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; r 2 = 0.84, slope = 1.21) and uptake velocity (v f ; r 2 = 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.

Quantifying ambient nitrogen uptake and functional relationships of uptake versus concentration in streams: a comparison of stable isotope, pulse, and plateau approaches

“…Some previous studies compared plateau and pulse additions to each other (e.g. Payn et al 2008;Gooseff et al 2008;Powers et al 2009;Alvarez et al 2010), or plateau additions to stable isotopes (e.g. Dodds et al 2002;Mulholland et al 2002;Payn et al 2005;Earl et al 2007).…”

Sensitivity analysis of a pulse nutrient addition technique for estimating nutrient uptake in large streams

“…Total nutrient mass loss is the product of the uptake rate and total nutrient mass, regardless of any hydrogeomorphological influences on the degree of spread of the nutrient pulse. Nutrient uptake measures that rely on nutrient mass loss to calculate spiraling metrics should give reasonable uptake estimates if there is first-order uptake, but Powers et al (2009) also noted that the TPNA technique might yield different results if uptake was nonlinear, as we have shown in this study. When nutrient uptake follows Monod kinetics, nutrient uptake increases with a decreasing rate as nutrient concentration increases, uptake is less than the first-order uptake kinetics, and this uptake difference increases at higher nutrient concentration.…”

“…Therefore, nutrient saturation is a concern. Powers et al (2009) tested the TPNA technique under an assumption of first-order nutrient uptake kinetics. As an extension of that study, we tested the TPNA technique under the Monod kinetics nutrient uptake assumption.…”

“…Therefore, a pulse nutrient addition tech-nique (PNA technique) was suggested as an alternative for uptake measurement in large streams. This technique was first used by Meals et al (1999) and more recently used to estimate nutrient uptake parameters by Tank et al (2008) and Powers et al (2009). Using the PNA technique, a slug of nutrient and biological conservative tracer is released, and their concentrations are monitored at several downstream stations.…”

Nitrogen and phosphorus dynamics in a large river estimated by an in situ Lagrangian tracking approach

“…Nutrient spiralling metrics are generally estimated by adding biologically active tracers or isotopically labelled tracers and monitoring the longitudinal decline of such added nutrients at multiple stationary locations downstream from the injection points (Powers, Stanley, & Lottig, 2009;Stream Solute Workshop, 1990;Webster & Valett, 2006). The nutrient uptake metrics estimated by such short-term tracer addition experiments are considered to represent gross uptake of a stream ecosystem (Stream Solute Workshop, 1990).…”