Uptake Length Research Articles

The seasonal and spatial variability of ammonium uptake in a hilly watershed

Elevated nutrient loading can cause deleterious impacts on aquatic ecosystems such as eutrophication. Seasonal variability and land use change often lead to varied nutrient uptake from streams. However, the impacts of seasonal and spatial variation on stream nutrient transport within the same watershed haven’t been fully understood. Here, we conducted nutrient addition experiments using the Tracer Additions for Spiraling Curve Characterization (TASCC) approach within the Xiaogang Watershed, Zhejiang Province, China. Six of the experiments were conducted in one stream every other month and eight releases in eight different streams to quantify ammonium uptake kinetics across different seasons and land uses. Our findings suggest that the uptake capability increases with both discharge and ambient concentration (C0): seasonal variability of discharge shows small impacts on uptake metrics, except the ambient uptake length (Sw-amb), which increases with discharge; while the change in C0 could have more significant effects on both ambient areal uptake (Uamb) and maximum areal uptake rate parameter (Umax). Downstream the river network, the increase in discharge led to a significant increase in Uamb, ambient uptake velocity (Vf-amb), and Umax. On the other hand, the change in C0 is less influential than discharge along the river network, which may be correlated with the hydraulic geometry. Additionally, there is an optimal temperature (T) for uptake in our study region around 17°C, which may be explained by the growth of chlorophyll a. The positive correlation between Vf-amb and Q at the spatial scale may be attributed to the increase of dissolved organic carbon (DOC) and river chlorophyll a downstream with stream width. In addition, land use had an important effect on ion concentration in streams, and the proportion of agricultural land was positively correlated with nutrient concentration. Our findings could help provide scientific support for land use management and water quality regulation.

Importance of Dense Aquatic Vegetation in Seasonal Phosphate and Particle Transport in an Agricultural Headwater Stream

AbstractAgricultural headwater streams and ditches commonly host dense stands of aquatic vegetation that grow and decay over seasons and exert physical and biological controls on the transport of nutrients from cropland to larger rivers. This study examined changes in the transport of phosphorus (P) in an agricultural drainage ditch in the Maumee River Basin (Ohio, USA) by conducting constant rate injections of a novel tracer mixture (conservative salt [Cl as NaCl], dissolved P [KH2PO4], and a fluorescent fine particle) in spring, summer, and fall. We quantified transport behavior for solutes and particles using a traditional transient storage modeling framework consisting of mobile and immobile storage zones connected by a first‐order exchange rate constant. Transient storage was greatest during the spring, when thicker vegetation caused more pooling and flow stagnation, and decreased through fall, as vegetation thinned. Soluble P uptake lengths were 8.7 times longer in fall than spring, likely due to declines in biological uptake rates with colder temperatures and immobile zone storage with thinning vegetation. Particle capture lengths also decreased by a factor of 4.3 from fall to spring. With the increasing eutrophication of Lake Erie and waterbodies around the world that lie downstream from agricultural landscapes, it is beneficial to understand nutrient transport across watersheds, including small agricultural streams. This study highlights the physical and biological roles that aquatic vegetation plays in small agricultural streams by creating seasonally variable immobile zones that slow the flow of nutrients, providing surface area for biofilms, and capturing particles that bind nutrients.

Nutrient uptake in tropical rivers receiving wastewater treatment plant discharge: High mass removal but low nutrient uptake efficiencies

