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 approximately 2.5 microM, gravel approximately 6.5 microM, and stones approximately 65.0 microM (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 approximately 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.

Seasonal nutrient dynamics in a chalk stream: the River Frome, Dorset, UK.

Chalk streams provide unique, environmentally important habitats, but are particularly susceptible to human activities, such as water abstraction, fish farming and intensive agricultural activity on their fertile flood-meadows, resulting in increased nutrient concentrations. Weekly phosphorus, nitrate, dissolved silicon, chloride and flow measurements were made at nine sites along a 32 km stretch of the River Frome and its tributaries, over a 15 month period. The stretch was divided into two sections (termed the middle and lower reach) and mass balances were calculated for each determinand by totalling the inputs from upstream, tributaries, sewage treatment works and an estimate of groundwater input, and subtracting this from the load exported from each reach. Phosphorus and nitrate were retained within the river channel during the summer months, due to bioaccumulation into river biota and adsorption of phosphorus to bed sediments. During the autumn to spring periods, there was a net export, attributed to increased diffuse inputs from the catchment during storms, decomposition of channel biomass and remobilisation of phosphorus from the bed sediment. This seasonality of retention and remobilisation was higher in the lower reach than the middle reach, which was attributed to downstream changes in land use and fine sediment availability. Silicon showed much less seasonality, but did have periods of rapid retention in spring, due to diatom uptake within the river channel, and a subsequent release from the bed sediments during storm events. Chloride did not produce a seasonal pattern, indicating that the observed phosphorus and nitrate seasonality was a product of annual variation in diffuse inputs and internal riverine processes, rather than an artefact of sampling, flow gauging and analytical errors.

Cadmium absorption and retention by rats fed durum wheat (Triticum turgidum L. var. durum) grain.

A whole-body radioassay procedure was used to assess the retention and apparent absorption by rats of Cd in kernels of durum wheat (Triticum turgidum L. var. durum) harvested from plants grown hydroponically in 109Cd-labelled nutrient solution. Wholegrain wheat, containing 5 micromol Cd (570 microg)/kg dry weight labelled intrinsically with 109Cd, was incorporated into test meals fed to rats that had been maintained on diets containing marginally adequate, adequate or surplus levels of Zn (0.12 mmol (8 mg), 0.43 mmol (28 mg) or 1.55 mmol (101 mg) Zn/kg respectively), and either 0 or 50 g durum wheat/kg. Regardless of diet, all rats consumed about 99 % of the test meal offered. In rats fed diets without wheat, initial Cd absorption averaged 7.7, 4.6 and 2.4 % of the dose when the diet contained 0.12 mmol (8 mg), 0.43 mmol (28 mg) or 1.55 mmol (101 mg) Zn/kg diet respectively. In rats fed wheat-containing diets, initial Cd absorption averaged 3.8 and 2.6 % of the dose when dietary Zn concentration was 0.12 mmol (8 mg) and 0.43 mmol (28 mg)/kg diet respectively. The amount of Cd retained in the body at 15 d postprandial was <2 % of the dose in all rats, and decreased as Zn in the diet increased. Even at 15 d postprandial, 32 to 44 % of the Cd retained in the body was still in the gastrointestinal tract. The results show that: (1) the bioavailability to rats of Cd in wholegrain durum wheat was depressed when wholegrain wheat was part of the regular diet; (2) increased intake of dietary Zn lowered Cd absorption and retention; (3) retention of Cd in the body at 15 d postprandial from diets containing adequate Zn was <1.3 %.

The phosphorus budget of the Thame catchment, Oxfordshire, UK: 1. Mass balance.

The relative magnitudes of annual diffuse and point source loads of phosphorus (P) to the River Thame were estimated from daily and monthly measurements of discharge and concentration. Existing data from gauging and monitoring sites on the river network and at point sources were supplemented by survey data at a range of spatial scales. Results showed that during low flow periods most of the P could be attributed to point sources, while at high flows the figure was less than 10%. The introduction of P stripping at Aylesbury, a major sewage treatment works in the catchment, was estimated to have reduced the annual load of P from the sewage treatment works by approximately 45 t, with a similar reduction in loss from the catchment. This gave a reduction in low flow concentrations of soluble reactive phosphorus (SRP) from 2.5 to 1.7 mg l(-1). Concentrations of SRP in river water remain above eutrophication thresholds because of the influence of other STWs in the catchment and insufficient natural discharge to dilute this.

