What immediately comes to mind when many of us think of an atoll is a desolate, circular array of coral reefs with white, sandy islands populated by a few lonely, swaying palm trees and perhaps a castaway or two. Were we to consider them more closely, however, we would find that this standard perception just skims the surface. Atolls are, in fact, some of the most complex and vibrant structures on the planet. Built diligently over thousands of years by tiny, sea anenome-like coral polyps, these ring shaped coral structures can be tens of kilometers in diameter with individual reefs large enough to support lush tropical islands and even small cities. As is the case with any living coral structure, countless species of fish and invertebrates can be found inhabiting the waters in and around an atoll. But unlike the fringing reefs along Florida’s coast or even the barrier reefs off the shore of Australia, atolls do not border anything. Instead, they sit on a coral base that often rises thousands of meters from the ocean’s floor in some of the most remote areas of the tropical oceans.

Though scientists have been studying atolls at least since the time of Charles Darwin in the mid-1800s, many mysteries remain about exactly how they form and what factors determine their shape. One such question centers on the degree to which climate conditions affect the growth of the coral reefs that make up an atoll. Some researchers believe that the weather acts primarily to erode and diminish the underlying structure of fully formed reefs. Others believe that given the right conditions, waves and currents shape the reefs by actually stimulating growth. Resolving this debate one way or the other hasn’t been easy though, as most atolls are in remote areas of the ocean and are hard to get to, let alone map or fully analyze.
 

Two scientists at Dalhousie University in Halifax, Nova Scotia, may now be on their way to solving this riddle of the atolls. Using satellite imagery collected by Landsat 7, marine ecologist Bruce Hatcher and Maldivian doctoral student Abdulla Naseer are mapping out the reefs of the atoll archipelago that make up the Maldives in the Indian Ocean. By comparing the maps to wind and wave data from the region, they are attempting to discern if the monsoons that blow regularly from the east and the west played a role in shaping the Maldives. And they believe such knowledge may have a practical application as well. Understanding how the coral reefs grow could help the Maldives people shield themselves from the rising sea levels that may occur as a result of global warming.

 Blowing in the Wind

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

Blowing in the Wind

"Our primary reason for this study is to address one of the old chestnuts in coral reef science," says Hatcher. He explains that one of the ways in which atolls form is the result of a change in sea level. Atolls begin as fringing reefs surrounding a volcanic island (Darwin 1842). Through the process of global warming, glacial melting and/or island subsidence, the level of the sea gradually rises relative to the seabed and water begins to overtake the island. Since most reef-building corals cannot grow easily at depths of more than 150 feet (45 m) below the ocean’s surface, they will begin constructing their protective calcium carbonate encasements on top of one another at a rate fast enough to keep up with the sea level rise. At the same time the corals at the surface grow laterally to stay abreast of the ever-diminishing coastline. Provided that the sea does not rise too rapidly, the corals will continue to push upward and outward well after the volcanic island is completely submerged (Fagerstrom 1987).

The final shape that an atoll takes isn’t one of a giant tube ascending from the ocean floor. Rather, assorted detritus and dead coral will pile up on the inside of the coral ring and fill the void where the island used to be. From above, most atolls end up resembling an elliptical array of coral reefs with steep sides surrounding a relatively shallow lagoon in the center. If one could dive into the ocean and view an atoll from the side at a distance, it would resemble a very steep volcano with the topmost portion of the crater grazing the ocean’s surface (Huxley 1873).

Hatcher explains that these basic truths of coral atoll formation and structure have been accepted in one form or another for the past century. Yet, many of the finer points remain open for debate. One such dispute has arisen over the effect wind and wave activity in a region has on the width of the reefs that make up an atoll at sea level. "The dispute dates back to the middle of the nineteenth century and involves a disagreement between Charles Darwin and a James Dwight Dana [an American coral reef pioneer]," says Hatcher.

While atolls generally take the shape of the volcanic island they originally surrounded, the widths of the reefs that make up an atoll can vary quite a bit. Often times the reefs on one section of an atoll may be much wider than the rest, giving the atoll an irregular, oftentimes ocular shape when viewed from above. Charles Darwin believed that such differences in reef width were wholly due to the topography of the island’s coastline where the reef first took root. Gently sloping seafloors around an island would produce wider reefs and steep seafloors would produce thin reefs. Unless the atoll experienced significant erosion as it grew, he professed that the reefs would retain their original shape as they metamorphosed into an atoll. "With this vertical growth argument, the body of the atoll now apparent at the ocean’s surface is nothing but a direct reflection of the shape that was apparent when the current reefs started growing years before," says Hatcher.
 

