HIGH NITRATE, MUDDY ESTUARIES AS NITROGEN SINKS - THE NITROGEN BUDGETOF THE RIVER-COLNE ESTUARY (UNITED-KINGDOM)

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 significant t ransformations 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 estuar y were large sinks for NO3- and major sites of denitrification. The fl ux of NO3- into the sediment at these sites was correlated with the wa ter column NO3- concentration, and there was a large capacity for the sediments to respond to increases in the water column NO3- concentrati on. A seasonal cycle occurred with maximum denitrification during earl y winter, when water column NO3- was greatest and low temperatures fav oured denitrification over NO3- ammonification. Highest unit area rate s of denitrification were measured by acetylene inhibition at the uppe rmost site in the estuary, but when allowance was made for the area of sediment surface in each sector the middle reaches of the estuary wer e more significant to the estuarine nitrogen budget. Approximately 50% of the NO3- flux through the estuary was denitrified during 1993-1994 . In addition, measurements of denitrification by the (NO3-)-N-15 isot ope pairing technique suggested that coupled nitrification-denitrifica tion within the sediment was also important, and when this was also al lowed for the sediments removed by denitrification between 18 and 27 % of the total nitrogen flux through the estuary. There was some questi on, however, of whether the coupled nitrification-denitrification was overestimated if the anammox reaction was occurring in the highly orga nic, high NO3- sediments at the river end of the estuary. It is conclu ded that in these turbid, muddy estuaries the sediments are not only m ajor attenuators of the flux of NO3-, but are also very effective trap s for organically bound nitrogen. This suggests that the loads of nitr ogen through these estuaries to the North Sea, which are usually deriv ed from river gauging above the high tide mark, significantly overesti mate the real load as they do not take into account attenuation of nit rogen flux within the estuary. While this attenuation may decrease the nitrogen loads, it implies that any environmental impact in coastal w aters may be the result of much lower loads of nitrogen than hitherto assumed.