What is Coral Bleaching?

Mass coral bleaching is one of the leading threats facing coral reefs worldwide, yet few people in the general public actually understand what coral bleaching is, or how it affects the long-term health and sustainability of marine resources. During the 2010 mass bleaching event in the South China Sea I helped perform socioeconomic surveys with divers in the Gulf of Thailand for a collaboration project being led by CISIRO and the Nature Conservancy. Divers returning from bleached reefs were asked what they thought of the dive, and few of the divers actually knew anything was different, or just noticed the fish.; Other divers commented that all the white corals were so beautiful, or that the reefs reminded them of a nice winter scene. Some of the divers, slightly more aware of the problem, commented erroneously that the corals were all dead. So what is coral bleaching, how is it affecting our planet’s reefs, what can we do, and is there going to be another big bleaching event in 2014?

Mass coral bleaching in 2010, Chalok Bay, Koh Tao

Mass coral bleaching in 2010, Chalok Bay, Koh Tao

What is coral bleaching?

Coral bleaching refers to the loss of symbiosis between the coral animal, and the unicellular algae (zooxanthallae) that live inside them. The coral animal itself is actually clear, and the color we normally see is the color of this algae, which also provide 85-95% of the energy (in the form of sugars and carbohydrates) that the coral uses. In return, the algae benefits from having a safe place to live where conditions are always the same, and nutrient ‘fertilizers’ are provided by the coral animal. Normally everybody benefits, and it is this symbiosis that forms the base of the reef environment, sustaining an estimated 6-9 million species.

When temperatures of the water warm up by just 1-3°C, the chemistry inside the coral animal changes, (same as when you get a high fever) which begins to attack the algae. At the same time, the algae produce more by-products of photosynthesis (ozone, hydrogen peroxide, etc) and this happy symbiosis becomes competition. The corals will then digest or expel the algae, thus losing their color (and 85-95% of their energy supply), allowing us to see the white skeleton underneath.

Heathly Branching coral on the left showing thier dark brown color. Bleached corals on the right without internal zooxanthallae appear white, but are still alive.

Healthy Branching coral on the left showing their dark brown color. Bleached corals on the right without internal zooxanthallae appear white, but are still alive.


Is a bleached coral dead, or can it come back?

Map of coral mortality around Koh Tao, 2010. Based on survey data collected through the EMP Program by the NHRCP

Map of coral mortality around Koh Tao, 2010. Based on survey data collected through the EMP Program by the NHRCP

A bleached coral is thus a coral in a very stressed out state. It has lost most of the energy it needs to keep up with daily functions (immune system, tissue repair, mucus production, growth, calcification, etc), but it is NOT dead. If the coral has enough reserved energy stored in the form of lipids and oils then it may be able to last a few weeks or months in the state, and possibly get algae back in its tissues and return to good health. If the coral does not have enough reserved energy or is hit by some other problems (predation, breakage, algae overgrowth, sediment, etc) then it will die.

This is where the public media often confuses things, because there are thus two types of recovery from such an event. The first, Recovery from Bleaching, means that the corals bleached, but then resequestered zooxanthallae (algae) back in to its tissues, its color returned, and aside from being depleted of reserves is healthy again. This can happen in weeks or months, as seen on Koh Tao in 2010 with 98.2% of the corals bleached in June, but by September no corals bleached. The second type, Recovery from Bleaching Induced Mortality, means that the corals died, and new coral larvae will have to come into the ecosystem, and successfully settle down and grow, before the coral coverage will return. This process can take years or decades, if ever.

The amount of coral that die from bleaching really depends on many factors, and will vary greatly between locations, depths, and species types. In General, corals with thick tissues (massive and submassive corals or mushroom corals) will outlast those with thinner tissues (fast growing corals like the branching or bushy ones). Very important is the level of stress on the corals before and during the bleaching event, in such a stressed out state it only takes a tiny push for the corals to die.


So is this a natural or anthropogenic problem?

The fact that corals exposed to higher temperatures and other stresses will bleach has been known for a long time, and was described in scientific literature even at the turn of the 20th century. But until 1979-1980 nobody had ever recorded an entire reef bleaching. In that year, corals from many different species across several 10’s of kilometers bleached at the same time, and a new term for it was coined; Mass Coral Bleaching.

Since 1979, there have been mass coral bleaching events in almost every coral reef ecosystem on the planet, at least once every decade. In 1998, the hottest year for oceans in 800,000 years, there was a global mass bleaching event, which killed 16% of the world’s hard corals. In 2010, tied for second hottest year on record, corals throughout the South China Sea bleached, especially in the Andaman Sea, which had largely escaped the 1998 event.

