Equilibrium Phosphate Concentration Research Articles

Phosphorus release potential and pollution characteristics of sediment in downstream Nansi Lake, China

The research aimed to evaluate present and potential phosphorous pollution due to high sedimentary phosphorus load and release from sediment, when external phosphorus was reduced in downstream Nansi Lake. Pollution load of the sediment and overlying water was investigated. Kinetics and isotherms of adsorption/release of sedimentary phosphorus were studied to determine equilibrium phosphate concentration (EPC0) and release potential. Kinetics of phosphorus adsorption on sediment and release from sediment were well described by both the pseudo-first-order rate equation and the pseudo-second-order rate equation, but more appropriate to the pseudo-second-order rate equation with the adsorption/release capacity more close to the measured values, suggesting that the processes were chemically rate controlled and dependent on adsorption capacity. Soluble reactive phosphorus (SRP) sorption isotherms on sediment were best fitted by the modified Langmuir model indicating a monolayer adsorption. By comparing EPC0 and SRP of water, the status (adsorption, releasing or in equilibrium) of sediment phosphorus could be determined. The sediments at site S1, S3, S4, S5, and S7 where the EPC0s were greater than the SRPs, had a potential to release phosphorus into the water column. However, those sediments at S9, S10, and S12, where the EPC0s were approximately equal to the SRPs, were in impermanent equilibrium with overlying water in status of phosphorus, the sediments can be likely to release phosphorus to the water column once the equilibrium was broken. Therefore, sedimentary phosphorus can be a secondary pollution source in downstream Nansi Lake.

Phosphorus sorption in soils and sediments: implications for phosphate supply to a subtropical river in southeast Queensland, Australia

Phosphorus (P) is often a key limiting nutrient in freshwater systems, and excessive P can result in algal blooms, with flow-on effects to aquatic food webs. P sorption is an important process in aquatic and terrestrial ecosystems whereby phosphate (PO4 3−) is exchanged between liquid and solid phases. This study shows that differences in the concentration of PO4 3− in a subtropical river system during high and low flow can be attributed to differences in P sorption characterises of its catchment soils and sediments. The sediments have lower Equilibrium Phosphate Concentrations (EPC0) and higher binding energy (Kd); the surface soils have higher EPC0 and higher easily desorbed P (NH4Cl–P). A comparison of filterable reactive phosphorus (frP) in water samples collected at high and low flows, with soil and sediment EPC0, suggested that during event flows, the high EPC0 and NH4Cl–P of surface soils is producing a net movement of PO4 3− from the soil/sediment system into runoff and stream flow. At baseflow, there is more likely a net movement of PO4 3− into the riverbed sediments. This has important implications for management actions aimed at reducing P loads to river systems and downstream water storages, namely the need to increase the infiltration of rainfall to decrease the amount of PO4 3− being flushed from the surface soil.

The effects of drying on phosphorus sorption and speciation in subtropical river sediments

The present study investigated whether drying and desiccation substantially increase the biologically available phosphorus (P) in riverbed sediments from a dry subtropical river. Sequential extraction and batch equilibrium experiments were undertaken on sediments with contrasting organic matter content, percentage fines and P content. The response of sediments to drying differed predominately as a result of drying time rather than as a result of the different physiochemical properties and total P content of the sediments. For both in situ and laboratory drying, major changes in P speciation occurred in the surface-layer sediment (0–2 cm) where NH4Cl-P (loosely sorbed P) was higher in partially dried and desiccated sediments than in wet sediments. Conversely, NaOH-nrP (labile organic and poly-P) was significantly lower (P < 0.05) in partially dried and desiccated sediments than in wet sediments, suggesting that a substantial transformation from a relatively unavailable organic form to a readily available inorganic form of P had occurred with drying. The equilibrium phosphate concentration (EPC0), which is a measure of the potential for sediments to function as a source or a sink of phosphate (PO43–), was higher in sediments desiccated in situ and in the laboratory than in submerged and partially dried sediments. Together with the speciation results, the higher EPC0 indicates that the potential for sediments to release P during the next flow event is substantially increased as a result of desiccation. The lower EPC0 in partially dried sediments suggested that the degree of drying may be an important factor in terms of the long-term potential for sediments to act as a source or a sink of PO43– after rewetting. The results from the present study have important implications because natural or anthropogenic processes which lead to riverbed drying may increase the flux of bioavailable P from bed sediments when flow returns.

