EVER since Charles Darwin described coral reefs as oases in the desert of the ocean, scientists have been struck by a peculiar irony. Coral reefs are among the richest ecosystems on the planet, yet they grow in crystal-clear waters that contain hardly any nutrients.
Now a team of Australian and American coral experts say they have solved this enigma, which has come to be known as 鈥淒arwin鈥檚 paradox鈥�.
The key, it turns out, is that corals have extremely rough surfaces. Objects under water are usually surrounded by a small boundary layer of still water, which effectively acts as a barrier to the rest of the ocean. But when waves break, the ultra-rough edges of corals amplify the water turbulence at a microscopic level. This disrupts the boundary layer, allowing the corals to hoover up what sparse nutrients there are.
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Marlin Atkinson of the Hawaii Institute of Marine Biology in Kaneohe developed this idea after pumping water over corals housed in long, narrow fish tanks. With Cliff Hearn from the University of New South Wales in Canberra, he discovered that the corals take up nutrients more rapidly when water is pumped faster or the coral reef is rougher. Last year, they worked out an equation describing how energy released by the resulting increase in water turbulence alters the boundary layer and hence the coral鈥檚 nutrient uptake (Coral Reefs, vol 20, p 347).
The two researchers have now backed up their theory with experiments on the coral reef in Biosphere 2, the huge glass and steel bubble in Arizona that houses miniature versions of some of the Earth鈥檚 main ecosystems.
There, the characteristics of the waves that crash onto the corals can be altered simply by shifting a lever. The researchers used a device called an acoustic doppler velocimeter to monitor minute changes in water velocity 50 times per second, and used these measurements to calculate the energy released by the varying turbulence. Sure enough, the changes in turbulence accurately predicted the corals鈥� rate of nutrient uptake. The researchers are now preparing the results for publication.
鈥淚t fits. Reefs grow best when there is a fair bit of turbulence 鈥� you get rapid growth, lots of branching,鈥� says Clive Wilkinson of the Australian Institute of Marine Science in Townsville.
Corals may also find other ways to squeeze a decent meal out of the oceans. Rocks at the base of coral reefs could provide some nutrients. And corals are great recyclers, as one organism鈥檚 waste often become another鈥檚 food. However, without the adaptation of rough surfaces, even the tightest recycling would leave them short of nutrients like nitrogen and phosphate, claim the researchers.