Quantifying phosphorus uptake using pulse and steady‐state approaches in streams

Abstract

Steady‐state approaches to the study of stream nutrient processing have several limitations. Dynamic (time series) approaches are more flexible, and allow interpretation of nutrient additions introduced as unsteady slugs (pulses). We compared soluble reactive phosphorus (SRP) uptake metrics from experimental nutrient pulses modeled dynamically with those from continuous injections modeled with a steady‐state approach. For six southern Wisconsin streams, uptake metrics from these two methods were similar despite low nutrient demand. Linear regression of paired‐pulse versus steady‐state estimates of the first‐order uptake coefficient (λ; r2 = 0.84, slope = 1.21) and uptake velocity (vf; r2 = 0.95, slope = 1.03) were highly significant. There was a tendency for slightly higher uptake with pulses, possibly due to P sorption. Sampling across five stations of one stream yielded a similar longitudinal pattern between experimental pulse SRP flux and steady‐state (plateau) SRP concentration. Conservative transport parameters for pulse and continuously injected tracer data were also similar. These results suggest that unsteady nutrient amendments can provide usable nutrient uptake values, even in low‐uptake situations for which uncertainty is high. The flexibility of dynamic approaches to nutrient spiraling facilitates research in poorly understood situations, including conditions of high water residence time, high discharge, and changing discharge or background chemistry.