Soluble Reactive Phosphorus Uptake Research Articles

Leaves stimulate aquatic phosphorus uptake by dark‐grown but not by light‐grown microbial communities in sediments: A laboratory study

Abstract It is a widely discussed topic in restoration ecology to use organic carbon (OC) management to increase nutrient uptake in chronically loaded agricultural streams. However, although nitrate uptake has been shown to correlated closely with the availability of dissolved OC, this relationship is less clear for phosphate. Likewise, the role of particulate OC (POC) on stimulating nutrient uptake has to be clarified. We aimed at investigating the effects of leaf additions on the net uptake of soluble reactive phosphorus and nitrate‐nitrogen from the water column by benthic biofilms in combined laboratory flume and batch experiments. Fine sediments were colonised under nutrient‐enriched conditions in the dark (dark‐grown heterotrophic biofilms) and under a 12/12‐hr dark/light cycle (light‐grown photoautotrophic biofilms) in flumes. In each light treatment, half of the biofilms were grown under strong OC‐limitation, the other half under near‐optimal C:N:P ratios. For the uptake experiments, the colonised sediments were incubated in nutrient‐enriched water to which (1) fresh alder leaves, (2) pre‐leached leaves, or (3) no leaves were added. Net nutrient uptake (or release) rates were determined via the change in nutrient concentrations in the water column over time. Net phosphorus uptake by dark‐grown biofilms was significantly increased by the addition of leaves. Despite their high initial nutrient leaching, fresh leaves showed stronger stimulating effects on the net P uptake than leached leaves over the entire experiment. Furthermore, biofilms grown under near‐optimum C:N:P ratios responded stronger to the POC supply than biofilms grown under C‐limited conditions. In contrast to dark‐grown biofilms, leaf additions had no significant effects on the net P uptake by light‐grown biofilms. We also found no effects of leaf additions on net nitrate uptake. Our study shows that an increased POC supply in nutrient‐impacted streams may boost heterotrophic phosphate uptake. However, the stimulating effect of the POC addition depends on a variety of POC‐dependent (e.g. respiration, heterotrophic assimilation, denitrification) and independent (e.g. adsorption, photoautotrophic assimilation) processes, making nutrient control via POC management challenging. We suggest application of management approaches that focus on restoration of riparian areas and wetlands to increase the amount of natural, complex POC with a moderate degradability, thus keeping the heterotrophic metabolism low but potentially stimulating in‐stream N and P retention.

Riverine macrophytes control seasonal nutrient uptake via both physical and biological pathways

Abstract In low‐gradient, macrophyte‐rich rivers, we expect that the significant change in macrophyte biomass among seasons will strongly influence both biological activity and hydraulic conditions resulting in significant effects on nutrient dynamics. Understanding seasonal variation will improve modelling of nutrient transport in river networks, including annual estimations of export, which could optimise decision‐making and management outcomes. We explored the relationships among seasonal differences in reach‐scale nutrient uptake, macrophyte abundance, solute transport and transient storage in the River Gudenå (Denmark), a large macrophyte‐rich river. We used the minimal pulse addition technique to measure uptake of ammonium, nitrate, soluble reactive phosphorus, as well as reach‐scale metabolism, and surface transient storage in spring, summer, and autumn. We found that riverine uptake changed among seasons and was linked to macrophyte biomass via both biological activity, reflected in reach‐scale metabolism, and through physical processes, as solute transport was influenced by longitudinal dispersion. In this macrophyte‐rich river, seasonal changes in macrophyte biomass affected contact time between the water and biota, which influenced ammonium and soluble reactive phosphorus uptake. Using stoichiometric scaling of reach‐scale metabolism, we found that seasonal variation also influenced the relative contributions of autotrophic and heterotrophic biota in assimilatory uptake. In summary, riverine nutrient uptake was not static, highlighting the importance of seasonality, with significant implications for modelling of nutrient export in river networks. Moreover, current management strategies that remove macrophyte biomass (i.e. weed cutting and dredging) will short‐circuit the positive effects of enhanced nutrient uptake resulting from abundant macrophytes in rivers.

