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GEOCHEMISTRY

The third bank of the river

Organic matter transported by the Amazon travels nearly 600 kilometers into the Atlantic, reaching the Caribbean

056-057_rioamazonas_233Images Norman – Kuring - NASA/GSFC / OBPGwwEvery year, the Amazon River transports as much as 27 million metric tons of terrestrial organic matter into areas of the Atlantic Ocean far from the coast. The matter consists of compounds produced by burning, as well as the remains of plants, animals and microscopic creatures from the forest that are deposited into the river by wind and rain. The waters of the Amazon discharge all of this material into the ocean, where it serves as food for marine organisms. The amount of organic matter, recently estimated by Brazilian and American researchers, and the extent of its reach into the ocean, took the researchers by surprise.

In a paper published in Global Biogeochemical Cycles, the researchers also analyzed the transformations undergone by organic matter as the river water mixes into the ocean water. The Amazon alone accounts for 15% to 20% of the volume of freshwater discharged into the world’s oceans. The organic matter, suspended or dissolved in the river water, collects in the Amazon Basin and reaches the Atlantic at Marajó Island, in the state of Pará. In the ocean, the freshwater plume from the river extends for 600 kilometers and stretches as wide as 200 kilometers. Between 2010 and 2012, about 40 researchers took three cruises to South America and collected water samples at hundreds of points between Óbidos, in Pará, 800 kilometers from the mouth of the river, and the vicinity of Barbados, in the North Atlantic, after the Amazon plume has traveled nearly 600 kilometers towards the Caribbean, driven by currents along the Brazilian coast. “The studies of the plume and the river were done separately,” says American oceanographer Patricia Yager, a researcher at the University of Georgia and principal investigator of the project, entitled River-Ocean Continuum of the Amazon (ROCA). “The goal of ROCA is to think about the system in an integrated fashion,” she says.

Using an ultra-high resolution mass spectrometer, the researchers identified at least 4,400 organic compounds in the Amazon plume. The molecules contain four chemical elements (carbon, hydrogen, oxygen and nitrogen) that combine in various proportions. As the river water advances towards the ocean, the compounds undergo a series of transformations: they are degraded by bacteria and other microorganisms (biodegradation) or by light (photodegradation) and can also produce more complex molecules. Although they occur simultaneously, some processes are more active in certain sections of the journey.

Near the mouth of the Amazon, bacteria digest the complex organic molecules and recycle sugars, amino acids and lipids. In the ocean, microscopic algae extract nitrogen-containing by-products, such as amino acids and urea. After passing the continental shelf, which extends 80 kilometers out from the coast, the water becomes less muddy, and sunlight helps break down complex organic compounds.

From complex to simple
These biological and photochemical changes leave detectable traces in the water samples. The material from the river contains a lower percentage of hydrogen than of carbon, indicating that the organic molecules there are more complex. In the samples collected from the ocean, the situation is reversed, with a larger concentration of hydrogen than of carbon—a sign of simpler organic molecules that formed from the breakdown of complex molecules.

These data help scientists understand what is happening chemically and biologically. “The lower hydrogen-to-carbon ratio in the compounds found in the river suggests an influx of terrestrial matter, characterized by the presence of aromatic rings [formed by six carbon atoms],” Yager says. These rings, she says, are harder to degrade, especially for marine bacteria.

The proportion of hydrogen and carbon in the water samples from the plume collected in the ocean indicates the presence of aliphatic compounds, formed by long open chains of carbon. These compounds are indicative of the activity of algae, which during photosynthesis transform small molecules of carbon into larger molecules that are easier to digest and that serve as an energy source for marine bacteria.

Despite the action of microorganisms and sunlight, a large amount of terrestrial organic matter resists the changes and travels far beyond the mouth of the Amazon. During high discharge periods, more than 70% of the terrestrial organic matter transported by the river is found near French Guiana. During drier periods, the percentage drops to 50%. When converted into absolute numbers, these data suggest that the Amazon plume discharges between 13 million and 21 million metric tons—on some occasions 27 million metric tons—of terrestrial organic matter into the Atlantic. “These estimates can contain biases caused by the heterogeneity of the plume or its extensive length,” says Brazilian oceanographer Patricia Medeiros, first author of the paper in Global Biogeochemical Cycles.

Studies conducted in the United States indicate that 50% of the terrestrial organic matter transported by the Mississippi River is degraded near the coast. “We know rapid degradation occurs in the Mississippi and other rivers. We were surprised that it’s not happening in the Amazon,” she says.

Medeiros has two theories to explain how so much organic matter reaches the open ocean. “Since much of the degradation occurs during transport along the river, it is possible that the material that reaches the ocean is more resistant,” she says. Another possible explanation is the speed of transport. Estimates indicate that organic matter in the Mississippi takes months to get from the mouth to the open ocean. In the Amazon, the journey takes 30 to 60 days—not enough time for degradation of terrestrial organic matter to occur.

Scientific article
MEDEIROS, P. M. et al. Fate of the Amazon River dissolved organic matter in the tropical Atlantic Ocean. Global Biogeochemical Cycles. V. 29, p. 677-90. April 25, 2015.

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