A Matter of Perspective
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.
"While it is true that many reefs being monitored are
deteriorating rapidly, many of the worlds 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 dont 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 worlds 10,000 plus reefs
havent 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 healthsuch as this one developed by the World Resources Instituteare 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 reefs health. (Map courtesy World Resources Institute Reefs at Risk Indicator)
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 worlds 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 NASAs 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 planets 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 reefs 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)
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 Landsats 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 dont always stick out in these images. Its 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. "Wed 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 worlds coral reefs."
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 Worlds Coral
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 worlds 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 reefs 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 Earths history.
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
|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 reefs
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
Dustan explains that hes used this technique on reefs on the Carysfort Reef in Florida, which was once one of Floridas 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 worlds 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 Worlds Coral Reefs. World Resources Institute, Washington, DC.
4. Pockley, P., 2000: Global Warming Identified as Main Threat to Coral Reefs. Nature, 407 (6807), 932.
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)