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.

The waters there are lacking in iron, a condition that causes diatoms to build bigger and bigger opal shells, according to Sarmiento. In the process of building these big shells, the diatoms strip the waters of silicic acid.

"It is that relative depletion of silicic acid which limits diatoms from becoming even more dominant than they already are," Sarmiento said.

According to Nelson, the iron deficiency in the Southern Ocean "prevents [the diatoms] from producing as much organic matter as they would if there was plenty of iron present," he said.

The diatoms keep building their shells as they slowly grow enough organic matter to divide and grow a new cell. By the time they have enough organic matter to divide, their shells are bigger than normal, Sarmiento said.

Climate Sensitivity

Sarmiento and his colleagues say their analysis of how this ocean current from the Southern Ocean delivers nutrients to the world's oceans has important implications for understanding the sensitivity of oceanic biological productivity to climate change.

Conceptually, biological productivity is defined as the ability of a water body to support life, such as plants, fish, and wildlife. Scientifically, it is defined as the rate at which organic matter is produced.

Since diatoms and other types of phytoplankton need nutrients to grow, the nutrient supply is essential to oceanic biological productivity.

If the Southern Ocean became enriched with iron—for example, as a result of more dust blowing over it—the diatoms would be able to take up more nitrate along with the large amounts of silicic acid, making the waters similar to those in the rest of the ocean, Nelson said.

Such a scenario, said Sarmiento, would in turn "have a really dramatic effect on global biological production." Instead of the nutrients being shipped around the world from the Southern Ocean, they would be stripped out of the water in the Southern Ocean.

As well, several climate change models predict that warming of the Earth will cause ocean circulation patterns to change. While the impacts of such changes are not well understood, Sarmiento said it is safe to say they would be significant.

Sarmiento and his colleagues are now investigating the details of this nutrient circulation pattern to understand how it might respond to global warming.

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