Like the circulatory system that helps move blood, carry nutrients and filter waste in the human body, the planet’s river networks are in a very real sense similar conduits that help keep the planet alive.
One of a river’s important functions is removing some of the pollution that ends up in its waters — from roads, lawns, septic systems and sewage treatment plants, agriculture (and more) — before those waters reach sensitive downstream ecosystems like estuaries and oceans.
New research has found that watershed size plays a major role in a river network’s ability to do this work. The findings both further our understanding of which estuaries and coastal areas will be more impacted by human development in their watersheds and shed light on the intricacies of the global carbon cycle.
Using a model that integrates what is known about how streams and rivers function, the team of scientists found that when the area of land (the “watershed”) that drains into an aquatic system increases, the rate at which rivers filter pollution doesn’t just increase at a linear rate — it increases even faster, thanks to the larger rivers that commonly go hand-in-hand with larger watersheds.
“It is not well-known what controls how much pollutant filtration river networks can do, or whether it occurs primarily in small versus large rivers,” says Wilfred Wollheim, professor of natural resources and the environment, principal investigator at UNH’s Earth Systems Research Center, and lead author of the article about the research that was recently published in Nature Communications. “To use an analogy, when body size increases, the amount of energy it needs to do its work (metabolism) also increases. However, for body metabolism it is well known that as body size increases the metabolism increases at a slower rate. We wanted to see if something similar happens to aquatic metabolism or — as we discovered — something different.”
Wollheim and his team describe what they uncovered about watershed size and river function as superlinear scaling, and he says it occurs because larger rivers contribute disproportionately to the pollution-filtering function of entire networks of aquatic ecosystems, which includes lakes, streams, rivers and wetlands.
To keep as much pollution as possible out of estuaries and oceans, says Wollheim, the research indicates that it is more important to manage land use and mitigate non-point source pollution — like runoff carrying fertilizers, herbicides, insecticides, and toxic chemicals — in smaller watersheds, which are less able to filter pollutants, than larger watersheds. It is also important to mitigate nonpoint pollution in parts of the watershed that are closer to an estuary or coastal area, where the system will have less of a chance to filter the pollutants before it reaches those critical areas, than those that are further away.
The research also reveals new information about the role of rivers in the global carbon cycle.
“Land is known to be a net carbon sink, but recent research has found that a large proportion of this carbon actually ends up in rivers,” says Wollheim. “Our research shows that due to superlinear scaling, aquatic ecosystems of larger watersheds potentially release the carbon that makes its way into the water from land (and thought to be stored there) back to the atmosphere, while this would not be as evident in smaller watersheds.”
The team hopes this new information about behavior of aquatic ecosystems and rivers in particular will help stakeholders design better pollution management strategies and improve our understanding of the feedback loop between the Earth’s ecosystems and its atmosphere and how it impacts the rate of climate change.
Wollheim’s collaborators are Tamara K. Harms from the University of Alaska, Andrew L. Robison from UNH, Lauren E. Koenig and Ashley M. Helton from the University of Connecticut, Chao Song from Michigan State University, William B. Bowden from the University of Vermont and Jacques C. Finlay from the University of Minnesota.
