Phosphorus release from resuspended sediment in the shallow and wind-exposed Lake Arresø, Denmark

Wind-induced sediment resuspension occurs frequently in the shallow and eutrophic Lake Arresø, Denmark. The impact of resuspension on internal phosphorus loading was investigated by laboratory experiments studying P-release from the undisturbed sediment surface and by experiments simulating resuspension events.Phosphorus release from undisturbed sediment sampled in May and August was 12 mg and 4 mg m^–2 d^–1, respectively. During experimental simulation of resuspension, soluble reactive phosphate (SRP) increased by 20–80 µg l^–1, which indicates that a typical resuspension event in the lake would be accompanied by the release of 150 mg SRP m^–2. The internal P loading induced by resuspension is estimated to be 60–70 mg m^–2 d^–1, or 20–30 times greater than the release from undisturbed sediment.SRP release during simulation of resuspension was mainly dependent on the equilibrium conditions in the water column and was basically independent of the increase in suspended solids and the duration of resuspension. A second simulation of resuspension conducted 26 hours later, did not result in any further release of SRP from sediment sampled in May. In contrast, there was an additional SRP release from sediment sampled in August, indicating that an exchangable P pool, capable of altering equilibrium conditions, is built up between resuspension events.It is concluded that resuspension, by increasing the P flux between sediment and water, plays a major role in the maintenance of the high nutrient level in Lake Arresø. A relatively high release rate is maintained during resuspension because of the low Fe:P ratio and the high concentration of NH₄Cl-extractable P in the sediment.     

Phosphorus release from sediments in a river-valley reservoir in the northern Great Plains of North America

Aside from a companion investigation to this study, there are currently no peer-reviewed phosphorus (P) release rate data for northern North American (i.e., Canadian) reservoirs. Using Lake Diefenbaker, Saskatchewan, Canada as a case study, we tested the effect of variation in overlying water DO conditions on the P release rates from sediment cores. Sediment cores from four down-reservoir locations in Lake Diefenbaker were incubated under high (>8 mg l⁻¹), low (2–3 mg l⁻¹), or anoxic (<1 mg l⁻¹) DO concentrations. Sediment cores were then analyzed for total P (TP) and three geochemical P fractions to assess how the DO regime influenced sediment P inventory. Maximum P release rates were highest under anoxic conditions and similar among sites (15.0–20.3 mg m⁻² day⁻¹), with the low-DO rates intermediate to the high-DO and anoxic P fluxes. Predictive internal P loading models considering only hypolimnetic anoxia may therefore oversimplify and thus underestimate P mobilization in situ. Non-apatite inorganic P (54 ± 10% across sites) from the top 1 cm of the sediment profile was the main source of P released during incubations, indicating that sampling on a coarser scale of resolution could obscure the relationship between sediment geochemistry and short-term P flux.

     

High variability in iron-bound organic carbon among five boreal lake sediments

Being both stable carbon sinks and greenhouse gas sources, boreal lake sediments represent significant players in carbon (C) cycling. The release of dissolved organic carbon (DOC) into anoxic water is a widespread phenomenon in boreal lakes with impact on sediment C budgets. The association of OC with iron (Fe) is assumed to play an important role for this anoxic OC release via the dissimilatory reduction of Fe, but also to influence the stabilization of OC in sediments. To investigate the role of Fe–OC association for OC dynamics in different boreal lake sediments, we compared the content of Fe-bound OC [Fe–OC, defined as citrate bicarbonate dithionite (CBD) extractable OC] and the extent of reductive dissolution of solid-phase Fe and OC at anoxia. We found high among-lake variability in Fe–OC content, and while the amount of Fe–OC was high in three of the lakes (980–1920 µmol g^?1), the overall contribution of Fe–OC to the sediment OC pool in all study lakes was not higher than 11%. No linkages between the amount of the Fe–OC pool and lake or sediment characteristics (e.g., pH, DOC concentration, sediment OC content, C:N ratio) could be identified. The observed release of OC from anoxic sediment may be derived from dissolution of Fe–OC in the lake sediments with high Fe–OC, but in other lake sediments, OC release during anoxia exceeded the sediment Fe–OC pool, indicating low contribution of reductive Fe dissolution to OC release from these lake sediments. The range of the investigated boreal lakes reflects the high variability in the size of the sediment Fe–OC pool (0–1920 µmol g^?1) and CBD-extractable Fe (123–4050 µmol g^?1), which was not mirrored in the extent of reductive dissolution of Fe (18.9–84.6 µmol g^?1) and OC (1080–1700 µmol g^?1) during anoxia, suggesting that Fe-bound OC may play a minor role for sediment OC release in boreal lakes. However, studies of redox-related OC cycling in boreal lake sediments should consider that the amount of Fe–OC can be high in some lakes.     

