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CORALS ARE “IN HOT WATER”: HOW NOAA SATELLITES ARE HELPING

Bleached elkhorn coral near St. Croix, US Virgin Islands, in October 2005. Please credit NOAA Center for Coastal Monitoring and Assessment.July 31, 2006 — August is the start of the coral bleaching season in the Caribbean. Average water temperatures reach a maximum in August and September, putting corals at risk. August 2005 saw the beginning of a record-breaking coral bleaching event throughout the Caribbean region, from Belize to the Bahamas and from Tobago to Texas. The U.S. Virgin Islands were hit particularly hard: up to 95 percent of the corals bleached, and some areas saw 40 percent of the coral area killed — an astonishing loss in just a few months. (Click NOAA image for larger view of photo showing bleached elkhorn coral near St. Croix, U.S. Virgin Islands, in October 2005. Click here for high resolution version. Please credit "NOAA Center for Coastal Monitoring and Assessment.")

Mass coral bleaching events like this are caused by unusually high water temperatures and high light that stress corals. The coral polyps will expel the symbiotic algae that live in their tissues, exposing the white skeleton underneath. Corals can recover from mild bleaching, gradually recovering their color by repopulating their algae. However, if the bleaching is severe or prolonged, individual polyps or whole colonies will die.

The connection might not be immediately obvious, but NOAA’s polar-orbiting satellites that circle the Earth are a crucial tool in combating coral bleaching. How can a satellite that orbits 850 km (530 miles) above the Earth help the coral colonies that grow just below the ocean surface? On board each satellite is a sensor called the Advanced Very High Resolution Radiometer, or AVHRR. Among other things, this sensor can measure the heat that radiates from the surface of the Earth, including the ocean, allowing NOAA to monitor the sea surface temperature around the globe every day.

Bleaching HotSpots from August 27, 2005. Yellow to orange colors indicate areas above the bleaching threshold. Note the warm waters around the entire Caribbean region. Please credit NOAA Coral Reef Watch.The NOAA Coral Reef Watch program customizes products from these satellite temperature readings to look for areas at risk for coral bleaching. The key is to focus in on regions where the SST is higher than normal — especially where it’s warmer than the mean temperature during the hottest month. For example, take Sombrero Reef in the Florida Keys: the mean water temperature reaches its maximum value in August and September, peaking at around 29°C (84°F). The NOAA Coral Reef Watch program monitors this site to see if its SST goes above 29°C, creating what is known as a HotSpot. When this same methodology is applied to all global satellite SST measurements, it produces a global chart of coral bleaching HotSpots. (Click NOAA image for larger view of photo showing bleaching HotSpots from August 27, 2005. Yellow to orange colors indicate areas above the bleaching threshold. Note the warm waters around the entire Caribbean region. Please credit "NOAA Coral Reef Watch.")

Degree Heating Weeks from August 27, 2005, at the beginning of the bleaching event. Green to red colors indicate areas where heat stress has accumulated, and corals are at risk for severe bleaching. Please credit NOAA Coral Reef Watch.HotSpots are just the first step in pinpointing areas at risk for bleaching. High temperatures are important, but scientists have discovered that coral stress builds up if the high temperatures are sustained for weeks at a time. Thus, another important factor that the NOAA Coral Reef Watch program measures is the “cumulative thermal stress” the corals have been exposed to over the past three months — also known as Degree Heating Weeks. DHWs are a combined measurement of how high the HotSpots have been, and how long they have lasted. If DHWs rise above four, coral bleaching is expected; values above eight indicate that mass bleaching and some coral mortality is likely. The situation in the Caribbean was so bad in 2005 that some areas reached DHWs over 16 — four times the level of stress needed to initiate coral bleaching — resulting in widespread coral bleaching and death in 2005. (Click NOAA image for larger view of photo showing Degree Heating Weeks from August 27, 2005, at the beginning of the bleaching event. Green to red colors indicate areas where heat stress has accumulated, and corals are at risk for severe bleaching. Please credit "NOAA Coral Reef Watch.")

The latest tool in the Coral Reef Watch suite of operational data products are the Satellite Bleaching Alerts. For select locations around the world, reef managers and researchers — and even you — can sign up for free automatic e-mail alerts. The e-mails warn subscribers when ocean temperatures rise to dangerous levels and corals may be at risk.

The data products that the NOAA Coral Reef Watch program produces are applicable to a wide range of users: coral reef researchers, climate scientists, managers of coral reef parks and reserves, students and the interested public. Therefore, it is important to provide the data in a variety of different formats to meet the varying needs. Images have always been the most popular format, since they are so widely used, but the NOAA Coral Reef Watch program also offers the raw data, so that a more technical audience can process the data to meet their own individual needs. Recently, NOAA also started making the data available to Google Earth users — providing a flexible and educational new way to see where reefs are currently at risk from coral bleaching. (Click NOAA image to see the latest HotSpot images for the Caribbean region — updated twice-weekly. Please credit "NOAA.")

