N uptake as a function of concentration in streams

Abstract

Detailed studies of stream N uptake were conducted in a prairie reach and gallery forest reach of Kings Creek on the Konza Prairie Biological Station. Nutrient uptake rates were measured with multiple short-term enrichments of NO 3 Ϫ and NH 4 ϩ at constant addition rates in the spring and summer of 1998. NH 4 ϩ uptake was also measured with 15 N-NH 4 ϩ tracer additions and short-term unlabeled NH 4 ϩ additions at 12 stream sites across North America. Concurrent addition of a conservative tracer was used to account for dilution in all experiments. NH 4 ϩ uptake rate per unit area (U t) was positively correlated to nutrient concentration across all sites (r 2 ϭ 0.41, log-log relationship). Relationships between concentration and U t were used to determine whether the uptake was nonlinear (i.e., kinetic uptake primarily limited by the biotic capacity of microorganisms to accumulate nutrients) or linear (e.g., limited by mass transport into stream biofilms). In all systems, U t was lower at ambient concentrations than at elevated concentrations. Extrapolation from uptake measured from a series of increasing enrichments could be used to estimate ambient U t. Linear extrapolation of U t assuming the relationship passes through the origin and rates measured at 1 elevated nutrient concentration underestimated ambient U t by ϳ3-fold. Uptake rates were saturated under some but not all conditions of enrichment; in some cases there was no saturation up to 50 mol/L. The absolute concentration at which U t was saturated in Kings Creek varied among reaches and nutrients. Uptake rates of NH 4 ϩ at ambient concentrations in all streams were higher than would be expected, assuming U t does not saturate with increasing concentrations. At ambient nutrient concentrations in unpolluted streams, U t is probably limited to some degree by the kinetic uptake capacity of stream biota. Mass transfer velocity from the water column is generally greater than would be expected given typical diffusion rates, underscoring the importance of advective transport. Given the short-term spikes in nutrient concentrations that can occur in streams (e.g., in response to storm events), U t may not saturate, even at high concentrations.

The saturation of N cycling in Central Plains streams: 15N experiments across a broad gradient of nitrate concentrations

“…This response may differ from responses to acute pulses of NO 3 -concentration, which may not follow the same pattern observed here. Dodds et al (2002) reported a 1st order response to short NO 3 -additions on Kings Creek, while O'Brien (2006) observed saturation of NO 3 -uptake in response to short-term addition experiments in three prairie streams. Thus the response of an individual stream to temporal increases in NO 3 -may follow the Michaelis-Menten or 1st order models, but the response across streams with chronic NO 3 -inputs is best described with the Efficiency Loss model.…”

“…But phosphorus uptake or growth experiments in streams (Bothwell 1989;Mulholland et al 1990) and lakes (Dodds 1995) reveal half-saturation constants for uptake or growth for assemblages that are far greater than those exhibited by individual organisms in laboratory culture. The rates of ammonium (NH 4 + ) uptake, NO 3 -uptake, nitrification, and denitrification do not always saturate in prairie streams (Dodds et al 2002;Kemp and Dodds 2002b). Furthermore, even if short-term increases in N cause saturation, the influence of chronic N increases is not well characterized across systems.…”

“…An example is the relationship between NH 4 + uptake and concentration in streams observed by Dodds et al (2002). This model is described by a power relationship in which the exponent (or order) is less than one (U t = k · [N] m , where m < 1).…”

Longitudinal and seasonal variation of stream N uptake in an urbanizing watershed: effect of organic matter, stream size, transient storage and debris dams

“…Similar to empirical methods (e.g., Webster and Ehrman 1996), we determined S w by regressing plateau concentrations against distance, after correcting for background and dilution. A total of 6 simulations were performed with increasing levels of solute addition, from which we estimated S w at background concentration through extrapolation (similar to the approach suggested by Dodds et al 2002). Uptake rate constant (K c ), uptake rate (U) and uptake velocity (V f ) were calculated subsequently using Eq.…”

Saturation of NO3−uptake in prairie streams as a function of acute and chronic N exposure

“…However, the observed K s values for NH 4 + and NO 3 2 uptake in our study fall within the range of values that have been reported in the literature. Dodds et al (2002) reported a K s of 64 mg NH 4 + -N/L for a prairie reach of Kings Creek, whereas Kemp and Dodds (2002b) projected whole-stream K s of 12.3 mg/L for NO 3 2 and 6.7 mg/L and NH 4 + based on uptake kinetics of different substratum types within the stream. Payn et al (2005) reported K s in forested streams of 6 mg NH 4 + -N/L for Ball Creek, North Carolina (USA), and 14 mg NH 4 + -N/L for Walker Branch, Tennessee (USA).…”