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."

next Keeping an Eye on the World’s Coral
next Corals in Crisis

 

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)

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