Uptake Length Research Articles

Transport‐based method for estimating in‐stream nitrogen uptake at ambient concentration from nutrient addition experiments

Nutrient addition experiments are commonly used for reach‐scale quantification of in‐stream nitrogen uptake. Traditional, short‐term nutrient addition experiments are subject to nutrient saturation effects, which influence the measured uptake rates, and are not transport based, which limits their ability to distinguish between channel and hyporheic processes. We developed a transport‐based nutrient addition approach adapted to account for the effect of nutrient saturation. We incorporated the Michaelis‐Menten saturation‐limited uptake function into the One‐dimensional Transport with Inflow and Storage (OTIS) solute transport model and specifically designed the field experiments for this modified version of the model (called OTIS‐MM). Cross‐method evaluations were performed for estimates of NH4+ and NO3− uptake lengths (Sw). For NH4+, OTIS‐MM and sequential, multiple‐level traditional experiments produced almost identical Sw values (124 versus 122 m), and the slopes of the regression lines relating Sw to added concentration were similar (4% difference). For NO3−, the methods produced similar Sw values (872 versus 909 m), but the slopes of the regression lines relating Sw to added concentration were different (23% difference). We complemented the cross‐method evaluations with sensitivity analyses for both NH4+ and NO3−, which confirmed that the estimates of the OTIS‐MM uptake parameters were robust. Our field‐modeling approach can easily be designed to cover multiple, sequential reaches in one single addition; hence, it allows for increased spatial coverage.

Nitrogen uptake and denitrification in restored and unrestored streams in urban Maryland, USA

There is growing interest in rates of nitrate uptake and denitrification in restored streams to better understand the effects of restoration on nitrogen processing. This study quantified nitrate uptake in two restored and two unrestored streams in Baltimore, Maryland, USA using nitrate additions, denitrification enzyme assays, and a 15N isotope tracer addition in one of the urban restored streams, Minebank Run. Restoration included either incorporation of stormwater ponds below a storm drain and catch basins to attenuate flow or hydrologic “reconnection” of a stream channel to its floodplain. Stream restoration was conducted for restoring aging sanitary and bridge infrastructure and introducing some stormwater management in watersheds developed prior to current regulations. Denitrification potential in sediments was variable across streams, whereas nitrate uptake length appeared to be significantly correlated to surface water velocity, which was low in the restored streams during summer baseflow conditions. Uptake length of NO3−–N in Minebank Run estimated by 15N tracer addition was 556 m. Whole stream denitrification rates in Minebank Run were 153 mg NO3−–N m−2 day−1, and approximately 40% of the daily load of nitrate was estimated to be removed via denitrification over a distance of 220.5 m in a stream reach designed to be hydrologically “connected” to its floodplain. Increased hydrologic residence time in Minebank Run during baseflow likely influenced rates of whole stream denitrification, suggesting that hydrologic residence time may be a key factor influencing N uptake and denitrification. Restoration approaches that increase hydrologic “connectivity” with hyporheic sediments and increase hydrologic residence time may be useful for stimulating denitrification. More work is necessary, however, to examine changes in denitrification rates in restored streams across different seasons, variable N loads, and in response to the “flashy” hydrologic flow conditions during storms common in urban streams.

Nitrate removal in stream ecosystems measured by 15N addition experiments: Total uptake

We measured uptake length of 15NO3− in 72 streams in eight regions across the United States and Puerto Rico to develop quantitative predictive models on controls of NO3− uptake length. As part of the Lotic Intersite Nitrogen eXperiment II project, we chose nine streams in each region corresponding to natural (reference), suburban‐urban, and agricultural land uses. Study streams spanned a range of human land use to maximize variation in NO3− concentration, geomorphology, and metabolism. We tested a causal model predicting controls on NO3− uptake length using structural equation modeling. The model included concomitant measurements of ecosystem metabolism, hydraulic parameters, and nitrogen concentration. We compared this structural equation model to multiple regression models which included additional biotic, catchment, and riparian variables. The structural equation model explained 79% of the variation in log uptake length (SWtot). Uptake length increased with specific discharge (Q/w) and increasing NO3− concentrations, showing a loss in removal efficiency in streams with high NO3− concentration. Uptake lengths shortened with increasing gross primary production, suggesting autotrophic assimilation dominated NO3− removal. The fraction of catchment area as agriculture and suburban‐urban land use weakly predicted NO3− uptake in bivariate regression, and did improve prediction in a set of multiple regression models. Adding land use to the structural equation model showed that land use indirectly affected NO3− uptake lengths via directly increasing both gross primary production and NO3− concentration. Gross primary production shortened SWtot, while increasing NO3− lengthened SWtot resulting in no net effect of land use on NO3− removal.