The phosphorus budget of the Thame catchment, Oxfordshire: 2. Modelling.

The phosphorus budget of the River Thame was modelled at a daily time scale, using estimates of diffuse and point source contributions of discharge. The model simulated suspended sediment (SS), soluble unreactive phosphorus (SUP), soluble reactive phosphorus (SRP) and particulate phosphorus (PP) concentrations within the main river and major tributaries. Diffuse source estimates of phosphorus loads were based on characteristic losses from identified main landscape classes, with hydrology described by a simple conceptual storage model. In-stream flow was modelled using a kinematic wave equation. Transfer of suspended sediment and phosphorus components was approximated by advection. In-stream sources and sinks included uptake and release of soluble reactive phosphorus by bed sediment, instant equilibration between SRP and the PP concentration on suspended sediment, and flow-related entrainment and deposition of suspended sediment. Simulations at sites within the catchment were compared with measurements made in 1998-1999. Results showed the P budget is dominated by mixing of diffuse and point source water, but some within-river processes have been shown to be capable of significantly influencing SRP concentrations. The development of a sediment entrainment and deposition component of the model has proved particularly valuable in emulating the hysteretic relationship between discharge and suspended sediment concentration in the river. It also provides a measure of available bed sediment.

Phosphorus loading in the Frome catchment, UK: seasonal refinement of the coefficient modeling approach.

This paper describes the results of an export coefficient modeling approach to predict total phosphorus (TP) loading in the Frome catchment, Dorset, UK from point and diffuse sources on a seasonal (monthly) basis in 1998 and on an annual basis for 1990-1998. The model predicted an annual TP load of 25 605 kg yr(-1), compared with an observed (measured) value of 23400 kg yr(-1). Monthly loads calculated using the export coefficient model agreed well with monthly observed values except in months of variable discharge, when observed values were low, probably due to infrequent, and therefore unrepresentative, sampling. Comparison between filterable reactive phosphorus (FRP) and TP concentrations observed in the period 1990-1997 showed that trends in FRP could be estimated from trends in TP. A sensitivity analysis (varying individual export coefficients by +/-10%) showed that sewage treatment works (STWs) (3.5%), tilled land (2.7%), meadow-verge-seminatural (1.0%), and mown and grazed turf (0.6%) had the most significant effect (percent difference from base contribution) on model prediction. The model was also used to estimate the effect of phosphorus stripping at STWs in order to comply with a pending change in the European Union wastewater directive. Theoretical reduction of TP from the largest STW in the catchment gave a predicted reduction in TP loading of 2174 kg yr(-1). This illustrates the value of this seasonal export coefficient model as a practical management tool.

Comparison of sample storage protocols for the determination of nutrients in natural waters.

There have been several reports on storage protocols for the determination of nutrients in natural waters but each one has been limited to a particular sample matrix and they have reached different, matrix specific conclusions. The aim of this study was therefore to systematically apply the various recommended storage protocols to a range of natural water matrices. Samples from four contrasting sites in the UK, collected in late winter (February, 1999), were filtered and stored under different conditions (-80 degrees C, -20 degrees C, 4 degrees C and at 4 degrees C and -20 degrees C with 0.1% (v/v) chloroform) for up to 247 days prior to analysis. The sites were the River Frome in Dorset (a chalk stream catchment) and three sites from the Tamar Estuary (draining a non-chalk catchment) with salinities of 0.5 per thousand, 10 per thousand and 34 per thousand, Samples and controls were analysed for total oxidised nitrogen (TON) and filterable reactive phosphorus (FRP) using a segmented flow analyser with spectrophotometric detection. To investigate possible seasonal effects (particularly changes in biological and chemical matrix composition). a second sampling campaign was undertaken in early autumn (October, 1999). The results showed that the optimum storage conditions for the determination of TON and FRP were highly matrix dependent. with significant differences in FRP stability between the Frome and Tamar catchments (due to different calcium concentrations) and between samples of different salinities (due to different bacterial populations and/or dissolved organic matter). General guidelines for sample handling and storage are listed and matrix specific recommendations presented for samples rich in calcium and dissolved organic matter.