Phosphorus sorption characteristics of sediment in the Simiyu and Kagera River basins: implications for phosphorus loading into Lake Victoria

As part of a larger study to assess the influence of land use on riverine and atmospheric phosphorus (P) loading to Lake Victoria, P sorption characteristics of eight composite bottom sediment samples from the Simiyu and Kagera rivers were determined using the Langmuir equation. The samples had low to medium Langmuir adsorption maxima (Γ_m), ranging from 107 to 201μg g^?1. Langmuir binding energy co-efficient (K) ranged from 60 to 181μg l^?1 and the equilibrium P concentration at zero sorption (EPC₀) from 0.1 to 2.75μg g^?1. By using Langmuir co-efficients derived from P sorption experiments and soluble reactive phosphorus (SRP) concentrations measured in rivers as well as the in-shore waters of Lake Victoria, it was possible to determine the potential release of SRP into the lake by sediment from the two catchments. For the 2000 water-year, it was estimated that about 28.65 ± 0.89 (mean ± SD) and 66 ± 6.76 tons of SRP were released into Lake Victoria by sediment deposited by the Simiyu and Kagera rivers, respectively. The implications of these results to future management of cultural eutrophication in Lake Victoria are also discussed.     

Soluble reactive phosphorus (SRP) transport and retention in tropical, rain forest streams draining a volcanic landscape in Costa Rica: in situ SRP amendment to streams and laboratory studies

Soluble reactive phosphorus (SRP) transport/retention was determined in two rain forest streams (Salto, Pantano) draining La Selva Biological Station, Costa Rica. There, SRP levels can be naturally high due to groundwater enriched by geothermal activity within the surfically dormant volcanic landscape, and subsequently discharged at ambient temperature. Combined field and laboratory approaches simulated high but natural geothermal SRP input with the objective of estimating the magnitude of amended SRP retention within high and low SRP settings and determining the underlying mechanisms of SRP retention. First, we examined short-term SRP retention/transport using combined SRP-conservative tracer additions at high natural in situ concentrations. Second, we attempted to observe a DIN response during SRP amendment as an indicator of biological uptake. Third, we determined SRP release/retention using laboratory sediment assays under control and biologically inhibited conditions. Short-term in situ tracer-SRP additions indicated retention in both naturally high and low SRP reaches. Retention of added SRP mass in Upper Salto (low SRP) was 17% (7.5 mg-P m⁻² h⁻¹), and 20% (10.9 mg-P m⁻² h⁻¹) in Lower Salto (high SRP). No DIN response in either nitrate or ammonium was observed. Laboratory assays using fresh Lower Salto sediments indicated SRP release (15.4 ± 5.9 μg-P g dry wt.⁻¹ h⁻¹), when incubated in filter sterilized Salto water at ambient P concentration, but retention when incubated in filter sterilized river water amended to 2.0 mg SRP l⁻¹ (233.2 ± 5.8 μg-P g dry wt.⁻¹ h⁻¹). SRP uptake/release was similar in both control- and biocide-treated sediments indicating predominantly abiotic retention. High SRP retention even under biologically saturated conditions, absence of a DIN response to amendment, patterns of desorption following amendment, and similar patterns of retention and release under control and biologically inhibited conditions all indicated predominantly abiotic P flux.     