Nitrate removal in stream ecosystems measured by 15N addition experiments: Denitrification

We measured denitrification rates using a field 15NO3− tracer‐addition approach in a large, cross‐site study of nitrate uptake in reference, agricultural, and suburban‐urban streams. We measured denitrification rates in 49 of 72 streams studied. Uptake length due to denitrification (SWdenn) ranged from 89 m to 184 km (median of 9050 m) and there were no significant differences among regions or land‐use categories, likely because of the wide range of conditions within each region and land use. N2 production rates far exceeded N2O production rates in all streams. The fraction of total NO3− removal from water due to denitrification ranged from 0.5% to 100% among streams (median of 16%), and was related to NH4+ concentration and ecosystem respiration rate (ER). Multivariate approaches showed that the most important factors controlling SWden were specific discharge (discharge / width) and NO3− concentration (positive effects), and ER and transient storage zones (negative effects). The relationship between areal denitrification rate (Uden) and NO3− concentration indicated a partial saturation effect. A power function with an exponent of 0.5 described this relationship better than a Michaelis‐Menten equation. Although Uden increased with increasing NO3− concentration, the efficiency of NO3− removal from water via denitrification declined, resulting in a smaller proportion of streamwater NO3− load removed over a given length of stream. Regional differences in stream denitrification rates were small relative to the proximate factors of NO3− concentration and ecosystem respiration rate, and land use was an important but indirect control on denitrification in streams, primarily via its effect on NO3− concentration.

Assessment of in-stream phosphorus dynamics in agricultural drainage ditches

The intensive agricultural systems in the Midwestern United States can enrich surface waters with nutrients. Agricultural drainage ditches serve as the first and second order streams throughout much of this region, as well as other highly productive agricultural areas in humid regions throughout the world. This project was conducted to evaluate in-stream processing of soluble P (SP) in agricultural drainage ditches. Soluble P injection studies were conducted at seven sites along three drainage ditches (298 to 4300 ha drainage area), and one site on a third-order stream that receives the discharge from the agricultural ditches (19,000 ha drainage area) by increasing the SP concentration in the ditch water by approximately 0.25 mg L − 1 . Sediments collected from smaller watersheds contained greater amounts of Mehlich 3 and exchangeable P (ExP), silt and clay size particles, and organic matter. Phosphorus uptake lengths ( S net) ranged from 40 to 1900 m, and SP uptake rates ( U) ranged from 0.4 to 52 mg m − 2 h − 1 . Phosphorus S net was correlated with ditch geomorphological (i.e. width) and sediment properties (i.e. organic matter, ExP, and equilibrium P concentration; r 2 = 1.00, P < 0.001), indirect drainage in the watershed ( r 2 = 0.92, P < 0.001), and the amount of small grains, forest, urban area, alfalfa and corn ( r 2 = 1.00, P < 0.0001). Agricultural drainage ditches actively process nutrients and could potentially be managed to optimize this processing to minimize SP export from these landscapes.

Nitrate retention and removal in Mediterranean streams bordered by contrasting land uses: a &amp;lt;sup&amp;gt;15&amp;lt;/sup&amp;gt;N tracer study

