On a warm summer afternoon, wading into Fish Creek near the Kent State campus, you might not notice anything unusual about the water running clear over smooth rocks. But David Costello, Ph.D., sees something that most scientists have spent decades overlooking: a stream that is quietly iron-deficient.
Costello, an associate professor of biological sciences at 5X, has spent years studying how human activities affect stream ecosystems. His latest research, published March 8 in the journal Ecology Letters, challenges a foundational assumption in stream ecology — that algae and other primary producers at the base of aquatic food webs are almost exclusively limited by nitrogen and phosphorus. His team’s findings suggest that trace metals, particularly iron and zinc, play a far more significant and previously underappreciated role.
DRAFT “We’ve known for decades that iron can limit plant growth in the ocean, but the prevailing assumption has always been that freshwater streams have plenty of metals,” said Costello. “This study shows that assumption is wrong — and that has real implications for how we think about managing our streams and rivers.”
Testing the Assumption Across 41 Streams
Working with a team of 10 co-authors from Kent State, Oakland University, the University of Georgia, and other institutions, Costello conducted nutrient and metal enrichment experiments in 41 streams spanning 14 degrees of latitude across the eastern United States. The research was supported by a National Science Foundation grant.
The study sites included streams well-known to Northeast Ohio residents: Fish Creek and Breakneck Creek in Kent, Tinkers Creek, the Rocky River, the Chagrin River, and streams at the Holden Arboretum in Lake County. Additional sites extended into both the upper and lower peninsulas of Michigan.
In each stream, the research team deployed small substrates that released controlled amounts of nitrogen, phosphorus, iron, zinc, and other metals — then measured how algae and microbial communities responded. The fieldwork unfolded across two summers, 2021 and 2022, with each in-stream experiment running two to four weeks.
The effort involved three Kent State graduate students — Jordyn Stoll, Renn Schipper, and Olufemi Akinnifesi — as well as post-baccalaureate researcher Paisley Kostick, at least eight Kent State undergraduates, and two undergraduate summer interns through the NSF Research Experience for Undergraduates program.
What They Found: Iron Was the Biggest Limiter
The results were striking. Iron limitation was the most widespread and consistent finding, affecting 50 percent of the streams studied. Zinc limitation was documented in 33 percent of streams — the first time zinc limitation of stream biofilms has been demonstrated at this spatial scale.
Metals were rarely acting alone. They were frequently co-limiting alongside nitrogen and phosphorus, revealing a more complex nutritional picture than the field had previously recognized. Different organisms also responded differently: diatoms proved more responsive to zinc, while cyanobacteria thrived with nitrogen and phosphorus enrichment.
“Nutrient limitation — whether by metals or major nutrients like nitrogen and phosphorus — is actually the normal, healthy status of a stream,” Costello explained. “The opposite is very obvious. When a stream has plenty of nitrogen, phosphorus, and trace metals, the rocks will be covered in slimy green algae.”
A New Management Lever for Lake Erie?
For Ohioans familiar with harmful algal blooms in Lake Erie, the implications of Costello’s research extend well beyond the science pages. Excessive nutrient runoff — particularly nitrogen and phosphorus from agricultural and urban sources — has long driven eutrophication in Lake Erie and its tributary rivers. Costello’s findings suggest the picture may be more nuanced.
“Eutrophication and harmful algal blooms are a big concern in Lake Erie, but these kinds of problems can also happen in rivers and streams,” Costello said. “Algae clean up nutrient pollution from water and provide a food source for fish and other stream animals — but algae can’t clean up excessive pollution, and that leads to the excessive algae you don’t want.”
His team also developed predictive models showing that iron and zinc limitation can be forecasted from broader environmental variables — a finding that could eventually help land managers and regulators better target their interventions.
“Controlling iron and zinc might be an additional management lever we could pull to address harmful algal blooms in rivers and Lake Erie,” Costello said. “Nutrient management is a top priority for Ohio rivers that feed into Lake Erie. By knowing that metals like iron and zinc might also be critical nutrients for algae, it opens up new possible management actions. There’s still a ways to go, but this is a promising direction.”
Building on a Growing Body of Work
The new study is the latest in a series of high-profile publications from Costello’s lab focused on the health of freshwater ecosystems. In 2024, Costello and many of the same collaborators — including co-authors Scott Tiegs of Oakland University and Krista Capps of the University of Georgia — published a landmark study in the journal Science mapping how human activities are accelerating organic matter decomposition in rivers worldwide, with direct implications for global carbon emissions. That study also relied on field data from streams near Kent, including Breakneck Creek.
Taken together, the two studies reflect a sustained effort to understand freshwater ecosystems in their full complexity — one that is producing findings with real consequences for water quality, climate, and the communities that depend on healthy rivers and streams.
DRAFT: “The work we’re doing here in Northeast Ohio is part of a larger effort to understand how human activities are reshaping freshwater ecosystems — and what we can do about it,” Costello said.
The study, “Anaemic Streams: Iron and Essential Trace Metals Frequently Limit Primary Producer Biomass,” was published in Ecology Letters (Volume 29, Issue 3, 2026) and is available open access at