Mapping the Decline of Coral Reefs


Anyone who has strapped on diving gear and glided past a healthy coral reef knows that few sights in nature are as breathtaking. From the intricately embroidered patchwork of the corals themselves to the myriad of multicolored creatures that live in the reefs’ crevices to the shimmering schools of fish that seem to move as one, every cubic inch of a thriving coral habitat appears to be alive and teaming with complexity. In truth, coral reef habitats represent some of the densest and most varied ecosystems on Earth. Though they cover only 0.2 percent of the ocean’s floor, scientists estimate that nearly one million species of fish, invertebrates, and algae can be found in and around the world’s reefs.

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While reef habitats appear to be robust enough to withstand almost anything, they are extremely fragile. Not only are most corals brittle, but they usually need pristine, clear, warm, relatively nutrient-free waters to survive. Over the past 50 years, humans have put an enormous amount of pressure on coral reef environments by altering their waters and tearing up their foundations. From dynamite fishing to global warming, we are rapidly sending the world’s reefs into oblivion. The latest reports state that as much as 27 percent of monitored reef formations have been lost and as much as 32 percent are at risk of being lost within the next 32 years.

For marine biologists, the destruction of the reefs has proven to be as frustrating as it is heartbreaking. Because reef habitats are so complex, and because worldwide reef monitoring and mapping efforts only began a little over a decade ago, scientists simply do not have enough information to keep tabs on the destruction of the reefs, let alone come up with an effective solution. At the rate the reefs are disappearing, they may be beyond repair by the time a comprehensive plan to save reefs can be put into place.

  Coral reefs rival the tropical rainforests as the most diverse ecosystems on Earth. With a wide variey of plant, animal, and microbial life, they are not only beautiful destination for divers, but an important indicator of ocean health. (Photograph copyright Corel Corporation)

Tuanake Atoll

Scientists at NASA’s Goddard Space Flight Center and at several universities around the world, however, may have at least a partial solution to this problem. They have been examining detailed images of the ocean collected by the Landsat 7 and other high-resolution remote sensing satellites. While these types of satellites were primarily launched to observe land-based change, they have also been found to produce detailed images of shallow waters around the ocean’s margins. Using these images, the scientists have been able to map reefs in a fraction of the time it takes to map them by boat or airplane. With funding, the researchers believe they could have a comprehensive map of the world’s reefs within three years. This map would not only be useful for identifying large-scale threats to the reefs, but would allow the researchers to locate those reefs that are in the most trouble.

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The data used in this study are available in one or more of NASA's Earth Science Data Centers.


Tuanake Atoll in French Polynesia is one of many of the remote reefs recently mapped with the help of satellite data. This true-color image was acquired by Landsat 7. (Image courtesy Serge Andrefouet, University of South Florida)


Corals in Crisis

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Phil Dustan, a marine biologist at the College of Charleston, has been lending his expertise to the satellite monitoring effort. For the past five years, he has been working as a principal investigator with the Environmental Protection Agency (EPA) in the Coral Reef Monitoring Project to study the reefs of the Florida Keys.


Dustan explains that corals in many respects are very thin amounts of tissue on top of a rock that they build (Dustan 1999). Though they may appear to be jagged stone plants sprouting from the ocean floor, only a very thin layer of polyps on the coral’s surface is actually alive. These polyps, which resemble tiny sea anemones, build interconnected tubes around themselves as protection against predators. Each time new polyps are born, they will construct their shells on top of their predecessors’ empty encasements. Stacks upon stacks of the limestone shells pile up on top of one another through the years to create coral branches and heads and ultimately giant reefs (Miller and Crosby 1998). Over millions of years, thousands of species of sea creatures have come to rely on the habitat that the reefs provide. So far, researchers have identified nearly 4,000 kinds of fish and tens of thousands of invertebrates that thrive and depend on some 800 types of known coral. Some scientists speculate that there may be hundreds of thousands of species of reef-dwelling animals that have not even been cataloged yet (Bryant et al. 1998).

