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

Nitrogen (N) and phosphorus (P) are widely recognized as primary drivers of eutrophication, yet translating ambient nutrient concentrations into predictive assessments of benthic algal limitation remains challenging. In particular, statewide analyses that couple in‑stream chemistry to nutrient limitation outcomes and inform management thresholds are lacking. This gap constrains the ability to design nutrient‑management strategies tailored to specific regions or watersheds. To address this need, we evaluated nutrient limitation using nutrient‑diffusing substrate (NDS) bioassays in 56 wadeable streams across Wisconsin, located within the Midwestern United States, a region characterized by mixed agricultural, urban, and forested land uses. Sites encompassed a broad range of ambient total nitrogen (TN) and total phosphorus (TP) concentrations (TN: 0.22-15.17 mg/L; TP: 7-919 µg/L) with only weak correlation between TN and TP. Limitation responses varied: 41% of sites showed no limitation, 23% were N-limited, 20% co-limited by N and P, and 9% were P-limited. Mixed-effects models indicated that N and combined N × P treatments significantly stimulated algal accrual, whereas P alone had weak effects. Logistic regression identified ambient total N thresholds for a 50% probability of N limitation at 1.37-1.93 mg/L across total P gradients, while P limitation thresholds (14-29 µg/L) were well below Wisconsin's TP standard. These findings suggest that N often exerts stronger control on algal growth and provides a plausible range of TN thresholds needed to constrain excess benthic algal growth in wadeable streams. These results underscore the need for dual-nutrient management strategies in Midwestern streams.