Benthic nutrient fluxes in a eutrophic,polymictic lake

Sediment release rates of soluble reactive phosphorus (SRP) and ammonium (NH₄) were determined seasonally at three sites (water depth 7, 14 and 20 m) in Lake Rotorua using in situ benthic chamber incubations. Rates of release of SRP ranged from 2.2 to 85.6 mg P m⁻² d⁻¹ and were largely independent of dissolved oxygen (DO) concentration. Two phases of NH₄ release were observed in the chamber incubations; high initial rates of up to 2,200 mg N m⁻² d⁻¹ in the first 12 h of deployment followed by lower rates of up to 270 mg N m⁻² d⁻¹ in the remaining 36 h of deployment. Releases of SRP and NH₄ were highest in summer and at the deepest of the three sites. High organic matter supply rates to the sediments may be important for sustaining high rates of sediment nutrient release. A nutrient budget of Lake Rotorua indicates that internal nutrient sources derived from benthic fluxes are more important than external nutrient sources to the lake.     

Phosphorus dynamics during decomposition of mangrove (Rhizophora apiculata) leaves in sediments

Rhizophora apiculata leaf litter decomposition and the influence of this process on phosphorus (P) dynamics were studied in mangrove and sand flat sediments at the Bangrong mangrove forest, Phuket, Thailand. The remaining P in the mangrove leaf litter increased with time of decomposition to 174% and 220% of the initial amount in the litter in sand flat and mangrove sediment, respectively, although about 50% of the dry weight had been lost. The incorporation of P into the litter was probably associated with humic acids and metal bridging, especially caused by iron (Fe), which also accumulated in considerable amounts in the litter (5-10 times initial concentration). The addition of leaves to the sediment caused increased concentrations of dissolved reactive phosphate (DRP) in the porewater, especially in sand flat sediment. The DRP probably originated from Fe-bound P in the sediment, because decomposition of buried leaf litter caused increased respiration and reduced the redox potential (E_h) in the sediments. Binding of P to refractory organic material and oxidized Fe at the sediment-water interface explains the low release of DRP from the sediment. This mechanism also explains the generally low DRP concentration in the mangrove porewater, the low nutrient content of the R. apiculata leaves, but also the higher total sediment P concentration of the mangrove sediment as compared to sediments outside the mangrove. Both the low release rates for DRP from the sediment and the accumulation of P associated with leaf litter decomposition tend to preserve P in the sediments.     

Dissolved phosphorus concentrations and sediment interactions in effluent–dominated Ozark streams

Phosphorus (P) loads from point sources have a significant influence on dissolved P concentrations in streams and sediment-water column dynamics. The goal of this study was to quantify dissolved P concentrations and sediment-P interactions in Ozark (USA) headwater streams with high point source P loads. Specifically, the objectives were to: (1) compare soluble reactive P (SRP) upstream and downstream from wastewater treatment plant (WWTP) effluent discharges; (2) examine longitudinal gradients in SRP downstream from WWTPs; (3) evaluate the effect of WTTP P inputs on sediment-water column P equilibrium and sediment exchangeable P. Water and sediment samples were collected, extracted and analyzed from July 2002 through June 2003 at these Ozark streams. Mean SRP concentrations in the select Ozark streams were significantly greater downstream from effluent discharges (0.08–2.10 mg L⁻¹) compared to upstream (0.02–0.12 mg L⁻¹). Effluent discharge from the WWTPs increased equilibrium concentrations between stream sediments and the water column; mean sediment equilibrium P concentration (EPC₀) was between 0.01–0.07 mg L⁻¹ upstream from WWTP and the increase downstream was proportional to that observed in water column SRP. Sediment exchangeable P (EXP) was greater downstream from the effluent discharges (0.3–6.8 mg kg⁻¹) compared to upstream (0.03–1.4 mg kg⁻¹), representing a substantial transient storage of P inputs from WWTPs. Furthermore, P was generally not retained in these stream reaches when dilution was considered using a hydrologic tracer and was released in one stream reach where effluent P concentrations decreased over the study period. Thus, the effect of the WWTPs was profound in these streams increasing water column and sediment-bound P, and also reducing the ability of these stream reaches to retain P. In P-enriched streams, effluent P discharges likely regulate sediment and aqueous phase P equilibrium and sediment bioavailable P, not the sediments.     

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.     