Phosphorus Removal by Sediment in Streams Contaminated with Acid Mine Drainage

Acid mine drainage (AMD) affects thousands of stream miles in the Appalachian region of the USA and results in elevated concentrations of iron and aluminum in the stream water and sediments and wide ranging pH values. It was hypothesized that these conditions would lead to increased P buffering capacity of the sediments which in turn would cause a decrease in dissolved reactive phosphorus (DRP) in the water column. In the lab fresh Fe, Al, and Mn oxide precipitates all adsorbed DRP strongly but over different pH ranges. Sulfate and calcium ions inhibited adsorption of DRP with Fe oxides but the effect was less apparent with Al oxides. In the field DRP concentration was reduced 54–90% just downstream of an AMD input compared to upstream of the input. In addition the sediment buffering capacity increased and equilibrium phosphate concentration decreased dramatically downstream of the AMD inputs. The strength of the effect and the widespread occurrence of AMD suggest that AMD could be altering the P dynamics of streams and rivers throughout the Appalachian region.

Sorption of phosphate ions on iron(III), zirconium(IV), and chromium(III) oxyhydroxides from aqueous electrolyte solutions

The adsorption of phosphate ions from aqueous solutions with an ionic strength of 0.5 on iron(III), zirconium(IV), and chromium(III) oxyhydroxide hydrogels has been studied as influenced by chloride and sulfate ions. Despite the high concentrations of chloride and sulfate ions, they do not inhibit phosphate adsorption on the hydrogels; they only slightly change the isotherm shape. In the range of equilibrium phosphate concentrations equal to 30–50 mmol/l, all isotherms for iron and zirconium oxyhydroxide gels signify the appearance of a second adsorption layer (two-step isotherms). Both steps are satisfactorily fitted by the Langmuir equation. The maximum adsorptions and adsorption constants have been calculated. For chromium oxyhydroxide gels, the intraduction of an electrolyte dramatically decreases the equilibration rate.

Sediment phosphorus cycling in a large shallow lake: spatio-temporal variation in phosphorus pools and release

Sediment and water column phosphorus fractions were recorded monthly for one year (April 2004–April 2005) in a shallow lake recovering from nutrient pollution (Loch Leven, Scotland). Equilibrium phosphate concentration (EPC0) and gross sediment phosphorus (P) release rates were estimated from laboratory experiments. Pore water and organic P pools were lowest during warm water periods whereas bottom water P was lowest during cold water periods. Reductant-soluble, organic, metal oxide-adsorbed, residual and sediment total phosphorus pools all varied significantly with overlying water depth. Short-term, high magnitude, redox initiated P release events occurred in late summer and winter as a result of anoxic sediment conditions. Lower magnitude long-term release conditions were maintained for most of the year, most likely as a result of organic P cycling and maintenance of high concentration gradients between the pore and bottom water P pools. Estimates of summer P uptake/release rates, across an intact sediment-water interface, suggested that maximum gross internal release was ~12 mg SRP m−2 lake surface area d−1 with EPC0 values ranging between 180 and 270 μg P L−1. This study highlights the biological mediation of internal loading in shallow eutrophic lakes, and in particular, the role of sediment algae in decreasing, and sediment bacteria in enhancing, sediment P release.

Relative Importance of Solid-Phase Phosphorus and Iron on the Sorption Behavior of Sediments

Of all the metal oxide particles, amorphous iron oxides have the greatest adsorption capacity for phosphate. Coastal sediments are often coated with terrigenous amorphous iron oxides, and those containing high iron are thought to have a high adsorption capacity. However, this conventional wisdom is based largely upon studies of phosphate adsorption on laboratory-synthesized minerals themselves containing no phosphorus. Using natural sediments that contain variable phosphorus and iron, our results demonstrate thatthe exchangeable phosphate rather than the iron oxides of sediments governs the overall sorption behavior. The iron oxide content becomes important only in sediments that are poor in phosphorus. A total of 40 sampling sites across the Florida Bay provide detailed spatial distributions both of the sediment's zero equilibrium phosphate concentration (EPC0) and of the distribution coefficient (Kd) that are consistent with the distribution of the exchangeable phosphate content of the sediment. This study provides the first quantitative relationships between sorption characteristics (EPC0 and Kd) and the exchangeable phosphate content of natural sediments.

