Colvin Recipient Studies Nutrient Uptake in the Flood Plains of the Mississippi River

Nick DiUlio for the Princeton Environmental Institute ・ Princeton Environmental Institute
Sunset over the Mississippi River
Sunset over the Mississippi River. (Photo credit: Katherine Smith)

For many years Katherine Smith has been fascinated by the complexities of riparian ecology, but as she approached her senior year at Princeton University the ecology and evolutionary biology major felt the need to transition from the classroom to the real world. So when it came time to choose a senior thesis project, Smith turned her attention to the Lower Mississippi River watershed to study a pressing problem of water pollution.

“I thought this was incredibly interesting and important because people have been altering the floodplains of major rivers for a while now, especially the Mississippi,” said Smith. “And there are extensive repercussions that we need to better understand.”

According to Smith, the Mississippi’s watershed is the largest in the United States, comprising about one third of the nation’s land area and producing the majority of its corn, soybean, wheat, cattle, hogs and chickens. For nearly five decades, farmers have applied massive amounts of fertilizer to increase agricultural productivity.

The fertilizers farmers apply to crops often contain high concentrations of nitrogen which in high concentrations can be toxic to aquatic life. When it rains, that fertilizer—along with its nitrogen—flows into nearby streams that feed the Mississippi River and eventually enter the Gulf of Mexico.

The problem of nitrogen runoff is already well documented said Smith, but less is known about nitrogen uptake.

Apparently different floodplain environments— including secondary channels, backwater lakes, and flooded forests—absorb nitrogen in the Lower Mississippi River, slowing its approach to the Gulf and acting as a natural filtration system for the massive river.

(left to right) Cliff Ochs, associate professor of biology at the University of Mississippi with Colvin recipient Katherine Smith ’15 (middle) and Alex Dominguez ’14 after a long day in the field.
(left to right) Cliff Ochs, associate professor of biology at the University of Mississippi with Colvin recipient Katherine Smith ’15 (middle) and Alex Dominguez ’14 after a long day in the field. (Photo credit: Katherine Smith)

Smith wanted to figure out whether that process—called nutrient uptake—occurs primarily through algal and plant assimilation or through the conversion of nitrate into nitrogen gas, a process called denitrification.

“What made this project unique was looking at the role of these lesser-studied floodplain environments,” said Smith. “It’s important to know what biological processes are capturing some of these nutrients and preventing them from flowing further south, because if we knew more about these processes perhaps we could manage and support this system so nutrients don’t flow south and are instead removed along the way.”

Smith said the Mississippi used to have many more meanders than it does today. Throughout the late 19th and early 20th centuries, however, dozens of these diversions—which served as natural filters for the river—were cut off to create deeper, more navigable channels. Now, however, the Army Corps of Engineers is adding weirs to various river channels in order to increase the flow of water into the floodplain adding a degree of timeliness to Smith’s project.

“I think the fact that the Army Corps is already thinking about a floodplain expansion made this study even more important from a management perspective,” Smith said. “We need to assess just how valuable the floodplain is for nutrient uptake and the region’s food web.”

In June 2014 Smith traveled to Oxford, Mississippi, where she met Clifford Ochs, a professor of biology at the University of Mississippi who would serve as her fieldwork guide and advisor for the ensuing 10 weeks. During that time, Smith embarked on many days of exhausting sample collection and cataloguing funded by the Becky Colvin ’95 Memorial Award. The award is presented annually by the Princeton Environmental Institute (PEI) and the Department of Ecology and Evolutionary Biology to support field research projects critical to the senior thesis.

“Once we decided how we were going to approach the research, Katie threw herself at it with great dedication,” Ochs said. “It took a lot of time and it was often physically demanding, but she was always completely engaged, no matter how challenging it was.”

View of the lake where Smith conducted her field work
View of the lake where Smith conducted her field work. (Photo credit: Katherine Smith)

Over the course of her fieldwork, Smith collected water and mud from floodplain environments including the main river channel, two side channels, two backwater lakes and two flooded forests. The purpose was to test the hypothesis that nitrogen uptake differs across these different floodplain environments as a function of several factors.

The fieldwork was taxing at times, but Smith said she was well prepared by two prior academic excursions. The first was a semester spent studying tropical ecology in Panama’s rain forests during the spring of her junior year. The second was a 2013 Development Grand Challenge internship in Kenya where she spent the summer studying the impact of grazing on a riparian ecosystem.

“Both of those trips were amazing experiences and without them I don’t think I’d have any idea how to do a thesis project like this,” Smith said. “My experience in Panama, in particular, was like ecology boot camp. It was invaluable.”

Aided by her thesis adviser, ecology and evolutionary biology professor Lars Hedin, Smith spent the past several months analyzing the water samples she obtained in Mississippi. Her next goal is to analyze the sediment.

“The most challenging part of this project is that a Princeton undergrad was placed in the real world with an urgent real world question to answer alongside top scientists,” Hedin said. “But that’s precisely what a Princeton senior thesis experience should be about. It’s daunting and it takes a lot of confidence and risk taking, but Katie has not been afraid of ambitiously combining her years of knowledge at Princeton with issues facing the real world.”

In writing her thesis, Smith found that there is evidence of denitrifictation across all floodplain environments but that nitrogen flux is most measurable in the backwater lakes and flooded forests where the hydrology is less complex. In the river’s main and secondary channels Smith said it is difficult to accurately quantify changes in nitrate concentrations over short distances because the water moves so quickly through the system.

Looking ahead, Smith said she wants to share her final results with the Army Corps of Engineers as it prepares to tackle the same questions about the river’s natural filtration system—albeit on a much larger scale.

View of the flooded forest research site
View of the flooded forest research site. (Photo credit: Katherine Smith)

“ I think it’s pretty exciting that the Corps is now conducting a project on the same stretch of river Katie and I studied last summer,” Ochs said. “What we did in a rather small way is now being followed up by the Corps in a multi-year project involving at least a dozen different scientists. This is not to say they’re doing this because we were out there last summer, but it highlights the fact that the work Katie was doing is considered very important.”

After graduation, Smith, who is from Haddonfield, New Jersey, will work for the regulatory branch of the Army Corps of Engineers in Manhattan. While this position does not involve the Mississippi River, Smith will apply what she has learned through her senior thesis research to tackle permitting, policy, and regulatory affairs regarding water bodies in New York. In the future, she also plans to pursue graduate work in environmental policy or environmental law.

“Obviously I think science is incredibly important, but I also feel like society has some catching up to do in terms of translating what we’ve learned about our environment into action,” Smith said. “And I’d really like to help be a part of that.”