Phosphorus dynamics and loading in the turbid Minnesota River (USA): controls and recycling potential

Phosphorus (P) dynamics in the agriculturally-dominated Minnesota River (USA) were examined in the lower 40 mile reach in relation to hydrology, loading sources, suspended sediment, and chlorophyll to identify potential biotic and abiotic controls over concentrations of soluble P and the recycling potential of particulate P during transport to the Upper Mississippi River. Within this reach, wastewater treatment plant (WWTP) contributions as soluble reactive P (SRP) were greatest during very low discharge and declined with increasing discharge and nonpoint source P loading. Concentrations of SRP declined during low discharge in conjunction with increases in chlorophyll, suggesting biotic transformation to particulate P via phytoplankton uptake. During higher discharge periods, SRP was constant at ~0.115 mg l⁻¹ and coincided with an independently measured equilibrium P concentration (EPC) for suspended sediment in the river, suggesting abiotic control over SRP via phosphate buffering. Particulate P (PP) accounted for 66% of the annual total P load. Redox-sensitive PP, estimated using extraction procedures, represented 43% of the PP. Recycling potential of this load via diffusive sediment P flux under anoxic conditions was conservatively estimated as ~17 mg m⁻² d⁻¹ using published regression equations. The reactive nature and high P recycling potential of suspended sediment loads in the Minnesota River has important consequences for eutrophication of the Upper Mississippi River.     

Eight years of internal phosphorus loading and changes in the sediment phosphorus profile of Lake Søbygaard,Denmark

During each of the first 8 years following an 80–90% reduction in external phosphorus loading of shallow, hypertrophic Lake Søbygaard, Denmark in 1982, phosphorus retention was found to be negative. Phosphorus release mainly occurred from April to October, net retention being close to zero during winter. Net internal phosphorus loading was 8 g P m^–2 y^–1 in 1983 and slowly decreased to 2 g P m^–2 y^–1 in 1990, mainly because of decreasing sediment phosphorus release during late summer and autumn. The high net release of phosphorus from Lake Søbygaard sediment is attributable to a very high phosphorus concentration and to a high transport rate in the sediment caused by bioturbation and gas ebullition. Sediment phosphorus concentration mainly decreased at a depth of 5 to 20 cm, involving sediment layers down to 23 cm. Maximum sediment phosphorus concentration, which was 11.3 mg P g^–1 dw at a depth of 14–16 cm in 1985, decreased to 8.6 mg P g^–1 dw at a depth of 16–18 cm in 1991. Phosphorus fractionation revealed that phosphorus release was accompanied by a decrease in NH₄Cl-P + NaOH-P and organic phosphorus fractions. HCl-P increased at all sediment depths. The Fe:P ratio in the superficial layer stabilized at approximately 10. Net phosphorus release can be expected to continue for another decade at the present release rate, before an Fe:P ratio of 10 will be reached in the sediment layers from which phosphorus is now being released.     

Pore water dynamics in the sediment of a shallow and hypertrophic lake

Seasonal variations in pore water with main stress on pH and phosphate were investigated in the sediment of the shallow and hypertrophic Lake Søbygaard, Denmark. The purpose was to evaluate factors affecting the internal phosphorus loading. Pore water was obtained by in situ incubation of ceramic cups, sampled anaerobicaly from a fixed position in the sediment. The method is evaluated. During summer, pH and phosphate concentrations increased in the upper 8–10 cm of the sediment. Increased pH was most pronounced in the upper 5 cm, where pH increased to between 9 and 10. This is believed to be caused by the photosynthetically elevated pH in the above lake water. Phosphate concentrations increased with depth, from 0–2 mg P 1^–1 in the upper 5 cm to 3–6 mg P 1^–1 in 6–10 cm depth. Average phosphate gradient in the upper 6–8 cm was 1.0 mg P 1^–1 cm ^–1 in the summer decreasing to 0.2 mg P 1^–1 cm ^t1 in the autumn/winter. In spite of low redox potential, Fe(II) was not present in the upper 20 cm. The seasonal variation in pore water phosphate is believed mainly to be due to the variations in pore water pH inducing a substitution of phosphate ions with hydroxyl ions on ironhydroxides during summer. A considerable sedimentation of organic bound phosphorus and decomposition in the sediment is also considered important. Phosphorus release from the sediment is facilitated by bio- and gas turbation and by the frequent occurrence of resuspension caused by windaction. Net release rate is highly variable during the season. The summer average is 40 mg P m^–2 d^–1.     