Immediate and legacy effects of urban pollution on river ecosystem functioning: A mesocosm experiment

Effluents from urban wastewater treatment plants (WWTP) consist of complex mixtures of substances that can affect processes in the receiving ecosystems. Some of these substances (toxic contaminants) stress biological activity at all concentrations, while others (e.g., nutrients) subsidize it at low concentrations and stress it above a threshold, causing subsidy-stress responses. Thus, the overall effects of WWTP effluents depend mostly on their composition and the dilution capacity of the receiving water bodies. We assessed the immediate and legacy effects of WWTP effluents in artificial streams, where we measured the uptake of soluble reactive phosphorus (SRP) by the biofilm, biomass accrual, benthic metabolism and organic matter decomposition (OMD). In a first phase (32 d), the channels were subjected to a gradient of effluent contribution, from pure stream water to pure effluent. WWTP effluent affected the ecosystem processes we measured, although we found no clear subsidy-stress patterns except for biofilm biomass accrual. Instead, most of the processes were subsidized, although they showed complex and process-specific patterns. Benthic metabolism and OMD were subsidized without saturation, as they peaked at medium and high levels of pollution, respectively, but they never fell below control levels. SRP uptake was the only process that decreased with increasing effluent concentration. In a second phase of the experiment (23 d), all channels were kept on pure stream water to analyse the legacy effects of the effluent. For most of the processes, there were clear legacy effects, which followed either subsidy, stress, or subsidy-stress patterns. SRP uptake capacity was stressed with increasing pollution legacy, whereas algal accrual and benthic metabolism continued being subsidized. Conversely, biofilm biomass accrual and OMD showed no legacy effects. Overall, the WWTP effluent caused complex and process-specific responses in our experiment, mainly driven by the mixed contribution of subsidizers and stressors. These results help improving our understanding of the effects of urban pollution on stream ecosystem functioning.

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.

Uptake of ammonium and soluble reactive phosphorus in forested streams: influence of dissolved organic matter composition

Many microbes responsible for inorganic nutrient uptake and transformation utilize dissolved organic matter (DOM) as a nutrient or energy source, but little is known about whether DOM composition is an important driver of nutrient uptake in streams. Our goal was to determine whether incorporating DOM composition metrics with other more commonly considered biological, physical, and chemical variables improved our ability to explain patterns of ammonium ( $${\text{NH}}_{4}^{ + }$$ –N) and soluble reactive phosphorus (SRP) uptake across 11 Lake Superior tributaries. Nutrient uptake velocities (Vf) ranged from undetectable to 14.6 mm min−1 for $${\text{NH}}_{4}^{ + }$$ –N and undetectable to 7.2 mm min−1 for SRP. Logistic regressions suggested that DOM composition was a useful predictor of where SRP uptake occurred (4/11 sites) and $${\text{NH}}_{4}^{ + }$$ –N concentration was a useful predictor of where $${\text{NH}}_{4}^{ + }$$ –N uptake occurred (9/11 sites). Multiple regression analysis revealed that the best models included temperature, specific discharge, and canopy cover, and DOM composition as significant predictors of $${\text{NH}}_{4}^{ + }$$ –N Vf. Partial least squares revealed fluorescence index (describing the source of aquatic fulvic acids), specific ultraviolet absorbance at 254 nm (an indicator of DOM aromaticity), temperature, and conductivity were highly influential predictors of $${\text{NH}}_{4}^{ + }$$ –N Vf. Therefore, streams with higher temperatures, lower solute concentrations, more terrestrial DOM signal and greater aromaticity had greater $${\text{NH}}_{4}^{ + }$$ –N Vf. Our results suggest that DOM composition may be an important, yet often overlooked, predictor of $${\text{NH}}_{4}^{ + }$$ –N and SRP uptake in deciduous forest streams that should be considered along with commonly measured predictors.

