Until now, scientists have been unsure as to exactly how productive river plumes were because they contain very low concentrations of nitrate – an essential nutrient for most phytoplankton. But according to new sampling experiments in the Northwestern Tropical Atlantic, nitrogen-fixing cyanobacteria play a crucial role in enhancing the productivity of large tropical river plumes. The result could have implications for carbon sinks associated with river plumes around the world, making it important for regional carbon budgets.
Ajit Subramaniam of Columbia University in New York and colleagues from the US and the UK showed that the Amazon River Plume supports nitrogen fixation far from the river mouth and provides important pathways for atmospheric carbon dioxide sequestration in the western tropical North Atlantic. The researchers found that the dominant species change as the water moves away from the mouth of the river.
"Near the mouth, where there is just enough nitrate, the waters are dominated by typical coastal diatoms that are sustained by the nitrates and silicates found in the plume," explained Subramaniam. "When the nitrate runs out, within a few hundred km of the mouth, the dominant species change to diatoms that contain symbiotic cyanobacteria that can fix nitrogen gas," he told environmentalresearchweb.
Diatoms, which are large chain-forming cells, sink rapidly when they die and carry organic carbon – incorporated into their cells when they were alive as part of the photosynthesis process – down to the sea floor. Phytoplankton thus becomes a vector, or long-term sink for atmospheric carbon dioxide.
Before this work, scientists had not considered river plumes as regions where nitrogen "fixers" were present. This has now changed. "We also think that the carbon associated with nitrogen fixers in river plumes is almost three times as much as that due to coastal phytoplankton near the mouth of a river," said Subramaniam.
The researchers obtained their results by sampling the northwestern Tropical Atlantic during summer 2001, winter 2001 and spring 2003. They collected water samples and studied the species present in phytoplankton, nutrient concentrations and the rates of nitrogen and carbon fixation by phytoplankton. Real-time satellite data guided the ship to where the plume was on any given day.
The team calculated that the total "new" primary production supported by nitrate coming out of the river is 0.6 Tmol of carbon (about 7.2 million tonnes) per year. What’s more, about 1.7 Tmol of carbon (20.4 million tonnes) per year is supported by nitrogen fixation, for a total sink of 2.3 Tmol of new carbon fixed in the plume. This is in complete disagreement with recent models that predict a source of 2.5 Tmol of carbon to the atmosphere.
Subramaniam added that man-made activities, such as using fertilizers in agriculture and removing mangroves, could change the chemistry of the river bed and alter nutrient ratios there. In turn, this could alter phytoplankton species compositions and the amount of nitrogen fixed by the plume. Building dams may also reduce water discharge and hence the size of the plume, which could affect the carbon sink associated with it.
"This carbon sink may have implications for regional carbon budgets, especially in a Kyoto Protocol carbon cap and trade policy for individual countries," he added. "For example, countries considering building dams for 'green power' might want to consider the loss of the carbon sink associated with the river plume." Indeed, climate modellers are starting to take this factor into account to improve regional predictions and to reconcile model results with observations.
The team also has preliminary evidence that the same processes are at work in other tropical rivers, such as the Congo, Orinoco and the Mekong. Subramaniam has just come back from two cruises in the Congo River, where he found high nitrogen fixation rates in plumes. "We now hope to make more comprehensive surveys of these other large tropical river systems both to obtain a better global estimate of the contribution of river plumes to carbon sequestration as well as to understand the impact of anthropogenic activity on this process," he revealed. "The Mekong River is undergoing very rapid change even as we speak and studying that system rigorously may help us better understand the process."
The work was published in PNAS.