"It’s a real tragedy," says Dustan. "But over the past twenty years, we’ve seen a rapid decline in the vitality of coral reefs and their ecosystems worldwide." Dustan explains that corals evolved in warm, clean, still waters with stable levels of sunlight and salinity. In order for the corals to survive, these waters must remain pristine and relatively undisturbed. A delicate balance also has to be maintained between the animals that feed on and live among the reefs. If any of these factors are thrown out of whack, the polyps in the reef will be eaten away by predators, devastated by disease, or simply become so stressed that they die. If this top living layer of coral does not reassert itself, then the reefs will collapse and the creatures that dwell on the reefs will vanish from the area (Bryant et al. 1998).


Coral Polyps
Coral reefs are built one thin layer at a time by tiny animals. If the coral polyps are harmed, then their reefs will die. (Photograph courtesy Phillip Dustan, College of Charleston)


Carysfort Reef, 1975 - 1995

Since the late 1970s, reefs across the world have been dying at an unprecedented rate, and it only seems to be getting worse. Dustan points out that in the Florida Keys alone extensive reef monitoring studies conducted by the EPA and other agencies have shown that the reefs lost more than 38% of their living coral cover from 1996 to 1999. Carysfort Reef lost over 90% of its coral cover from 1974 to 1999. The Global Coral Reef Monitoring Network, the single largest coral reef monitoring effort in the world, reported in October 2000 at the 9th International Coral Reef Symposium in Bali, Indonesia, that of all the reefs they monitor worldwide, 27 percent have been lost and another 32 percent could be lost in the next 20-30 years (Pockley 2000). Another report published by the World Resources Institute states that 58 percent of all reefs are at serious risk from human development. All of these reports point to human activity as the primary reason for the decline of the reefs. With half a billion people now living within fifty miles of reef habitat and more on their way, it’s likely only to get worse (Bryant et al. 1998).


The health of Carysfort Reef off the coast of Florida has declined dramatically in the past 25 years. The photographs at left show this decline. Coral that was healthy in 1975 are visibly sick by 1985, and dead and broken by 1995. (Photographs courtesy Phillip Dustan, College of Charleston)


Reefs and Urbanization


"The problem is human activities assault reefs on many different levels," says Dustan. In Florida people drain their septic tanks directly into the ocean. The additional nitrates in the human waste cause algae to grow on top of the coral structures and deprive the coral polyps of sunlight. In the Indian Ocean around Sri Lanka, fishermen often use dynamite to catch fish and in the process end up blowing the reefs to bits. Around the islands of the Philippines and Japan, over fishing of natural predators has allowed the Crown of Thorns starfish to run rampant and devastate the coral in the area (Miller and Crosby 1998). The world over, global warming, which many believe to be caused by human greenhouse gas emissions, is warming the top layers of the seas in the tropics and causing the coral to turn white and lose their polyps—a condition known as "bleaching" (Pockley 2000).

Unfortunately, the damage that humans cause to reefs seems to be long lasting and may be permanent. "I’ve seen what happens when natural disasters such as hurricanes hit the coral reefs. They fully recover. They’ve adapted themselves to recover from storms," says Dustan. The ecological pressures put on reefs by humans, such as sewage and dynamite fishing, are ongoing and tend to wear the reefs down to the point where they can no longer bounce back. Even when people try to plant new reefs in these troubled areas, they only last a year or two in many instances and then die. The irony is that the people who are oftentimes killing the reefs rely on them for tourism income and for the food the fish provide (Miller and Crosby 1998).

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The growth of coastal populations is one of the primary causes of damage to coral reefs. Some fishing techniques, careless divers, and boat traffic harm the reefs directly, while pollutants and nutrients from sewage and runoff disrupt the food chain. This true-color Landsat 7 image shows dense development (left) near coral reefs (right) in the Florida Keys. (Image courtesy Serge Andrefouet, University of South Florida)


A Matter of Perspective

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Despite the bleak picture that these reports paint, there is still quite a lot of debate among the scientific community as to how bad the problem really is and what should be done to solve it. Serge Andrefouet, a remote sensing specialist at the University of South Florida (USF), has been observing reefs around the world for the past five years, and he is concerned about many of the conclusions the media and the public are drawing. There is no doubt that the situation is severe and optimism would be foolish, but Andrefouet believes that there is simply not enough long-term data on reefs to come to a judgement about their future or what exactly is causing them to die.