The potential of agricultural headwater streams to retain soluble reactive phosphorus

The study focuses on the capacity of agricultural headwater streams to retain soluble reactive phosphorus (SRP). In-stream phosphorus uptake was determined via short-term SRP additions in 14 reaches differing in channel morphology and riparian vegetation. In addition, zero equilibrium phosphorus concentrations (EPC0) were estimated for 8 reaches based on adsorption experiments. Average SRP uptake lengths amounted to 3.8 km in channelized sections, 1.9 km in forested sections, and 0.5 km in open meanders. Mass transfer coefficients were highest in open meanders (0.1 cm min−1), followed by forested (0.05 cm min−1) and channelized sections (0.04 cm min−1). EPC0 ranged from 20 to 1,600 µg SRP l−1 and correlated positively with inorganic P and reductant-soluble P concentrations of the sediments. In 50% of the reaches, phosphorus was released from the sediments at initial water concentrations of up to 500 µg SRP l−1, indicating a high release potential. Although EPC0 did not correlate with in-stream SRP uptake, sediments probably play a significant role for the P retention in agricultural headwater streams as they supply the benthic community with phosphorus from the subsurface. Thus, it is crucial that sediment–water interactions are considered in the restoration and management of agricultural headwater streams.

Nutrient Retention in Restored Streams and Rivers: A Global Review and Synthesis

Excess nitrogen (N) and phosphorus (P) from human activities have contributed to degradation of coastal waters globally. A growing body of work suggests that hydrologically restoring streams and rivers in agricultural and urban watersheds has potential to increase N and P retention, but rates and mechanisms have not yet been analyzed and compared across studies. We conducted a review of nutrient retention within hydrologically reconnected streams and rivers, including 79 studies. We developed a typology characterizing different forms of stream and river restoration, and we also analyzed nutrient retention across this typology. The studies we reviewed used a variety of methods to analyze nutrient cycling. We performed a further intensive meta-analysis on nutrient spiraling studies because this method was the most consistent and comparable between studies. A meta-analysis of 240 experimental additions of ammonium (NH4+), nitrate (NO3−), and soluble reactive phosphorus (SRP) was synthesized from 15 nutrient spiraling studies. Our results showed statistically significant relationships between nutrient uptake in restored streams and specific watershed attributes. Nitrate uptake metrics were significantly related to watershed surface area, impervious surface cover, and average reach width (p < 0.05). Ammonium uptake metrics were significantly related to discharge, velocity, and transient storage (p < 0.05). SRP uptake metrics were significantly related to watershed area, discharge, SRP concentrations, and chl a concentrations (p < 0.05). Given that most studies were conducted during baseflow, more research is necessary to characterize nutrient uptake during high flow. Furthermore, long-term studies are needed to understand changes in nutrient dynamics as projects evolve over time. Overall analysis suggests the size of the stream restoration (surface area), hydrologic connectivity, and hydrologic residence time are key drivers influencing nutrient retention at broader watershed scales and along the urban watershed continuum.

Effects of benthic algae on release of soluble reactive phosphorus from sediments: a radioisotope tracing study

To evaluate the effect of benthic algae on soluble reactive phosphorus (SRP) release from sediments in shallow lakes, experiments on SRP release with and without benthic algae in sediment cores and an experiment on SRP uptake by benthic algae were conducted using the radioisotope (32P) tracing method. The dissolved oxygen (DO) concentration in sediment cores was also investigated. The results show that benthic algae effectively reduce the release of SRP from sediments to overlying water. The uptake of SRP by benthic algae, which is the direct way in which benthic algae affect the SRP release from sediments, is low in filtered water and increases with the SRP concentration. However, in the experiment, the increased uptake rate lasted for a short time (in one hour), and after that it returned to a low rate. Benthic algae make the DO concentration and the oxic layer thickness increased, which can indirectly reduce the SRP release from sediments. These findings indicate that benthic algae can reduce the SRP release from sediments in both direct and indirect ways. It seems that the indirect way also plays an important role in reducing the SRP release from sediments.