Phosphorus release from sediments in a treatment wetland: Contrast between DET and EPC0 methodologies

Wetlands are capable of reducing nutrient loadings to receiving water bodies, and hence many artificial wetlands have been constructed for wastewater nutrient removal. In this study, diffusive equilibrium in thin films (DETs) and equilibrium phosphorus concentration (EPC₀) analysis were used to examine the role of sediment as a nutrient source or sink in a constructed treatment wetland in summer. The effect of dredging on sediment-water nutrient exchange was also studied. Soluble reactive phosphorus (SRP), ammonium (NH₄⁺) and sulphate (SO₄²⁻) concentration profiles were measured by DET across the sediment-water interface (SWI) in both a settling pond and iris reed bed within the wetland. The SRP concentrations in the sediment pore-waters of the settling pond were extremely high (up to 29,500 μg l⁻¹) near the SWI. This is over an order of magnitude higher than the levels found in the water column, which in turn are over an order of magnitude higher than environmental levels proposed to limit eutrophication in rivers. The profiles demonstrated an average net release of SRP and NH₄⁺ from the settling pond sediment to the overlying water of 58 mg m⁻² d⁻¹ (±32 mg m⁻² d⁻¹ (1 sd)) and 16 mg m⁻² d⁻¹ (±25 mg m⁻² d⁻¹ (1 sd)), respectively. The DET SO₄²⁻ concentration profiles revealed that the sediment was anoxic within 2 cm of the SWI. Dredging of the reed bed made no significant difference to the P release characteristics across the SWI. The EPC₀s were much lower than the SRP concentration of the overlying water, indicating that the sediment had the potential to act as a phosphate sink. The apparent contradiction of the DET and EPC₀ results is attributed to the fact that DET measurements are made in situ, where as EPC₀ measurements are ex situ. These results show that substantial releases of P can occur from wetland sediments, and also highlight the need for caution when interpreting ex situ EPC₀ analytical results.     

Seasonal phosphorus dynamics in the surficial sediment of two shallow temperate lakes: a solid-phase and pore-water study

This study examined phosphorus (P) dynamics by surveying the solid-phase and pore-water of the surficial sediment in a mesotrophic and eutrophic shallow lake in Maine, USA. Both lakes were dimictic, developed hypolimnetic anoxia, and released sedimentary P. We examined the controls on sedimentary P release by considering two possible mechanisms; mineralization and release of sedimentary organic P, and dissolution of Fe hydroxide following the onset of anoxia. The temporal investigation of solid-phase included sequential chemical extraction and ³¹P NMR analysis, and the pore-water included use of equilibrium samplers. In both lakes, the relative contribution of organic P to total sedimentary P release was minor compared to Fe hydroxide-associated P. The eutrophic lake, however, had more evidence of microbial uptake of sedimentary P and a higher degree of rapid P transformation in the water column. Sediment polyphosphates were dynamic and possibly contributed to hypolimnetic P accumulation. The pore-water Fe and P profiles exhibited similar temporal patterns as the solid-phase results. Together, they showed an upward migration of the redoxcline in the sediment, from winter to summer, resulting in the accumulation of Fe-bound P at the sediment?Cwater interface in the winter followed by its release into the summer.     

Seasonal Variations in Chlorophyll a Concentrations in Relation to Potentials of Sediment Phosphate Release by Different Mechanisms in a Large Chinese Shallow Eutrophic Lake (Lake Taihu)