Abstract. We used 15N-labelled nitrate (NO3−) additions to investigate pathways of nitrogen (N) cycling at the whole-reach scale in three stream reaches with adjacent forested, urban and agricultural land areas. Our aim was to explore among-stream differences in: (i) the magnitude and relative importance of NO3− retention (i.e. assimilatory uptake) and removal (i.e. denitrification), (ii) the relative contribution of the different primary uptake compartments to NO3− retention, and (iii) the regeneration, transformation and export pathways of the retained N. Streams varied strongly in NO3− concentration, which was highest in the agricultural stream and lowest in the forested stream. The agricultural stream also showed the lowest dissolved oxygen (DO) concentration and discharge. Standing stocks of primary uptake compartments were similar among streams and dominated by detritus compartments (i.e. fine and coarse benthic organic matter). Metabolism was net heterotrophic in all streams, although the degree of heterotrophy was highest in the agricultural stream. The NO3− uptake length was shortest in the agricultural stream, intermediate in the urban stream, and longest in the forested stream. Conversely, the NO3− mass-transfer velocity and the areal NO3− uptake rate were highest in the urban stream. Denitrification was not detectable in the forested stream, but accounted for 9% and 68% of total NO3− uptake in the urban and the agricultural stream, respectively. The relative contribution of detritus compartments to NO3− assimilatory uptake was greatest in the forested and lowest in the agricultural stream. In all streams, the retained N was rapidly regenerated back to the water column. Due to a strong coupling between regeneration and nitrification, most retained N was exported from the experimental reaches in the form of NO3−. This study provides evidence of fast in-stream N cycling, although the relative importance of N retention and removal varied considerably among streams. Results suggest that permanent NO3− removal via denitrification may be enhanced over temporary NO3− retention via assimilatory uptake in heterotrophic human-altered streams characterized by high NO3− and low DO concentrations.

Modeling Phosphorus in the Lake Allatoona Watershed Using SWAT: I. Developing Phosphorus Parameter Values

Lake Allatoona is a large reservoir north of Atlanta, GA, that drains an area of about 2870 km2 scheduled for a phosphorus (P) total maximum daily load (TMDL). The Soil and Water Assessment Tool (SWAT) model has been widely used for watershed-scale modeling of P, but there is little guidance on how to estimate P-related parameters, especially those related to in-stream P processes. In this paper, methods are demonstrated to individually estimate SWAT soil-related P parameters and to collectively estimate P parameters related to stream processes. Stream related parameters were obtained using the nutrient uptake length concept. In a manner similar to experiments conducted by stream ecologists, a small point source is simulated in a headwater sub-basin of the SWAT models, then the in-stream parameter values are adjusted collectively to get an uptake length of P similar to the values measured in the streams in the region. After adjusting the in-stream parameters, the P uptake length estimated in the simulations ranged from 53 to 149 km compared to uptake lengths measured by ecologists in the region of 11 to 85 km. Once the a priori P-related parameter set was developed, the SWAT models of main tributaries to Lake Allatoona were calibrated for daily transport. Models using SWAT P parameters derived from the methods in this paper outperformed models using default parameter values when predicting total P (TP) concentrations in streams during storm events and TP annual loads to Lake Allatoona.

Net Changes in Antibiotic Concentrations Downstream from an Effluent Discharge

Many studies have shown the occurrence of antibiotics and degradation products in streams; however, relatively little work has applied a functional perspective to antibiotic transport and uptake. This study examined net changes in antibiotic concentrations downstream from a wastewater treatment plant (WWTP) effluent discharge and estimated net uptake length (Snet), net uptake velocity (v(f-net)), and net areal uptake rate (Unet) of antibiotics over a 3-km stream reach at Mud Creek, northwest Arkansas, USA, during June, September, and December 2006. Ten antibiotics and one degradation product (azithromycin, ciprofloxacin, erythromycin, erythromycin-H2O, ofloxacin, sulfachloropyridazine, sulfadimethoxine, sulfamethoxazole, tetracycline, oxytetracycline, and trimethoprim) were found at least once at Mud Creek downstream from the effluent discharge. All chemicals persisted in measurable concentrations in the water column over the 3-km stream reach, and we observed significant net retention of some antibiotics and one degradation product across the sampling events. Antibiotics that were significantly retained traveled kilometer-scale distances (Snet: 1.8 to 51.5 km) with relatively low uptake velocities (v(f-net): 1.6 to 33.9 x 10(-6) m s(-1)) and rates (Unet: 0.01 to 38.4 x 10(-6) microg m(-2) s(-1)). This study illustrates that some antibiotics do not travel conservatively in streams and that uptake processes occur over the scale of kilometers, linking upstream effluent sources to downstream processing over large spatial scales.

