Intensive measurements of nutrient dynamics in the River Swale

The results of a nutrient stability study, pilot studies and three major 100-h monitoring campaigns measuring phosphorus (soluble reactive, total dissolved and total) and dissolved nitrogen (nitrate, nitrite and ammonium) concentrations in the R. Swale catchment in Yorkshire, UK are reported. The nutrient stability studies showed that although nitrite showed considerable stability during 24-h storage, both ammonium and phosphorus were found to be less stable. These nutrients must be analyzed as soon as possible after sampling to avoid changes caused by processes in solution and interactions of soluble compounds with colloids and suspended material during storage. The results of the intensive measurements indicate the importance of point-inputs of dissolved phosphorus to the main river and major tributaries during low-flow conditions in the autumn and spring. This was demonstrated using a simple dilution model to estimate the flux of point-inputs. However, during a storm event, diffuse inputs are also important leading to increases in soluble unreactive phosphorus and particulate phosphorus concentrations in the river. Nitrate concentrations are more influenced by diffuse inputs and previous weather conditions with the highest concentrations measured after a period of low rainfall. Nitrite and ammonium were at similar concentrations during low-flow conditions. Although nitrite concentrations generally increased with river flow, the changes were less marked when compared with nitrate. Ammonium also increased in the main river during high-flow conditions. All nutrients showed hysteresis in their concentration–discharge relationship during a major storm which indicated the importance of diffuse inputs. The effect was consistent with a depletion of nutrients during the falling limb of the hydrograph.