Study of dissolved silicon and nitrate dynamics in a freshwater stream.

Dissolved reactive silicon and nitrate were measured at weekly intervals over a 3 year period (1991-94) on a 1.2km reach of a gauged Dorset Mill Stream. In addition, dissolved nitrite was measured over a 8 month period from the spring to late autumn in 1992. Two intensive studies with sampling at 2h intervals were also completed in low and high riverflow conditions. The results were analysed using a mass-balance approach with the loss and gains in nutrients dissolved in the water expressed in terms of areal rates. Losses of both nutrients occurred during periods of low streamflow in spring and summer. Losses of silicon are attributed to growth of epilithic diatoms whereas nitrate losses are consistent with a number of processes including the growth of aquatic plants, the development of epilithic biofilms and nitrogen transformations, such as denitrification, in bed-sediments. Stream water gained dissolved nitrite during its passage through the section. Silicon losses from the stream amounted to between 52 and 63 mmol m(-2) d(-1) (expressed per area of bed-sediment) for the spring periods in 1992-94. Nitrate losses were more variable with overall rates between 24 and 89 mmol m(-2) d(-1) for the summer periods in 1991-93.

Seasonal export of phosphorus from a lowland catchment: upper River Cherwell in Oxfordshire, England.

The export of phosphorus from a lowland catchment, the River Cherwell in southern England, was measured over a period of 1 year. The results describe total phosphorus concentrations in the water and river discharge at 4-day intervals. These were used to estimate the load of total phosphorus exported from the catchment. These annual loads were compared with exports estimated from sewage inputs and diffuse inputs calculated from land-coverage data, with assigned phosphorus export coefficients for particular land uses. The method was further developed to examine seasonal changes in phosphorus exports by predicting monthly losses using annual export coefficients normalised with respect to the relative hydraulic runoff for a particular month. The results show a strong seasonal dependence of total exports, with retention of phosphorus in the river system in the spring to early autumn and release of stored material during the winter. This pattern remained true, even with 50% increase in the main land-cover export and a similar increase in treated sewage exports.

Genetic selection for enhanced bioavailable levels of iron in bean (Phaseolus vulgaris L.) seeds.

The bioavailability of Fe from 24 select genotypes of bean (Phaseolus vulgaris L.) seeds containing a range of concentrations of Fe, myo-inositol pentaphosphate plus phytic acid (IP5+IP6), and tannins was studied using a rat model. Bean accessions, selected from field trials for their variations in Fe, phytate, and tannin seed concentrations, were grown in a greenhouse in nutrient solutions radiolabeled with (59)Fe. Mature seeds were autoclaved and lyophilized. Test meals (containing 1 g of dried bean, 0.5 g of sucrose, and 1 g of basal Fe-deficient diet) were fed to marginally Fe-depleted weanling rats over a 3-h period; rats were radioassayed in a gamma-spectrometer immediately after feeding and daily thereafter for the next 10 d. Radioiron retention data were used to calculate percent Fe absorption (i.e., Fe bioavailability) from the meals. Seed Fe concentrations ranged from 52 to 157 microg g(-)(1) dry weight. There was a tendency to also select for higher Zn concentrations in the beans when selecting for high Fe concentrations. The Fe bioavailability to rats from test meals depended on the genotype and varied from 53% to 76% of the total Fe. Bean genotypes with higher seed Fe concentrations resulted in increased amounts of bioavailable Fe to rats. There was no significant correlation between the Fe concentration in different bean genotypes and Fe bioavailability to rats attributable to variations in IP5+IP6 or tannins, even though these antinutrients varied widely (i.e., from 19.6 to 29.2 micromol of IP5+IP6 g(-)(1) and from 0.35 to 2.65 mg of tannins g(-)(1)) in the test meals. Other unknown seed factors (i.e., antinutrients or promoter substances) may be contributing factors affecting Fe bioavailability from bean seeds.

