Confounding factors in algal phosphorus limitation experiments

Area of Science:

• Stream ecology
• Aquatic primary production
• Nutrient cycling

Background:

• Nutrient diffusing substrate (NDS) experiments are key for assessing algal nutrient limitation in streams.
• Unexpectedly, phosphorus (P) additions in NDS experiments sometimes decrease algal biomass, suggesting potential inhibition.

Purpose of the Study:

• To investigate the causes of decreased algal biomass observed in NDS experiments with phosphorus (P) additions.
• To differentiate between direct P toxicity, experimental artifacts, and environmental factors causing P-inhibition.

Main Methods:

• Quantified inhibition frequency in published NDS experiments.
• Conducted a meta-analysis on heterotrophic competition and selective grazing.
• Performed field experiments varying phosphate cation, form, and NDS preparation.

Main Results:

• 12.9% of published NDS P experiments showed significant algal inhibition.
• Meta-analysis found no evidence for enhanced heterotrophy or selective grazing explaining P-inhibition.
• Field experiments indicated lower productivity with monobasic vs. dibasic phosphate salts due to pH, not direct P toxicity.

Conclusions:

• Algal growth inhibition in NDS P experiments is likely due to artifacts like altered pH from phosphate form, not direct P toxicity.
• Recommendations are provided to minimize experimental artifacts and ensure accurate assessment of nutrient limitation in streams.