Studying Carbon in Rivers

Researcher explores physical, chemical and biological interactions.

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By Marlene Cimons, National Science Foundation

Carbon gets a lot of attention, largely because of its impact on climate change.  But its behavior in the atmosphere is only one part of the carbon cycle.  The biological and physical changes to carbon on the earth also are key to how fast carbon moves through its cycle, as well as the form it takes.  Thus, “it’s important to study all aspects of the loop,” says scientist Hilairy Hartnett.

Carbon is one of the most abundant elements in the universe, and carbon compounds form the basis for all known life. “Carbon is one of the main currencies of most living things on the planet,’’ says Hartnett, associate professor in the school of earth and space exploration, and in the department of chemistry and biochemistry at Arizona State University.  “Almost everything that is alive needs organic carbon for energy.”

Hartnett is studying what happens to carbon in the Colorado River, a large and heavily managed river that flows 1,450 miles from the Rocky Mountains to the Gulf of California, and serves as the main water supply for the desert Southwest, including Arizona, Southern Utah,  Nevada and Southern California.  Her goal is to understand how organic carbon moves from the land to the ocean, and how it changes, or doesn’t change, along the way.

“The types of molecules that get to the ocean are really important in understanding how much of that carbon gets preserved,” she says.  “I would like to know how biogeochemical processes in rivers affect the type of carbon that gets to the ocean.  When you look at the ocean, you’re not looking at pieces of tree. I want to know what happens to that material before it gets to the ocean. We want to know how much carbon makes it all the way to the end of the river, and how is it different from the carbon at the beginning?”

Understanding the processes that affect carbon as it travels along a major river, particularly one that provides water and hydro electric power to major agricultural and urban areas, has important implications for the ecosystem at large, as well as for water managers who care about water quality.  “If you are interested in the food web, who eats what, and in plants, invertebrates, fish, etc., then water chemistry matters a lot,” Hartnett says. 

Hartnett is studying carbon in the Colorado River under a National Science Foundation (NSF) Faculty Early Career Development (CAREER) award, which she received in 2009 as part of NSF’s American Recovery and Reinvestment Act. The award supports junior faculty who exemplify the role of teacher-scholars through outstanding research, excellent education and the integration of education and research within the context of the mission of their organization. NSF is funding her work with about $574,000 over five years.

“As someone who studies organic matter and carbon cycles, I moved to Arizona, looked at a map and said: this river has important implications for people who live here,” she says. “I decided that this river was ripe for biogeochemical investigation.”

Under the grant, which has a strong education component, 17 students also are working with Hartnett in the field and in the lab.  “One of the things that is most exciting to me is taking my students into the field to do these projects,” she says, adding: “There are simple tests and experiments that these students can do. Furthermore, it is interdisciplinary, involving so many fields, including chemistry, earth sciences, life sciences, as well as aspects of civil and chemical engineering.”

Carbon typically enters the river one of two ways. “Terrestrial” carbon originates from the landscape, that is, from plants, animals and soil, carried there by rain, snow melt and wind. “Riverine” carbon comes from algae and plants in the water that make their own carbon.  Hartnett wants to know how each type of carbon changes as it makes its way along the river, starting in the highest portions of the watershed all the way to the bottom, including the reservoirs, as it reaches the ocean.  The goal is to examine specific processes in the river in specific places, including spots along the river where the flow is faster, slower or nonexistent.