True-color image of the Amazon River outflow, which extends thousands of kilometers into the Atlantic Ocean. Click on image for full size (146 Kb) Image Courtesy of Norman Kuring/NASA

Nutrients from the Amazon River's outflow spread well beyond the continental shelf and drive carbon cycling in the tropical ocean, say scientists who conducted a multi-year study. They will publish their results this week online in the journal Proceedings of the National Academy of Sciences (PNAS) .

The researchers discovered a significant and surprising drawdown of carbon dioxide from the atmosphere into the tropical ocean by microorganisms living in the Amazon River's outflow. The finding reveals the surprisingly large role of tropical oceans and major rivers in the oceans' total carbon uptake.

"This work has led to an important discovery about the source of nitrogen that fuels the productivity of tropical ocean waters, especially those into which large rivers flow," said David Garrison, director of the National Science Foundation (NSF)'s biological oceanography program. NSF's Biocomplexity in the Environment program funded the research.

The Amazon River is the largest river in the world by volume; it also has the largest drainage basin on the planet, accounting for some one fifth of Earth's total river flow. Because of its vast dimensions, it's sometimes called "the river sea."

The Amazon River's outflow covers an area more than twice the size of the state of Texas for several months each year, said Ajit Subramaniam, a biological oceanographer at Columbia University and lead author of the PNAS paper. (Subramaniam is currently on leave from Columbia, now serving as a rotating program director at NSF.)

The tropical North Atlantic had been considered a net emitter of carbon from the respiration of ocean life. A 2007 study estimated the tropical Atlantic Ocean's carbon contribution to the atmosphere at 30 million tons annually.

The new study finds that the respiration is offset by phytoplankton, most of which belong to a group of organisms called diazotrophs.

Diazotrophs take nitrogen and carbon from the air and use them to make organic solids that sink to the ocean floor.

Diazotrophs "fix" nitrogen, enabling them to thrive in nutrient-poor waters. They also require small amounts of phosphorus and iron, which the Amazon River brings to ocean waters far offshore.

The microscopic life forms responsible for this carbon drawdown change along the river outflow, said Subramaniam. "These organisms are regulated by the biogeochemistry of the river, and are sensitive to land-use alterations and climate change. Activities such as dam construction and changing agricultural practices will alter the magnitude of this drawdown."

Other large tropical rivers of the world also may contribute to carbon capture, said Doug Capone, a marine scientist at the University of Southern California and co-author of the PNAS paper, adding that studies on such rivers are in progress.

Polar seas are still responsible for most of the oceans' carbon uptake. But though carbon dioxide dissolves more easily in cooler waters, the warm oceans may be where a permanent carbon sink is more likely, said Capone.

"The important places are probably not the high latitudes, but rather the low latitude areas where nitrogen fixation is a predominant process," Capone said.

The other authors of the paper are researchers from the University of Georgia, Athens; San Francisco State University; the University of Liverpool; the University of Hawaii, Honolulu; Rutgers University; Georgia Institute of Technology; and the University of California, Los Angeles.

Text above is courtesy of the National Science Foundation

True-color image of the Amazon River outflow, which extends thousands of kilometers into the Atlantic Ocean. Click on image for full size (146 Kb) Image Courtesy of Norman Kuring/NASA

A large plume of nutrient-rich waters flows from the Amazon River far out into the ocean. Ajit Subramaniam, a biological oceanographer, and Doug Capone, a marine scientist, have found that microorganisms living in the Amazon River's outflow take in a significant amount of carbon dioxide from the atmosphere into the tropical ocean. This finding reveals the surprisingly large role of tropical oceans and major rivers in the oceans' total carbon uptake.

Scientists knew that the tropical North Atlantic Ocean emitted carbon from the respiration of ocean life. The new study finds that the respiration is offset by a group of organisms called diazotrophs, which are phytoplankton. Diazotrophs take nitrogen and carbon from the air and use them to make organic solids that sink to the ocean floor. Diazotrophs "fix" nitrogen so that they can thrive in the nutrient-poor waters of the deep ocean. These processes are part of the carbon cycle.

The Amazon River is the largest river in the world by volume; it also has the largest drainage basin on the planet, including about one fifth of Earth's total river flow. Because it is so large, it is sometimes called "the river sea." Other large tropical rivers of the world also may contribute to carbon capture, said Doug Capone. He added that studies on such rivers are in progress.

Polar seas are still responsible for most of the oceans' carbon uptake. But though carbon dioxide dissolves more easily in cooler waters, the warm oceans may be where a permanent carbon sink is more likely, said Capone.

True-color image of the Amazon River outflow, which extends thousands of kilometers into the Atlantic Ocean. Click on image for full size (146 Kb) Image Courtesy of Norman Kuring/NASA

A large plume of nutrient-rich waters flows from the Amazon River far out into the ocean. Scientists have found that microorganisms living in the flow of water coming out of the Amazon River into the ocean take in a lot of carbon dioxide from the atmosphere. This finding shows that the tropical oceans and major rivers play a large role in the amount of carbon the ocean takes in from the atmosphere.

Scientists knew that ocean life in the tropical Atlantic Ocean put out carbon through respiration. The new study shows that the opposite also happens because a group of organisms called diazotrophs take nitrogen and carbon from the air and use them to survive in the nutrient-poor waters of the deep ocean. These processes are part of the carbon cycle.

The Amazon River has about one fifth of Earth's total river flow. Because it is so large, it is sometimes called "the river sea." Other large tropical rivers of the world may also capture carbon from the atmosphere and studies on such rivers are in progress.