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.