Wastewater treatment plants (WWTPs) have eliminated many problems related to sewage inputs to aquatic systems. However, the treated effluents still affect river and stream ecosystems. Yet, very limited information is available about the fate of treated effluents in tropical receiving water bodies. Here, we examined the longitudinal changes in ammonium (NH4-N), nitrate (NO3-N), and soluble reactive phosphorus (SRP) concentrations in reaches downstream from chronic WWTP inputs in three receiving rivers in São Paulo state (Southeastern Brazil). The studied WWTPs were not designed or operated to have specific tertiary treatment for nutrient removal at the time of the study. We conducted a total of five sampling campaigns between 2019 and 2021. We applied the nutrient spiraling approach to these high-magnitude nutrient additions to examine net nutrient uptake. Changes in nutrient loads along the reaches were also examined. Overall, nutrient concentrations and loads were considerably increased by WWTP discharges. Net uptake lengths (SW-net) for NO3-N, NH4-N, and SRP ranged from 1.2 to 13.6, 1.5–infinity (uptake coefficient < 0), and 1.2–7.5 km, respectively. Net uptake velocities (Vf-net) varied between 0.7–2.5, −0.4 to 4.1, and 0.2–10.2 mm/min, respectively. Areal net uptake rates (Unet) ranged from 0.78 to 10.7, −20.08–45.8, and 0.18–4.4 g/m2/min for NO3-N, NH4-N, and SRP, respectively. High export (long SW-net) and limited net uptake along the reaches (low Vf-net) indicated that nutrients were transported downstream for long distances without efficient removal (nutrient availability higher than demand) despite the high mass removal (high Unet). These results suggest that the export of nutrients is substantial, potentially creating water quality impairments to downstream ecosystems. Our study highlighted that WWTPs with limited nutrient removal have considerable effects on water chemistry, nutrient cycling and loading in tropical receiving freshwaters.

Comparing spiraling‐ and transport‐based approaches to estimate in‐stream nutrient uptake length from pulse additions

AbstractThe distance that a nutrient travels as a solute before its removal from the stream water column is known as the uptake length (SW), which is a functional indicator of environmental quality and integrity. Among nutrient enrichment methods, instantaneous nutrient addition (e.g., slug or pulse) have been proposed as an alternative to plateau and labeled nutrient approaches. Two approaches have been commonly used to estimate SW and its associated metrics (i.e., areal uptake rate, U; and uptake velocity, Vf) from pulse additions: the spiraling approach, based on the longitudinal variation in nutrient concentrations, and the transport modeling approach, based on the advective and dispersive transport of solutes. However, little is known in how the choice of such analytical methods impacts the estimation of stream uptake parameters and the conclusions we draw from them. Here, we estimated the SW and Vf of ammonium‐nitrogen (NH4‐N) and soluble reactive phosphorus (SRP) from 16 pulsed additions conducted in four low‐order streams in southeastern Brazil. We compared metrics estimated by the Tracer Additions for Spiraling Curve Characterization (TASCC) and the One‐Dimensional Transport with Inflow and Storage (OTIS) methods, based on the spiraling‐ and transport‐based approaches, respectively. The TASCC:OTIS SW ratio averaged 0.71 for NH4‐N and 1.01 for SRP, whereas the mean of TASCC:OTIS Vf ratio was 2.04 for NH4‐N and 1.03 for SRP. The results showed that both SW and Vf estimates differed significantly between methods for NH4‐N, but no statistical differences were observed in SRP estimates. In our study, we highlighted the significant effects of transient storage and variable nutrient concentration on pulsed enrichments. Such information should be considered when choosing which method is appropriate to use for a particular site. Differences between modeling approaches must be addressed when comparing methods to expand our knowledge on broad temporal and spatial patterns of in‐stream nutrient uptake.

Excretion from the benthic macrofauna covers little of spring nutrient uptake in a small Danish forest stream