Reef Risk Map


"While it is true that many reefs being monitored are deteriorating rapidly, many of the world’s reefs are not monitored at all. They are located in places where no one goes very frequently," he says. He explains that over the past decade, reef monitoring programs such as the Global Coral Reef Monitoring Network and ReefCheck have managed to set up a network of volunteers around the globe to monitor reefs. These organizations provide volunteers with a set of protocols to assess the health of nearby reefs. The work usually involves strapping on scuba gear and snorkels and observing any damage directly. Since most of these volunteers don’t stray too far off the beaten path, the reefs they assess are generally near populated areas and prone to damage by humans. Acquiring the necessary boats and hiring personnel to check on all the reefs in the remote areas of the world would simply be too costly for these monitoring networks. Consequently, a large part of the world’s 10,000 plus reefs haven’t been assessed or mapped. Only a little more than 10 percent of the reefs in the Pacific, for instance, are monitored for health (Bryant et al. 1998).


Existing maps of coral reef health—such as this one developed by the World Resources Institute—are based on potential threats such as coastal development and inland erosion. (Blue dots represent reefs with a low risk of damage, yellow dots indicate medium risk, and red dots represent high risk.) The maps lack actual measurements of a reef’s health. (Map courtesy World Resources Institute Reefs at Risk Indicator)


Reef Survey

Another problem is that scientists do not have long-term data for reefs. Researchers did not begin assessing reef habitats on anything approaching a global scale until a few decades ago. No one knows for certain how much of what appears to be global destruction of coral reef habitats is a result of natural, long-term cycles and how much is caused by human expansion and development. "While we have more and more data that show the decrease of reef health, we lack the background data to understand long-term cycles and check if what we see now has happened in the past," says Andrefouet.

To understand fully what is occurring to the reefs around the world, how bad the problem is, and what should be done to correct it, Andrefouet explains that a more comprehensive method to monitor the world’s reefs would have to be put into place. Ideally such a system would allow scientists to assess individual reefs, observe worldwide trends affecting reefs, such as global warming and pollution, and maintain a consistent historical record of the reefs.

Though there may not be a cost-effective way to set up such an extensive monitoring effort on Earth, Andrefouet says there may be one in orbit around our planet. He and his colleagues Frank Muller-Karger, David Palandro, Chuanmin Hu, and Kendall Carder at the University of South Florida have teamed up with Josh Gash, Terry Arvidson, and Darrell Williams at NASA’s Goddard Space Flight Center to use Landsat 7 data to examine possible ways to address this problem.

Launched in 1999, Landsat 7 moves in a nearly circular obit approximately from pole to pole around the Earth and measures the infrared and reflected solar radiation from the surface of our revolving planet. It beams these readings in the form of data to receiving stations on the ground where scientists can convert them into meaningful images of the Earth. With a resolution on the order of 30 x 30 meters and up, the images are not well suited for viewing details of the planet’s surface any smaller than an office building. They are, however, extremely useful for mapping and monitoring large features.


Direct examination of reefs provides a very accurate picture of a reef’s health. However, it is difficult and expensive to map reefs over a large area and for long periods of time without the help of satellites. (Photographs courtesy Phillip Dustan, College of Charleston)


Mangareva pseudo atoll


Traditionally, Landsat 7 has been used to track changes in land cover such as deforestation. Based upon the scientists’ formal request, NASA agreed to modify the Landsat 7 image acquisition strategy to begin monitoring shallow ocean regions. Using these new Landsat images (taken by Landsat’s Enhanced Thematic Mapper Plus (ETM+) instrument), Andrefouet and his colleagues have begun mapping the locations of reefs all over the world.

