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 Floridas 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 oceans 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 hasnt 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.
The Maldives are a group of coral islands resting on top of an ancient volcanic mountain range off the coast of India.
For more about using satellites to study coral reefs with satellites, read: Mapping the Decline of Coral Reefs
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 oceans 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 isnt 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 oceans 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
islands 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
oceans surface is nothing but a direct reflection of the shape
that was apparent when the current reefs started growing years
before," says Hatcher.
The site Oceanography from the Space Shuttle has further examples of the evolution of coral reefs.)
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 Danas time, the debate has never been put to rest. The
problem hasnt 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 Danas 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.
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 Hatchers 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."
Seawalls may be able to limit damage caused by rising sea level in the Maldives. To be effective, they must be placed on islands with broad reefs. (Photograph Copyright Ismail Faiz)
|Blowing in the Wind|
The Maldives are a particularly complex archipelago. Individual atolls may be composed of many islands and reefs, and islets and reefs often fill the atolls' interior. (Map courtesy Abdula Nasseer)