In October 2016, an obituary was posted in dedication to the Great Barrier Reef, sending the world into frenzy that the reef was gone forever due to anthropogenic impacts. The media honed-in on this, with a mass of articles released from news companies all over the world stating that everything was doom and gloom for one of the world’s most famous and diverse ecosystems. But that wasn’t quite the truth, with most articles lacking an understanding of the biology of coral reefs and how they function. Their only true value was to scare-monger the rest of the world into misbelief. The Great Barrier Reef isn’t dead yet, but it isn’t in the greatest condition.

The Basics on Corals

The anatomy of an individual coral polyp. (Source: NOAA via Wikicommons).

Contrary to popular belief most corals aren’t actually a single organism, but rather a colony of sessile individual polyps of the same species, that come together to form one assumed individual. These individual organisms can be produced through fragmentation (a type of asexual reproduction) of the original settler or through spawning. They attach to the sea floor or other hard substrate. Corals are from the class Anthozoa and can come in a variety of forms; from stony corals like staghorn corals, to soft corals like the snowflake coral. They can be found from the tropics to cold water climates, where species like Lophelia pertusa can reach depths of 3,000m.

Corals often form a symbiotic relationship with zooxanthellae (a type of single celled dinoflagellate). The zooxanthellae live safely within the corals cells, consuming the CO2 and nitrogen produced by the coral. In exchange, they photosynthesise and provide energy for the coral, aiding in the calcification of the coral skeleton. Corals also gain energy by feeding on organisms from zooplankton to small fish, using nematocysts to immobilise their prey.

Some corals can be found without zooxanthellae, and some can be found as individual polyps known as solitary corals like species within Cycloseris and Fungia. These solitary coral polyps are usually larger than individual colonial coral polyps.

Bleaching

Coral bleaching has been known of and researched by scientists since the 1970s, and reported in every region where coral reefs exist. Corals are highly sensitive organisms. Many live near their maximum temperature tolerance, making them vulnerable to rising sea surface temperatures. Under stress such as a temperature change or elevated irradiance, corals expel the zooxanthellae from their cells. This is due to thermal temperature changes altering the photosynthetic system of the zooxanthellae, which produces too many free oxygen radicals that are toxic to the coral and can cause tissue damage. The loss of zooxanthellae gives the coral their white ‘bleached’ appearance, as the zooxanthellae held the pigments typically associated with coral colouration.

Bleaching can occur when usual summer temperatures are just 1° over their usual maximum, or due to weather changing events like El Niño warming waters. Three mass-bleaching events have been reported on the Great Barrier Reef (GBR) over 18 years, occurring in 1998, 2010 and 2015/16. A long-term study was undertaken on the GBR spanning 18 years over 2,300km. The study showed that over the past 18 years bleaching events appear to be affecting more corals each year, with >60% of corals showing signs of bleaching in 2016 (out of 1,156 reefs surveyed). Only 8.9% of the 2016 surveyed reefs were not affected by bleaching, in comparison to 42.4% in 2002 (out of 631 reefs surveyed) and 44.7% of reefs in 1998 (out of 638 reefs surveyed).

A diver filming bleached coral at Heron Island for the XL Catlin reef survey (Source: The Ocean Agency / XL Catlin Seaview Survey / Richard Vevers.)

Can they Recover?

Without zooxanthellae, corals are deprived of their main source of energy, which can result in a lack of growth or starvation of corals. Additionally, corals also become more susceptible to disease and overgrowth by algae. The longer the coral is without zooxanthellae, the more its health will decline, and the chances of mortality will increase. The relationship between coral and its ‘associates’ can become affected by bleaching too. Corals act as host to many species, such as crustaceans who feed on the mucus and detritus. The lack of food increases mortality rates for such organisms.

The good news is that corals can survive bleaching events if they gain more zooxanthellae, and the symbiotic relationship continues. There can however be long term damage caused with key implications being issues with reproduction, growth, the regeneration of tissues and calcification. The overall effect usually depends on the species with some species being more adaptable in the long term than others.

Other Impacts Affecting the Reef

Bleaching isn’t the only thing that’s affecting coral reefs. Ocean acidification is expected to affect a coral’s (primarily stony coral) ability to calcify, with the saturation of calcium and aragonite in the ocean expected to decrease. This will result in declined growth of corals as these elements are necessary for the skeletal composition. This could result in overall substandard health, decreased diversity and a decline in coral populations. Corallivores such as butterfly fish and in particular the Crown-of-Thorns (COTS) star fish, both of whom feed on all types of coral, can decimate large amounts of coral tissue faster than corals are able to grow. Other factors include but are not limited to; eutrophication, which increases nutrients within the water resulting in algal blooms; tourism, with ineffective education on what not to touch; removal of fish and corals for the aquarium trade.

Healthy coral found on Flynn Reef, part of the Great Barrier Reef near Cairns, Queensland, Australia. (Source: Toby Hudson via Wikicommons)

What can be Done

Scientists around the world are looking at ways in which our reefs can be protected and what can be done to prevent total loss of our coral ecosystems. One fundamental method currently used is assisted evolution. This is the idea that naturally, through evolution, the corals would adapt to stressors. With assisted evolution, scientists go to areas of mass bleaching, for example the most northern parts of the GBR where bleaching is at its highest. They find surviving corals, with a higher tolerance and more resistance to the change. They then propagate them artificially, letting them spawn, and cultivate the nubbins for replantation back on the reef. This results in the development of a more resistant population faster.

The coral reefs aren’t dead yet, but they are under extreme threat due to natural and anthropogenic impacts. The best thing any one individual can do is be wary about your carbon footprint, as climate change is believed to be a leading cause in global warming.

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