The effect of sewage effluent on denitrification in Grasmere (English Lake District)

Abstract

SUMMARY. Denitrification processes in Grasmere were studied using several techniques. Sediments were analysed for nitrate reductase activity and the accumulation of nitrogenous gases in samples incubated in the laboratory. Changes in the concentrations of inorganic nitrogen species in the water column and the release of gas bubbles into gas traps suspended above the sediment were also measured. Sewage effluent entered the lake via the main inflow and faecal bacteria were used as tracers to map the path of the river (and effluent) across the lake. The main river flow was west of the island in the centre of the lake and across the smaller of the two main basins. A preliminary study indicated that this western basin was the site of most active denitrification and this was confirmed in the following year when a seasonal survey showed that deoxygenation, nitrate reduction and ammonium accumulation was more advanced at this site. In addition, this basin was the only area where consistent discharge of nitrogen and methane from the profundal zone was observed. The nitrate reductase assay demonstrated that the sediment was the major site of denitrification, and that activity in the water column was higher in the anoxic hypolimnion than in the euphotic zone. Gas evolution from incubated sediment samples was analysed by gas liquid chromatography (GLC) and indicated that maximum denitrifying activity occurred below the mud surface in oxidized sediments. There was reasonable agreement between the enzyme assay and the GLC technique but neither predicted events in the field satisfactorily. Viable, most probable number (MPN) counts of denitrifying bacteria had similar limitations. A bimodal seasonal pattern of nitrate reductase activity was observed at four sites during thermal stratification, but this could not be explained fully. The major site of denitrification was identified and it was calculated that the nitrogen gas discharged was equivalent to c. 54% of the nitrate removed from the water column.