The relationship between chlorophyll a and fractionation of sediment phosphorus, inorganic phosphate-solubilizing bacteria (IPB), and organic phosphate-mineralizing bacteria (OPB) was evaluated in a large Chinese shallow eutrophic lake (Lake Taihu) and its embayment (Wuli Bay). At the three study sites, the increase of chlorophyll a concentrations in April paralleled those of the iron bound phosphate accounting for major portion of sediment inorganic phosphate, and in June significantly higher OPB and IPB numbers (especially OPB) in sediment were main contributors to the peaks of chlorophyll a concentration. Even though IPB peaked from February to June, it should serve as an unimportant P source due to the irrelevancy with chlorophyll a and soluble reactive phosphorus (SRP). By contrast, at the other site in the embayment, the calcium-bound phosphate was predominant and solid, which was difficult to be released, and neither IPB nor OPB were detectable in the sediment, indicating weak potential for phosphorus release from the sediment, which was reflected in the small seasonal variation in SRP concentration in water column. Hence, the extents to which the three general mechanisms behind phosphate release from sediment (desorption of iron bound phosphate, solubilization by IPB and enzymatic hydrolysis by OPB) operated were different depending on seasons and sites in Lake Taihu, they may jointly drive phosphate release and accelerate the eutrophication processes.     

Sulphide release from anoxic sediments in relation to iron availability and organic matter recalcitrance and its effects on inorganic phosphorus recycling

This research aims to analyse the sediment capacity to buffer free sulphide release in three coastal lagoons which differ in terms of eutrophication level, tide influence and primary producer communities. A preliminary estimate of soluble reactive phosphorus (SRP) regeneration coupled with sulphide fluxes is also made. Sediment profiles of ferrous and ferric iron and reduced sulphur pools were determined in three stations in the Bassin d'Arcachon (South West France), in one site in the Etang du Prévost lagoon (Southern France), and in three stations in the Sacca di Goro lagoon (Northern Italy). Laboratory experiments were also conducted by incubating sediment slurries. Slurries from the French lagoons were also enriched with about 2% d.w. of organic detritus obtained from the dominant macrophytes of each site, namely Zostera noltii and Ruppia cirrhosa (Bassin d'Arcachon), and Ulva rigida (Etang du Prévost). In the Sacca di Goro, slurry experiments were conducted at two sites with different salinity range, sediment composition and hydrodynamics.Field data showed that concentrations of available iron (Fe(II)+Fe(III)) ranged from a minimum of 28.5 µmol cm^–3 (Etang du Prévost) to a maximum of 275.7 µmol cm^–3 (Sacca di Goro). Moreover, in the French lagoons, acid volatile sulphide (AVS) accumulation in the superficial sediment was related to ferrous iron concentrations. Laboratory experiments showed that in spite of strong reducing conditions, sulphide and SRP release was weaker in iron-rich sediments and in those enriched with the most refractory organic matter. The highest fluxes were detected in sediment slurries from the Etang du Prévost, which had the lowest iron content, supplied by 2% of the labile detritus from Ulva rigida. In this case, SRP release was directly related to sulphide production.Two factors seem significant to evaluate the buffer capacity against free sulphide and SRP release from anoxic sediment: organic matter biodegradability, which forces sediment toward reducing conditions, and iron availability, which can affect sulphide mobility as well as the iron hydroxide-phosphate-sulphide system.     

Factors influencing phosphate exchange across the sediment-water interface of eutrophic reservoirs

The results of a survey of the sediment chemistry of 7 East Anglian reservoirs are presented as part of a regional study on the assessment and control of eutrophication. The influence of water quality (dissolved oxygen, pH, temperature) on phosphate (PO₄) adsorption by sediment from hypertrophic Ardleigh Reservoir is also examined. Extractable phosphate-P (extr.-P) varied between 92 and 383 mg kg^–1 dry matter. Extractable P varied between 5.3 and 16.6% of the total phosphate-P (Tot. P) content and increased with the concentration of dissolved reactive phosphate-P (DRP) in the overlying water column. Organically complexed iron (organic Fe) was the determinand which correlated most closely with phosphate adsorption capacity, PAC (r = 0.8). Organic Fe was also related inversely to Extr. P. The rate and extent of PO₄ adsorption by Ardleigh Reservoir sediment increased with the initial concentration of DRP and adsorption equilibria were reached after 24 h. The equilibrium DRP concentration, [DRP], was 0.7 mg P 1^–1 under aerobic conditions indicative of a high potential for PO₄ exchange. The rate and extent of PO₄ adsorption was greater at 7 °C than at 22 °C PO₄ adsorption increased markedly with dissolved oxygen status. Ardleigh sediment exhibited a marked buffering capacity to a change in pH; however, PO₄ adsorption was greatest at an equilibrium pH of 5.6 and decreased progressively either side of this pH value.Options for the artificial control of sediment PO₄ release are discussed in relation to the seasonal variation in sediment PO₄ exchange observed for Ardleigh Reservoir.     

