A single leaf dropping into a stream has just a tiny impact, but, together, the billions of leaves that drop into waterways every year help keep global ecosystems going. How this works and why it’s so important are two questions addressed in a massive, new study in Science Advances co-authored by UMBC’s Chris Swan and over 150 researchers across all seven continents.
Leaves are mostly made of carbon, a primary building block for all living things. As leaves fall, organisms that live in or near rivers and streams are “supported by this pulse of carbon,” either directly or indirectly, says Swan, professor of geography and environmental systems.
“At any one point in time there’s not a lot of carbon there,” Swan explains, “but rivers serve as the plumbing system of the planet when it comes to how much carbon flows through.” So what factors drive how all that carbon is processed?
The new landmark study reveals how environmental factors drive the flow of carbon through the world’s waterways. “The study looked across the globe to learn what drives the rate of carbon decay in rivers and streams,” Swan says. “How is it transformed into energy as it travels up through the food web or transported downstream?”
The researchers first determined the overall carbon flow in waterways “across all continents and across a serious range of latitudes,” Swan says. They found that temperature was the biggest driver of carbon flow.
“That’s important because of climate change,” Swan says. “This study suggests that if temperature goes up, the rate at which carbon degrades will also go up.” This means that temperature change could reshape how carbon flows “either up the food chain or downstream.”
For example, bacteria, fungi, and aquatic invertebrates (like crayfish) are all responsible for breaking down some of the carbon in rivers. With climate change, “Bacteria and fungi that consume carbon are probably going to be more responsible for degrading it than invertebrates, because invertebrates aren’t able to evolve as fast” to adapt to the changing temperature, Swan explains. As some species succeed and others struggle, that could eventually lead to shifts in the makeup of aquatic ecosystems.
Development and dead zones
Increasing human development near waterways may also play an important role in carbon flow. Development “snips off” the smallest streams, but it’s at those abundant small streams where there’s an “intimate connection between streams and the forest,” Swan says. Those small streams are “part of the water purification process,” he explains.
Without as many small streams to break down the carbon, it ends up on pavement and runs off into larger streams and rivers. That creates higher than normal carbon concentrations, which can lead to low-oxygen “dead zones” in those larger waterways that are dangerous, even lethal, to aquatic life.
The value of partnership
The results of this study provide an important baseline for research on carbon flow in waterways moving forward. The experimental design at each site was simple, “but to manage and deploy it across the globe to all seven continents was a Herculean task that could only be done with a network of colleagues,” says Swan. “The key here is true partnerships and global coverage.”
To come to their conclusions, the scientists all used the same protocol: They each placed identical cotton squares provided by the lead author, Scott Tiegs at Oakland University, in a nearby stream. Then they carefully measured variables like the temperature, chemical makeup, flow rate, and width of the stream, plus the percentage canopy cover (how shaded the stream is). Everyone sent their cotton squares back to Tiegs, and he measured the cotton’s integrity at the end of the experiment.
Leaf chemistry can play a significant role in how quickly leaves decay—oak leaves are very slow, and ash leaves are fast, for example. But by using the cotton squares instead of real leaves, “we held all of that leaf chemistry constant and just looked at the environmental drivers,” Swan says.
“What I’m most proud about is being part of a group that was able to document the patterns on such a large scale using a consistent approach,” Swan says. Because of its extensive scope and scale, Swan expects the paper to be used in classrooms for a long time to come. As a benchmark study, it could teach generations of ecology students the fundamentals of carbon processing in waterways.
“The study is a lesson in stream ecology,” Swan says, “but the bigger lesson is that if you have partnerships you can do big things, and come up with big patterns.”
Image: Chris Swan. Photos by Marlayna Demond ’11 for UMBC.