ABSTRACT Freshwater consumers play a role in ecosystem nutrient cycling and dynamics by excreting metabolic waste products containing ammonium (NH4) and phosphate (PO4). In this study we quantified excretion of a selection of macroinvertebrate taxa and measured nutrient uptake metrics in a small Danish forest stream. Macroinvertebrate excretion was measured in situ from February to April 2019, and uptake metrics were measured using short-term nutrient addition experiments on 2 dates in March and June. To calculate area-specific excretion, quantitative benthic samples were taken on the 3 dominating substratum types: sand, gravel-pebbles, and leaf packs. No significant temperature dependency of invertebrate excretion was found over the measured range (0.3–10.3 °C). Average NH4-N excretion rates ranged from 0.027 (Ptychoptera spp.) to 0.097 µg mg−1 h−1 (Paraleptophlebia submarginata) while PO4-P excretion ranged from 0.0077 (Ptychoptera spp. and Nemoura spp.) to 0.0261 µg mg−1 h−1 (P. submarginata). The uptake length increased from 75 to 117 m for NH4 and from 28 to 71 m for PO4 from March to June. Uptake velocity, by contrast, decreased from March (0.63 m h−1 for NH4 and 1.72 m h−1 for PO4) to June (0.13 m h−1 for NH4 and 0.22 m h−1 for PO4). The estimated contribution from excretion from the entire benthic macrofauna to whole-stream net uptake was 3.4% and 8.9% of NH4 and 0.5% and 2.0% of PO4 in March and June, respectively, suggesting that excretion from benthic macroinvertebrates plays a minor role in meeting whole-stream nutrient demand.

Evaluating Instream Restoration Effectiveness in Reducing Nitrogen Export from an Urban Catchment with a Data‐Model Approach

AbstractUrbanization increases stormwater runoff into streams, resulting in channel erosion, and increases in sediment and nutrient delivery to receiving water bodies. Stream restoration is widely used as a Best Management Practice to stabilize banks and reduce sediment and nutrient loads. While most instream nutrient retention measurements are often limited to low flow conditions, most of the nutrient load is mobilized at high stream flows in urban settings. We, therefore, use a process‐based stream ecosystem model in conjunction with measurements at low flows and focus on estimation of stream nitrogen retention over the full streamflow distribution. The model provides a theoretical framework to evaluate the geomorphic, hydrologic, and ecological factors that are manipulated by stream restoration, and drive nitrogen retention. We set a model for a pool‐riffle sequence restored stream (190 m) in Baltimore County, Maryland and calibrated the model to the in situ measured primary production (Nash–Sutcliffe model efficiency coefficient [NSE] NSE = 0.89), respiration (NSE = 0.74), and nitrate uptake lengths (R2 = 0.88). At the daily scale, simulations showed low nitrogen retention during high flows due to high transport rates, mobilization of stored hyporheic nitrogen, and scouring of periphyton biomass. This result underscores the need to reduce contributing watershed runoff flashiness to promote aquatic nutrient cycling and retention. At monthly and yearly time scale, model predicted a higher percent reduction in summer than in winter and estimated 5.7%–9.5% of annual nitrate reductions. While the model was tested in a pool‐riffle sequence restoration design, the approach can be adapted to evaluate a range of channel restoration design characteristics, and the effects of upland watershed restoration to mitigate stormwater loading through both restored and unrestored streams.

Nitrate uptake enhanced by availability of dissolved organic matter in tropical montane streams

Tropical forests store large amounts of Earth’s terrestrial C, but many tropical montane streams have low dissolved organic matter (DOM). This low availability of energy likely limits certain pathways of inorganic N uptake, as evidenced by the high rates of nitrification and predominance of nitrate (NO3−) in the total pool of dissolved N seen in many tropical montane forests. To explore the influence of DOM availability on tropical stream N cycling, we performed nutrient pulse additions of NO3− with or without an added C source (acetate or urea) in streams of the Luquillo Experimental Forest, Puerto Rico. In the absence of added DOM, NO3− uptake was either undetectable or had very long (>3000 m) uptake lengths (Sw). When DOM was added with NO3−, Sw values for NO3− were much shorter (97–1500 m), with the shortest lengths resulting from additions of acetate. Comparing uptake metrics of the added C sources, there was greater demand for acetate compared to urea, and measurable urea uptake was detected much less frequently. During NO3−-only additions, ambient concentrations of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) decreased in some cases, suggesting increased metabolic demand for energy from the ambient organic matter pool under elevated levels of inorganic nutrients. Collectively, these results demonstrate that pathways of inorganic N cycling are tightly tied to energy availability at this tropical site. The response of ambient DOC and DON to increases in NO3− concentrations points to important feedbacks between inorganic N and DOM including organic N. Understanding the controls on NO3− processing in these streams is important to predicting network-scale exports of N from tropical ecosystems.

