An enormous plume of freshwater leaves the Amazon River and spreads across the tropical Atlantic Ocean, setting the stage for the symbiosis of some bacteria and algae on a scale that appears to subtly influence the climate of the Earth.
USC researchers have studied the freshwater plume for more than 10 years, and two USC faculty members have new support from the National Science Foundation to study the importance of symbioses that apparently carry carbon dioxide down into the deep sea and trap some of it on the ocean floor.
Douglas Capone and William Berelson of USC College will study the plume on research cruises supported by the foundation in 2010 and 2011. Capone and Berelson will pay special attention to the symbioses of certain cyanobacteria (a photosynthetic and nitrogen-fixing bacterium) and diatom algae (marine phytoplankton with silica outer shells) that occur in nitrogen-poor surface waters of the world’s oceans.
Nitrogen is an essential building block for all life on Earth, and the ability to fix nitrogen in these marine environments can provide a competitive advantage for cyanobacteria and diatoms.
“The Amazon has a profound effect in promoting these kinds of organisms,” said Capone, professor of biological sciences at USC College and the principal investigator on the National Science Foundation award.
The Amazon supplies critical nutrients to the open ocean including phosphorus, iron and silicon, all needed by the symbioses to proliferate and grow. Capone has participated in studies of the Amazon plume since 1994, and he said the symbioses are strongly connected to reduced levels of dissolved carbon dioxide in the freshwater plume as it spreads over the surface of the Atlantic Ocean.
“Our recently published data suggest that CO2 is being drawn down and sequestered,” Capone said. “Our new program aims to better quantify this sequestration.”
Capone and Berelson will measure the carbon dioxide that is fixed into organic matter by assemblages of cyanobacteria and diatoms.
Diatoms are negatively buoyant and when they die, they sink into the depths of the water column. On previous trips, Capone and colleagues from other universities used sediment traps to measure the movement of these assemblages as they fell toward the bottom. On the upcoming cruises, Capone and Berelson will go a step further and measure what actually reaches the bottom.
“The chemistry of the sea floor provides a signal of what has fallen through the water column,” said Berelson, professor of earth sciences at USC College. He said the water that is trapped between the grains of sediment on the sea floor holds an amplified signal of carbon and silica that has been sinking for millennia.
During their research cruises, Berelson and Capone will draw cores of sediment from the sea floor to recover this seawater, and they will analyze it to discern the pattern and history of carbon and silica deposition in the region.
Diatoms and cyanobacteria are not the only things that cause carbon to sink toward the bottom in these tropical waters. Microscopic and macroscopic animals known as zooplankton feed on organic material at the surface and produce fecal pellets that also sink rapidly to the bottom, carrying carbon with them.
“One of the first-order questions in oceanography is how the oceans interact with atmospheric CO2,” Capone said. “Most people think the tropical ocean releases CO2 into the atmosphere. We’re showing that in certain places, where you have sufficient river-borne nutrients to support nitrogen fixers, it can be the other way around – that the tropical ocean can be a sink for CO2, not a source of it.”
The Amazon has the largest watershed of any river in the world, and it delivers about 20 percent of all the freshwater that is carried by all the world’s rivers to the ocean. The effect of the Amazon on the salinity of Atlantic Ocean’s surface water extends about 200 miles from the river’s mouth. Capone said it is possible that human activities in the Amazon watershed could change the processes that will be measured during the upcoming research cruises, but assessing that possibility is outside the scope of this three-year project.
The upcoming study of the Amazon River plume continues previous work that was published in July 2008 in the Proceedings of the National Academy of Sciences.
The lead author of the article was Ajit Subramaniam, a former postdoctoral researcher in Capone’s USC lab and now a research scientist at the Lamont-Doherty Earth Observatory at Columbia University. Capone was the senior author of the paper, and Sergio Sañudo-Wilhelmy, a professor of biological sciences and earth sciences at USC College, contributed to the research and analysis.
The upcoming research by Capone and Berelson is supported by a new program at the National Science Foundation called “Emerging Topics in Biogeochemical Cycles” managed by the foundation’s directorate for geosciences and directorate for biological sciences.
The project also involves investigators from seven other institutions – San Francisco State University, the University of Georgia, the Georgia Institute of Technology, the College of William & Mary (Virginia), Virginia Polytechnic Institute and State University, the University of Maryland and the Bigelow Laboratory for Ocean Sciences (Maine).