"There is no formula to mapping out the reefs. We have to look at each reef on a case-by-case basis," says Andrefouet. Unfortunately, the coral reefs don’t always stick out in these images. It’s often difficult to distinguish between the coral reefs and the rocks, sand, silt, algae, and other things covering the ocean floor and floating in the water. The depth of the reef and the turbidity of the water can also make reefs appear different from area to area.

Much of the work the researchers have done over the past few years has been simply in refining techniques to locate the reefs in a given image. In some instances, the process is as easy as having a trained expert outline the area that looks like a reef. In other instances, complex computer programs involving fuzzy logic and neural networks are used to bring out the reef. Oftentimes the scientists will send someone to the reef site to sample the area and quickly verify what they see in the image. In the end, the researchers typically end up with a colorful map of the reefs and all that surrounds them. The whole process takes only a fraction of the time that it would take to map the reef by boat or airplane and costs a whole lot less.

With the help of local governments and researchers, the USF team has mapped out a number of reefs, including many of the atolls of French Polynesia and the reefs that border Belize, Honduras, and Mexico. The group is in the process of applying for additional NASA funding to use this technique to map all the reefs in the world. Andrefouet believes that it would take approximately 1,000 Landsat 7 images of the tropical and sub-tropical oceans and three years of work. "We’d require one year to collect all the images, one to analyze the data and classify the reefs, and another to publish and distribute the results to the public," says Andrefouet. "This would be the first-ever high-resolution baseline map made of the world’s coral reefs."

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This Landsat 7 image shows the Gambier Islands in French Polynesia. A coral reef surrounds an inner lagoon with several islands, forming a pseudo atoll. Each island is itself surrounded by coral reefs. Remote islands like these are more easily monitored by satellite rather than by labor-intensive manual surveys. (Image courtesy Serge Andrefouet, University of South Florida)


Keeping an Eye on the World’s Coral

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Andrefouet says at the very least this high-resolution map would provide an accurate resource for anyone wanting to study or monitor reefs. The ultimate goal of the NASA and USF scientists is to use this map to assess the health of the world’s reefs on a number of different levels. On a global scale, such a map would provide scientists with a way to monitor large-scale potential threats to the reefs. The reef map, for instance, could be compared to worldwide sea surface temperature maps to determine which reefs are most likely to be bleached by global warming. Maps demarcating pollution from coastal run-off areas could alert researchers as to which reefs were in danger of being inundated with pollution. "With existing data, it is possible to observe these phenomena almost on a day-to-day basis," says Andrefouet. "It also provides a baseline to prioritize acquisition of very high resolution (few meters) images, such as those provided by IKONOS and the future hyperspectral Orbview-4 commercial satellites, as well as for optimized sensors operated by NASA on the International Space Station."

The satellite map could also be employed to observe long-term trends in reef mortality. After a section of reef has been dead for a number of years, the reef’s underlying structure will begin to disintegrate. "Over a period of time the compact platforms undergo a shift to platforms with holes and breaks," says Andrefouet. The Landsat 7 can pick up on these changes in the reef's structure. By constantly keeping updated images of the reefs over the decades, scientists could develop an archive of images that would allow them to see if any of these reef deaths are part of a larger, natural cycle. They could then compare these present day trends to fossil cores taken from ancient coral reefs (known as paleorecords) to see if similar changes have occurred before in the Earth’s history.


Black Band Disease
Black band disease attacks coral that have been weakened by sedimentation, excess nutrients, toxic chemicals, and warmer than normal temperatures. Coral colonies can be killed in a matter of months by the fast-moving disease. (Photograph courtesy Phillip Dustan, College of Charleston)


Carysfort Reef, Florida


Unfortunately, Andrefouet says that given the current level of technology researchers cannot discern if a reef is dying or has just died using remote sensing data alone. The instruments on Landsat 7, or for that matter any commercial satellite, simply are not powerful enough to observe if a reef has recently lost its thin layer of polyps. Usually the change is subtle since the algae that typically covers a reef en masse soon after the polyps die creates the illusion of a healthy reef in a satellite image. As reefs can die in a matter of months, networks of volunteers will still have to assess the immediate health of reefs they believe to be in danger. Andrefouet points out, however, that remote sensing maps aid in these rapid assessments.

