Hawaiian Coral Reef Ecosystem a Unique Treasure
By John Roach
for National Geographic News
|July 14, 2003|
Editor's Note: Jean-Michel Cousteau's Ocean Futures Society is on a film expedition in the remote Northwestern Hawaiian Islands to raise awareness about the need to protect the region's unique biodiversity for future generations. This is the second National Geographic News feature to showcase the islands and the expedition.
For the past 50 million years the Pacific Plate has slowly crept over a stationary plume of magma deep in the Earth's mantle, allowing the formation of a chain of islands that today comprise the most remote, large scale coral reef ecosystem on the planet.
The region, known as the Northwestern Hawaiian Islands, stretches 1,200 miles (1,930 kilometers) towards Asia from the main Hawaiian Islands and is home to a thriving marine ecosystem full of unique, or endemic, species. Many of the corals, fishes, sea birds, and mammals that are found there are found nowhere else in the world.
"You can't go anywhere else in the world and find a large scale coral reef ecosystem as intact as you see in the Northwestern Hawaiian Islands. It makes it really unique," said Dave Gulko, a coral reef ecologist with the Hawaii Department of Land and Natural Resources in Honolulu.
Gulko was one of the primary investigators on a multi-agency and research institution expedition to survey the region in 2000 to determine its management needs. Now, Jean-Michel Cousteau's Ocean Futures Society is on a film expedition in the region to raise awareness about the need to protect it for future generations.
"If protected it would be a major accomplishment on the part of this country to preserve the largest coral reef [system] that exists in the U.S.," said Cousteau in an interview prior to his departure. The region is proposed to become the country's 14th National Marine Sanctuary.
Cousteau and his crew of 22 are currently in the islands, exploring the mid-ocean ecosystem formed millions of years ago and left largely undisturbed by humankind. The adventure can be followed along via the website of the Ocean Futures Society (www.oceanfutures.org).
Island and Reef Formation
The Northwestern Hawaiian Islands are part of the Hawaiian Ridge-Emperor Seamounts chain that extends some 3,700 miles (6,000 kilometers) from the island of Hawaii to the Aleutian Trench off the coast of Siberia, according to Ken Rubin, a geologist at the University of Hawaii in Honolulu.
A stationary plume of magma called a hotspot forms the chain as the Pacific Plate creeps over it in a northwesterly direction at a rate of about 4 inches (10 centimeters) per year.
The hotspot is presently somewhere underneath the vicinity of the southern end of the island of Hawaii, causing molten lava to ooze out of the Kilauea and Mauna Loa volcanoes and the underwater seamount Lo'ihi, which is off the southern end of the island.
"It is believed that the hotspot feeds all three roughly vertically from below but the details of the plumbing and the circumference of the hotspot column as it rises through the mantle to the base of the crust are not well established," said Rubin.
The island of Hawaii will continue to grow until it, like all the islands in the chain to its northwest, eventually moves off the hotspot. A new volcano, and potentially the next Hawaiian island, will form to the southeast.
The Northwestern Hawaiian Islands range in age from the 7.2 million year old Nihoa at the southeastern end of the chain to Kure at the northwestern end which, although not officially dated, is probably 30 million years old or older, said Gulko.
"Midway, which is 60 miles [97 kilometers] to the east [of Kure], is 27.7 million years old. Pearl and Hermes Atoll, 90 miles east-southeast of Midway, is aged at 20.6 million years. You go 90 miles to the east and gain 7 million years, so Kure could be considerably older," he said.
Animals begin to colonize the basalt volcanoes when they are still deep beneath the sea surface. As they grow, the types of species found there continually change until they reach the depth where light penetrates, which is about 330 feet (100 meters) in the central Pacific, said Rubin. Then plants join the system.
It is the skeletons of these latter creatures and sediments from algae, coral, snails, urchins, and other calcium rich organisms which glom together over hundreds of thousands of years to form reefs on the island fringe. The reefs are alive, colonized within and on top of this agglomeration by new organisms, which help them adapt to changes in climate and sea level.
Most of the main Hawaiian Islands have reefs of this type, but as time goes on and the islands move further from the hotspot the islands begin to sink and erode.
"If the island subsides and the coast moves landward, the reef may be moved offshore somewhat in a relative sense, since it is the island shoreline that is moving forming a lagoon between the island and reef," said Rubin. "At some point the volcanic island subsides enough for it to submerge, leaving an atoll behind."
An atoll is a ring-shaped coral reef enclosing or nearly enclosing a shallow lagoon. All that remains above the surface is living reef and accumulated sediment. Many of the outer Northwestern Hawaiian Islands are at this stage. The inner Northwestern Hawaiian Islands are still basalt islands.
"You've got a wide range of ecosystems that make up the Northwestern Hawaiian Islands," said Gulko.
Further northwest from the islands, coral growth can no longer keep up with the rate of the sinking volcano. No new reef is added and the older reef erodes. What remains is an underwater mountain called a guyot.
The Hawaiian Islands are isolated from the rest of the coral reef ecosystems in the Pacific Ocean. Of the few species that have reached the islands, several have mutated and evolved to creatures that are unique to Hawaii. Generally speaking, 25 percent of all coral reef species in the islands can be found nowhere else, said Gulko.
"They are not the most biodiverse, but the rates of endemism are the highest for any large-scale coral reef ecosystem on the planet," he said. Many of these species may be as old as 60 million years old, having evolved when the seamounts further to the northwest were islands and hopped to the newer islands as they formed.
Gulko likens the level of endemism to walking into a supermarket and noting that 25 percent of the food sold there carries the brand of that supermarket. That brand of food can only be found in that brand of a supermarket.
Fifty to sixty percent of the biomass of the fish in the lagoons at the outer atolls of the Northwestern Hawaiian Islands is endemic, said Gulko. In keeping with the supermarket analogy, that is the equivalent of weighing all the food in a supermarket and discovering that 50 to 60 percent of the weight comes from the supermarket brand.
On the "Voyage to Kure," Cousteau said that he hopes "to discover new territories, new groups, new species." Given the islands' isolation, rate of endemism, and relatively unexplored waters, Cousteau's film should indeed be full of discoveries.
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