Spatial and seasonal variation in denitrification in the Swale–Ouse system, a river continuum

Rates of denitrification were studied in the sediments at five sites along the river continuum of the Swale–Ouse river system, NE England, together with one site on a highly eutrophic tributary (R. Wiske). Intact cores were taken on a monthly basis for 17 months and used to assess denitrification rates under simulated ambient conditions using acetylene blockage methodology. Within the headwater site (2.5 km upstream of start of main river) rates rarely exceeded 20 μmol N m−2 h−1. At the most downstream site (145.0 km down the main river and at the tidal limit) the maximum was 659 μmol N m−2 h−1. The highest rates on all sampling occasions were for the Wiske. Both spatial and seasonal trends were evident. A general trend for the rate to increase moving downstream can be explained largely by two factors: increase in ambient nitrate concentration and changes in sediment composition. Seasonally, a spring peak was evident at all sites, which can again be explained largely by two factors: temperature and nitrate concentration.