Factors affecting water quality in degraded sewage receivers — case study of the phosphorus dynamics in the Ner River

No adequate reaction has been observed of the decreased contaminant loads discharged by Łodź, particularly the loads of phosphorus, on its concentration in the Ner River. That’s why the impact of sediment on phosphorus content in the water was evaluated. Not only was the amount of phosphorus taken under consideration but also the equilibrium phosphate concentration (EPCo). The meaning of EPCo is that any phosphate concentration in the water below this value will lead to phosphorus release from sediments. Performed study shows that in the Puczniew cross-section EPCo is higher then phosphorus concentration in water, thus with mean concentration of PO4 equal to 9.5 mg PO4·dm phosphorus could be released from sediments. This concentration in Lutomiersk crosssection, however, equals 1.2 mg PO4·dm.

Phosphate adsorption on synthetic goethite and akaganeite

Low crystalline iron hydroxides such as goethite ( α-FeOOH) and akaganeite ( β-FeOOH) were synthesized, and the selective adsorption of phosphate ions from phosphate-enriched seawater was examined. The results of the distribution coefficients ( K d ) of oxoanions in mixed anion solutions at pH 8 follow the selectivity order Cl −, NO − 3, SO 2− 4 ≪ CO 2− 3, HPO 2− 4 for goethite, and Cl −, CO 2− 3 < NO − 3 < SO 2− 4≪ HPO 2− 4 for akaganeite. In seawater, both adsorbents show high selectivity for phosphate ions despite the presence of large amounts of major cations and anions in seawater. The adsorption isotherms fitted better with the Freundlich equation and the maximum uptake of phosphate from phosphate-enriched seawater was 10 mg P/g at an equilibrium phosphate concentration of 0.3 mg P/L on both adsorbents. The phosphate adsorption/desorption cycles show that akaganeite is an excellent adsorbent even after 10 cycles and its chemical stability is good.

Spatial and historical variation in sediment phosphorus fractions and mobility in a large shallow lake

Temporal and spatial variation in sediment P composition and mobility were investigated in Loch Leven. Little change was observed in total sediment P (surface sediment at 4 m depth), in comparison to a previous study (1990), despite significant reduction of external point sources of P. Labile P and residual P have both increased (0.007–0.039 mg PO 4–P and 0.121–0.420 mg PO 4–P per gram dry weight of sediment, respectively) since 1990. An analysis of P fractions, along a depth transect, indicated elevated labile P concentrations in shallow water sediment (<12 m overlying water depth). Regression analysis showed that spatial variability in reductant-adsorbed P was significantly related to sediment chlorophyll a concentration ( R 2 = 0.7 3 3 , p < 0.0 5 ). This may be linked to the production of oxygen, by benthic algae, resulting in the maintenance of an oxygenated layer at the sediment surface. Variation in labile P was best explained by overlying water temperature and equilibrium phosphate concentration (EPC0).

The effects of sediment size fraction and associated algal biofilms on the kinetics of phosphorus release

Experiments using flumes containing sediment of three different size fractions, from two sites on the River Tame, investigated the influences of sediment particle size, and an associated biofilm, on sediment–water exchanges in heterogeneous sediment deposits. This is the first study undertaken to understand the kinetics of the release of soluble reactive phosphorus from sediments of natural systems to identify which of the size compartments affected those fluxes most. Samples of fine material (< 2 mm), gravel (2–20 mm), and stones (> 20 mm) were collected over a period of several weeks and brought to a fluvarium where they were placed in artificial, controlled flow, and flume channels. Synthetic solutions of similar ionic strength to the river were prepared using calcium chloride. Temperature, pH, and dissolved oxygen of the solution overlying the sediment were monitored automatically whilst filtered samples were obtained at 2 h intervals over 48 h. The biomass, expressed as mg m − 2 chlorophyll a, of the algal component of the biofilm from the surface of the sediment was estimated using methanol extraction. Differences in the responses were observed between the sediment size fractions and the two sites, where contaminant concentrations varied. The equilibrium phosphate concentration and a phosphorus transfer index were used to establish that there was a net uptake of phosphorus by all three sediment size fractions, from both sites, at the time of sampling. The kinetic results showed very fast initial reactions of phosphorus release from the larger size fractions with a well-developed filamentous algal growth present implying a different mechanism than diffusion being involved. The stones and associated biofilms also released more phosphorus than the fine fraction, e.g. final release concentrations for the most contaminated site were: fines ∼ 2.5 μM, gravel ∼ 6.5 μM, and stones ∼ 65.0 μM (expressed as soluble reactive phosphorus). Phosphorus fluxes, calculated assuming the concentration of phosphorus in the sediment was less than the equilibrium concentration, were a maximum at the most contaminated site, e.g. fines 6.4 nmol m − 2 s − 1 , gravel 27 nmol m − 2 s − 1 , and stones 109 nmol m − 2 s − 1 (normalised with respect to the river bed area). These results confirm that sediment having a biofilm and associated particulate material results in a greater flux than fine sediment, which does not support a filamentous biomass. Removal of the fine particulates trapped in the algal growth reduced soluble phosphorus release. These factors demonstrate that both gravel and stone substrates have an important control over the release of soluble reactive phosphorus.