Effects of plant morphology on root–soil hydraulic interactions ofSchefflera heptaphylla

Root water uptake induces additional soil suction by soil water extraction. However, induced soil suction restricts root water uptake when beyond a threshold (st) and tends to stop water uptake at a limiting suction (sl). This process is called root–soil hydraulic interactions, whose relationship with plant morphology (e.g., plant height, leaf area, and root length) is unclear. This study aims to investigate the effects of plant morphology on response of the sink term (water uptake intensity; WUI) to soil suction for Schefflera heptaphylla. Laboratory tests were conducted on 26 individuals in five different height groups. A new empirical model was derived to consider effects of plant morphology on root water uptake distribution in the full soil suction range. The locations of maximum root length density (RLD) and WUI for taller individuals were relatively deeper and farther from centreline. Before soil suction reached st, water uptake length ability (WULA) remained at a maximum level, positively related to LR ratio (ratio between leaf area, LA, and root length, RL), but less affected by plant height. For taller plants, the decreasing rate of stwith LR ratio was larger, while the decreasing rate of WULA with soil suction beyond stappeared smaller. slappeared independent of height, LA, and RL.

Ecosystem responses to channel restoration decline with stream size in urban river networks.

Urban streams are often severely impaired due to channelization, high loads of nutrients and contaminants, and altered land cover in the watershed. Physical restoration of stream channels is widely used to offset the effects of urbanization on streams, with the goal of improving ecosystem structure and function. However, these efforts are rarely guided by strategic analysis of the factors that mediate the responsiveness of stream ecosystems to restoration. Given that ecological gradients from headwater streams to mainstem rivers are ubiquitous, we posited that location within a river network could mediate the benefits of channel restoration. We studied existing stream restorations in Milwaukee, Wisconsin, to determine (1) whether restorations improve ecosystem function (e.g., nutrient uptake, whole-stream metabolism) and (2) how ecosystem responses vary by position in the urban river network. We quantified a suite of physicochemical and biological metrics in six pairs of contiguous restored and concrete channel reaches, spanning gradients in baseflow discharge (19-196L/s) and river network position (i.e., headwater to mainstem). Hydrology differed dramatically between the restored and concrete reaches; water velocity was reduced 2- to 13-fold while water residence time was 50-5,000% greater in adjacent restored reaches. Restored reaches had shorter nutrient uptake lengths for ammonium, nitrate, and phosphate, as well as higher whole-stream metabolism. Furthermore, the majority of reaches were autotrophic (i.e., gross primary production>ecosystem respiration), which is not common in stream ecosystems. The difference in ecosystem functioning between restored and unrestored reaches was generally largest in headwaters and declined to equivalence in mainstem restorations. Our results suggest that headwater sites offer higher return on investment compared to larger downstream channels, where ecosystem responsiveness is low. If this pattern proves to be general, the scaling of ecosystem responses with river size could be integrated into planning guidelines for urban stream restorations to enhance the societal and ecological benefits of these expensive interventions.

Pulse of dissolved organic matter alters reciprocal carbon subsidies between autotrophs and bacteria in stream food webs