Nitrogen cycling and metabolism in the thalweg of a prairie river

Nutrient dynamics in rivers are central to global biogeochemistry. We measured ammonium (NH4+) uptake, metabolism, nitrification, and denitrification in the thalweg, the river region of greatest flow, of the Kansas River (discharge = 14,360 L/s). We estimated gross and net uptake with a depleted 15N‐NH4+ release, metabolism with diel O2 measurements, and denitrification with dissolved N2 measurements. Net ecosystem production was negative. Net NH4+ uptake length was 2.1 km when concentrations were elevated, and gross uptake length was 1.9 km at ambient concentrations. Gross uptake rate measurements were comparable to estimates made using extrapolations from data obtained from streams (systems with 1/10th or less the discharge). Calculated lengths were maximal because the isotope pulse was primarily confined to the thalweg and not the shallow side channels or backwaters. Denitrification and nitrification rates were below detection. In the Kansas River, rates of N cycling are driven by heterotrophic processes, and considerable processing of N, particularly NH4+ uptake, occurred over a few kilometers of river length, with net uptake rates of NH4+ increasing with greater NH4+ concentrations.

Functional measures and food webs of high elevation springs in the Swiss alps

We examined the ecosystem functioning and food webs of high elevation springs in or near the Swiss National Park. Springs originated from silicate or carbonate geologies and were near or above treeline. One iron-sulphur and three temporary springs were also included in the study. Ecosystem function was assessed in four springs via measures of bacterial abundance, sediment respiration, nutrient uptake, and ecosystem metabolism. Food webs were assessed in all 20 springs using nutrient content (C, N, P) and stable isotopes of carbon and nitrogen. Bacteria counts ranged from 1.8 to 3.4×108 cells mL−1 sediment with no significant differences between sites. Sediment respiration rates ranged from 0.13 to 0.46 mg O2 h−1 and did not differ between springs. Uptake lengths for N ranged from 11 to 63 m, and for phosphorus from 4 to 60 m. Nitrogen uptake rates (U) ranged from 57 to 266 μg m−2 h−1 and those for P from 0.11 to 4.2 μg m−2 h−1. Gross primary production (GPP) ranged from 0.7 to 7.1 g O2 m−2 d−1, but reached values of 59.1–70.9 g O2m−2 d−1 in the iron-sulphur spring. Ecosystem respiration (ER) ranged from 1.3 to 10.3 g O2m−2 d−1, but was 91.1–101.8 g O2m−2 d−1 in the ironsulphur spring. All four springs were net-heterotrophic with production to respiration ratios (P/R) ranging from 0.48 to 0.72. The percentage C, N, and P varied significantly among the different food web compartments. The molar ratios (C:P,C:N,N:P) of the different nutrients were consistent within compartments, although varying among the different compartments. Stable isotope signatures (δ13C, d15N) were related to the specific spring types, although food webs were relatively simple with most benthic invertebrates showing omnivory. Riparian spiders partially used aquatic insects in the diet. The results suggest that these alpine springs are complex but functionally similar to forested headwater streams with simple food webs.

NO EVIDENCE OF GENETIC ASSOCIATION BETWEEN DRD3 VARIANTS AND TARDIVE DYSKINESIA IN KOREAN SCHIZOPHRENIC PATIENTS

Stream nutrient uptake is an important ecosystem service, because it supplies human needs such as water purification. Uncertainty in nutrient uptake measurement determines, in combination with effect magnitude, whether effects of land use, stream restoration or climate change can be detected by this method. However, measurement uncertainty for nutrient uptake metrics is rarely reported. Here, we compared whole-stream phosphate and nitrate uptake among three pristine and three agricultural tropical Cerrado streams. We quantified uncertainty in uptake metrics estimated by kinetic nutrient-addition experiments and evaluated their potential to detect land-use effects. Ambient phosphate and nitrate uptake lengths (SW) ranged from 11 to 106 m and from 29 to 357 m, respectively, in pristine streams and from 41 to 586 m and from 76 to 1372 m in agricultural streams. Moderate measurement uncertainty in SW allowed for the detection of land-use impacts with this metric. However, derived nutrient uptake metrics, such as ambient uptake rates and uptake velocities, as well as parameters of whole-stream saturation kinetics, were associated with high measurement uncertainties that prevented the detection of potential land-use effects. Sensitivity analyses suggested that avoiding high enrichment levels in kinetic addition experiments and increasing sampling effort for plateau nutrient and tracer concentration at the first and last sampling stations of investigated stream reaches are the most promising strategies to reduce measurement uncertainty. Analyzing nutrient uptake without considering measurement uncertainty might lead to poor interpretation in case studies and meta-analyses, such as incorrect evaluations of human impacts or comparisons among systems.

