Out of Balance: What’s Limiting Freshwater Stream Growth
What controls how much life a stream can support? For decades, scientists have pointed to nutrients like nitrogen and phosphorus as the main drivers of algal growth, which forms the foundation of aquatic food webs. This study takes a closer look at that assumption, asking whether other, less visible elements may also limit how these systems function.
Researchers from the University of Georgia’s Savannah River Ecology Laboratory (SREL), in collaboration with researchers from multiple universities, examined how trace metals influence algal growth in freshwater streams. Trace metals are elements like iron and zinc that are needed in very small amounts but are still essential for life. The study was led by David M. Costello, a professor at Kent State University, and includes contributions from Krista A. Capps, an associate professor at SREL and with the Odum School of Ecology, Raven L. Bier, an assistant research scientist at SREL, and Dean E. Fletcher, a research professional at SREL.
To test how trace metals influence algal growth in freshwater streams, researchers conducted experiments across 52 streams in the eastern United States, with 41 included in the final analysis, using small in-stream devices that slowly released nutrients and trace metals. By comparing how algal communities responded to different combinations of elements, the team evaluated whether growth depended on a single nutrient or on several working together.
Growth limits were common across the study streams, and they extended beyond nitrogen and phosphorus alone. In more than 80% of streams, algae did not reach their full potential because at least one key element was in short supply. While nitrogen remained a major factor, trace metals were also frequently involved, showing that they are not just minor components but regular drivers of productivity.
“The inspiration for this work came from an observation by Dave Costello, who led the study, that these trace metals are essential for core microbial and algal processes. They’re part of the enzymes that drive photosynthesis and nutrient cycling, so there was a strong reason to expect they might matter,” Capps shares. “But they’ve been largely overlooked because freshwater ecology has focused so heavily on nitrogen and phosphorus, and there’s been a long-standing assumption that metals in streams are abundant enough that they wouldn’t limit growth. What changed our thinking was seeing that biology line up with field evidence.”
Trace metals played a substantial role, and they often worked alongside more familiar nutrients. Iron influenced about half of the streams studied, while zinc played a role in roughly one-third, marking one of the first large-scale indications that zinc can limit growth in freshwater systems. Rather than acting alone, these metals were often part of a combination of elements that together controlled how much algae could grow.
Adding a single nutrient often had little effect, but combining nutrients and metals led to much stronger responses. This reflects how biological systems function. Nitrogen and phosphorus help build new cells, while metals like iron and zinc support the internal processes that allow those cells to use energy and take up nutrients, so growth depends on having both the materials and the ability to use them.
“To grow, organisms require many different elements, each at a different amount. When only one nutrient is provided, there is an imbalance as it alone cannot provide everything needed for living biomass. Rather, micronutrients such as metals are essential for building and operating those cellular structures in combination with macronutrients like nitrogen,” says Bier.
Algal responses also varied by group, and different types of algae showed distinct nutrient needs. Some groups increased when nitrogen and phosphorus were added, while others responded more strongly to metals like zinc. These differences help explain why changes in nutrient availability can shift not only how much algae grows, but also which groups become most common in a stream.
Patterns across sites added another layer of insight, and streams with higher phosphorus levels were more likely to show signs of metal limitation. This suggests that as some nutrients increase, the demand for other elements can also rise and shape how ecosystems respond.
“Concentrations of the macronutrients nitrogen and phosphorus have long been studied and measured throughout freshwater stream systems,” Fletcher says. “Identifying relationships between trace elements such Fe and Zn with these macronutrients is providing valuable insight to the likelihood of these trace elements limiting stream primary production.”
Taken together, these findings point to a broader understanding of how streams function. Nitrogen and phosphorus remain important, but they are part of a larger network of elements that regulate biological activity, and overlooking trace metals may leave key processes unexplained. Recognizing how these elements interact helps clarify why streams respond differently to environmental change and may improve how freshwater systems are studied and managed.
When Costello was asked about the future direction of this research, he responded: “This research has sparked many new questions. Many of the same trace metals that organisms need are also in high demand for batteries other advanced technologies, so understanding their natural availability is increasingly important” Costello shares. “We’re now taking a look at long-term water-quality records and encouraging agencies to included trace metals in routine monitoring. We’re also collaborating with ecotoxicologists, because these elements can become harmful at high concentrations, and we want a full picture of how they shape stream ecosystems.”
The full study, Anaemic Streams: Iron and Essential Trace Metals Frequently Limit Primary Producer Biomass, was published in Ecology Letters. Authors include David M. Costello, Olufemi J. Akinnifesi, Renn C. Schipper, Paisley Kostick, Jordyn T. Stoll, Scott D. Tiegs, Amy M. Marcarelli, Sally A. Entrekin, Raven L. Bier, Krista A. Capps, and Dean E. Fletcher.