Conclusions to the special issue of Science of the Total Environment concerning 'The water quality of UK rivers entering the North Sea'.

This paper presents an overview of the water quality functioning of eastern UK rivers draining into the North Sea. It summarises the results of three special issues of Science of the Total Environment and related work and complements a companion bibliography (Neal and Turner, 2000, this volume). The rivers are described in straightforward broad terms, and the reader is guided via the bibliography to the special volumes for the detailed findings. The water quality is shown to be highly variable over space and time. The changes over the past 300 years reflect first the transition from a rural society to the major impact of the industrial revolution and subsequently to one of the worlds' most successful environmental cleanups. The present water quality reflects the relative impacts of diffuse agricultural sources, urban and industrial point source discharges together with background inputs associated with geology: all these can be modified by within-river processes associated with physical mixing, biology and inorganic reactions. The work is set within the context of water quality, modelling and flux outputs to the North Sea and contemporary and future environmental research and management needs. A range of environmental impact studies is included. The detailed information is now available on a major and easily accessible database.

Kinetics of zinc metabolism: variation with diet, genetics and disease.

Kinetic studies are used to investigate metabolic processes. By adding an isotope to a system and measuring its movement in the system over time, pool sizes and transport rates can be determined by mathematically modeling the data. This approach enables rate differences to be determined in conditions that have been modified by diet, environment, genetics or disease. Kinetic studies in humans have shown that there are multiple pools of zinc that turnover from minutes to years and that processes, including zinc absorption and excretion, are regulated to maintain tissue levels when zinc intake varies. Animal studies allow for greater understanding of kinetics because more tissues can be sampled and environmental and genetic factors can be controlled. Kinetic studies in animals will provide information on the overexpression or the deletion of genes coding for specific proteins involved in zinc transport and metabolism. The advances that have been made in our understanding of the role of zinc in metabolism have been aided by the development of techniques for measuring isotopes in biological materials. In the future, the kinetics of zinc bound to different compounds will be measured. Modeling will enable this information, at the molecular level, to be integrated with knowledge of zinc metabolism at the cellular, organ and whole body level. To understand more fully the role of zinc in human health, kinetic studies are needed in healthy and disease states to identify differences in metabolic processes. This knowledge can be used as a basis for dietary and therapeutic recommendations.

Phytase improves iron bioavailability for hemoglobin synthesis in young pigs.

Dietary phytase supplementation improves bioavailabilities of phytate-bound minerals such as P, Ca, and Zn to pigs, but its effect on Fe utilization is not clear. The efficacy of phytase in releasing phytate-bound Fe and P from soybean meal in vitro and in improving dietary Fe bioavailability for hemoglobin repletion in young, anemic pigs was examined. In Exp. 1, soybean meal was incubated at 37 degrees C for 4 h with either 0, 400, 800, or 1,200 units (U) of phytase/kg, and the released Fe and P concentrations were determined. In Exp. 2, 12 anemic, 21-d-old pigs were fed either a strict vegetarian, high-phytate (1.34%) basal diet alone, or the diet supplemented with 50 mg Fe/kg diet (ferrous sulfate) or phytase at 1,200 U/kg diet (Natuphos, BASF, Mt. Olive, NJ) for 4 wk. In Exp. 3, 20 anemic, 28-d-old pigs were fed either a basal diet with a moderately high phytate concentration (1.18%) and some animal protein or the diet supplemented with 70 mg Fe/kg diet, or with one of two types of phytase (Natuphos or a new phytase developed in our laboratory, 1,200 U/kg diet) for 5 wk. In Exp. 2 and 3, diets supplemented with phytase contained no inorganic P. In Exp. 1, free P concentrations in the supernatant increased in a phytase dose-dependent fashion (P<.05), whereas free Fe concentrations only increased at the dose of 1,200 U/kg (P<.10). In Exp. 2 and 3, dietary phytase increased hemoglobin concentrations and packed cell volumes over the unsupplemented group; these two measures, including growth performance, were not significantly different than those obtained with dietary supplemental Fe. In conclusion, both sources of phytase effectively degraded phytate in corn-soy diets and subsequently released phytate-bound Fe from the diets for hemoglobin repletion in young, anemic pigs.