Modelling sediment deposition and phosphorus retention in a river floodplain

Phosphorus (P) is one of the major limiting nutrient in many freshwater ecosystems. During the last decade, attention has been focused on the fluxes of suspended sediment and particulate P through freshwater drainage systems because of severe eutrophication effects in aquatic ecosystems. Hence, the analysis and prediction of phosphorus and sediment dynamics constitute an important element for ecological conservation and restoration of freshwater ecosystems. In that sense, the development of a suitable prediction model is justified, and the present work is devoted to the validation and application of a predictive soluble reactive phosphorus (SRP) uptake and sedimentation models, to a real riparian system of the middle Ebro river floodplain. Both models are coupled to a fully distributed two-dimensional shallow-water flow numerical model. The SRP uptake model is validated using data from three field experiments. The model predictions show a good accuracy for SRP concentration, where the linear regressions between measured and calculated values of the three experiments were significant (r2 ≥ 0.62; p ≤ 0.05), and a Nash–Sutcliffe coefficient (E) that ranged from 0.54 to 0.62. The sedimentation model is validated using field data collected during two real flooding events within the same river reach. The comparison between calculated and measured sediment depositions showed a significant linear regression (p ≤ 0.05; r2 = 0.97) and an E that ranged from 0.63 to 0.78. Subsequently, the complete model that includes flow dynamics, solute transport, SRP uptake and sedimentation is used to simulate and analyse floodplain sediment deposition, river nutrient contribution and SRP uptake. According to this analysis, the main SRP uptake process appears to be the sediment sorption. The analysis also reveals the presence of a lateral gradient of hydrological connectivity that decreases with distance from the river and controls the river matter contribution to the floodplain. Copyright © 2014 John Wiley & Sons, Ltd.

Solute-specific scaling of inorganic nitrogen and phosphorus uptake in streams

Abstract. Stream ecosystem processes such as nutrient cycling may vary with stream position in the network. Using a scaling approach, we examined the relationship between stream size and nutrient uptake length, which represents the mean distance that a dissolved solute travels prior to removal from the water column. Ammonium (NH4+) uptake length increased proportionally with stream size measured as specific discharge (discharge/stream width) with a scaling exponent = 1.01. In contrast, uptake lengths for nitrate (NO3−) and soluble reactive phosphorus (SRP) increased more rapidly than increases in specific discharge (scaling exponents = 1.19 for NO3− and 1.35 for SRP). Additionally, the ratio of inorganic nitrogen (N) uptake length to SRP uptake length declined with stream size; there was relatively lower demand for SRP compared to N as stream size increased. Finally, we related the scaling of uptake length with specific discharge to that of stream length using Hack's law and downstream hydraulic geometry. Ammonium uptake length increased less than proportionally with distance from the headwaters, suggesting a strong role for larger streams and rivers in regulating nutrient transport.

Greater phosphorus uptake in forested headwater streams modified by clearfell forestry

Clearfell, burn and sow (CBS) forestry can potentially alter stream environments by increasing available light and the input of woody debris. However, little is known about how CBS forestry affects in-stream processes such as nutrient uptake. We evaluate whether short-term (2–7 years) environmental changes (e.g. light availability and woody debris) associated with CBS forestry lead to differences in nutrient uptake metrics. To do this, we measured in-stream uptake of soluble reactive phosphorus (SRP) and ammonium (NH4) in three old growth (OG) and four CBS-affected headwater stream reaches. The abundance of fine woody debris and light availability were significantly greater in CBS-affected than in OG reaches. Uptake velocities varied from 0.0880 to 0.951 mm min−1 for NH4 and from 0.0383 to 1.06 mm min−1 for SRP across all sites. The mean uptake of SRP, but not NH4, was significantly greater (i.e. higher uptake velocities and lower uptake lengths) in CBS-affected than in OG reaches. These results suggest that CBS forestry altered the stream environment enabling greater SRP uptake relative to OG reaches. Our findings highlight the tight linkage between headwater streams and their surrounding terrestrial environment, which has direct implications for catchment-scale biogeochemical processes.