Effects of residence time on summer nitrate uptake in mississippi river flow‐regulated backwaters

AbstractNitrate uptake may be improved in regulated floodplain rivers by increasing hydrological connectivity to backwaters. We examined summer nitrate uptake in a series of morphologically similar backwaters on the Upper Mississippi River receiving flow‐regulated nitrate loads via gated culverts. Flows into individual backwaters were held constant over a summer period but varied in the summers of 2003 and 2004 to provide a range of hydraulic loads and residence times (τ). The objectives were to determine optimum loading and τ for maximum summer uptake. Higher flow adjustment led to increased loading but lower τ and contact time for uptake. For highest flows, τ was less than 1 day resulting in lower uptake rates (Unet &lt; 300 mg m−2 day−1), low uptake efficiency (U% &lt; 20%) and a long uptake length (Snet &gt; 4000 m). For low flows, τ was greater than 5 days and U% approached 100%, but Unet was 200 mg m−2 day−1. Snet was &lt; half the length of the backwaters under these conditions indicating that most of the load was assimilated in the upper reaches, leading to limited delivery to lower portions. Unet was maximal (384–629 mg m−2 day−1) for intermediate flows and τ ranging between 1 and 1.5 days. Longer Snet (2000–4000 m) and lower U% (20–40%) reflected limitation of uptake in upper reaches by contact time, leading to transport to lower reaches for additional uptake. Uptake by ∼10 000 ha of reconnected backwaters along the Upper Mississippi River (13% of the total backwater surface area) at a Unet of ∼630 mg m−2 day−1 would be the equivalent of ∼40% of the summer nitrate load (155 mg day−1) discharged from Lock and Dam 4. These results indicate that backwater nitrate uptake can play an important role in reducing nitrate loading to the Gulf of Mexico. Copyright © 2008 John Wiley &amp; Sons, Ltd.

An empirical regression model of soluble phosphorus retention for small pristine streams evaluating tracer experiments

Models of in-stream phosphorus retention either lack high spatial and temporal resolution, or need a high number of input parameters.We provide a simple new approach that deals with these deficits. Soluble reactive phosphorus (SRP) tracer studies based on the nutrient spiralling concept were evaluated to derive a simple model that explains SRP retention. SRP uptake length (SRP-Sw) was considered to be a measure of transient SRP storage and was transformed to load-weighted retention (R) using an exponential relationship. Stream order (so) and flow velocity (u) were considered as input parameters to explain SRP uptake length. Model validation showed significant correlation with measured uptake lengths. The model explained 46% of SRP retention, and simulated and measured retention were in the same order of magnitude. Our model may act in concert with emission models to account for lateral SRP sources within the catchment. Although our empirical model does not describe biological processes and is not a substitute for detailed biogeochemical studies, it provides an efficient tool to predict load-weighted soluble nutrient retention and nutrient transport to downstream systems and is applicable in most small pristine streams.

Combined effects of leaf litter inputs and a flood on nutrient retention in a Mediterranean mountain stream during fall

This study examined the effect of increasing in‐channel leaf standing stocks on hydrologic transient storage and nutrient retention in a Mediterranean mountain stream. A flood at the end of the leaf fall period provided the opportunity to examine the effect of abrupt removal of much of the leaf material. Twenty‐one chloride additions were performed from October to December 2004. In 13 of these, we also added ammonium and phosphate to estimate nutrient uptake lengths and uptake velocities to assess nutrient retention. The one‐dimensional transport with inflow and storage (OTIS) model was used to estimate transient water storage parameters. Although discharge remained constant during leaf fall, water residence time increased because of in‐channel litter accumulation, as did nutrient uptake velocity. Flooding reduced leaf benthic standing stocks by 65% and dramatically altered hydraulic and nutrient retention properties of the channel. After recession, the stream rapidly recovered in terms of nutrient retention, especially for phosphate. Abrupt changes in discharge under flood conditions largely determined the variability in stream nutrient retention. However, leaf litter inputs played an important role in nutrient dynamics during constant flow. Because both the flood regime and the timing of leaf fall are being regionally altered by climate change, our results have implications for stream nutrient dynamics under climate change scenarios.