Nutrient dynamics in a lowland stream impacted by sewage effluent: Great Ouse, England.

Nutrient and ancillary chemical changes in a stretch of the Great Ouse river near Brackley in Northamptonshire, UK, were measured on a seasonal basis over one year with an initial pilot study in the spring of 1994. River bed-sediments were characterized by their physical, adsorptive and chemical properties and a batch returned to the laboratory for experiments in a fluvarium channel. These experiments involved studies of the release and uptake of soluble reactive phosphorus to or from the overlying water in oxic conditions and release when the dissolved oxygen concentration was reduced to near zero. There was no impact of the point-sources on the concentration of nitrate in the river, a slight effect on the concentration of silicon during low-flow conditions in the summer and net uptake in the spring caused by the growth of diatoms. However there was a substantial impact on phosphorus concentrations, particularly during the summer sampling when the river was in low flow. The results for the winter showed little impact of point discharges because of the high dilution of the treated effluent. The bed-sediments at this time were found to be close to equilibrium with respect to the concentration of soluble reactive phosphorus in the overlying water. Both the fluvarium channel and field measurements obtained in the autumn are consistent with a lower net uptake of phosphorus and degradation of vegetation in the river. In the spring and summer visits, the phosphorus concentrations increased immediately downstream of the main point input and then decreased in concentration at the next downstream site. This effect was particularly noticeable in the summer and was consistent with a large uptake of phosphorus to the bed-sediment and associated vegetation. The contribution of the bed-sediment was estimated using a chemical model describing the uptake kinetics by the Elovich equation and also a parabolic equation. The stability of the waters with respect to calcite and calcium phosphate minerals was assessed in detail. Seasonal changes in the sediment composition were consistent with the deposition of calcite and coprecipitation of inorganic phosphate in the lattice of calcite, either abiotically, or in association with algal biofilms in the sediment. Good correlation between the total phosphorus and calcium contents of the sediments were evident, particularly at the sites furthest from the main sewage input. Measurements of the equilibrium phosphate concentrations of the surface sediments showed that they did not respond quickly to the higher concentrations of dissolved phosphorus found in the summer. It is also evident that the use of the equilibrium phosphate concentration to predict the magnitude of the release of soluble reactive phosphorus becomes less reliable as the solution concentration approaches the equilibrium phosphate concentration. Perturbations may arise because of changes in the surface micro-layer caused by a number of processes such as particle-size fractionation, biological activity or changes in the local redox conditions. However bearing these constraints in mind, with an equilibrium phosphate concentration of the sediments generally below 5 mumol dm-3, the release of phosphorus to the overlying water is not expected until the concentration is below this value. The results also show that phosphorus is not accumulating in the surface sediment and that much of the phosphorus in the sediment is not easily desorbed.

The role of insulin in the regulation of milk protein synthesis in dairy cows.

We examined the role of insulin in milk protein synthesis using the hyperinsulinemic-euglycemic clamp approach in combination with abomasal infusion of casein. The two experimental periods consisted of abomasal infusion of water or 0.5 kg/d of casein. An insulin clamp was conducted over the last 4 d of each period. During the insulin clamp, circulating insulin was elevated fourfold, and euglycemia was maintained by the infusion of exogenous glucose. Casein infusion increased milk yield so that milk protein yield was 10% greater than baseline values. Use of the insulin clamp combined with casein infusion increased milk protein yield by 230 g/d (28% greater than baseline values). Milk protein composition was not altered, but content was increased from 3.13% during the baseline period to 3.44% by d 4 of the clamp; calcium concentration in milk increased about 10% to 1.2 g/kg. During the clamp, circulating concentrations of essential amino acids were dramatically reduced. The most pronounced effects were noted for branched-chain amino acids (64% reduction from baseline values). The insulin clamp resulted in alterations in circulating insulin-like growth factor (IGF)-I concentrations (increase) as well as IGF-II and IGF-binding protein-2 concentrations (decreases). Overall, results indicated that the ability of the mammary gland to synthesize milk protein does not function at maximum capacity, and there is a previously unrecognized potential to enhance milk protein percentage and yield.

Compartmental analysis of zinc kinetics in mature male rats.

