Development of a compartmental model of zinc kinetics in mice.

To investigate zinc (Zn) kinetics in mice, tracer ((65)Zn) was administered orally to 9-wk-old female mice in the fed state and tracer and Zn concentration were measured in 21 tissues over the following 8 d. Data were analyzed by compartmental modeling using WinSAAM. A published model for Zn kinetics in rats was modified to fit the data from mice and to calculate transfer rates and pool sizes of Zn. Parallel studies were performed in mice lacking genes for metallothionein (MT), MT-I and MT-II (MT-/-), to quantify differences in Zn kinetics in the absence of these proteins in vivo. We confirmed that tracer time course in most tissues was similar in wild-type mice and those lacking MT, except for the pancreas of MT-/-, which retained less tracer. By fitting tissue and intestinal data simultaneously, we found that intestinal tracer could be explained by unabsorbed isotope and loss of Zn from pancreas went through plasma. Differences in pancreatic data in MT-/- were explained by Zn turning over twice as fast in this tissue (4 h) compared with wild type (9 h). These kinetic studies provide parameter values for normal, fed mice that can be used to assess Zn kinetics in abnormal conditions, as demonstrated by the higher turnover of Zn in the pancreas of MT knockout mice.

Potential for improving bioavailable zinc in wheat grain (Triticum species) through plant breeding.

A "whole-body" radioassay procedure was used to assess retention and absorption by rats of Zn in mature kernels of whole grain wheat harvested from 28 genotypes (Triticum spp.) grown in nutrient solution supplied with 2 microM ZnSO4 radiolabeled with 65Zn. Grain-Zn concentration differed among genotypes and ranged from 33 to 149 microg g(-1) of dry weight (DW); similarly, grain-Fe concentration varied approximately 4-fold, from 80 to 368 microg g(-1) of DW. Concentrations of Zn and Fe in the grain were positively correlated. Therefore, selecting genotypes high in grain-Zn also tends to increase grain-Fe concentration. Concentrations of myo-inositolhexaphosphate (phytate) in the wheat grain varied from 8.6 to 26.1 micromol g(-1) of DW. Grain intrinsically labeled with 65Zn was incorporated into test meals fed to Zn-depleted rats. All rats readily ate the test meals, so that Zn intake varied directly with grain-Zn concentration. As determined by the percentage of 65Zn absorbed from the test meal, the bioavailability to rats of Zn in the wheat genotypes ranged from about 60 to 82%. The amount of bioavailable Zn (micrograms) in the grain was positively correlated to the amount of Zn accumulated in the grain. There was a significant negative correlation between grain-phytate levels and percentage of Zn absorbed from the wheat grain, but the effect was not large. These results demonstrate that concentrations of Zn in whole-wheat grain, as well as amounts of bioavailable Zn in the grain, can be increased significantly by using traditional plant-breeding programs to select genotypes with high grain-Zn levels. Increasing the amount of Zn in wheat grain through plant-breeding contrivances may contribute significantly to improving the Zn status of individuals dependent on whole grain wheat as a staple food.

Phosphorus-discharge hysteresis during storm events along a river catchment: the River Swale, UK.

Variations in the concentration of determinands in rivers during storms often result in a hysteresis effect with different concentration during the rising and falling limb of the hydrograph. This is investigated here by measuring total phosphorus, particulate phosphorus and soluble reactive phosphorus at 3-h intervals at three points along the River Swale. Phosphorus concentration-discharge hysteresis from 10 storm events were quantified using an empirical model. The size and direction of the hysteresis loops were described by a response factor, and the slope of the loop quantified by a gradient constant. The modelled loops produced acceptable agreement with the field measurements. Hysteresis patterns for all phosphorus fractions changed markedly downstream, with predominantly anticlockwise trajectories in the upland moors (indicating a slow diffuse phosphorus delivery to the river) and clockwise in the intensively farmed lowland (indicating mobilisation of within-channel and riverbank phosphorus, and rapid inputs from field drains). The size of the hysteresis loops increased downstream, indicating an increased capacity for phosphorus storage and mobilisation within the lower catchment. During a succession of storms, lowland hysteresis loops decreased in magnitude, tending towards anticlockwise behaviour, indicating a depletion of mobile phosphorus from the river channel and margins. The modelling of hysteresis trajectories offers a convenient method of determining the relative contributions of diffuse and within-channel phosphorus sources.

