Nitrogen Saturation in Stream Ecosystems

Abstract

Abstract. The concept of nitrogen (N) saturation has organized the assessment of N loading in terrestrial ecosystems. Here we extend the concept to lotic ecosystems by coupling Michaelis-Menten kinetics and nutrient spiraling. We propose a series of saturation response types, which may be used to characterize the proximity of streams to N saturation. We conducted a series of short-term N releases using a tracer ( 15 NO 3 -N) to measure uptake. Experiments were conducted in streams spanning a gradient of background N concentration. Uptake increased in four of six streams as NO 3 -N was incrementally elevated, indicating that these streams were not saturated. Uptake generally corresponded to Michaelis-Menten kinetics but deviated from the model in two streams where some other growth-critical factor may have been limiting. Proximity to saturation was correlated to background N concentration but was better predicted by the ratio of dissolved inorganic N (DIN) to soluble reactive phosphorus (SRP), suggesting phosphorus limitation in several high-N streams. Uptake velocity, a reflection of uptake efficiency, declined nonlinearly with increasing N amendment in all streams. At the same time, uptake velocity was highest in the low-N streams. Our conceptual model of N transport, uptake, and uptake efficiency suggests that, while streams may be active sites of N uptake on the landscape, N saturation contributes to nonlinear changes in stream N dynamics that correspond to decreased uptake efficiency.

Inferring nitrogen removal in large rivers from high‐resolution longitudinal profiling

“…We note that v f from Alexander Creek (our lowest concentration site) on 20 March 2013 is much larger than any other value (even from this stream on other dates); removing this outlier strengthens the correlation (p 5 0.02). This suggests removal efficiency declines with concentration, a widely observed phenomenon (Earl et al 2006;O'Brien et al 2007;Hall et al 2009). This is supported by the cross-site relationship between removal and concentration.…”

“…In this ''efficiency-loss model,'' removal is a power function of concentration. Other recent work using plateau (Earl et al 2006) and pulse additions (Covino et al 2010) to assess reaction kinetics has suggested Michaelis-Menten (MM) kinetics where removal becomes completely saturated at high concentration. Empirically resolving controls on removal kinetics in time and space, with network position and in a way that is pathway specific (i.e., assimilation vs. denitrification), is an important priority, with implications for understanding river responses to nutrient enrichment (Mulholland et al 2008).…”

“…Both applications fit a single trend line to the entire profile and did examine the spatial variability. Furthermore, the nature of the fitted trend line implicitly assumed first-order kinetics, an assumption subsequently challenged by multiple studies (Earl et al 2006;O'Brien et al 2007;Covino et al 2010). In this work, we adopt a Lagrangian reference frame to assess river N removal in a manner that is passive and inexpensive (i.e., performed at ambient conditions without isotope or nutrient additions), repeatable, and scalable across river sizes.…”

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

“…Although we do not have statistical evidence of an enrichment effect on uptake, enrichment magnitude almost certainly influences the biological assimilation of a nutrient pulse to some degree. Because lower nutrient concentrations are known to yield higher (approaching ambient) measures of uptake velocity (e.g., Earl et al 2006), local uptake velocity will increase longitudinally along a stream as a nutrient pulse disperses if all other factors are held constant. This means that pulse approaches likely underestimate uptake velocity at the upper end of a study reach.…”

Tracer Additions for Spiraling Curve Characterization (TASCC): Quantifying stream nutrient uptake kinetics from ambient to saturation

“…This area of research has expanded as N loading to terrestrial and aquatic ecosystems has increased (Vitousek et al 1997;Turner and Rabalais 2003), and concerns over deleterious impacts on receiving water bodies have grown (Rabalais et al 2009). Stream reach nutrient export is impacted by both biological (i.e., uptake) and physical (i.e., hydrologic loss) retention (Triska et al 1989;Hart et al 1992;Covino et al 2010), and research has shown that in-stream uptake (i.e., biological retention) can be strongly influenced by nutrient concentration (Dodds et al 2002;Earl et al 2006;O'Brien et al 2007). The relationship between biological uptake and nutrient concentration is of particular concern because nutrient uptake efficiency has been shown to decrease with elevated concentrations (Mulholland et al 2002).…”