A compartmental model of zinc kinetics in mature male rats was developed. The model was based on zinc and radioisotopic zinc measured in samples of plasma, skeletal muscle, kidneys, testes, spleen, bone, and intestinal segments collected at various times for up to 4 days after 65Zn was injected intravenously. Zinc intake, excretion of zinc and 65Zn in urine and feces, and the whole body retention of 65Zn were also determined. Other data used to develop the model included published information on zinc concentration and zinc kinetics in tissues that were not sampled. In the model, the intestinal tract was represented by five compartments. Plasma, spleen, kidneys, and testes were each represented by an individual compartment. In contrast, two compartments each were used to represent exchangeable zinc in liver, bone, skeletal muscle, skin, and red blood cells. The present model extends earlier models of zinc kinetics, describes the distribution of zinc in the whole body, and may provide a means to evaluate the influence of either pathophysiological conditions or dietary extremes on the metabolism of zinc.

An investigation of the effects of water velocity on inorganic phosphorus influx to a sediment.

Measurements of the net influx of soluble reactive phosphorus (SRP), to a river bed-sediment, illustrate the importance of the water velocity and hydrodynamics in controlling the transfer rates. Experiments are reported using a characterised bed-sediment, with associated fauna, contained in a flowing-water channel. The results show a systematic increase in the net influx of SRP with increasing water velocity. A mathematical description of the influx was sought by modelling the experimental results using the Elovich equation, a boundary-layer model and a parabolic rate equation. In fact all three kinetic equations produce a good representation of the experimental data and it is concluded that further research is needed, in well-defined hydrodynamic conditions, to distinguish between the boundary-layer model and the parabolic equation. The boundary-layer model leads to an inverse relationship between the boundary-layer thickness (z/microm), and the water velocity (v/cm s(-1), viz z approximately 2500/v). In comparison, the parabolic equation of the form: influx of SRP (mol m(-2) = kp [SRP-EPC0]2, where EPC0 is the concentration at which the influx is zero prior to the sorption of phosphorus by the sediment and kp is the rate constant which leads to a velocity dependence, kp* = 0.714v + 1 where kp* is the reduced rate constant, kp* = kp(v)/kp(0). The semi-empirical Elovich equation in the form: influx of SRP (mol m(-2)) = (1/b) ln(1 + abt) where a and b are the Elovich parameters and t the time, gives a convenient description of the net influx of SRP to bed-sediments downstream of a point-source of pollution. The parameters calculated from the results obtained from the experimental channel are used to estimate the SRP flux to the sediment for a distance of up to 5 km downstream of a point-input of SRP.

SDS-degrading bacteria attach to riverine sediment in response to the surfactant or its primary biodegradation product dodecan-1-ol.

A laboratory-scale river microcosm was used to investigate the effect of the anionic surfactant sodium dodecyl sulphate (SDS) on the attachment of five Pseudomonas strains to natural river-sediment surfaces. Three of the Pseudomonas strains were chosen for their known ability to express alkylsulphatase enzymes capable of hydrolysing SDS, and the other two for their lack of such enzymes. One strain from each category was isolated from the indigenous bacterial population present in the river sediment used; other isolates were from soil or sewage. The alkylsulphatase phenotypes were confirmed by gel zymography of cell extracts. Addition of SDS to mixed suspensions of river sediment with any one of the biodegradation-competent strains stimulated the attachment of bacteria to the sediment particles. In contrast, the attachment of biodegradation-incompetent strains was weak and, moreover, was unaffected by SDS. The SDS-stimulated attachment for competent organisms coincided with rapid biodegradation of the surfactant. The primary intermediate of SDS biodegradation, dodecan-1-ol, accumulated transiently, and the numbers of attached bacteria correlated closely with the amount of dodecan-1-ol present. Direct addition of dodecan-1-ol also stimulated attachment but the effect was more immediate compared with SDS, when there was a lag period of approximately 2 h. To account for these observations, a model is proposed in which SDS stimulates the attachment of biodegradation-competent bacteria through its conversion to dodecan-1-ol, and it is hypothesized that the observed reversibility of the attachment is due to the subsequent removal of dodecan-1-ol by further bacterial metabolism.