Dana put forth another theory ten years or so after Darwin. He argued that the current shape of an atoll had more to do with the patterns of wind and wave activity. Specifically, more rapid movement of waters around the outer edges of a living reef provides the reef-building coral polyps and their symbiotic algae (zooxanthellae) with more of the calcium, nutrients, and food particles they need to grow. The net effect is that the reefs become wider and denser over time near the locations where the water flow is the greatest. "So, in essence, this horizontal growth argument goes that the density and thickness of a reef are determined by the amount of nutrients and minerals delivered by the waters flowing around it," says Hatcher.

Despite all the research on corals that has taken place since Darwin and Dana’s time, the debate has never been put to rest. The problem hasn’t been one of intelligence or of effort on the part of the scientists, but of logistics. In order to settle this issue, a researcher would need evidence of many atolls’ interactions with the waves and currents over a long period of time. Since atolls grow upwards at a maximum rate of only 1.2 centimeters per year, witnessing the process first hand is out of the question (Nature Conservancy 2000). Attempting to discern the development of the reefs from current clues can be very difficult. Most atolls simply are not mapped to the level of precision needed to understand the nuances of how they formed. In the few instances where atolls have been mapped, the effort was both pricey and time consuming.
 

Hatcher and Naseer became intrigued by this debate while observing the health of the coral reefs of the Maldives. These islands are made up of a cluster of 22 atolls grouped in an elongated formation stretching across roughly 823 kilometers (511 miles) north-south in the Indian Ocean south of Sri Lanka. Each of the atolls is made up of several dozen reefs. Rubble and sediments derived mainly from dead coral have piled up on some of these reefs to form low, flat islands where roughly a quarter of a million people now live.

Here the researchers saw a pattern that seemed to confirm Dana’s theory on coral reef formation. "There is a strong consistency in the asymmetry of the reefs that form the atolls in the Maldives," says Naseer. Specifically, the reefs that face the open ocean to the west and to the east tend to be a little wider and denser than those reefs that face the atoll lagoons or those that are protected in the inner lagoons between atolls. These are the same directions from which the characteristic monsoons blow. In the winter they come in from the east and then for the remaining three seasons they blow even harder from the west. The monsoons bring larger waves and stronger currents to the margins of the unshielded reefs and stir up cooler, nutrient-rich waters.

 Analyzing Atolls
 Mapping the Maldives’ Amazing Atolls

Analyzing Atolls

To prove that the monsoons shaped the coral reefs, Hatcher and Naseer would have to demonstrate that a relationship exists between the two across the entire atoll cluster. Moreover, they would have to compare the monsoon weather data such as wind speed, rainfall, and wave height over several decades to detailed maps of the reefs. But when they went looking for this information, they ran into the same old problem. Though the climate data were there, detailed maps of the atolls were non-existent. The only maps of the Maldives were English admiralty charts from 1896 and modern maps drawn to a scale of one to three hundred thousand. "They were all maps that were designed for navigating the waters amongst the islands. We needed maps with enough detail to see the submerged reef habitats at the scale of about one to ten thousand," says Hatcher.
 

The researchers eventually found a solution to this largely academic dilemma in a seemingly unlikely place–the Landsat 7 satellite. Launched in 1999, Landsat 7 orbits approximately from pole to pole around the Earth. An instrument on board, known as the Enhanced Thematic Mapper Plus (ETM+), measures the infrared and reflected solar radiation from the surface of our revolving planet. These readings are beamed as digital 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 by 30 meters per pixel, the images are not well suited for viewing details on our planet’s surface any smaller than an office building. They are, however, extremely useful for mapping and monitoring large features such as coral atolls.

Traditionally, Landsat 7 has been used to track change in land cover such as deforestation. At the request of a group of scientists at the University of South Florida and NASA’s Goddard Space Flight Center, NASA agreed to modify the Landsat 7 image acquisition strategy to begin monitoring shallow ocean regions. For the past two years the group has been using these images as part of an effort to map and monitor the health of coral reefs around the globe. They passed their images of the Maldives along to Hatcher, who saw them as the perfect means with which to test their hypothesis. Not only were the images relatively inexpensive, but they were detailed enough to make out the coral reef habitat and extensive enough to cover the entire atoll archipelago.

Of course, the satellite images did not come classified and labeled. The researchers had to take the raw data and demarcate the individual reefs and any other features in the image that would be helpful in uncovering the effects of the monsoons on the atoll reefs. More specifically, they needed to know the dimensions and orientation of each reef’s growth features, which include the reef crest, the reef slope, the shallow and deep reef lagoons, the sand flats, and the vegetated islands.
 

