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.
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 ironfor example, as a result of more dust blowing over itthe 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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