Scientists Track Nutrients Around Oceans

John Roach
for National Geographic News
June 17, 2004

The glass-hoarding behavior of single-celled plants called diatoms that dominate the surface layer of the ocean around Antarctica has allowed scientists to map the delivery of ocean nutrients around the world.

"Diatoms basically come to dominate wherever there is enough silicic acid and other nutrients around," said Jorge Sarmiento, a professor of ocean and atmospheric sciences at Princeton University in New Jersey.

Silicic acid is the dissolved chemical that diatoms use to make their cell walls, or shells, out of a kind of glass known as opal. There are over 10,000 varieties of diatoms, accounting for a large fraction of the ocean's single-celled plant life, known as phytoplankton.

"This tracer we used, this tag we used, is a measure of the amount of silicic acid in the water relative to another nutrient called nitrate," Sarmiento said.

Given adequate light for photosynthesis and an ample supply of nutrients, diatoms take up an equal amount of silicic acid and nitrate. But in the Southern Ocean they take up four times as much silicic acid than nitrate.

As a result, the waters flowing from the Southern Ocean have an unusually low amount of silicic acid relative to waters from other parts of the ocean, giving the Southern Ocean waters a unique signature.

David Nelson, a biological oceanographer at Oregon State University in Corvallis, said this signature "is a very useful new way to evaluate the cycling of nutrients within the three-dimensional circulation field of the ocean."

By using this signature, Sarmiento and his colleagues were able to trace nutrients brought from the deep ocean to upper layers in the Southern Ocean as they were distributed everywhere around the world except for the North Pacific.

The finding, reported in the January 1 issue of the science journal Nature, indicates that this single circulation pattern is vital to maintaining a nutrient supply for three quarters of ocean life.

Iron Deficiency

Sarmiento and his colleagues were able to trace this nutrient supply because of the peculiar behavior of diatoms in the Southern Ocean.

Continued on Next Page >>


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