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Coral bleaching describes a situation in which corals appear to turn white. In other words, they look like they've been bleached.

How does this happen? What causes it? Let's find out.

Bleaching Basics

Coral bleaching happens when corals expel their zooxanthellae (symbiotic algae) as the result of some kind of stress event, often high water temperatures. Without the algae, the coral polyps are mostly clear, allowing you to see through to their white skeletons below. Hence, the bleached appearance.

Closeup of bleached star coral polyps
Close-up of bleached star coral polyps (Montastraea sp.)
Photo: Joyce & Frank Burek 2005

The good news is that coral bleaching is not necessarily a death sentence. If stress conditions are alleviated in time, the corals may take on new algae and return to a healthy state. Even so, recovery may take weeks to months and recently stressed corals may be more susceptible to coral diseases.

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Partially bleached coral showing some brownish color amid the white.
A couple of colonies of bleached star coral (Montastraea cavernosa). Photo: Joyce & Frank Burek 2005

The bad news is that by the time we can see bleaching, the process has already been taking place for some time. In addition, without the algae there to provide a majority of their food, the corals are beginning to starve.

A partially bleached colony of coral.  The middle section appears completely bleached while the outer sections still have color.
Only some of this ten-ray star coral (Madracis decactis) is completely bleached. Photo: Joyce & Frank Burek 2005

If the stress event continues for too long, the corals will eventually die, leaving just their skeletons behind.

Infographic explaining coral bleaching
Click on this infographic from NOAA's Coral Reef Conservation Program to see a larger image.

For a better understanding of how corals and zooxanthellae interact, please visit our Coral Basics page.

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Environmental Stresses

Stresses that may lead to bleaching include extremes in salinity, pollution, sedimentation, and temperature. The key word here is "extreme." Any of these factors may fluctuate on a given day, but when the changes are severe and/or last for a long time bleaching may occur.

A bleached colony of fire coral amid other healthy corals.
The white of this bleached fire coral colony (Millepora sp.) creates a stark contrast to the normal reef colors around it.
Photo: Joyce & Frank Burek 2005

Most coral bleaching is the result of water temperatures that go beyond the corals' level of tolerance for too long. This usually means temperatures that are too high, but can also mean temperatures that are too low.

A large, spreading coral colony that is mostly white, but still has some color in places.
A partially bleached colony of star coral (Orbicella (Montastraea) faveolata). Photo: Joyce & Frank Burek 2005

Reef-building corals survive within specific temperature ranges that vary slightly by region. At Flower Garden Banks National Marine Sanctuary, the preferred temperature range is about 68-86F (20-30C).

A large boulder of coral with the bottom half mostly white and the top half mostly brownish.  To the left is a large colony of another type of coral in its normal greenish color.
There was a lot of bleaching on sanctuary reefs in 2005.
Photo: Joyce & Frank Burek 2005

When sea surface temperatures in the sanctuary stay above 86F (30C) for too long, the corals begin to stress and may start to get rid of their zooxanthellae.

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The longer elevated temperatures continue, the more zooxanthellae leave and the paler the coral color becomes. When enough zooxanthellae leave, the coral looks bleached.

A large brain coral colony that is mostly white surrounded by some colored coral colonies and some white fire coral.
A partially bleached brain coral colony with some bleached fire
coral colonies nearby.
Photo: Joyce & Frank Burek 2005

Bleaching may affect an entire colony or only part of it. It may affect only some species and not others. In fact, it may even affect only certain colonies of a particular species while leaving other colonies of the same species untouched.

A partially bleached colony of spiny flower coral with a bleached fire coral alongside.
Some parts of this spiny flower coral (Mussa sp.) have already died and are overgrown with algae. Other parts are completely or partially bleached. Photo: Joyce & Frank Burek 2005

Sea surface temperatures were consistently high in the Gulf of Mexico during the summer of 2005, causing a massive bleaching event in the sanctuary. Similar problems were seen on coral reefs around the world that year.

.A reef scene with some coral colonies turned white.
In 2005, as much as 50% of sanctuary corals bleached.
Photo: Joyce & Frank Burek 2005

So far, we haven't seen any bleaching in the sanctuary as a result of cold water temperatures. However, in February and March of 2010 water temperatures fell below 60F (16C) for several days in Florida Keys National Marine Sanctuary and caused quite a bit of coral bleaching in shallow reef areas there.

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Measuring Temperature

How do we know what water temperatures are on the reef when we're in our offices in Galveston?

We have water quality instruments on the sea floor at East and West Flower Garden and Stetson Banks throughout the year. These sensors continuously record data that we can download to a computer each time the sensors are retrieved and maintained. Ideally, this happens about once a month.

Tubular data instruement attached to the upright pieces of a large railroad wheel partly buried in the sandy sea floor.  A diver is kneeling next to the equipment.
A diver prepares to change out a datasonde at
East Flower Garden Bank.

In between data downloads, we can track daily temperatures in the sanctuary using satellite measurements of sea surface temperature (SST). While this doesn't tell us the temperature down on the reef, it does give us an estimate.

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Remote Sensing System

Satellite data on Sea Surface Temperature (SST) is the basis of a Remote Sensing System coordinated by NOAA Coral Reef Watch in the Caribbean and Gulf of Mexico.

Through this system, scientists monitor SST at over 30 sites, including Flower Garden Banks National Marine Sanctuary, then compare that data to historic temperature averages to determine the likelihood of coral bleaching at each location.

Map of the Caribbean and Gulf of Mexico with numbered red circles marking the locations of different coral reefs.
Map from the Coral Reef Watch remote sensing web site
showing all of the sites currently being monitored.

Data graphs on that web site will give you a first-hand look at how close temperatures are to the bleaching thresholds and average monthly temperatures. You can also sign up for bleaching alert emails to receive notifications of any change in status at the sanctuary or one of the other sites.

For more information on how this system works and what the temperature graphs tell you, please visit the Satellites & Bleaching module on the Coral Reef Watch web site.

More education materials associated with remote sensing are available from the Coral Reef Watch Satellite Education and Outreach web page.

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Pale yellow sea slug with bright orange and blue ruffle down its back and bright orange spots on the end of its antennae.  Sea slug looks like a snail without its shell and is only a couple inches long.
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