Hydrology, Cryosphere & Earth Surface News

The Shape of Watersheds

Streams in flatter watersheds have carbon cycles more sensitive to temperature increases.

By

Historically, freshwater systems have been given short shrift in research on biogeochemical cycles.

“For a long time, the freshwater contributions to the global carbon cycle were assumed to be negligible,” said Daniel Schindler, a limnologist at the University of Washington in Seattle. “But we’ve known for a few decades now that relative to their area, freshwater systems are remarkably important to the global carbon cycle.”

Yet despite this importance, we still know very little about how these freshwater systems will respond to climate change.

A recent study by Schindler and fellow limnologist Kathi Jo Jankowski of the U.S. Geological Survey sheds new light on the matter.

Scientists know that much of the organic carbon in streams and rivers gets transported downstream to the ocean, but a lot of organic carbon gets processed by local ecosystems instead. The organisms in these ecosystems convert the organic carbon into carbon dioxide, which bubbles away into the atmosphere. This process is known as ecosystem respiration.

Temperature has a significant effect on ecosystem respiration. In part, the reason is microbial decomposition of organic matter generally happens faster at warmer temperatures, said Schindler.

“It’s just like putting your leftovers in the fridge versus in the closet,” he said. “If you put them in the closet, they stay warm and they decompose really rapidly.”

But even for streams in the same area of the world, there are big differences in how different streams respond to changes in temperature. Schindler and Jankowski wanted to figure out why.

The Impact of Topography

The answer lies in the shape of the landscape.

Over the course of 4 years, Jankowski and Schindler gathered data from streams in the pristine wilderness of the Wood River system in southwest Alaska. They measured the temperature, ecosystem respiration, and chemical composition of the streams and gathered data about the geomorphology of the stream’s watershed—in other words, the shape of the area of land drained by the stream.

They found that the geomorphology of the watershed had a huge impact on the temperature sensitivity of carbon processing in stream ecosystems. Ecosystem respiration was much more sensitive to temperature in streams that drained flat watersheds compared to streams that drained steep watersheds.

Scientists think that this difference in sensitivity is because watershed morphology affects the quality of organic material that ends up in freshwater systems.

“We know that streams carry a signature of the organic matter that they either receive from the surrounding watershed or that they produce in situ through algal growth,” said Schindler.

Schindler said that most of the organic carbon in streams that drain steep watersheds comes from algae in the streams themselves. This type of organic carbon is really easy for microbes to break down. And because it’s so easy, higher temperatures don’t really affect how fast the microbes can process the carbon.

On the other hand, organic matter in streams that drain flat watersheds tends to come from the soils in the watersheds, said Schindler. Flat watersheds produced streams with organic carbon of lower quality. This organic carbon is harder for microbes to decompose, but higher temperatures can really give microbes a boost when they’re tackling this tricky (or recalcitrant) organic matter.

Jim Hood, an aquatic ecologist at The Ohio State University in Columbus, said that this paper has important implications for our understanding of freshwater ecosystem processes and biogeochemistry on a larger scale. “The really fantastic thing is that by linking [temperature dependency of ecosystem respiration] to geomorphology and then linking it again to the quality of the carbon, they allow us to extend their findings to other systems.” Hood was not involved in the research.

He said that a better understanding of these admittedly complex biological processes is crucial for improving our models of global carbon cycles and our predictions of how they will be affected by climate change.

—Hannah Thomasy (@HannahThomasy), Freelance Science Writer

Citation: Thomasy, H. (2020), The shape of watersheds, Eos, 101, https://doi.org/10.1029/2020EO138861. Published on 21 January 2020.
Text © 2020. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.