ABSTRACT: Measurements in the estuary of the River Colne, Essex, UK, showed strong gradients of nitrate and ammonium concentrations, increasing upriver due to inputs from the river and a large sewage treatment works. In the low salinity region (<10 to 12 psu) nitrate concentrations in the water column were sometimes >1 mM, but rapidly decreased within the top 1 cm of sediment. Concentrations of N₂O in the water column correlated with nitrate concentrations, due to denitrification and N₂O formation in the surface sediment. There was no formation of N₂O in the water column. N₂O in the estuary water was always supersaturated with respect to N₂O in air, and in the low salinity/high nitrate part of the estuary could be up to 50 times air equilibration. Water-air N₂O fluxes, calculated from a thin film model, also showed marked gradients up the estuary. The emission fluxes of N₂O from the surface of tidally exposed sediments decreased with time after exposure. This was the result of the fast turnover (<40 min) of the sedimentary nitrate pool, and its depletion when it was no longer recharged by transport of nitrate from the water column. The sediments were, therefore, extremely important processors of nitrate from the water column, but only when covered by the tide. Water-air emission fluxes in each sector of the estuary were calculated and showed that maximum N₂O emission at high tide was from the sector below that where peak unit area emission fluxes were detected, because of the greater area of water surface in the sector. Total N₂O emission from the estuary did not vary with tidal state because the high nitrate/high N₂O water remained within the estuary even at low tide. Integration of the emission fluxes with time suggested that about 1.2 x 10⁵ mol of N₂O-N were emitted to the atmosphere in a year, of which 83% was from the water surface. This emission accounted for 0.5% of the Total Oxidised Nitrogen load to the estuary, or 0.3% of the Total Nitrogen load. Benthic N₂O production corresponded to <2% of the nitrate denitrified in the bottom sediments, but could nonetheless give rise to significant export of N₂O to the atmosphere. If the N₂O fluxes in the Colne estuary were extrapolated globally, the total estuarine N₂O production would be equivalent to between 0.13 and 0.45 Tg N₂O-N yr^-1. This is equivalent to the source strengths of adipic acid and nitric acid productions, but less than the hypothesised missing global source(s) of N₂O.

KEY WORDS: Eutrophication · Denitrification · Nitrous oxide · Greenhouse gas

Published in MEPS Vol. 164 (1998) on April 9
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1998