ABSTRACT: The muddy estuary of the River Colne, east coast UK, is hypernutrified, with strong gradients of NO₃^- and NH₄⁺ up the estuary due to inputs from the river and sewage treatment works. There were no significanttransformations of nitrogen detected in the water column. In the sandy sediment at the mouth of the estuary nitrification occurred with NO₃^- export from the sediment, but the muddy sediments higher up the estuary were large sinks forNO₃^- and major sites of denitrification. The flux of NO₃^- into the sediment at these sites was correlated with the water column NO₃^- concentration, and there was a large capacity for thesediments to respond to increases in the water column NO₃^- concentration. A seasonal cycle occurred with maximum denitrification during early winter, when water column NO₃^- was greatest and low temperaturesfavoured denitrification over NO₃^- ammonification. Highest unit area rates of denitrification were measured by acetylene inhibition at the uppermost site in the estuary, but when allowance was made for the area of sediment surface ineach sector the middle reaches of the estuary were more significant to the estuarine nitrogen budget. Approximately 50% of the NO₃^- flux through the estuary was denitrified during 1993-1994. In addition, measurements ofdenitrification by the ¹⁵NO₃^- isotope pairing technique suggested that coupled nitrification-denitrification within the sediment was also important, and when this was also allowed for the sediments removed bydenitrification between 18 and 27% of the total nitrogen flux through the estuary. There was some question, however, of whether the coupled nitrification-denitrification was overestimated if the anammox reaction was occurring in the highly organic, highNO₃^- sediments at the river end of the estuary. It is concluded that in these turbid, muddy estuaries the sediments are not only major attenuators of the flux of NO₃^-, but are also very effective traps fororganically bound nitrogen. This suggests that the loads of nitrogen through these estuaries to the North Sea, which are usually derived from river gauging above the high tide mark, significantly overestimate the real load as they do not take into accountattenuation of nitrogen flux within the estuary. While this attenuation may decrease the nitrogen loads, it implies that any environmental impact in coastal waters may be the result of much lower loads of nitrogen than hitherto assumed.