Whole-stream phosphorus cycling: Testing methods to assess the effect of saturation of sorption capacity on nutrient uptake length measurements

The cycling rate of nutrients such as phosphorus (P) is a fundamental parameter in stream ecology. In whole-stream ecosystem experiments, cycling rates are often assessed using continuous short-term nutrient addition studies. While several simplifying assumptions are generally recognised, these are rarely, if ever, fully tested under field conditions. One principal assumption is that uptake (sorption) processes do not become saturated during periods of nutrient addition, which is perhaps questionable from laboratory studies of soluble reactive phosphorus (SRP) sorption kinetics. Three approaches were developed and tested, which bridged the gap between laboratory-based net (sum of uptake and release) sorption kinetics and whole-stream assessments of P uptake. These were applied to a short-term (three times mean travel time of water in the studied reach) whole-stream multiple-rate P addition. The results were then tested independently with a whole-stream long-term (15 times mean travel time) SRP addition. The net sorption kinetics were not altered during the short-term addition with low SRP additions, 9–16μgL−1 (two to three times the ambient concentration). Although this may not be the case at higher added concentrations (as possibly hinted at five times the ambient concentration), the long-term addition showed no change in P uptake length with a P addition (39μgL−1) 16 times higher than the ambient concentration.

Untangling the complex issue of dissolved organic carbon uptake: a stable isotope approach

Summary1. We estimated uptake of stream water dissolved organic carbon (DOC) through a whole‐stream addition of a 13C‐DOC tracer coupled with laboratory measurements of bioavailability of the tracer and stream water DOC.2. The tracer, a leachate of 13C‐labelled tree tissues, was added to the head waters of White Clay Creek, Pennsylvania, U.S.A., over a 2‐h period and followed 1.27 km downstream to generate mass transfer coefficients for DOC lability classes within the tracer.3. From the longitudinal 13C uptake curve, we resolved labile and semi‐labile DOC classes within the 13C‐DOC tracer comprising 82% and 18% of the tracer respectively.4. Plug‐flow laboratory bioreactors colonized and maintained with stream water were used to determine the concentration of stream water DOC fractions that had a similar lability to the labile and semi‐labile classes within the tracer and we assumed that stream water DOC and tracer DOC with comparable lability fractions in the bioreactors behaved similarly in the stream, i.e. they had the same mass transfer coefficients.5. A small fraction (8.6%) of the stream water DOC was labile, travelling 238 m downstream before being taken up. The remaining bioavailable stream water DOC was semi‐labile and transported 4.5 km downstream before being taken up. These uptake lengths suggest that the labile DOC is an energy source within a stream reach, while the semi‐labile DOC is exported out of the reach to larger rivers and the downstream estuary, where it may provide energy for marine microbial communities or simply be exported to the oceans.

Contribution of sediment fluxes and transformations to the summer nitrogen budget of an Upper Mississippi River backwater system

Routing nitrate through backwaters of regulated floodplain rivers to increase retention could decrease loading to nitrogen (N)-sensitive coastal regions. Sediment core determinations of N flux were combined with inflow-outflow fluxes to develop mass balance approximations of N uptake and transformations in a flow-controlled backwater of the Upper Mississippi River (USA). Inflow was the dominant nitrate source ((95%) versus nitrification and varied as a function of source water concentra- tion since flow was constant. Nitrate uptake length increased linearly, while uptake velocity decreased linearly, with increasing inflow concentration to 2m g l -1 , indicating limitation of N uptake by loading. N saturation at higher inflow concentration coincided with maximum uptake capacity, 40% uptake efficiency, and an uptake length 2 times greater than the length of the backwater. Nitrate diffusion and denitrification in sediment accounted for 27% of the backwater nitrate retention, indicating that assimilation by other biota or denitrification on other substrates were the dominant uptake mecha- nisms. Ammonium export from the backwater was driven by diffusive efflux from the sediment. Ammo- nium increased from near zero at the inflow to a maximum mid-lake, then declined slightly toward the outflow due to uptake during transport. Ammonium export was small compared to nitrate retention.

