RESEARCHERS MAP AQUATIC-BASED, HUMAN-DRIVEN IMPACTS ON CLIMATE CHANGE

A study published in the journal Science uses a global experiment and predictive modeling to illustrate how human impacts to aquatic ecosystems are contributing to the global climate crisis.

The study estimates rates of organic-matter decomposition in freshwater ecosystems (a source of carbon emissions) across the world, including areas such as the tropics that have been underrepresented in prior studies. Researchers Scott Tiegs (Oakland University), Krista Capps (University of Georgia) and David Costello, Ph.D. an associate professor in the Department of Biological Sciences at Kent State University, are the study’s co-lead authors.

The researchers collected field data from sites around the world and used predictive modeling and machine-learning algorithms to fill in gaps. The field study involved 550 rivers from around the globe, including four in northeastern Ohio. Three of them were located near Kent and Ravenna and one at the Holden Arboretum in Lake County. More than 150 researchers in 40 countries contributed samples using a standardized field assay based on the decomposition of small pieces of cotton fabric.

“Through a large, collaborative effort using a standardized, experimental approach, we were able to generate the first estimates of decomposition rates in streams throughout the world, including understudied areas such as the tropics,” Capps said. “Integrating experimental data collected at such a large scale with modeling is a powerful approach to examine global patterns in important ecological processes like decomposition.”

The study reported the highest decomposition rates among densely populated, agricultural areas, including parts of the United States, Europe and Southeast Asia. Agricultural and urban runoff, the authors point out, is a major contributor to the increase in carbon emissions responsible for climate change.

“When we think of greenhouse gas emissions, we tend to think of tailpipes and factories, but a lot of carbon dioxide and methane can come from aquatic ecosystems,” Tiegs said. “This is a natural occurrence, but when humans add nutrient pollution (like fertilizer) to fresh waters and elevate water temperatures, we increase the decomposition rates and direct more carbon dioxide (CO₂) into the atmosphere.”

One of the study’s key takeaways is that humans are impacting decomposition rates in rivers on a global scale – a finding illustrated by the maps presented in the study. These maps are also freely accessible through a website built by the authors and hosted on a server at Kent State.

“The online mapping tool lets anyone see how fast we think different kinds of leaves will decompose in their local rivers,” Costello said. “Even if no researcher has made measurements in a river, we have an estimate for how fast leaves should decompose.”

Using predictive modeling, the researchers also identified key environmental variables responsible for increased decomposition rates, notably temperature and nutrient concentrations.

The primary variables driving decomposition rates are impacted by human activities, such as nutrients and temperature. The authors noted that reducing human impacts on decomposition will keep more carbon in rivers, preventing it from being released into the atmosphere as CO₂.

“We need to minimize human impacts on fresh waters to more effectively manage our global carbon cycle.” Tiegs said.

Kent State’s Costello discusses the project further
“There are really large areas of the world that we haven't been able to do a lot of research in, in particular in streams and rivers, which take up a really tiny slice of the landscape,” Costello said. “They're thin little ribbons through forests and plains and mountain areas, and it's hard to figure out what's going on using things like remote sensing or GIS or satellite measurements. It takes being on the ground to make these measurements, which means there are areas that have not been well studied. Our approach with these tiny pieces of fabric is really powerful in that we can put them in the mail and ship them to someone in a remote location, and they can do this experiment embedded within this collaborative network and we can learn a lot about things at really big spatial scales.”

This study has implications for climate change and mitigation, in particular for storing carbon. Ecosystems capture carbon naturally. Trees suck carbon out of the air, and then they turn it into biomass which can either be buried, move into other organisms or it can go right back to the atmosphere.

“This is really an important natural carbon storage mechanism that we're studying here,” Costello said. “So, if humans are speeding up rates of decomposition, they add a lot of nutrients to the water and decomposition rates are faster. That means the carbon doesn't end up in fish or buried in the bottom of lakes, but it goes right back to the atmosphere and then can contribute to global warming.”

In the northern hemisphere, specifically around northern Ohio in the fall, all the leaves that were produced during the summer fall off the trees and a large amount of them get into the rivers and streams. Then a couple of different things can happen.

“Leaves around here can eventually make their way to Lake Erie, in either big pieces or little pieces,” Costello explained. They’ll either be buried in the bottom of the lake, get consumed by insects in the stream and eaten by a fish and turned into part of the fish, or they could get blown off by microbes as carbon dioxide and go back to the atmosphere.”

Citizen scientist contributions on the horizon?
Currently the online mapping tool is static, but Costello is hopeful that citizen scientists (such as school groups, universities, or other citizens) will be able to contribute to this project in the future by uploading data and starting to fill in some of those gaps and continue to build and refine the map.

“We're not there yet,” Costello said. “This is the baseline, just demonstrating it's something we can do. I have students doing other projects with cotton strips all the time. I've had at least four undergrads over the last five years that have worked on related projects, and it's only going to continue. We've had a lot of grad students through our lab working on similar projects to this and they're involved in all aspects of it including going to different sites and placing these things in water and later pulling them out and helping with the analysis.

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Media Contacts: 
Dave Costello, Ph.D., 330-672-2035, dcostel3@kent.edu
Jim Maxwell, 330-672-8028, JMAXWEL2@kent.edu

WRITTEN BY: JIM MAXWELL