Evaluation of phosphorus concentrations in relation to annual and seasonal physico-chemical water quality parameters in a UK chalk stream.

The aim of this paper was to examine historical physico-chemical water quality parameters (1990-1997) in the River Frome, East Stoke (NGR SY867868), in order to show both annual and seasonal (monthly) trends. EpCO2 (defined as the partial pressure of CO2 in natural water divided by the equilibrium partial pressure of CO2) levels ranged from mean values of 6.32+/-0.41 in spring/summer to 7.86+/-1.17 in autumn/winter. A decreasing trend in mean annual EpCO2 was also observed, with a high of 9.61 in 1990 and a low of 5.22 in 1996. The variations were attributed to changes in pH, which showed an inverse relationship with river discharge (r2=0.47). Both pH and EpCO2 levels were strongly linked to biological activity with increases caused by primary productivity. Filterable reactive phosphorus (FRP) and total phosphorus (TP) concentrations correlated with river discharge. The results showed that the majority of the phosphorus load was transported during storm events, which agrees with results from an export coefficient model predicting phosphorus loading in the Frome catchment. Recent River Frome monitoring campaigns using an in situ flow-injection (FI)-based monitor were in agreement with phosphorus concentration and related physico-chemical trends observed during historical sampling and laboratory analysis.

Total phosphorus content of river sediments in relationship to calcium, iron and organic matter concentrations.

Data on the total concentrations of phosphorus, calcium, iron and organic matter in surface bed-sediments taken from rivers in the Thames catchment (Wey, Blackwater, Thame and Kennet), the River Swale in Yorkshire (data excludes the organic matter content) as well as the headwaters of the Great Ouse, are collated and compared. Total concentrations of phosphorus, iron and calcium range from 1.7-649, 12-8,333 and 9-4,605 micromol g(-1) (dry weight), respectively, with organic matter in the range of 0.6-19% by dry weight. For the Wey, Blackwater and Great Ouse, sewage inflows had no detectable effect on the sediment concentrations of total calcium, iron and organic matter whereas for the Blackwater and Great Ouse, the total phosphorus contents of the sediment were higher downstream of the effluent input in comparison with a less impacted upstream location. Relationships between the total phosphorus content of the sediments and contents of iron, calcium and organic matter indicated marked differences between the rivers. Although the organic matter content of the sediments was found to be a significant predictor for the total phosphorus concentration for the Blackwater and Great Ouse, the total iron content was also useful for the Blackwater and total calcium for the Great Ouse. It is postulated that this difference is a result of the sediment processes that are known to occur in these two systems, i.e. co-precipitation of phosphate with calcite in the Great Ouse and the formation of vivianite in anoxic sediments of the Blackwater.

Phosphorus dynamics along a river continuum.

Changes in phosphorus concentration and form along 110 km of the River Swale in Northern England were examined over a 2-year period during 1998-2000. This study aimed to use these data to identify the importance of within-channel storage on phosphorus dynamics and to determine the changes in longitudinal transport of phosphorus along a river continuum. The catchment was divided into three contrasting zones: the upland, dominated by sheep farming; a transitional zone, and an intensively-farmed lowland, impacted by sewage inputs. Samples, taken at the downstream extent of each zone at approximately 2-day intervals, were analysed for total phosphorus (TP), total dissolved phosphorus (TDP) and soluble reactive phosphorus (SRP), all of which increased in concentration downstream. SRP concentrations were highest in summer and during low flows, although 92% of phosphorus was exported between autumn and spring. The TDP in the upper and transitional zones consisted of both soluble reactive and un-reactive phosphorus, but in marked contrast was almost entirely in soluble reactive form in the lowland. The majority (85%) of phosphorus exported from the catchment was generated within the lowland, due to sewage inputs and losses from intensive agricultural land. It was predominantly particulate-bound, due to interactions of dissolved phosphorus with suspended sediment. The upland contributed less than 5% to the TP annual budget. Intensive river water monitoring highlighted that the lowland dominated phosphorus export during the rising stage of storms (indicating a rapid mobilisation of fine phosphorus-rich sediment), whereas the transitional zone became dominant on the falling stage (due to greater diffuse phosphorus input).

