A MASS-BALANCE APPROACH TO QUANTIFYING THE IMPORTANCE OF IN-STREAM PROCESSES DURING NUTRIENT TRANSPORT IN A LARGE RIVER CATCHMENT

The importance of riverine processes in the River Swale catchment in Y orkshire, UK, have been assessed during three intensive monitoring cam paigns of approximately 100-h duration in the autumn, spring and winte r 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 wat er discharge data from gauging stations on the main river and major tr ibutaries as well as manual measurements of the minor tributaries, hav e been used to calculate mass-balances. The difference between the inp uts and outputs from the section, enable within-reach losses and gains to be estimated and compared with the water-balance for the river. Di ssolved silicon was relatively conservative compared with calcium whic h did show losses from the river water in the Spring and Autumn, proba bly associated with precipitation of calcite. Relatively large decreas es in soluble phosphorus were found which may be associated with uptak e by bed-sediments and riverine flora during conditions of low-flow. U ptake of soluble phosphorus by suspended sediments was also important in a storm event as shown by increases in the mass-balance of particul ate phosphorus. Mass-balances for nitrate indicated smaller changes co mpared with dissolved phosphorus, with losses in the autumn, gains in the spring and little evidence of riverine processes in the winter sto rm. Ammonium was also lost from the water during all three campaigns b ut not in sufficient amounts to account for concomitant increases in n itrate or nitrite by nitrification. Exports of soluble reactive phosph orus, total phosphorus and nitrate were calculated for each of the cam paigns. The export of total phosphorus was compared with predicted val ues using suitable export coefficients for different land-use in the U K. The results are consistent, with the export predictions falling in the measured ranges for the catchments. The exports of nitrate were pa rticularly large during the storm event compared with literature data for annual exports. (C) 1998 Elsevier Science B.V.