# Anaemic Streams: Iron and Essential Trace Metals Frequently Limit Primary Producer Biomass

## TL;DR

This study shows that trace metals like iron and zinc often limit the growth of algae and other primary producers in freshwater streams, challenging previous assumptions.

## Contribution

The first large-scale evidence that zinc and iron frequently limit stream biofilm biomass, with interactions between trace metals and macronutrients.

## Key findings

- Iron limitation was observed in 50% of the studied streams, showing the strongest biomass response.
- Zinc limitation was found in 33% of streams, marking the first evidence of Zn limitation at this spatial scale.
- Diatoms responded more to zinc, while cyanobacteria thrived with nitrogen and phosphorus enrichment.

## Abstract

Metals are essential for microbial metabolism, yet their role as limiting nutrients in freshwater streams remains poorly understood. We quantified the prevalence of metal and nutrient (co‐)limitation of primary producers in 41 streams. Metal limitation was widespread with Fe limitation eliciting the strongest and most consistent biomass responses (50% of streams). Zn limitation was also common (33% of streams), marking the first evidence of Zn‐limited stream biofilms at this spatial scale. Metals were often co‐limiting with N and P, highlighting interactions between macro‐ and micronutrients. Diatoms were more responsive to Zn and cyanobacteria reached higher biomass with N and P enrichment, emphasizing divergent nutrient responses among taxa. Predictive modelling indicated that Fe and Zn limitation could be forecasted from environmental variables related to macronutrient supply. These findings challenge the long‐standing assumption that stream primary producers are rarely metal‐limited and suggest that trace metals may play an underappreciated role in regulating stream productivity, community composition and nutrient cycling.

Nutrient enrichment experiments in 41 streams across the eastern United States demonstrate that trace metals can limit the growth of primary producers. Trace metals are frequently co‐limiting with macronutrients and the availability of N and P in streams and watersheds are predictive of Fe and Zn limitation status.