Ocean "Conveyor Belt" Sustains Sea Life, Study Says

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The problem oceanographers faced is that any nutrients that sink past the barrier layer and into the abyss of the deep ocean have a hard time getting back up to the ocean surface, where they are of use to phytoplankton.

Without a mechanism to get the nutrients back to the surface, the oceans would lose about one-fiftieth of their nutrients to this sinking process each year, Sarmiento said.

Southern Ocean

What Sarmiento found was that the nutrient-rich deep waters are carried by deep ocean currents to the cold, hostile, storm-whipped Southern Ocean.

According to Gordon, who is an expert on the Southern Ocean currents, the westerly winds there create the greatest of all ocean currents, the Antarctic Circumpolar Current. The current "swirls water—around 140 million cubic meters (4.9 billion cubic feet) per second—around Antarctica."

For the sake of comparison, all of the rivers in the world carry only 1.3 million cubic meters (46 million cubic feet) per second, according to Gordon.

The Antarctic Circumpolar Current mixes waters from the Pacific, Atlantic, and Indian Oceans. Of equal importance, according to Gordon, is the vigorous circulation along the north-south plane across the Antarctic Circumpolar Current.

There, relatively warm waters of the deep-ocean rise to the cold sea surface just south of the current and balance the sinking of surface water along the edge of Antarctica and to the north. "As the deep ocean waters come to the surface, they bring with them all the nutrients that have been broken down and dissolved," Gordon said.

These waters are transported into the barrier layer, or thermocline, of the more northern oceans, where the nutrients are then absorbed by phytoplankton at the surface.

Climate Response

Since most scientists believe that ocean-circulation characteristics will change in response to global warming, Sarmiento's modeling results suggest that much of the world's marine life may be more susceptible to climate change than previously thought.

"If you kept water circulation going but were able to strip nutrients out before the water left the Southern Ocean, that would have a massive impact on global biological productivity," he said.

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 phytoplankton need nutrients to grow, the nutrient supply is essential to biological productivity.

Steve Rintoul, an oceanographer with the Commonwealth Scientific and Industrial Research Organization in Tasmania, Australia, said modeling results suggest the ocean conveyor belt—which allows cold, dense waters to sink and nutrient-rich waters to rise in both the Northern and Southern Hemispheres—will slow in response to climate change.

The conveyor belt would likely slow because in a warmer world more rain falls in the polar regions. The melting of sea ice and glaciers on land make surface waters fresher than they are now. Fresh water is buoyant—it doesn't sink.

"That slows down the whole pattern of currents that lead to upwelling in some regions and downwelling in others … we can speculate that this nutrient supply by the Southern Ocean would also slow down," he said.

Sarmiento and his colleagues, who published these findings in the January 1 issue of the science journal Nature, are now investigating the details of this nutrient-circulation pattern to understand how it might respond to global warming.

In future stories we'll learn how the researchers traced the ocean nutrients around the world.

For more news on oceans, scroll down

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