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.