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 a long the river continuum of the Swale-Ouse river system, NE England, t ogether with one site on a highly eutrophic tributary (R. Wiske). Inta ct cores were taken on a monthly basis for 17 months and used to asses s denitrification rates under simulated ambient conditions using acety lene blockage methodology. Within the headwater site (2.5 km upstream of start of main river) rates rarely exceeded 20 mu mol 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 mu mol N m(-2) h(-1). The highest rat es on all sampling occasions were for the Wiske. Both spatial and seas onal trends were evident. A general trend for the rate to increase mov ing downstream can be explained largely by two factors: increase in am bient nitrate concentration and changes in sediment composition. Seaso nally, a spring peak was evident at all sites, which can again be expl ained largely by two factors: temperature and nitrate concentration. ( C) 1998 Elsevier Science B.V.