Denitrification in sediments of the freshwater tidal Yorkshire Ouse

Studies were made on denitrification rate and nitrous oxide production in the 21-km freshwater tidal stretch of the Yorkshire Ouse. Denitrification was not detected in the water column at the most downstream site (Selby) under conditions likely to be the most favourable for this process. All but one sample of the sediment cores showed high activity per unit area, with values ranging up to 575 μmol N m−2 h−1. In contrast to all other measurements on the Swale–Ouse river system, values for denitrification rate at the most downstream site showed on three occasions a decrease in comparison with the next site upstream (Cawood). It is suggested that this may be due to sediment instability at this site. Experimental studies with sediment slurries made on two different dates showed an increase in denitrification rate at all sites in response to added nitrate, but none due to added glucose. However, a combination of nitrate and glucose added to slurries from the most upstream site (Naburn Weir) and assayed under low O2 conditions showed a significantly higher denitrification rate than with added nitrate alone.