Transition zones in small lakes: the importance of dilution and biological uptake on lake-wide heterogeneity

Stream inflows are frequently the dominant route for nutrients from catchments to lakes. Studies on large, deep reservoirs and lakes have shown the importance of the fate of inflow plumes for nutrient accessibility to phytoplankton. However, few studies have considered shallow water transition zones between streams and lakes, often a feature of small lakes. This study examined the spatial and temporal dynamics of phosphorus in a shallow stream-lake transition in a small lake to improve our understanding of how phosphorus reaches the pelagic zone. Despite the high discharge levels, and the importance of dilution in explaining observed spatial gradients in soluble reactive phosphorus (SRP), total phosphorus (TP) and chlorophyll a, we found evidence for significant biological uptake of SRP in the inflow embayment during the growing season. This may represent an additional mechanism for the dispersal of phosphorus from the embayment into the lake. The length scale for the transition zone was short (~150 m) which indicated that the direct influence of the inflow on the wider lake was small. However, SRP measurements taken only from the pelagic site underestimated mean lake-wide concentrations when including transition zones by up to 18% during the growing season.

Variation in stream C, N and P uptake along an altitudinal gradient: a space-for-time analogue to assess potential impacts of climate change

A space-for-time substitution approach was used to evaluate potential effects of climate change on stream nutrient uptake by examining the relationship between stream environmental parameters and carbon (C), nitrogen (N) and phosphorus (P) uptake along an altitudinal gradient. The study was carried out in 14 streams located in the Central Pyrenees (NE Spain) draining calcareous catchments that cover an altitudinal range of 700–2,100 m a.s.l. In these streams, uptake of inorganic (soluble reactive phosphorus (SRP), ammonium and nitrate) and organic (acetate and glycine) nutrients was estimated. Additionally, several physical, chemical and biological parameters were measured. Results showed higher uptake for both SRP, a potentially limiting nutrient in these streams, and glycine, a labile source of dissolved organic N, than for the rest of the nutrients. Uptake of SRP, nitrate, glycine and acetate varied along stream environmental gradients associated with changes in stream hydromorphology, SRP availability and epilithic biomass. However, these gradients did not vary with altitude. These results indicate that climate change effects on stream nutrient uptake are more likely to be driven by indirect effects on hydromorphology and nutrient availability induced by shifts in the precipitation and run-off regime than by direct modifications in the thermal regime.

Influence of suspended clay on phosphorus uptake by periphyton

We investigated the effect of suspended clay upon the phosphorus uptake rate exhibited by lotic periphyton communities. Suspended inorganic clays and periphyton are common to aquatic environments, and both can strongly influence physical and chemical water conditions. We used replicated artificial stream channels to test the prediction that suspended clay particles would affect the uptake of soluble reactive phosphorus (SRP) by periphyton. Commercially available kaolinite and bentonite clays were characterized for their aqueous suspension behavior and affinities for SRP. Periphyton was grown in a recirculating stream system and subjected to simultaneous suspended clay and SRP additions. SRP removal from solution, both in the presence and absence of suspended clays, was used to quantify SRP uptake parameters by periphyton. Clay type and concentrations of 20, 80, and 200 mg l−1 had no significant effect upon SRP uptake rate exhibited by periphyton during three 90-min experiments. Less than 1% of SRP removal was attributable to the suspended clay load or artificial stream construction materials, based on clay isotherm data and material sorption studies, indicating that 99% of SRP removal was attributable to biotic uptake. Removal of SRP (as KH2PO4) was described by a first-order equation with rate constants ranging between 0.02 and 0.14 min−1. Our results suggest that high turbidity conditions caused by suspended mineral clays have little immediate effect upon SRP removal from the water column by periphyton.

