was set up by a partnership between The Great Barrier Reef Marine Park Authority (GBRMPA) and Australian and Queensland Governments.27,28,29

Crown of Thorns Starfish

The Crown of Thorns Starfish, Acanthaster planci, is a ferocious predator, feeding on hard corals. Historically, it was never seen as an issue as it actually proves beneficial for the reef in small numbers. It prefers to feed on the fast-growing corals, allowing slower corals chance to grow, thus increasing diversity. However, although it has been native to The GBR for roughly 8,000 years, it has been known to almost wipe out reefs when in plague proportions. AIMS claims that A.planci was responsible for 42% of coral death on The GBR between 1985 and 2012. They further claims that it is still considered to be one of the largest threats to our natural wonder. Research suggests that had it not been for Crown of Thorns predation, the reef would have shown a net increase in coral cover over the last three decades.1,2,3,4,5


Outbreaks of Crown of Thorns seem to occur roughly every 17 years. Four episodes have occurred on The Great Barrier Reef since the 1960’s. The current outbreak began in 2010. If Crown of Thorns reach numbers above 15 per hectare, it is considered to be an outbreak. When present in these numbers, they can then consume coral at a faster rate than it can recover.

Outbreaks are triggered by two key factors, although it is likely that other factors come into play, too. Firstly, one of their main predators is The Giant Triton Shell, Charonias tritonis, which is a beautiful, large shell. This led to a large industry in the 60’s and 70’s that profited from selling these shells to tourists. There are very few left on the reef today.1,2,3,4,5 The Australian Institute of Marine Science are currently looking at reintroducing The Triton Shell into the wild.6

Another cause of outbreaks is the agricultural runoff of nutrients and sediments into the reef. Particular following large storms in the wet season, which coinciding with the spawning season of Crown of Thorns. These nutrients cause an increase in phytoplankton and the resulting algal blooms help the Crown of Thorns larvae to thrive.1,2,3,4,5


Currently, efforts are being made to reduce the numbers of Crown of Thorns, using a lethal injection method. The Association of Marine Park Tourism Operators (AMPTO) have two boats to take teams of divers to reefs where large numbers of Crown of Thorns are reported.  Some more good news for the reef is that the government have just released further funding to support this project (early 2018). This will  allow more vessels to join this fight. The divers inject the Crown of Thorns with bile salts, creating an allergic reaction in the animal. This leads to them to breaking apart and they die within 24 hours. Over 500,000 starfish have been culled since the project began and they recently broke their record. 47,000 Crown of Thorns were killed  in eight days at The Swains Reefs on the Southern end of The GBR.7,8,9

Ocean Acidification

Over the last 200 years, humans a greatly increased their production of Carbon Dioxide (CO2), largely due to the industrial revolution. Although initially released into the atmosphere, the oceans have now absorbed about 50% of this additional CO2. This has made them about 25% more acidic. This could make Ocean Acidification a great threat to the reef in the future. By 2100, predictions state that the pH of the oceans will drop from 8.2 to 7.8.19,20

This is extremely bad for many of the organisms on The Great Barrier Reef. First of all, many of these animals, coral included, make limestone (calcium carbonate) skeletons or shells. These will potentially dissolve in acidic conditions.

Ocean Acidification also means that there will be less free carbonate with which to make these skeletons and shells. As CO2 is absorbed by the ocean, it goes through a chemical reaction to produce carbonic acid. This then splits into a bicarbonate ion and a hydrogen ion. These hydrogen ions then bind to free carbonate in the water to create more bicarbonate. This makes the carbonate unavailable for organisms. Without carbonate, they cannot produce shells, but more importantly, coral cannot produce its skeleton. Without hard corals making calcium carbonate, there would be no structure on the reef for other animals to settle on. Therefore, the whole ecosystem could collapse.19


Pollution poses a major risk to not only The Great Barrier Reef, but oceans worldwide. Current studies suggest that 8 million tons of plastic enters the oceans each year, which transported and collected by currents.23 Even remote islands in The Far North of The GBR, within protected Pink Zones, are covered with plastic.

Many animals die each year from ingesting plastic. Turtles confuse plastic bags for jellyfish. National Geographic reports that 90% of sea birds contain plastic.25 This has drawn a lot of attention in mainstream media in recent years.

A recent study also looked at the impacts of plastics on corals. Corals are non-selective filter-feeders, meaning that they will ingest anything that comes their way – including micro plastics. This blocks their stomach cavity and starves the corals.24  Furthermore, larger plastics have been found to entangle corals, particularly spikey corals such as Staghorn. This increases the likelihood of disease. Coral disease has been shown to increase from 4% to 89% when they come in contact with plastics. A current estimate suggests that 11.1 billion plastic items are currently entangled in coral throughout the Asia-Pacific Region. This figure is only set to rise, with a 40% increase predicted by 2025.26

This is why it is more important than ever to refuse, re-use and recycle.