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 10(5) 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.