Kinetics of phosphorus release from a natural mixed grain-size sediment with associated algal biofilms

Experiments using flumes containing mixed grain-size sediment with an associated algal biofilm, from two sites on the R. Tame, investigated the sediment–water exchanges in heterogeneous sediment deposits. These results were considered in the light of findings of a companion study [Gainswin BE, et al. The effects of sediment size fraction and associated algal biofilms on the kinetics of phosphorus release. Sci Total Environ, this issue.] by considering this natural system in relation to the effects of the different sizes of material comprising the sediment. Sediment samples were collected in trays installed in the river over a period of one growth cycle (March 2001–April 2002) and placed in flume channels with controlled water flow. The temperature, pH, and dissolved oxygen of the solution overlying the sediment were monitored automatically whilst filtered samples were obtained at 2-0h intervals over 48 h. The biomass, expressed as chlorophyll a, of the algal component of the biofilm from the surface of the sediment was estimated using methanol extraction. The composition of the sediment, viz. size fractions, organic matter and porosity, were determined at the end of the experiments. The equilibrium phosphate concentration and a phosphorus transfer index were used to establish that a net uptake of phosphorus by some of the samples that occurred at the time of sampling. The results were modelled using a Diffusion Boundary Layer model and the maximum flux from the sediment (or limiting diffusion flux) compared for each of the samples. The limiting diffusion flux was highest at the most contaminated site—reaching ∼ 180 nmol m − 2 s − 1 (normalised with respect to the river bed area). The limiting diffusion flux calculated for the composite samples was in agreement with the flux estimated from the contributions expected from the individual size fractions [Gainswin BE, et al. The effects of sediment size fraction and associated algal biofilms on the kinetics of phosphorus release. Sci Total Environ, this issue.]. The dominance of the flux contribution from the stones size fraction (> 20 mm) confirms that sediment having a filamentous biofilm and associated particulate material results in a greater flux than a silt sediment without such a biomass.

Effects of particle size, organic matter and ionic strength on the phosphate sorption in different trophic lake sediments

The trophic status and development of lake system is significantly influenced by the phosphate sorption at the sediment–water interface. The effects of organic matter, particle size and ionic strength on the phosphate sorption by sediments were investigated in this study. The results show that maximum phosphate sorption capacity ( Q max), equilibrium phosphate concentration (EPC 0), phosphate sorption efficiency ( k) and phosphate sorption rate decreased as particle size increased for all the studied sediments. But the reliable desorbed phosphorus (RDP) increased. Q max, EPC 0, k and RDP of different particle size fractions varied and there were no obvious differences among different trophic lake sediments. Q max, EPC 0 increased while k and RDP decreased as the pollution level of lake sediment increased. The phosphate sorption mainly occurred within 0.5 h. Power function and simple Elovich models were the best kinetic models for the phosphate sorption of the different particle size fractions. Q max and phosphate sorption rate decreased with the decreasing of organic matter content of sediments and the increasing of ionic strength. This study suggests that ionic strength and organic matter had similar effects on the phosphate sorption for different trophic lake sediments.

