A mass-balance approach to quantifying the importance of in-stream processes during nutrient transport in a large river catchment

Abstract

The importance of riverine processes in the River Swale catchment in Yorkshire, UK, have been assessed during three intensive monitoring campaigns of approximately 100-h duration in the autumn, spring and winter of 1995/1996. This was done by monitoring dissolved silicon, calcium, soluble phosphorus, nitrate, nitrite, ammonium and total phosphorus concentrations at 2-h intervals at the main river and major tributary sites and daily at 15 minor tributaries. These data, together with water discharge data from gauging stations on the main river and major tributaries as well as manual measurements of the minor tributaries, have been used to calculate mass-balances. The difference between the inputs and outputs from the section, enable within-reach losses and gains to be estimated and compared with the water-balance for the river. Dissolved silicon was relatively conservative compared with calcium which did show losses from the river water in the Spring and Autumn, probably associated with precipitation of calcite. Relatively large decreases in soluble phosphorus were found which may be associated with uptake by bed-sediments and riverine flora during conditions of low-flow. Uptake of soluble phosphorus by suspended sediments was also important in a storm event as shown by increases in the mass-balance of particulate phosphorus. Mass-balances for nitrate indicated smaller changes compared with dissolved phosphorus, with losses in the autumn, gains in the spring and little evidence of riverine processes in the winter storm. Ammonium was also lost from the water during all three campaigns but not in sufficient amounts to account for concomitant increases in nitrate or nitrite by nitrification. Exports of soluble reactive phosphorus, total phosphorus and nitrate were calculated for each of the campaigns. The export of total phosphorus was compared with predicted values using suitable export coefficients for different land-use in the UK. The results are consistent, with the export predictions falling in the measured ranges for the catchments. The exports of nitrate were particularly large during the storm event compared with literature data for annual exports.