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.

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.

Within-river nutrient processing in Chalk streams: The Pang and Lambourn, UK

This work examines baseflow nutrient concentrations and loads along two rural Chalk streams, the Pang and Lambourn. Soluble reactive phosphorus (SRP) and boron (B) concentrations in these streams were heavily influenced by point-source inputs and the effects of downstream flow accretion and dilution. Unlike B (which is chemically conservative), SRP loads were also strongly influenced by in-stream processing resulting in uptake of SRP, particularly immediately downstream of sewage effluent discharges, where rates of SRP uptake were highest. For the upper River Pang, up to 80% of SRP loads were lost within 4 km downstream of Compton sewage treatment works (STW) and on the River Lambourn up to 55% of SRP loads were lost within 1.6 km downstream of East Shefford STW. In contrast, nitrate (NO3) concentrations at sites along the Pang and Lambourn were largely controlled by groundwater inputs and plant uptake during periods of high photosynthetic activity in spring and summer and silicon (Si) by diatom uptake in April/May. There were net gains in NO3 loads along the river reaches, as a result of volumetric increases in groundwater discharge, and, compared with SRP, the role of in-stream processing of NO3 appeared low. Examination of SRP exchange by bed sediment and uptake of SRP into algal biofilms indicated that biofilms accounted for only a very small percentage of in-stream P-uptake, but that bed sediment SRP-exchanges had a more important control on baseflow SRP concentrations and loads. Point source P remediation at East Shefford STW, by removal of P from final effluent (P-stripping), resulted in 70–90% reductions in river-water SRP loads. After introduction of P-stripping at East Shefford STW, bed sediments immediately downstream of the STW switched from being net sinks to net sources of SRP. Our results show that, in the immediate aftermath of P-stripping, bed sediment SRP-release was responsible for a 30 μg-P l−1 rise in river-water SRP along this reach. While this increase in SRP concentration, as a result of bed sediment SRP release, is potentially ecologically significant, it is small in relation to the increase in SRP concentrations from effluent prior to P-stripping, which resulted in increases in SRP concentration of up to 500 μg-P l−1. There was a six-month lag between the introduction of P-stripping at East Shefford STW and bed sediment EPC0 recovering to equilibrium levels with the overlying river water (and thus negligible SRP release). Recovery of bed sediments to equilibrium levels is likely to have occurred as a result of winnowing and removal of high-EPC0 sediment and delivery of lower EPC0 sediment from upstream. Under higher/more variable flow conditions and greater rates of in-channel sediment erosion/delivery, more rapid recovery of bed sediment EPC0 levels following P-stripping might be expected.

Role of river bed sediments as sources and sinks of phosphorus across two major eutrophic UK river basins: the Hampshire Avon and Herefordshire Wye

The Equilibrium Phosphorus Concentration (EPC 0) of river bed sediments has been measured for a wide range of agricultural subcatchments and main river sites across two major eutrophic river basins: the Hampshire Avon and Herefordshire Wye catchments, to examine whether bed sediments are acting as sources or sinks of soluble reactive phosphorus (SRP) under low flows and times of greatest eutrophication risk. A conceptual reach-based model of interactions between bed sediment and river water has been used to estimate relative differences in bed-sediment SRP flux transfers. In this model, processes of SRP uptake and release are assumed to occur within an operationally defined 0.1 m river water ‘boundary layer’ and the uptake and release of SRP is assumed to be driven by a differential between the EPC 0 of the sediment and SRP concentration in the boundary layer. Most of the river monitoring sites in the Wye and Avon catchments had elevated SRP and boron (B) concentrations in the water column at low flows, linked to sewage effluent discharges. At these sites, bed sediments consistently acted as net sinks for SRP, demonstrating the role of bed sediments in riverine ‘self-cleansing’ mechanisms. In contrast, bed sediments were found to act as net sources of SRP under three circumstances: (i) where there was minimal sewage influence (in headwater streams of the Avon), (ii) where sewage inputs were subject to large hydrological dilution by water of low SRP concentration (in the main River Wye), (iii) where EPC 0 values were relatively high, as a result of deposition of particulates with high exchangeable P concentrations from diffuse sources or from effluents (immediately downstream of sewage treatment works (STWs)). Under baseflow conditions, high SRP concentrations from sewage effluent in the tributaries appear to ‘swamp out’ any potential release of SRP from the bed sediments. For rivers that are subject to effluent P-stripping, reductions in SRP in the overlying water could potentially result in changes to the in-stream P-cycling mechanisms, with bed sediments possibly switching from net sinks to net sources of SRP. This feature is of potential importance in relation to environmental management and phosphorus mitigation operations.