So although coral bleaching is a natural phenomenon, the frequency and severity of these events is increasing rapidly as our planets warms. Over the last century, global ocean temperatures have increased by about 1°C, which doesn’t sound like much, but greatly exacerbates natural cycles of warming and cooling, known as the El Nino Effect. With 7 major bleaching events in the last two decades, and also 7 of the hottest years on record, it is clear to most scientists and reef researchers that human influenced climate change is a major factor. With more sever events, and less time for recovery and regrowth between them the outlook for our planet’s reefs is rarely optimistic.


Can we prevent corals from bleaching?

A bleached colony of Porites in Chalok Bay, May 2014.

A bleached colony of Porites in Chalok Bay, May 2014.

In aquariums, the temperature can be lowered and bleached corals can recover. Unfortunately in the ocean it is not possible to do anything on a realistic scale to help prevent or rehabilitate corals from bleaching. But this does not mean we cannot do anything. First and foremost we should be focused on reducing our consumption of fossil fuels and other resources and working towards alternatives or solutions to climate change. But since the likelihood of that happening before reefs are gone is small, we need to do something now. The most important thing that can be done is too reduce the other stresses that the corals will encounter, or improving the ‘resilience’ of the reefs. Coral reefs face a consortium of threats, and by properly managing our inputs to the oceans and activities in reefs we can increase the percentage of corals that will survive the event and recover from bleaching. Some examples of such activities include:

  • Reducing erosion and wastewater flow into the sea
  • Controlling divers, and not allowing new divers to go into coral reef areas
  • Preventing detrimental activities such as fishing, dredging, anchoring, fish feeding, reef walking, etc.
  • Shading corals, or relocating coral nurseries to deeper depths
  • Removing coral predators (Crown of Thorns, Drupella snails, etc)
  • Taking lots of data through focused research programs
  • Keeping the ideas of maximizing coral resilience in mind during restoration and artificial reef activities


2014 is an El Nino year, does that mean Koh Tao is in for a repeat of 2010?

The NOAA Coral Reef Bleaching Alert Area Map for the Gulf Region, May 12th, 2014

The NOAA Coral Reef Bleaching Alert Area Map for the Gulf Region, May 12th, 2014

Koh Tao has seen little rain and almost no waves throughout most of early 2014. Temperatures have been warm for a while, and in Mid-April NOAA issued a bleaching watch for the area. By the 12th of May that bleaching ‘watch’ has gone up to a bleaching ‘warning’ level (see the map at right). In the water, we have been seeing corals bleach in many sites around the island, and also seen an increase in coral diseases (also associated with bleaching events).

By mid-May, corals growing in shallow waters from several genera have become partially or fully bleached, mostly from the following genera: Porites, Acropora, Astreopora, Monitpora, and Pocilliopora. In our regular Ecological Monitoring Program Surveys, we have seen a decrease in the amount of healthy corals, and a rise in the amount of partially and fully bleached corals. More alarmingly we have also seen rises in the amount of Recently Killed or Dead Coral, mostly stemming from disease and predation of corals already stressed by temperatures (See the two graphs below).


Graph of coral health data from March 1st to June 30th of 2014 across Shallow EMP Lines (3-6 meters depth). Percentages are averaged from all corals assessed for the month during EMP Surveys; 386 corals from 5 sites, 277 corals from 3 sites, 556 corals from 7 sites, and 554 corals from 7 sites; March, April, May, June respectively.


Graph of coral health data from March 1st to June 3oth of 2014 across Deep EMP Lines (5-9  meters depth). Percentages are averaged from all corals assessed for the month during EMP Surveys; 277 corals from 5 sites, 187 corals from 3 sites,  223 corals from 5 sites, and 396 corals from 6 sites; March, April, May, June respectively.


During the 2010 coral bleaching event, about 35-55% of the corals had bleached by mid-May, so it would appear that this event is not going to be as bad as that one. But it is still too early to say for sure. The graphs above will be updated weekly, so you can follow along with the state of the reefs on Koh Tao.  Either way, this year will be detrimental to the all around health of abundance of our reefs, and highlights the pressing need for our communities and governments to start taking action. Coral reefs are one of our planet’s oldest, most productive, and most diverse ecosystems; we cannot afford to lose them.


Most current Temperature Data for Koh Tao

Koh Tao Water Temperature

Daily average temperature is calculated for a 24 hour period from data collected by in-situ Temp/Light probes. Probes are set to record at an interval of every 3 minutes, and are located at 4 depths (3, 5, 7, and 9 msw) in Chalok Ban Kao, Koh Tao. The maximum sea water temperature for the period was 33.33 °C on April 30th.


See current pictures from the 2014 bleaching event

Do you have other questions about coral bleaching globally or locally on Koh Tao? Please comment below using facebook and we will do our best to post answers on here.