LOGAN, UTAH, USA -- Algae is a ubiquitous feature in waterways throughout the globe, including western North America. Slippery, green epilithic algae is a familiar sight on river rocks. Toxic blue-green algae – cyanobacteria – is a visually interesting, yet worrisome phenomenon. Increasingly prevalent filamentous algae, with its long, voluminous green strands joins the picture, and is presenting new questions for scientists, recreationalists and land managers.
"In recent years, people have noted very large filamentous algae blooms in big, western rivers in the United States, including in Utah's Provo, Jordan and Bear Rivers," says Utah State University river ecologist and statistician Alice Carter. "They create a ton of biomass and grow filaments up to 12 meters long. Though not toxic, they're a nuisance interfering with traditional recreational river uses, including kayaking and fishing. Further, these huge blooms are essentially a food web dead end that don't support fisheries or macroinvertebrate communities."
Carter and colleagues from the University of Montana and Woods Hole Oceanographic Institution in Massachusetts have been studying these massive plant-like organisms in the Upper Clark Fork River in western Montana. They report findings in the Dec. 9, 2025 online issue of the journal Ecology. The research was supported by the National Science Foundation.
Carter, a professional practice assistant professor in USU's Department of Mathematics and Statistics and the USU Ecology Center, says the study revealed some unexpected aspects of these filamentous algae blooms – known as 'FABs.'
"While FABs are creating a significant shift in the ecosystem structure, we're seeing minimal change in the river's ecosystem metabolism and function," she says. "Instead, smaller, more fast-cycling epilithic algae appears to be quietly doing the heavy lifting of producing a healthy ecosystem good for the food web and driving the whole system."
FABs are producing roughly the same amount of carbon as slimy epilithic algae that occupies a much smaller footprint.
"This implies rivers might be an exception to our theory surrounding ecosystem ecology," Carter says. "In terrestrial ecosystems, structure and function are tightly linked. If you have a dense forest with tons of trees and biomass, it's also a productive forest. Whereas, If you have a small amount of vegetation, it will be a less productive environment."
In contrast, structure and function seem decoupled in rivers, she says.
"Rivers might not be playing by the rules, and that raises interesting management implications," Carter says. "If we're not seeing a large shift in function with big structural changes caused by FABs, then the inputs should be similar whether we have these big plants choking our waterways or not. It implies that something other than overabundance of nutrients has tipped us into a state where we're seeing the large algal blooms."
Finding that tipping point could help managers get rivers back to a healthier state without massive nutrient reduction campaigns.
"We might discover an easier-than-expected management solution," she says. "That might be overly optimistic, but it would be exciting to find out."
Carter will present the research at the 2026 Ecological Society of America Annual Meeting, to be held July 26-31 in Salt Lake City.