Some remarks on the presence of organic phosphates in sediments

This article describes a new method developed to assess the size and nature of the organic phosphate pool. Using sediment suspensions from the Rhone, Garonne and Po rivers, inorganic P compounds, Fe(OOH) and CaCO₃ were removed using mild extractants at sediment pH. The residual phosphate was then fractionated into an acid soluble organic phosphate pool and a residual organic phosphate pool by acid hydrolysis (0.5 M H⁺). Both pools were quantitatively important, accounting for between 16 and 54% and 16 and 51% of total phosphate respectively. Acid hydrolysis was chosen since it yielded a distinct plateau, with high reproducibility, within 30 minutes.This fractionation permits a further study of dynamics and bioavailability of sediment org-P, without interference of Fe(OOH) and CaCO₃.In many studies in which changes in the organic pool were examined after extraction of inorganic phosphate, 0.5 M HCl was used to extract apatite bound phosphate. The results presented here show that this is likely to result in a considerable underestimation of the organic phosphate pool.     

Modulation of transmitter release from the locust forewing stretch receptor neuron by GABAergic interneurons activated via muscarinic receptors

The role of muscarinic receptors in the down‐regulation of acetylcholine (ACh) release from the locust forewing stretch receptor neuron (fSR) terminals has been investigated. Electrical stimulation of the fSR evokes monosynaptic excitatory postsynaptic potentials (EPSPs) in the first basalar motoneuron (BA1), produced mainly by the activation of postsynaptic nicotinic cholinergic receptors. The general muscarinic antagonists scopolamine (10⁻⁶ M) and atropine (10⁻⁸ to 10⁻⁶ M) caused a reversible increase in the amplitude of electrically evoked EPSPs. However, scopolamine (10⁻⁶ M) caused a slight depression in the amplitude of responses to ACh pressure‐applied to the soma of BA1. These observations indicate that the EPSP amplitude enhancement is due to the blockade of muscarinic receptors on neurons presynaptic to BA1. The muscarinic receptors may be located on the fSR itself and act as autoreceptors, and/or they may be located on GABAergic interneurons which inhibit ACh release from the fSR. Electron microscopical immunocytochemistry has revealed that GABA‐immunoreactive neurons make presynaptic inputs to the fSR. The GABA antagonist picrotoxin (10⁻⁶ M) caused a reversible increase in the EPSP amplitude, which does not appear to be due to an increase in sensitivity of BA1 to ACh, as picrotoxin (10⁻⁶ M) slightly decreased ACh responses recorded from BA1. Application of scopolamine (10⁻⁶ M) to a preparation preincubated with picrotoxin did not cause the EPSP amplitude enhancement normally seen in control experiments; in fact, it caused a slight depression. This indicates that at least some of the presynaptic muscarinic receptors are located on GABAergic interneurons that modulate transmission at the fSR/BA1 synapse. © 1999 John Wiley & Sons, Inc. J Neurobiol 40: 420–431, 1999     

Phosphorus Fractions and Fluxes in the Water Column and Sediments of a Tropical Reservoir (Lobo‐Broa – SP)

Sedimentation rates of organic and inorganic matter, chlorophyll a, P fractions, Ca, Mn, Fe and Al, were determined by sediment traps in a tropical oligo‐mesotrophic reservoir of São Paulo (Brazil). Vertical profiles of the sediments were analyzed for organic content, metals, P and surface P fraction composition. Estimated mean sedimentation rates, corrected for resuspension were: total solids, 1068 g m^—2 y^—1 (OM = 44.7%); chlorophyll a, 2.1 g m^—2 y^—1 and total phosphorus, 2.9 g m^—2 y^—1. The predominant P fraction in the settling flux was associated with aluminum minerals while surface sediments were dominated by organic P. The reservoir exhibited low sediment retention of P (13.0%), Al (9.9%), Fe (9.9%), Mn (1.4%) and Ca (traces), compared to trap sedimentation. This feature was related with a high vertical dynamics (resuspension and bottom release) and with the low retention time of the system.