What is the Current Situation in the Caribbean?
NOAA Coral Reef Watch scientists are concerned that 2006 could be another rough year for Caribbean reefs. While NOAA can’t yet predict bleaching conditions for the future, there are some troubling indications of what the future might hold.

Hurricanes, like mass coral bleaching, have a relationship to ocean temperatures that are warmer than average. Looking at the past 20 years, there is a strong correlation between the number of named storms in the Atlantic and the percentage of the Caribbean basin at risk for coral bleaching (DHWs greater than four). The NOAA forecast for the 2006 hurricane season calls for a continuation of above-normal hurricane activity — which raises the possibility that corals may be at risk for bleaching again this year.

Bleaching watches (the lowest level of the Satellite Bleaching Alerts) were issued in early July for the Florida Keys, the Bahamas and Puerto Rico. Sea surface temperatures have decreased somewhat since then, and are currently just below the level where bleaching would be a concern. However, given the situation last year, scientists and reef managers will be extra-vigilant in the coming months, looking for signs of bleaching and thermal stress. (Click NOAA image to see the latest DHW images for the Caribbean region — updated twice-weekly. Please credit "NOAA.")

If temperatures do increase in the Caribbean again, it raises the question — can anything be done to prevent coral bleaching or to help corals recover? NOAA (in partnership with the Great Barrier Reef Marine Park Authority, the World Conservation Union, The Nature Conservancy, and others) has developed some guidelines to help marine park managers protect their corals from the long-term effects of coral bleaching. There are some mitigation measures that might help:

  • Restrict potentially stressful activities (such as diver access and fishing) on the reef during and after a bleaching event
  • Artificially shade or cool selected reefs to lessen the bleaching impacts
  • Remove coral predators from the area, to allow corals to recover
  • Enhance the overall health of the corals by reducing pollution, coastal runoff and over fishing — a healthy reef ecosystem is more able to recover

In addition, there are some areas that may be naturally protected from the conditions that cause bleaching or where corals have adapted to the stressful conditions. It is especially critical to identify and protect these areas.

Although researchers are still learning about the danger of bleaching events to coral reefs, NOAA has a plan of action. NOAA will continue its intense coral monitoring (both from space and in the ocean) during and after bleaching events, so researchers can learn more about the causes and consequences of coral bleaching. This knowledge will be critical for the long-term survival of the nation’s beautiful and valuable coral reefs. (Click NOAA image for larger view of photo showing healthy Acropora prolifera coral, photographed near Lee Stocking Island, Bahamas, February 2005. Click here for high resolution version. Please credit "NOAA Coral Reef Watch/Mark Eakin.")

Why Should We Care About Our Coral Reefs?

In many ways, coral reefs are critically important to the nation. They provide a large economic benefit — recreation and commercial fishing on coral reefs generate billions of dollars each year for local economies. Reefs also act as a natural barrier — a first line of defense — in protecting tropical coasts from storms and floods. In some areas, coral reefs even offered some protection against the devastating 2004 Indian Ocean tsunami (Global Coral Reef Monitoring Network report). Researchers are just beginning to explore the range of potential medicines that reef organisms can provide, including cancer treatments, painkillers and antivirals. But perhaps what is most important is the fact that coral reefs host some of the highest biodiversity on the planet — around a million species depend on the reefs for survival. Coral reefs are a vital, yet threatened national resource.

Read more about why we protect coral reefs, from the NOAA Coral Reef Conservation Program.

About Coral Bleaching

Reef-building corals require warm, tropical water — but how warm is too warm? Scientists have discovered that corals start getting stressed if the water is only one degree Celsius above the average temperature of the warmest summer month. This temperature is known as the “bleaching threshold.”

Corals have symbiotic algae, called zooxanthellae, living within their body tissues. The algae provide food and oxygen for the coral, and the algae get the benefit of nutrients and a safe growing environment in return. When corals are stressed, they expel the zooxanthellae. The algae give corals their distinctive colors, so when the coral loses their algae all you see is the white skeleton through the clear coral tissue — thus, the coral appears “bleached.”

If the temperatures don’t get too high or last too long, the corals can recover. They gradually rebuild their stock of zooxanthellae, and will likely go on to live healthy lives. However, if the temperature rises significantly above the bleaching threshold or remains high for several weeks, severe bleaching will occur. The corals are left without their main food source, and they can eventually die. Those that do survive are more likely to be infected with diseases, just like a stressed person is more likely to fall ill. These diseases can also kill corals, months after the bleaching occurs.

Read more about coral bleaching and coral diseases, from the NOAA Coral Reef Information System.

Relevant Web Sites
NOAA Coral Reef Watch Homepage

A Grade-School Curriculum about Remote Sensing and Coral Reefs

More about the 2005 Caribbean Bleaching Event

NOAA CCMA Biogeography Program’s Preliminary Report on the 2005 Caribbean Bleaching in St. Croix, U.S. Virgin Islands

Visit the Coral Reef Conservation Program, to Learn about all of NOAA’s Coral Reef Activities

Data and Publications from NOAA’s Coral Reef Activities are Available through the Coral Reef Information System

Media Contact:
Ben Sherman, NOAA Ocean Service, (301) 713-3066 ext. 178