Contribution of sedimentary resuspension to non-conservative fluxes of dissolved inorganic phosphorus in San Quintin Bay, Baja California: An experimental estimate

Abiotic processes like sedimentary resuspension may contribute to non-conservative fluxes of dissolved inorganic phosphorus (DIP) in shallow systems like San Quintín Bay (SQB), because suspended particles may adsorb or desorb inorganic phosphate. The contribution of suspended sediments to DIP concentrations in SQB was determined in the laboratory through adsorption-desorption experiments with two types of sediments (fine sand and sandy silt), and different concentrations of suspended particles, the initial concentration of DIP and resuspension time. The zero equilibrium phosphate concentration (EPC0) was observed during the first step of the adsorption process (fast step), which occurred before 10 h. The EPC0 was exceeded after 24 h of resuspension, indicating the diffusion of P from the surface toward the interior of particles (slow step). Adsorption was the dominant process in the experiments, and the intensity of adsorption-desorption was emphasized with an increase in the amount of resuspended particles. From the comparison of the non-conservative fluxes of DIP estimated using the LOICZ model with those estimated with resuspension experiments, we conclude that adsorption may lead to an underestimation of ~20% of net heterotrophy in SQB calculated with the LOICZ model, as excess respiration leads to a net release of DIP to the water column, but adsorption by particles masks this net release.

Exchange of inorganic phosphate between river waters and bed-sediments.

The kinetics of the release of Soluble Reactive Phosphorus (SRP) in oxic and anoxic conditions and uptake in oxic conditions by contaminated river sediments (River Blackwater in Southern England) were measured using a fluvarium channel operated to mimic environmental conditions. Release rates (from 1 to 10 nmol m(-2) s(-1)) and uptake rates were modeled successfully using a Parabolic equation and Diffuse Boundary Layer model. A SRP release experiment over 61 days showed that large gradients in SRP developed in the porewater as a result of diffusion, sorption, and in-situ generation of SRP in the anoxic zone. This was modeled using a new Triple Zone Model that incorporated diffusion through a liquid boundary layer and sorption/desorption in oxic and anoxic zones. The results highlighted the importance of the oxic zone in controlling the exchange of SRP between the sediment and water column. The model was also applied to explain why the Equilibrium Phosphate Concentration (EPC0) of the sediment measured in oxic conditions was constant (and equal to the value calculated from sorption isotherm measurements) during 2-day release experiments, and also why it increased in the uptake experiments. Measurements in anoxic conditions showed the importance of the sediment temperature in controlling the flux at the interface.

Desorption of phosphate from sugarcane soils into simulated natural waters

A laboratory-based study of the behaviour of phosphorus (P) was carried out on the soils of the lower Herbert River catchment, Queensland, Australia. The aim was to explore the potential for P sorption or desorption by Herbert soils in associated river and estuary waters, so that the extent of problems associated with sugarcane production and soil-derived inputs to streamwater could be defined. Anion exchange resin was used as a sink for P. The equilibrium phosphate concentration (EPC) measured in simulated soil pore water (0.01M CaCl2), and the EPC in the simulated river and estuary waters were strongly correlated. Based on this, and the close relationship between P sorption and selected soil properties, it was possible to estimate P desorption using commonly measured properties. Much less desorption of P took place in simulated estuary waters than in simulated river water of much lower ionic strength. This suggests that environmental degradation arising from the downstream export of soil-borne P from Herbert cane lands is likely to be concentrated in freshwater areas. Sorption properties of P in soils of the lower Herbert appear to be closely associated with aluminium-rich minerals, rather than with iron (hydr)oxides.

The Role of Macrophytes in the Retention of Phosphorus in the River Thame, England

The effects of the growth of the dominant species of macrophytes on phosphorus transport in the River Thame, a nutrient enriched lowland clay catchment in southern England, were assessed using a mass-balance approach. Macrophyte abundance was estimated between late March and early June at three sites along the river. the plant biomass of phosphorus and the total phosphorus content of sediments in the main part of the river channel and sediments associated with the plant roots were measured on each occasion. Total phosphorus concentrations in the river water were measured at weekly intervals at a gauging station at the lower end of the study reach. The results showed that the phosphorus content of plants at the site most impacted by sewage derived phosphorus was < 1% of the flux of total phosphorus estimated for the water column in April and May. Phosphorus contained in the total standing crop of macrophytes at the least impacted site was estimated as approximately 3% of the mean daily flux of total phosphorus in the water. Although no trends were found in the total phosphorus contents of the roots, shoots or sediments through the season, differences in the phosphorus content of the sediments were found between sites with different phosphorus loading. Higher porewater and Equilibrium Phosphate Concentrations (EPCoS) were recorded at the site most impacted by sewage effluent when compared to an upstream sampling site.