Transport and distribution of lindane and simazine in a riverine environment: measurements in bed sediments and modelling.

Aquatic sediments often remove hydrophobic contaminants from fresh waters. The subsequent distribution and concentration of contaminants in bed sediments determines their effect on benthic organisms and the risk of re-entry into the water and/or leaching to groundwater. This study examines the transport of simazine and lindane in aquatic bed sediments with the aim of understanding the processes that determine their depth distribution. Experiments in flume channels (water flow of 10 cm s(-1)) determined the persistence of the compounds in the absence of sediment with (a) de-ionised water and (b) a solution that had been in contact with river sediment. In further experiments with river bed sediments in light and dark conditions, measurements were made of the concentration of the compounds in the overlying water and the development of bacterial/algal biofilms and bioturbation activity. At the end of the experiments, concentrations in sediments and associated pore waters were determined in sections of the sediment at 1 mm resolution down to 5 mm and then at 10 mm resolution to 50 mm depth and these distributions analysed using a sorption-diffusion-degradation model. The fine resolution in the depth profile permitted the detection of a maximum in the concentration of the compounds in the pore water near the surface, whereas concentrations in the sediment increased to a maximum at the surface itself. Experimental distribution coefficients determined from the pore water and sediment concentrations indicated a gradient with depth that was partly explained by an increase in organic matter content and specific surface area of the solids near the interface. The modelling showed that degradation of lindane within the sediment was necessary to explain the concentration profiles, with the optimum agreement between the measured and theoretical profiles obtained with differential degradation in the oxic and anoxic zones. The compounds penetrated to a depth of 40-50 mm over a period of 42 days.

Diffusion of the synthetic pyrethroid permethrin into bed-sediments.

Bed-sediments are a sink for many micro-organic contaminants in aquatic environments. The impact of toxic contaminants on benthic fauna often depends on their spatial distribution, and the fate of the parent compounds and their metabolites. The distribution of a synthetic pyrethroid, permethrin, a compound known to be toxic to aquatic invertebrates, was studied using river bed-sediments in lotic flume channels. trans/cis-Permethrin diagnostic ratios were used to quantify the photoisomerization of the trans isomer in water. Rates were affected by the presence of sediment particles and colloids when compared to distilled water alone. Two experiments in dark/light conditions with replicate channels were undertaken using natural sediment, previously contaminated with permethrin, to examine the effect of the growth of an algal biofilm at the sediment-water interface on diffusive fluxes of permethrin into the sediment. After 42 days, the bulk water was removed, allowing a fine sectioning of the sediment bed (i.e., every mm down to 5 mm and then 5-10 mm, then every 10 mm down to 50 mm). Permethrin was detected in all cases down to a depth of 5-10 mm, in agreement with estimates by the Millington and Quirk model, and measurements of concentrations in pore water produced a distribution coefficient (Kd) for each section. High Kd's were observed for the top layers, mainly as a result of high organic matter and specific surface area. Concentrations in the algal biofilm measured at the end of the experiment under light conditions, and increases in concentration in the top 1 mm of the sediment, demonstrated that algal/ bacterial biofilm material was responsible for high Kd's at the sediment surface, and for the retardation of permethrin diffusion. This specific partition of permethrin to fine sediment particles and algae may enhance its threat to benthic invertebrates. In addition,the analysis of trans/cis-permethrin isomer ratios in sediment showed greater losses of trans-permethrin in the experiment under light conditions, which may have also resulted from enhanced biological activity at the sediment surface.

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.