  This true-color image of Carysfort Coral Reef, Florida was taken by the IKONOS satellite. Owned and operated by Space Imaging, the satellite collects data in some of the same wavelengths as Landsat 7, but at a higher resolution. (Image courtesy Serge Andrefouet, University of South Florida. Contains material copyright Space Imaging L.P.)


There are several efforts underway to develop techniques that will employ remote sensing to further identify reefs in immediate danger. Phil Dustan has been working with Andrefouet and Dave Palandro, a graduate student at USF, on just such a technique using Landsat 7 data. "By looking at the variability of the pixels’ brightness on a set of images, we can map how much change that reef has undergone," says Dustan. In the change from a polyp-covered reef to an algae-covered reef, there is a slight fluctuation in the reef’s color. He explains that while this may be too subtle to see outright on a satellite image, scientists may be able to identify the transformation by simply mapping out the variation in the brightness of the individual pixels in a satellite image over several months. The end product would resemble a three-dimensional relief of the reef with any peaks showing areas with the most variation in brightness and the flat sections depicting the areas with the least variation. This relief could then be set over a land cover classification map to see which areas of the reef fluctuated in color the most and are consequently in the most trouble.

  Carysfort Change Detection

Dustan explains that he’s used this technique on reefs on the Carysfort Reef in Florida, which was once one of Florida’s most vibrant reefs, replete with platforms of Elkhorn coral. The reef died over the course of the last five years. During the time it was dying, Dustan collected Landsat 7 and other satellite remote-sensing images of the reef even as he assessed the health of the reef firsthand underwater. Dustan along with the USF group constructed variable brightness maps of the region using the satellite data and overlaid them on an aerial map of the coral reef. Just as they suspected, the areas of reef that died showed up on the three-dimensional variable maps as tall peaks. The areas of reef that stayed the same were nearly flat.

Though the technique is in its early stages of development, Dustan feels that these variability maps could aid in a worldwide assessment of reefs. Once the scientists develop a comprehensive high-resolution map of the world’s reefs using Landsat 7, they could then use the same data to create the relief maps. The two could be placed on top of one another to pinpoint those reefs that are in the most danger of collapsing.

Of course, the best plan would be to put programs in place that stop us from destroying our reefs altogether. Sewage and water treatment systems need to be constructed in urbanized areas near coral reefs, dynamite fishing should be outlawed, and fishermen must realize what over fishing does to the reefs. "What we need to put in place is a list for cleaning up the oceans. Only then will coral reefs begin to recover," says Dustan.


1. Dustan, P., 1999: Testimony Presentation on Coral Reef Conservation Issues at the Senate Subcommittee Hearing on Oceans and Fisheries. Washington, DC.

2. Miller, S. L., and M. P. Crosby, 1998: The Extent and Condition of US Coral Reefs. NOAA's State of the Coast Report, National Oceanic and Atmospheric Administration (NOAA), Silver Spring, MD.

3. Bryant, D., L. Burke, J. McManus, and M. Spalding, 1998: Reefs at Risk: A Map-Based Indicator of Threats to the World’s Coral Reefs. World Resources Institute, Washington, DC.

4. Pockley, P., 2000: Global Warming Identified as Main Threat to Coral Reefs. Nature, 407 (6807), 932.

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This image of Carysfort Reef combines a snapshot of the reef (color) with the variability of the reef over time (height). The area of the reef is outlined. Researchers built this map by comparing the color of individual pixels representing a 30 by 30 meter area of the reef from 15 years of data. Increasing variability (height in this image) corresponds to a decrease in live coral cover accross time and highlights the shift from a healthy coral ecosystem to a rubble-algae dominated system. (Image courtesy Phillip Dustan, College of Charleston)