The importance of in-stream uptake for regulating stream concentrations and outputs of N and P from a forested watershed: evidence from long-term chemistry records for Walker Branch Watershed

Long-term, weekly measurements of streamwater nitrogen and phosphorus concentra- tions in the West Fork of Walker Branch, a 1st order forested stream in eastern Tennessee, were used to assess the importance of in-stream processes for controlling stream concentrations and watershed exports. Over the period from 1991 to 2002, there was a slight declining trend in watershed export of dissolved inorganic N via streamflow, despite relatively high and constant wet N deposition rates (5 kg/ha/y). The watershed retains >90% of N deposition inputs. Concentra- tions of NO3 � and soluble reactive phosphorus (SRP) showed distinct seasonal patterns with autumn and early spring minima and summer maxima. An end-member mixing analysis indicated that these seasonal concentration patterns were largely a result of seasonal variations in in-stream uptake processes, with net uptake of NO3 � and SRP having the greatest impact on streamwater concentrations in November (reductions of 29 lg N/l and 2.5 lg P/l, respectively). This was likely a result of high rates of uptake by microbes colonizing new inputs of leaf detritus. For NO3 � there was a secondary peak in net uptake in March and April (about 9 lg N/l) resulting from increased uptake by stream algae and bryophytes. Summer was a period of net release of NO3 � to stream- water (peaking at 9 lg N/l in July) and minimal net effects on SRP concentrations. On average, in-stream processes resulted in removal of about 20% of the NO3 � and 30% of the SRP entering the stream from the catchment annually. This study, as well as other recent work, suggests that in-stream processes are important buffers on stream nutrient concentrations and exports reducing the effects of changes in inputs and retention in terrestrial portions of watersheds.

Land use and stream ecosystem functioning: nutrient uptake in streams that contrast in agricultural development

Rates of nutrient uptake were measured in streams with varying amounts of agricultural development in their catchments. We conducted short-term (1- 3 h) relea- ses of nitrate and phosphate in seven streams in summer and five streams in winter, measuring uptake from downstream changes in nutrient concentrations after correction for dilution. Nitrate (NO3 - ) uptake lengths varied from 45 to 3560 m across sites and seasons, and soluble reactive phosphorus (SRP) uptake lengths from 80 to 1310 m. In streams monitored in both seasons, uptake lengths were generally longer in winter, when discharge was higher and temperatures were lower. Uptake lengths of both nutri- ents were positively related to discharge. NO3 - uptake velocity was related to nitrate concentration and benthic chlorophyll standing stock, which together explained 55 % of its variation. SRP uptake velocity was related to chlorophyll standing stock, and a multiple regression that included SRP concentration and chlorophyll concentration ac- counted for 34 % of the variation in uptake velocity. There was no clear evidence of saturation of nutrient uptake at high background concentrations in the streams. Nutri- ent uptake in these streams was not closely related to estimates of transient storage of water. Catchment land use influenced streamwater concentrations of nutrients and to a lesser extent chlorophyll standing stocks, and these two stream characteristics affected nutrient uptake in these streams.