AbstractSoils are currently leaching out dissolved organic matter (DOM) at an increasing pace due to climate and land use change or recovery from acidification. The implications for stream biogeochemistry and food webs remain largely unknown, notably the metabolic balance (biotic CO2 emissions) and carbon cycling between autotrophs and bacteria. We increased by 12% the flux of DOM in a stream for three weeks to mimic a pulse of natural DOM supply from soils rich in organic matter. We were able to track its fate into the food web through the use of a before and after control impact experimental design and the addition of DOM with a distinctive δ13C signature. We used whole‐stream metabolism to quantify carbon fluxes. Both photosynthesis and heterotrophic respiration increased rapidly following C addition, but this was short lived, likely due to nutrient limitations. Carbon exchange between autotrophs and bacteria in the control stream accounted for about 49% of bacterial production and 37% of net primary production, under stable flow conditions. Net primary production relied partly (19% in the control) on natural allochthonous dissolved organic carbon via the CO2 produced by bacterial respiration, intermingling the green and brown webs. The preferential uptake of labile carbon by bacteria and excess bacterial CO2 relative to nutrients (N, P) for autotrophs shifted the reciprocal carbon exchange between bacteria and autotrophs to a predominantly one‐way carbon flow from bacteria to autotrophs, increasing the C:N:P molar ratios of autotrophs, the latter likely to become less palatable to consumers. The bacterial response to sucrose addition shifted the metabolic balance toward heterotrophy increasing biotic CO2 emissions (+125%), shortened the average distance travelled by a molecule of organic matter (−40%), and thus provided less organic matter of lower quality for downstream ecosystems. Even a small increase in labile dissolved organic matter supply due to climate and land use change could significantly alter in‐stream carbon cycling, with large effects on stream food web and biogeochemistry in small streams draining catchments with soils rich in organic carbon.

Fate of bioavailable nutrients released to a stream during episodic effluent releases from a municipal wastewater treatment lagoon.

Municipal wastewater lagoons are common across North America and, unlike larger mechanical wastewater treatment plants, typically release nutrient-rich effluent directly to rivers in intermittent pulses. However, little is known about the fate of nutrients from these episodic events, which may happen under varying hydrologic or thermal conditions. We assessed fate of nitrogen (N) and phosphorus (P) from lagoon effluent during three releases to Deadhorse Creek, Manitoba, Canada. Using net nutrient uptake lengths and natural abundance stable isotope ratios of dissolved inorganic nitrogen (DIN) and primary producers, we found that DIN was processed during the summer releases though the dominant mechanism was unclear. However, nitrate was largely exported in autumn. Primary producers assimilated lagoon N but did not appear to reduce DIN concentrations. The longitudinal pattern of soluble reactive phosphorus (SRP) varied between releases and in summer 2019 the stream became a net source of SRP despite concomitant processing of DIN. We hypothesize that low demand for P in Deadhorse Creek, as suggested by upstream SRP > 0.05 mg P L-1, and nutrient ratios indicative of N limitation, reduced instream processing of P. Furthermore, our results indicated that cool or high flow conditions may result in the export of much of the lagoon nutrient load downstream. Our findings suggest the processes that transform wastewater nutrients are overwhelmed during effluent releases. Managers should consider increasing effluent dilution via continuous release of effluent rather than pulsed delivery. However, management of upstream nutrient supply may also be needed when relying upon the self-purifying capacity of rivers.

Agriculture influences ammonium and soluble reactive phosphorus retention in South American headwater streams

AbstractAgricultural activities can affect the delivery of nutrients to streams, riparian canopy cover, and the capacity of aquatic systems to process nutrients and sediments. There are few measures of nutrient uptake and metabolism from tropical or subtropical streams in general, and even fewer from tropical regions of South America. We examined ammonium (NH4+) and soluble reactive phosphorus (SRP) retention in streams in Brazil and Argentina. We selected 12 streams with relatively little or extensive agricultural activity and conducted whole‐stream nutrient additions and measurements of gross primary production and ecosystem respiration. We used multiple linear regression to determine potential drivers of nutrient uptake metrics across the streams. Nutrient concentrations and retention differed significantly between land use categories. Both NH4+ and SRP concentrations were higher in the agricultural sites (means of 161 and 495 μg l–1, respectively), whereas metabolic rates were slower and transient storage was smaller. Our analysis indicated that agriculture increased ambient uptake lengths and decreased uptake velocities. The regression models revealed that ambient SRP had a positive effect on NH4+ uptake and vice versa, suggesting strong stoichiometric controls. Drivers for nutrient uptake in streams with low‐intensity agriculture also included canopy cover, temperature, and ecosystem respiration rates. Nutrient assimilation in agricultural sites was influenced by a higher number of variables (gross primary production for SRP, discharge, and transient storage for both nutrients). Our results indicate agricultural activity changes both the magnitude of in‐stream nutrient uptake and the mechanisms that control its variation, with important implications for South American streams under agricultural intensification.