Nitrogen spiraling in streams: Comparisons between stable isotope tracer and nutrient addition experiments

A common method to quantify stream nutrient uptake length is to enrich the stream at an upstream point and monitor the downstream decline in the concentration of the added nutrient. However, increasing the nutrient concentration of the stream alters nutrient uptake, and uptake length quantified using this amendment approach does not necessarily reflect uptake length under ambient conditions. We conducted a series of 15NO3− tracer and 14NO3− nutrient amendment experiments to (1) assess the overestimation of amendment‐derived uptake length in streams spanning a gradient of ambient nitrogen concentration, and (2) evaluate a technique to estimate ambient uptake length using data from multiple amendments conducted at different concentrations. Single amendment‐derived uptake lengths were consistently longer than ambient uptake lengths quantified using the stableisotope tracer. The ratio of single amendment‐derived to ambient uptake length ranged more than sixfold across streams but was not related to ambient stream water nitrogen concentration or amendment concentration. Estimated ambient uptake length calculated from the multiple amendment technique was a better predictor of ambient uptake length than single amendment‐derived uptake length in three of five experiments. However, with this technique, calculated uptake length was negative in two streams. Our results suggest that nitrogen limitation was weak in these two streams and that the multiple amendment technique may be ineffective in streams where the nutrient of study is not limiting.

Nutrient Uptake in a Large Urban River1

Abstract: Small streams have been shown to be efficient in retaining nutrients and regulating downstream nutrient fluxes, but less is known about nutrient retention in larger rivers. We quantified nutrient uptake length and uptake velocity in a regulated urban river to determine the river’s ability to retain nutrients associated with wastewater treatment plant (WWTP) effluent. We measured net uptake of soluble reactive phosphorus (SRP), dissolved organic phosphorus, ammonium (NH4), nitrate, and dissolved organic nitrogen in the Chattahoochee River, Atlanta, GA by following the downstream decline of nutrients and fluoride from WWTP effluent on 10 dates under low flow conditions. Uptake of all nutrients was sporadic. On many dates, there was no evidence of measurable nutrient uptake lengths within the reach; indeed, on several dates release of inorganic N and P within the sample reach led to increased nutrient export downstream. When uptake occurred, SRP uptake length was negatively correlated with total suspended solids and temperature. Uptake velocities of SRP and NH4 in the Chattahoochee River were lower than velocities in less‐modified systems, but they were similar to those measured in other WWTP impacted systems. Lower uptake velocities indicate a diminished capacity for nutrient uptake.

The effect of transient storage on nitrate uptake lengths in streams: an inter‐site comparison

AbstractSurface water–groundwater interaction in the hyporheic zone may enhance biogeochemical cycling in streams, and it has been hypothesized that streams exchanging more water with the hyporheic zone should have more rapid nitrate utilization. We used simultaneous conservative solute and nitrate addition tracer tests to measure transient storage (which includes hyporheic exchange and in‐stream storage) and the rate of nitrate uptake along three reaches within the Red Canyon Creek watershed, Wyoming. We calibrated a one‐dimensional transport model, incorporating transient storage (OTIS‐P), to the conservative solute breakthrough curves and used the results to determine the degree of transient storage in each reach. The nitrate uptake length was quantified from the exponential decrease in nitrate concentration with distance during the tracer tests. Nitrate uptake along the most downstream reach of Red Canyon Creek was rapid (turnover time K−1c = 32 min), compared with nitrate uptake reported in other studies (K−1c = 12 to 551 min), but other sites within the watershed showed little nitrate retention or loss. The uptake length Sw‐NO−3 for the most downstream reach was 500 m and the mass transfer coefficient Vf‐NO−3 was 6·3 m min−1. Results from 15 other nitrate‐addition tracer tests were used to create a regression model relating transient storage and measures of stream flow to nitrate uptake length. The model, which includes specific discharge and transient storage area, explains almost half the variability in nitrate uptake length (adjusted R2 = 0·44) and is most effective for comparing sites with very different stream characteristics. Although large differences in specific discharge and storage zone area explain inter‐site differences in nitrate uptake, other unmeasured variables, such as available organic carbon and microbial community composition, are likely important for predicting differences in nitrate uptake between sites with similar specific discharge rates and storage zone areas, such as when making intra‐site comparisons. Copyright © 2007 John Wiley &amp; Sons, Ltd.