Factors influencing the measurement of equilibrium phosphate concentrations in river sediments

A river sediment was treated to obtain different oxic conditions, and the phosphorus status investigated by experiments in which KH 2PO 4 was added to suspensions in calcium chloride. The pH, redox and solution composition of dissolved Ca, Mg, Fe, Na, K, Mn, Cl and SO 4 and soluble reactive phosphorus were measured at the end of 24 h mixing. The equilibrium phosphate concentration was calculated from these measurements using the linear, Freundlich and dual-site Langmuir isotherm equations. The chemical speciation of the solutions was evaluated using PHREEQC and a model designed specifically for phosphorus speciation. These were used to estimate the solution saturations with respect to various iron and calcium containing minerals. The results show that at least a two-fold increase in the calculated equilibrium phosphate concentration of the sediment occurs with a moderate change in the electron activity from 8 to 4 as the sediments became reducing. Chemical precipitation and dissolution reactions were also found to be important during the experiments, with dissolved iron concentrations in oxic conditions determined by equilibration with amorphous iron hydroxide. Only in the suspensions with the highest soluble reactive phosphorus concentration was the solubility limit of ferric phosphate approached. There was also evidence for the loss of calcium from the solutions and oversaturation with respect to octacalcium phosphate. Vivianite, found in the sediment, was also expected to dissolve in the most oxic conditions. It is proposed that ancillary measurements of pH, redox potential, dissolved Ca, Fe, Mn and alkalinity during equilibrium phosphate concentration determinations will provide a better understanding to the buffering reactions controlling phosphorus in solutions in contact with sediments.

Interactions of phosphorus with sediments in the River Swale, Yorkshire, UK

The ultimate fate of inorganic phosphorus in natural waters is to a large degree determined by interactions with sediments. The interactions in river bed sediments have been investigated by a combination of methods using surface sediments, <2 mm in diameter, collected from a 55 km stretch of the River Swale in Yorkshire, UK. The sediments were analysed by digestion (Fe, Ca and P), clay mineralogy and their equilibrium phosphate concentration. All the equilibrium phosphate concentrations were below 2 μmol dm−3, which is much less than the concentration in the overlying water and indicates a net uptake of phosphorus by the sediments. Experiments in a fluvarium channel with a coarse sediment also showed that the release and uptake of soluble phosphorus was towards a steady-state concentration in the overlying solution, consistent with measurements of the equilibrium phosphate concentration. A kinetic model describing the net uptake of soluble reactive phosphorus by river sediments was applied to chemical data collected during an intensive monitoring exercise in the catchment. Copyright © 1999 John Wiley & Sons, Ltd.

SEDIMENT-PHOSPHORUS RELATIONSHIPS IN THREE NORTHCENTRAL OKLAHOMA STREAMS

Sediment characteristics related to their potential for P uptake and buffering capacity were examined in threeadjacent Oklahoma streams in order to determine if stream sediments were the dominant controllers of water column Pand if benthic sediments were capable of buffering growing P loads to these systems. Sediment and water samples werecollected from Council Creek, Feather Creek and Little Stillwater Creek in northcentral Oklahoma. Particle sizedistribution, equilibrium phosphate concentration (EPC0), phosphorus sorption index (PSI), and exchangeable-P weremeasured for a single date for benthic sediments from each site. There were no detectable differences in particle sizecomposition; however, sediment attributes related to P varied significantly among sites. Sediments from Little StillwaterCreek had higher exchangeable P content and EPC0 relative to the other two sites. There were no consistent relationshipsbetween exchangeable P and sediment or water characteristics, while EPC0 was significantly correlated with percent silt.Sediment and water phosphate pools were in equilibrium at only one site. At the other two sites, either the sediment/waterbalance was temporarily in disequilibrium, or sediments were not the major determinant of water column P. In contrast tothe other sediment metrics, PSI did not vary among sites. Total sorption potential was attributed to both abiotic and bioticuptake, with biotic removal becoming increasingly important for sediments with high PSI values. Disequilibrium betweensediment and water column P and the relative importance of biotic sorption to PSI measurements suggest that factorsother than, or in addition to, physical and geochemical processes associated with benthic sediments, are important incontrolling streamwater P concentration and retention.