Measurement uncertainty in stream nutrient uptake: Detecting land-use impacts on tropical streams

Abstract 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.

A Critical Assessment of Relating Resazurin–Resorufin Experiments to Reach‐Scale Metabolism in Lowland Streams

AbstractThe reactive tracer resazurin is frequently used to investigate metabolic processes and hyporheic exchange in streams, because its transformation to resorufin is considered a function of oxygen turnover in the presence of living cells and thus of ecosystem respiration. This has been investigated and confirmed in a number of laboratory and batch studies but so far not unanimously been verified in the field. In this study we conducted 13 tracer tests with resazurin in different small, low‐gradient streams typical of lowland regions. We determined tracer processing rates and ratios based on metrics commonly used to assess reactive‐tracer tests with resazurin and compared them to flow characteristics and respiration rates determined from diel‐oxygen recordings following a two‐station approach. We found no statistical relationship between tracer processing and parameters of transport, indicating that resazurin‐to‐resorufin transformation cannot be explained by hydrological parameters alone. The statistical relationship between resazurin processing and respiration rates based on diel‐oxygen profiles was strongest if resazurin‐to‐resorufin transformation ratios were used, and uptake lengths were also found to be a good proxy. However, the relationships were not linear, and weak for individual streams, contrary to what was expected based on previous laboratory and batch studies. We attribute this finding to a lack of process understanding of resazurin reactions under field conditions, and generally insignificant hyporheic exchange fluxes in lowland streams. Our findings highlight important limitations of the tracer technique under field conditions.

Combined Structural and Functional Imaging of the Kidney Reveals Major Axial Differences in Proximal Tubule Endocytosis.

The kidney proximal convoluted tubule (PCT) reabsorbs filtered macromolecules via receptor-mediated endocytosis (RME) or nonspecific fluid phase endocytosis (FPE); endocytosis is also an entry route for disease-causing toxins. PCT cells express the protein ligand receptor megalin and have a highly developed endolysosomal system (ELS). Two PCT segments (S1 and S2) display subtle differences in cellular ultrastructure; whether these translate into differences in endocytotic function has been unknown. To investigate potential differences in endocytic function in S1 and S2, we quantified ELS protein expression in mouse kidney PCTs using real-time quantitative polymerase chain reaction and immunostaining. We also used multiphoton microscopy to visualize uptake of fluorescently labeled ligands in both living animals and tissue cleared using a modified CLARITY approach. Uptake of proteins by RME occurs almost exclusively in S1. In contrast, dextran uptake by FPE takes place in both S1 and S2, suggesting that RME and FPE are discrete processes. Expression of key ELS proteins, but not megalin, showed a bimodal distribution; levels were far higher in S1, where intracellular distribution was also more polarized. Tissue clearing permitted imaging of ligand uptake at single-organelle resolution in large sections of kidney cortex. Analysis of segmented tubules confirmed that, compared with protein uptake, dextran uptake occurred over a much greater length of the PCT, although individual PCTs show marked heterogeneity in solute uptake length and three-dimensional morphology. Striking axial differences in ligand uptake and ELS function exist along the PCT, independent of megalin expression. These differences have important implications for understanding topographic patterns of kidney diseases and the origins of proteinuria.

