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

Area of Science:

• Ecology
• Environmental Science
• Microbiology

Background:

• Metals are crucial for microbial life but their role as limiting nutrients in freshwater streams is not well understood.
• Previous research has largely overlooked the potential for metal limitation in aquatic primary producers.

Purpose of the Study:

• To quantify the prevalence of metal and nutrient (co-)limitation of primary producers in 41 streams.
• To investigate the interactions between metal and macronutrients (N and P) in regulating stream productivity.
• To explore divergent nutrient responses among different microbial taxa.

Main Methods:

• Field surveys across 41 streams to assess primary producer biomass responses to nutrient additions.
• Quantification of metal (Fe, Zn) and macronutrient (N, P) limitation.
• Predictive modeling using environmental variables to forecast metal limitation.

Main Results:

• Metal limitation was widespread, with iron (Fe) limitation observed in 50% of streams and zinc (Zn) limitation in 33%.
• Zn limitation was documented in stream biofilms for the first time at this scale.
• Metals frequently co-limited with nitrogen (N) and phosphorus (P), indicating complex nutrient interactions.
• Diatoms responded more to Zn enrichment, while cyanobacteria biomass increased with N and P addition.
• Predictive models successfully forecasted Fe and Zn limitation based on macronutrient availability.

Conclusions:

• Trace metals, particularly Fe and Zn, play a significant and often underestimated role in regulating stream primary productivity and community structure.
• The assumption that stream primary producers are rarely metal-limited is challenged by these findings.
• Understanding metal-nutrient interactions is crucial for managing freshwater ecosystem health and nutrient cycling.