Coral Bleaching

Overhead view of reef with extensive coral bleaching
Extensive coral bleaching in 2010. (Image: FGBNMS/Schmahl)

Coral bleaching describes a situation in which corals appear to turn white. This happens when coral polyps expel their symbiotic algae (zooxanthellae) as the result of some kind of stress event. Without the algae, the coral polyps are mostly clear, allowing you to see through to their white skeletons beneath. This results in a bleached appearance.

Close up view of large, fleshy bleached polyps
The large polyps of Spiny Flower Coral (Mussa angulosa) show the drastic color change in bleached coral. (Image: FGBNMS/Hickerson)

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.

Three colonies of Great Star Coral showing bleached and unbleached areas.
Several bleached colonies of Great Star Coral (Montastraea cavernosa) with a few polyps still showing some color. (Image: FGBNMS/Joyce & Frank Burek)

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.

Bleached, partially bleached and dead sections of Spiny Flower Coral and a piece of bleached fire coral
This colony of Spiny Flower Coral (Mussa angulosa) shows sections that are partially bleached, completely bleached and even some dead sections already overgrown with algae. A bleached Fire Coral stands alongside. (Image: FGBNMS/Joyce & Frank Burek)

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.

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 too long, bleaching may occur.

A stark white section of coral with a Christmas Tree Worm in the middle
A completely bleached section of coral showing the distinct outline of a Christmas Tree Worm tube with the worm's gills protruding form the open end. (Image: FGBNMS/Schmahl)

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 partially bleached colony of Mountainous star Coral, where the peaks of the lumps still have color
Coral colonies don't necessarily bleach evenly, as seen in this Mountainous Star Coral (Orbicella faveolata) during a major bleaching event in 2005. (Image: FGBNMS/Joyce & Frank Burek)

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).

Varying levels of bleaching visible in coral colonies across the reef
Bleaching progresses at different rates across the reef. (Image: FGBNMS/Schmahl)

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 mound of coral with some colonies bleached and others not
The brain coral to the left is almost completely bleached, while the one to the right is paling (becoming bleached). The Boulder Star Corals in the middle don't appear to be bleaching at all. (Image: FGBNMS/Schmahl)

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 bleached brain coral colony next to an unbleached one
Not all corals respond equally to stress events as seen in these two brain coral colonies, side by side. One is completely bleached and the other is not. (Image: FGBNMS/Schmahl)

Sea surface temperatures were consistently high in the Gulf of Mexico during the summers of 2005, 2010, and 2016, causing major bleaching events in the sanctuary. Similar problems were seen on coral reefs around the world those same years.

A colony of brain coral that is half bleached and half not
A partially bleached brain coral in the Flower Garden Banks in 2010. (Image: FGBNMS/Schmahl)

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 and death in shallow reefs there.

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.

A diver kneeling next to equipment on the sea floor in a sand patch
A diver prepares to change out a datasonde at East Flower Garden Bank. (Image: FGBNMS)

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.

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.

A map of the Gulf of Mexico and Caribbean with remote sensing locations marked cnad overlayed colors showing sea surface temperatures in the region
Map from the Coral Reef Watch remote sensing website 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.

A graph showing sea surfact temperatures in FGBNMS from January 1, 2016 to May 8, 2017
A sample graph of the data from the FGBNMS point on the map above. Note the bleaching indicators in 2016.

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