Biotic control of in-stream nutrient retention in nitrogen-rich springs (Po Valley, Northern Italy)

Agriculture is a major contributor of nutrients to surface waters in large parts of the world and affects hydro-morphological conditions of rivers eventually altering their ecological status. Increasing attention has been given to those aspects of river functioning which refer to the processes able to modulate the concentration of nutrients exported downstream. This study aimed to evaluate the nutrient retention in agricultural groundwater-fed lowland streams, also known as “fontanili”, contaminated by elevated nitrate loadings. We used the nutrient spiraling method to assess ammonium (NH4) and phosphate (PO4) retention in two fontanili reaches, that significantly differed in presence/absence of aquatic macrophytes and tree canopy in the riparian zone. The results indicate that the overall nutrient retention was higher in the macrophyte-rich than in the unvegetated stream channel. The metric better reflecting the biological removal (i.e. the uptake velocity – Vf) was 9.8-fold and 4.4-fold higher, respectively for NH4 and PO4, in the vegetated than in the unvegetated reach. Temperature was an important driver of nutrient retention in vegetated condition, while discharge explained most variation of the uptake length of both nutrients in the reach where macrophytes were lacking. The role of the autotrophic compartment was further highlighted by the positive association between the uptake velocity (Vf-NH4 and Vf-PO4) and the gross primary production. The relationships observed between some retention metrics and the relative nutrient availability suggest that primary producers strongly controlled NH4 retention despite the extremely high N:P water ratio. The median NH4 uptake rate at the macrophyte-rich spring (30.5 µg m−2 min−1) was 5.5 fold higher than that at the unvegetated one, and was in the range reported for pristine streams. A few simple management actions are suggested that might be helpful to mitigate nitrogen and phosphorous pollution in the hydraulic networks of the Po Plain.

Nitrogen and Phosphorus Uptake Dynamics in Tropical Cerrado Woodland Streams

Pollution abatement through phosphorus and nitrogen retention is a key ecosystem service provided by streams. Human activities have been changing in-stream nutrient concentrations, thereby altering lotic ecosystem functioning, especially in developing countries. We estimated nutrient uptake metrics (ambient uptake length, areal uptake rate, and uptake velocity) for nitrate (NO3–N), ammonium (NH4–N), and soluble reactive phosphorus (SRP) in four tropical Cerrado headwater streams during 2017, through whole-stream nutrient addition experiments. According to multiple regression models, ambient SRP concentration was an important explanatory variable of nutrient uptake. Further, best models included ambient NO3–N and water velocity (for NO3–N uptake metrics), dissolved oxygen (DO) and canopy cover (for NH4–N); and DO, discharge, water velocity, and temperature (for SRP). The best kinetic models describing nutrient uptake were efficiency-loss (R2 from 0.47–0.88) and first-order models (R2 from 0.60–0.85). NO3–N, NH4–N, and SRP uptake in these streams seemed coupled as a result of complex interactions of biotic P limitation, abiotic P cycling processes, and the preferential uptake of NH4–N among N-forms. Global change effects on these tropical streams, such as temperature increase and nutrient enrichment due to urban and agricultural expansion, may have adverse and partially unpredictable impacts on whole-stream nutrient processing.

Seasonal Variation and Retention of Ammonium in Small Agricultural Streams in Central Thailand

Elevated nutrient concentrations are of concern in many agricultural streams. This study observed seasonal variation of inorganic nitrogen concentrations in small drainage streams that receive discharge predominantly from three different types of agricultural practices, swine farms, aquaculture, and paddy fields, in the Tha Chin River Basin in central Thailand. The drainage streams had high inorganic nitrogen concentrations, low dissolved oxygen concentrations (&lt;3 mg L−1), and ammonium was the dominant form of inorganic nitrogen. The highest ammonium concentrations were observed in the drainage streams through swine farm areas, exceeding 4 mg L−1 throughout the year. In-stream ammonium retention was examined using pulse co-injections of ammonium and chloride in a drainage stream, and the effects of in-channel vegetation on ammonium retention were evaluated. This study showed that a considerable amount of ammonium was retained with the presence of in-channel vegetation. After vegetation removal, both the hydraulic and ammonium uptake length increased by &gt;50 m, and the ammonium retention declined by approximately 50%. These results suggest that small agricultural streams experience a high concentration of inorganic nitrogen, similar or even more than larger streams, and that in-channel vegetation plays important role in nitrogen retention. The management of in-channel vegetation will have considerable effects on nitrogen transport.

