• Gina Lusardi

Coral Bleaching: What's really happening?


Lizard Island from above, The Great Barrier Reef
Lizard Island from above, The Great Barrier Reef

Think of tropical, shallow water coral reefs as the tropical rainforests of the ocean. They are not only incredibly productive; they are also the most biodiverse amongst marine ecosystems. Unlike rainforests which cover 2.3% of the Earth’s surface, coral reefs cover less than 0.1% (1); that’s about twice the size of Italy. The corals themselves are responsible for the structural composition of reef systems. Stony corals, such as large boulder-like corals and intricate branching corals, are the reef builders. Unlike other types of coral, stony corals produce a calcium carbonate skeleton which enables them to construct large limestone structures (2).

Healthy hard corals at Heron Island, The Great Barrier Reef, Australia - copyright Gina Lusardi
Healthy hard corals at Heron Island, GBR

Corals are invertebrates related to jellyfish and sea anemones in the phylum Cnidaria. When looking at corals it may seem like you are looking at one large individual, but actually a single structure can be made up of hundreds, if not thousands of independent polyps. The polyps themselves are the actual coral animal. Naturally colourless, polyps have a symbiotic relationship with photosynthetic algae called zooxanthellae (2). These algae are not only responsible for the corals’ vibrant colours, but also aid in sustaining them. In fact, 90% of a coral’s food source come from the zooxanthellae living within its tissues (3). When a coral is stressed, it expels the zooxanthellae. As it does so, the coral loses its colour which exposes the white skeleton underneath. This phenomenon has been coined “coral bleaching”. Corals become stressed for a number of reasons including: changes in light intensity, low and high temperatures, freshwater inundation, pollution and disease (4,5). It is possible that a coral can rebound after a bleaching event if the stress is short-term. However, if stressful conditions persist, the coral will die. For instance, just 4 weeks of temperature stress can trigger bleaching (5). If conditions persist for 8 weeks or longer, corals begin to perish (5). Of all the stressors that cause bleaching, unusually high sea temperatures are the primary cause of mass coral bleaching.


What is Mass Coral Bleaching?

Source: Great Barrier Reef Marine Park Authority (5)

Mass coral bleaching was first described in the early 1980’s and was attributed to warmer sea temperatures in conjunction with an El Niño event (6). Between 1982 and 1983, there was a 97% reduction of corals in the Galapagos, as well as, 75% in Panama and 50% in Costa Rica (6). Since then, there have been periodic events world-wide. In most recent years, the time between these events has been shrinking. If warmer sea temperatures are to blame then we need to identify the cause. Of course, the El Niño Southern Oscillation (ENZO) definitely plays a role due to its influence on global sea temperatures (7). However, climate change is also a key player. As the Earth heats up, the ocean absorbs around 90% (8) of that heat energy causing the ocean to warm at an increasingly fast rate (9).


In 2016, the news was flooded with articles about a mass bleaching event occurring on Australia’s Great Barrier Reef (GBR). One of the most shocking articles was Outside Magazine’s “Obituary: Great Barrier Reef (25 million BC-2016)”, which claimed that the reef was dead (10). Of course, it was a complete exaggeration, but there was indeed cause for concern. The Great Barrier reef was being devastated by a mass bleaching event triggered by record sea temperatures along the most Northerly section of the reef (11). In 2017, the bleaching continued to affect around 66% percent of the reef (12). Even in early 2018 a coral reef biologist by the name of Dr Selina Ward has seen some localised bleaching in the time before the “high risk” period (13).


Unfortunately, the GBR is only a small part of a bigger story. In 2016 alone, there was 29% loss of corals worldwide (14). As it stands, that year was the warmest year on record since 1880 (15). It is also important to note that 17 of the 18 warmest years have been since 2001 (15). In an article published in 2017 by Nature, 46 experts concluded that “securing a future for coral reefs, including intensively managed ones such as the Great Barrier Reef, ultimately requires urgent and rapid action to reduce global warming” (16). If things continue along this trend, the outlook will remain quite bleak. Fortunately, there is a lot of work being done around the world to help save coral reefs.


The global extent of mass bleaching of corals in 2015 and 2016

Symbols show 100 reef locations that were assessed: red circles, severe bleaching affecting >30% of corals; orange circles, moderate bleaching affecting <30% of corals; and blue circles, no substantial bleaching recorded. See table S1 for further details.

Source: Hughes, et al. (2018). Spatial and temporal patterns of mass bleaching of corals in the Anthropocene. Science, 359(6371), [Fig 3]. Available at: http://science.sciencemag.org/content/359/6371/80.full


What is Being Done to Save Coral Reefs?


Australia’s GBR may be one the best managed reefs in the world. The Great Barrier Reef Marine Park Authority (GBRMPA), who is in charge of managing the reef, recently launched a Coral Reef Initiative campaign called “The International Year of the Reef 2018” to try and spark world-wide public engagement in order to help save coral reefs (17). The Australian government also announced $2 million (Australian dollars) in funding for an international competition for research proposals to increase coral abundance (18). On the other side of the world, the United States of America has its own tools for protecting reefs. The National Oceanic and Atmospheric Administration’s (NOAA) Coral Reef Conservation Program was initiated in 2000 to help protect reefs from climate change, pollution and unsustainable fishing (19). In Central America, 60 organisations are collaborating across international borders in order to keep track of the health of the Mesoamerican Reef (20). Globally, a network of marine protected areas (MPAs) have been established to protect vulnerable ecosystems including: coral reefs. Some of the largest such as the Marae Moana Marine Park in the Cook Islands (21), the Papahānaumokuākea Marine National Monument in Hawaii (22) and the Natural Park of the Coral Sea in New Caledonia (23), include vast expanses of coral reef ecosystems.