Amazon Powers Major Carbon Sink

by Payal Parekh
Monday, September 15, 2008

Nutrients carried by the Amazon River into the Atlantic Ocean help absorb significant amounts of carbon dioxide (CO2) from the atmosphere, new research reveals. The nutrients fertilize a type of plankton that the researchers estimate to consume 27 million metric tons of CO2 annually.

Carbon-eating machine: The Amazon River meets the sea.
Carbon-eating machine: The Amazon River meets the sea.
Norman Kuring/NASA

Rivers are known to be an important source of nutrients to the coastal ocean, but the extent was not fully understood. Satellite images show that the Amazon River plume reaches hundreds to thousands of kilometers out into the ocean, covering an area about twice the size of Texas for several months each year. This plume supplies the nutrient-poor tropical Atlantic waters with silicon, phosphorous and iron, supporting a group of organisms called diazotrophs, which are bacteria capable of utilizing atmospheric nitrogen. During photosynthesis, atmospheric CO2 is converted into organic carbon. Normally, most of this carbon is quickly returned to the atmosphere due to respiration, the reverse reaction of photosynthesis. But in this case, the diazotrophs are hosted by diatoms, algae that are encased in heavy silica shells. They sink rapidly upon death, resulting in the long-term storage of carbon on the ocean floor.  

Previously, the tropical Atlantic had been considered a net emitter of carbon to the atmosphere. A 2007 study estimated the ocean emitted 30 million metric tons annually. The discovery of this new sink significantly offsets the amount of carbon believed to be added to the atmosphere from this region, and suggests that tropical oceanic regions may be more important carbon sinks than previously thought.

Although oceanic carbon uptake occurs primarily in the polar seas, other large tropical rivers of the world also may contribute to carbon capture, according to Dr. Douglas Capone, a marine scientist at the University of Southern California and co-investigator of the study. The lead author of the new study, Dr. Ajit Subramaniam of Lamont Doherty Earth Observatory of Columbia University, is planning to conduct similar research on other tropical rivers including the Congo, Orinoco and Mekong. "The Mekong River is undergoing very rapid change even as we speak and studying that system rigorously may help us better understand the process," Subramaniam said.

Subramaniam says that "activities such as dam construction and changing agricultural practices will alter the magnitude of this carbon drawdown." If the outflow of nutrients is reduced into the ocean due to the damming of tropical rivers, thus decreasing the effectiveness of this carbon sink, then the climate and environmental impacts of tropical hydro may be even greater than previously thought.

Scientists have already documented ecosystem changes in South China Sea caused by the damming of the Yangtze, Subramaniam notes. The reduction in nutrient flows resulted in an 86% drop in biological productivity and a shift in phytoplankton species.