SF State News {University Communications}

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Amazon River outflow fights greenhouse gasses

September 3 , 2008 -- The Amazon River plays a far more significant role in drawing carbon dioxide from the atmosphere than previously understood, according to a new study by a team of international scientists. Nutrients from the river's outflow travel thousands of miles past the continental shelf, influencing the carbon cycling in the tropical ocean.

A NASA Space Shuttle photograph shows a nutrient-rich, brown plume from the mouth of the Amazon River spreading out over the tropical Atlantic Ocean.

A large sediment plume from the mouth of the Amazon River expands outward into the tropical Atlantic Ocean. NASA

"We were surprised to find that a surface plume of fresh Amazon water about the size of Texas had traveled so far out to sea," said Ed Carpenter, professor of biology and Romberg Tiburon Center researcher, a co-author of the study. "Our research showed that nutrients and phosphorus from the Amazon outflow stimulate the production of nitrogen-fixing ocean organisms that convert gaseous nitrogen to amino acids and protein." This food boosts the production and number of the ocean organisms that absorb greenhouse gasses from the atmosphere and turn them into organic solids that sink to the bottom of the sea and die.

The Amazon River has the largest discharge of any of the world's rivers by volume and is the largest drainage basin on the planet. It accounts for 18 percent of all river output into the world's oceans. Previously scientists had only studied the movement of river outflow closer to shore.

"We are really excited about this discovery and are eager to learn if outflow from the Congo, the world's second largest river, has the same effect on ocean productivity," Carpenter said.

The team cautions that the quality of the river outflow plume could be negatively altered by development along the Amazon River. "These organisms are regulated by the biochemistry of the river and they are sensitive to land-use alterations and climate change," said Ajit Subramaniam, a co-author and biological oceanographer at Columbia University. He said activities such as dam construction and changing agricultural practices could alter the magnitude of the carbon cycling process.

The study, funded by the National Science Foundation's Biocomplexity in the Environment program, focused on the content in water samples from 82 stations in the western tropical North Atlantic Ocean collected between January 2001 and May 2003. Three years of analysis followed. Carpenter studied species composition and measured the amounts of chlorophyll in the plankton in his lab at SF State's Romberg Tiburon Center for Environmental Studies.

In addition to Columbia University, Carpenter's colleagues on the study represented the University of Georgia at Athens, University of Liverpool, University of Hawaii at Honolulu, State University of New Jersey, Georgia Institute of Technology, University of Southern California and University of California, Los Angeles.

-- Denize Springer


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