Rising stream temperatures may be weakening the foundation of river food webs by altering how carbon moves through these watery ecosystems.
In a new study published in the journal Ecosphere , researchers from Northern Arizona University found that when water temperatures increase, microbes and aquatic insects process fallen leaves, twigs and bark more rapidly, but a smaller fraction of that leaf litter supports their growth and a bigger fraction is released into the water and air as carbon dioxide.
The findings point to a shift in how river ecosystems retain energy under warming conditions, with implications for plants and animals in rivers across the western United States.
"Warming doesn't just speed up biological processes in streams—it changes how efficiently organisms turn carbon into biomass, with more of it being lost as CO₂," said Michael Zampini, a postdoctoral researcher at NAU and the lead author of the study.
A 'living laboratory' to track carbon flow
To examine how warming affects river processes, the NAU researchers built a controlled stream system at The Arboretum at Flagstaff , constructing 48 flow-through mini stream chambers inside a greenhouse. Using pond water, they manipulated the water temperature while maintaining natural light and water chemistry, simulating a range of stream conditions over two years.
"This system let us manipulate temperature while keeping everything else as close to a real stream as possible, which is critical for understanding how these processes actually play out in nature," said Zampini.
Within this system, the team used tracers to follow carbon from leaf litter—the primary energy source in many forested rivers—into microbes and caddisflies. By labeling leaves with a rare form of carbon, they directly measured how much carbon was retained as biomass, how much of it was released into the water and air as CO₂ and how much was transferred to microbes and insects, allowing them to quantify how effectively organisms converted food into growth.
Faster processing, lower retention in warming streams
The researchers found that as temperatures increased, decomposition rates rose, but a larger share of carbon was lost as CO₂ rather than incorporated into biomass. Caddisflies showed a distinct thermal response, with low temperatures limiting their activity, intermediate temperatures maximizing their efficiency, and higher temperatures increasing their consumption without corresponding gains in biomass. Together, these patterns indicate that warming releases more carbon into the atmosphere and converts less carbon into biomass.
"Even when consumption increases, the system becomes less efficient—more carbon goes to respiration and less to building the food web," said Jane Marks, professor in the Department of Biological Sciences and the Center for Ecosystem Science and Society (Ecoss) at NAU.
In rivers across the Southwest, where aquatic insects link leaf litter to animals higher on the food chain such as fish, this shift has broader implications. Declines in carbon use efficiency for microbes and aquatic insects mean a greater proportion of carbon entering rivers may be lost to the atmosphere, reducing energy available to support aquatic food webs.
"When less carbon is retained in biomass, there is less energy available to support aquatic life, which can ripple through the food web and ultimately affect fisheries, water quality and ecosystem stability that people depend on," Marks said.
Other researchers involved in the study included University of Alabama professor Steven Thomas and Northern Arizona University researchers George Koch, Benjamin Koch, Paul Dijkstra and Victor Leshyk at Ecoss. The research was funded by the National Science Foundation (DEB-1120343).
