The River Colne is a muddy macrotidal estuary on the east coast of England, with strong decreasing gradients of NO3- and NH4+ down the estuary due to inputs from the river and upstream sewage treatment works. The rates of uncoupled (D-w) and coupled (D-n) denitrification in the sediments at 4 sites were measured monthly by N-15 isotope-pairing technique from October 1996 to March 1998, There was a significant spatial and temporal variation in the rates of D-w and D-n. The highest rates were observed at Site 1 and the lowest at Site 4, corresponding to the highest NO3- concentrations in the water column and highest organic contents in the sediments at Site 1 and the lowest at Site 4. Short-term kinetic experiments showed that rates of D-w increased with increasing NO3- concentration in the water column, up to 500 mu M NO3- at Site 2 (the middle site along the estuary), and were not saturated up to 600 mu M NO3- at Site 1 or 180 mu M NO3- at Site 4. Field observations and kinetics experiments indicated a large sediment potential for D-w at all sites. Nitrate availability Limited the rates of D-w and was the main factor regulating the rates of D-w in this estuary. It was calculated that 32 to 44 % of total oxidised nitrogen inputs or 20 to 25 % of the total inorganic nitrogen inputs (total oxidised nitrogen + ammonium) into the estuary by the River Colne and the sewage treatment works were removed from the estuary by uncoupled denitrification (D-w) and total denitrification (D-w + D-n) respectively, in the sediments before they were transported into the North Sea. Microphytobenthos, mainly diatoms, assimilated 4.1 % of the ammonium inputs or 2.3 % of total inorganic nitrogen inputs into the estuary and temporarily retained the assimilated nitrogen in the estuary. They also affected the rates of denitrification in a complex manner, by regulating the depth of oxygen penetration by photosynthetic oxygen production and respiratory oxygen uptake. Photosynthesis inhibited the rates of D-w but stimulated D-n. The overall effect of microphytobenthos on denitrification was to increase the percentages of D-n in the total denitrification. The loads of nitrogen through the River Colne estuary to the North Sea was significantly attenuated by the removal of nitrate from the water column by D-w and of nitrate produced from ammonium via nitrification in the sediment by D-n.