As most of their work is carried out on a university campus in Nova Scotia, the researchers had to rely on their expertise regarding coral reefs, their prior knowledge of the Maldives, and an array of remote sensing techniques to map each atoll. In some instances, the process was as easy as outlining an area that looks like a reef or a sand flat. In other instances, complex computer programs involving fuzzy logic were used to bring out the various categories. In the end, they found they could obtain very detailed maps of the Maldives’ reefs from Landsat 7, and the whole endeavor took only a fraction of the time needed to map the reefs by airplane or boat and cost a great deal less. "Landsat 7’s ability to consistently and rapidly map reefs has given us the power to test hypotheses with a level of efficiency unheard before in marine geological research," says Hatcher.

The researchers will employ a Geographic Information System (GIS) to match their atoll maps point for point with the wind, rain, and wave height data taken in the Maldives over the last 20 to 30 years. Though only 20 percent of the reefs have been accounted for so far, the results seem to confirm the researchers’ hypothesis. The sections of the atolls facing in the direction of the monsoon winds–east and west–are wider and slope more gradually into the sea. Those reefs that were not exposed to the monsoons, either because they face another direction or because they were shielded by other reef formations, had more of wedge shape profile with narrow reef crests and steep slopes. "Though we are in the early stages of the project, the asymmetries are consistent with the geographic pattern of the monsoons," says Naseer.
 

Since the atolls do not show obvious signs of erosion, Hatcher says they likely maintained the same shape throughout the latest rise in sea level (125 m) that began with the end of the last ice age 12,000 years ago. Several lines of evidence suggest that the monsoons in that region of the world have blown with the same strength and direction for many tens of thousands of years. As these atolls grew above the volcanic mountain range that forms the backbone of the Maldivian archipelago, the monsoons acted to continually mold the size and shape of the reefs. "So in a way both [Darwin’s and Dana’s theories] were correct for the Maldives," says Hatcher. Though the atolls’ overall shape remains roughly the same for hundreds of feet under the ocean's surface, they have been continually shaped by the region’s dominant weather patterns.

 Battling a Rising Tide
 Blowing in the Wind

Battling a Rising Tide

Naseer explains this debate is not just purely academic, and such knowledge may help the Maldives to make better informed decisions about the management of its reefs. "The Maldives are being threatened by the rise in sea level due to global warming and increasingly violent weather," he says. As global warming seems to be more of an obvious reality, Maldivian scientists and government officials alike are concerned about the effects of rising sea levels. Since the Maldives islands are on average 5 feet (1.5 meters) above sea level, even a sea level rise of half a meter would cause severe problems for the more than 250,000 residents living there. Not only would flooding be a problem, but the seas may rise so quickly that they could erode the coral islands. If the reefs supporting an island fail to keep up with the rising waters, the island itself will inevitably disintegrate. To date the only recourse the Maldivians have against this potential catastrophe are concrete retainer walls. While such walls have effectively kept the sea at bay in a few key areas regularly struck by high waves, constructing them around dozens of inhabited islands would be an impossible undertaking for the relatively poor country. And no amount of retainer wall would completely stave off the erosion of an island.
 

Naseer and Hatcher’s research could help the government of the Maldives determine where they could best allocate their limited resources for shoreline protection. Since the reefs exposed to the monsoons are wider and grow more vigorously, the islands they support should have the best chance of surviving rapidly rising seas. "We would already predict from our observations that those islands sitting on gently sloping reefs with broad reef flats and extensive sand flats have a much better chance of staying above sea level than those perched on pinnacles," says Hatcher. By building retainer walls only around these islands, the Maldives would probably have a greater chance of surviving as a nation.

In the future, the researchers’ aim is to construct a scientific model that could help them predict just how these reefs will take shape as the sea levels rise in the future. But for now Hatcher and Naseer need to return to the Maldives and verify their initial results on the ground. Only then will they be able to tell if their data will be of any practical use to the Maldives. "We are just at the very beginning of this project," says Hatcher. "We still have a long way to go until we achieve an adequate understanding of the large scale factors that determine the development of coral reefs."

References

1. Darwin, C., 1842: On the Structure and Distribution of Coral Reefs. Smith, Elder & Co., London.

2. Fagerstorm, J. A., 1987: The Evolution of Reef Communities. John Wiley & Sons, New York.

3. Huxley, T. H, 1873: On Coral and Coral Reefs. In Critiques and Addresses, Macmillan, London.

4. Nature Conservancy, 2000: Coral Reef Facts, Arlington, VA.

 Analyzing Atolls