Using nutrient limitation experiments to inform nutrient management thresholds in wadeable streams

Area of Science:

• Environmental Science
• Aquatic Ecology
• Water Quality Management

Background:

• Eutrophication is driven by nitrogen (N) and phosphorus (P), but predicting benthic algal responses to nutrient levels is challenging.
• Statewide analyses linking in-stream chemistry to nutrient limitation and management thresholds are lacking, hindering effective nutrient management strategies.
• Wadeable streams in the Midwestern US face mixed land use impacts, necessitating region-specific nutrient assessments.

Purpose of the Study:

• To evaluate nutrient limitation in Wisconsin wadeable streams using nutrient-diffusing substrate (NDS) bioassays.
• To establish statewide nutrient thresholds for benthic algal growth and inform management strategies.
• To investigate the relationship between ambient nutrient concentrations and nutrient limitation outcomes.

Main Methods:

• Conducted nutrient-diffusing substrate (NDS) bioassays across 56 wadeable streams in Wisconsin.
• Collected data on ambient total nitrogen (TN) and total phosphorus (TP) concentrations.
• Utilized mixed-effects models and logistic regression to analyze nutrient limitation and establish thresholds.

Main Results:

• Nutrient limitation varied, with 41% of sites showing no limitation, 23% N-limited, 20% co-limited (N+P), and 9% P-limited.
• Nitrogen (N) and combined N×P treatments significantly stimulated algal accrual, while P alone had minimal effects.
• Identified ambient total N thresholds (1.37-1.93 mg/L) for 50% probability of N limitation, and P thresholds (14-29 µg/L) below the state standard.

Conclusions:

• Nitrogen often exerts stronger control on benthic algal growth in Midwestern wadeable streams.
• Established plausible total nitrogen thresholds to constrain excess algal growth.
• Highlighted the need for dual-nutrient management strategies to address N and P in these systems.