Rivers Studied for Clues to Global Carbon Cycle

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The debate over how carbon in the atmosphere is affecting global climate change continues to engage society. In order to contribute to the exchange, Aaron Packman, an associate professor of civil and environmental engineering in the McCormick School of Engineering, is collaborating with ecologists and microbiologists from around the world to study how organic carbon is processed in rivers.


The University press release says that Packman was a co-author of a paper on the topic published Jan. 20 online in the journal Nature Geoscience.

The lead author of the paper is Tom Battin of the Department of Freshwater Ecology at the University of Vienna in Austria. Other authors are Louis A. Kaplan, Stuart Findlay, Charles S. Hopkinson, Eugenia Marti, J. Denis Newbold, and Francesc Sabater.


The paper evaluates our current understanding of carbon dynamics in rivers and reaches two important conclusions; one it argues that carbon processing in rivers is a bigger component of global carbon cycling than people previously thought, and two it lays out the framework for how scientists should go about assessing those processes.

There is considerably more that is known about carbon cycling in the atmosphere and the oceans than in rivers. Evaluating large-scale material cycling in a river provides a difficult challenge – everything is constantly moving, and a lot of it moves in floods.

This has resulted in the fact that much of what we know about carbon processing in rivers is based on what flows into the ocean.

“But that’s not really enough,” Packman said. “You miss all this internal cycling.”

In order to understand how carbon cycles around the globe – through the land, freshwater, oceans, and atmosphere -- scientists need to understand how it moves around, how it’s produced, how it’s retained in different places and how long it stays there.

In rivers, carbon is both transformed and consumed. Microorganisms like algae take carbon out of the atmosphere and incorporate it into their own cells, while bacteria eat dead organic matter and then release CO2 back into the atmosphere.

“It’s been known for a long time that global carbon models don’t really account for all the carbon,” Packman said. “There’s a loss of carbon, and one place that could be occurring is in river systems.” Even though river waters contain a small fraction of the total water on earth, they are such dynamic environments because microorganisms consume and transform carbon at rapid rates.

“We’re evaluating how the structure and transport conditions and the dynamics of rivers create a greater opportunity for microbial processing.” Packman said.

While Packman is the first to admit that studying microorganisms, carbon and rivers appears to be more like ecology than engineering; however, such problems require work from all different areas, he said.

“We’re dealing with such interdisciplinary problems, tough problems, so we have to put fluid mechanics, transport, ecology and microbiology together to find this overall cycling of carbon,” he said. “People might say it’s a natural science paper, but to me it’s a modern engineering paper. To understand what’s going on with these large-scale processes, we have to analyze them quantitatively, and the tools for getting good estimates have been developed in engineering.”

Packman specializes in studying how particles and sediment move around in rivers and he was introduced to the co-authors of the paper -- ecologists who study how dead leaves and soil drive stream ecology and who come from as far away as Spain and Austria – about 10 years ago through the activity of the Stroud Water Research Center in Pennsylvania.

They have been working together since then on many similar projects around river structure and transport dynamics. Currently, they are working on a project funded by the National Science Foundation on the dynamics of organic carbon in rivers and trying to understand how carbon delivered from upstream areas influence the ecology of downstream locations.

“The broadest idea is really part of global change efforts to understand carbon cycling over the whole earth, which is an enormous challenge,” Packman said.