Nutrient Limitation and the Stoichiometry of Nutrient Uptake in a Tropical Rain Forest Stream

AbstractNutrient limitation assessment is important to understand stream ecosystem functioning. Aquatic primary producers are often limited by nitrogen, phosphorus, or both, as assessed by nutrient diffusing substrata (NDS), a common method for assessing nutrient limitation in streams. But little is known regarding how this method relates to patterns of nutrient uptake at the whole‐stream scale. We combined two techniques to examine nutrient limitation in a tropical stream. First, we conducted two NDS experiments using ammonium, nitrate, and phosphate alone and combined, to determine nutrient limitation at substrata scale over several weeks. Second, we conducted whole‐stream nutrient addition experiments over the course of a year, using nutrients alone and in combination, to test theoretical predictions about uptake characteristics of limiting and nonlimiting nutrients. NDS results consistently indicated N limitation. Ambient uptake length (SW‐amb) suggested either P limitation (shorter SW‐amb for P than N) or colimitation (similar SW‐amb for both nutrients). The relationship between N uptake and concentration when added alone or with P suggested P limitation, colimitation, or neither, depending on the date. We speculate that the different conclusions arise from differences in the spatial and temporal scale assessed by these techniques and the microbial processes involved, and the potential for physical processes influencing whole‐stream uptake estimates. We conclude that nutrient limitation is not as categorical as NDS results often imply, rather habitat, resource, and biotic diversity result in nutrient uptake rates that do not necessarily conform to predictions drawn from fine scale, process‐specific bioassays such as chlorophyll‐a accrual on NDS.

Nitrate Uptake Kinetics and Correlation Analysis in an Agricultural Drainage Ditch

To investigate the whole-reach nitrate (NO3--N) uptake dynamics in a headwater agricultural stream, we performed five pulse tracer additions of a reactive solute (as KNO3) and a conservative solute (as NaBr) in an agricultural drainage ditch in Hefei district, Chaohu Lake basin, from October 2016 to April 2017. The TASCC (tracer additions for spiraling curve characterization) approach and Michaelis-Menten (M-M) method were applied for the simulation of NO3--N uptake dynamics. Results showed that the ambient areal rate of total NO3--N uptake Uamb varied from 11.40 to 69.13 μg ·(m2 ·s)-1 with an average of 34.45 μg ·(m2 ·s)-1, and the ambient uptake velocity Vf-amb averaged 0.24 mm ·s-1 and varied from 0.07 to 0.43 mm ·s-1 across three well-mixed sub-reaches in the study. The ambient uptake length Sw-amb averaged 199.06 m with a range from 92.51 to 405.74 m, which was much smaller than the length of the drainage ditch (about 2.5 km), suggesting that the agricultural drainage ditch had a high potential for NO3--N retention. Generally, the M-M model fit the NO3--N uptake dynamics well, and the maximum uptake Umax ranged from 158 to 1280 μg ·(m2 ·s)-1 with a mean of 631.13 μg ·(m2 ·s)-1. The half saturation constant Km ranged from 0.16 to 5.52 mg ·L-1 with a mean of 1.46 mg ·L-1. According to correlation analysis, Sw-amb was negatively correlated with NO3--Namb, and Uambwas significantly positively correlated with NO3--Namb, while other nutrient spiraling metrics were not correlated with the NO3--N ambient concentration. Hydrological conditions had no distinct effect on the NO3--N retention, but both the width variability Фw and variability in cross-sectional area ФA were significantly correlated with most of the nutrient spiraling metrics, indicating that geomorphic features in the drainage ditch evidently impacted NO3--N uptake.