Analyzing Atolls

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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.
 

   
 

Overview of Atolls from Landsat

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.
 

 

Satellite imagery provides the wide-area images required to map the atolls of the Maldives. By comparing reef structures throughout the archipelago, scientists can determine what effect predominant weather patterns have on coral growth. (Image courtesy Abdulla Naseer, Dalhousie University)

 

High Resolution Landsat Image

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.
 

 

This high-resolution satellite image, a detaill of the one above, is suggestive of how the monsoons shape the reefs. The wide, outer edges of the reefs face the monsoon winds, while the thin, inner edge is protected from them. (Image courtesy Abdulla Naseer, Dalhousie University)

  Photographs of Wave Action

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.

next Battling a Rising Tide
next Blowing in the Wind

 

In the Maldives, monsoon winds drive waves, which stir up the nutrients needed by the corals. Preliminary research indicates that reefs exposed to the monsoons grow wider than those that are sheltered. (Photographs courtesy Bruce Hatcher, Dalhousie University)

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