Hypernutrified estuaries as sources of N2O emission to the atmosphere:the estuary of the River Colne, Essex, UK

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 nver and a large sewage treatment works. In the low salinity region (<l0 to 12 psu) nitrate concentrations in the water column were sometimes > l mM, but rapidly decreased within the top 1 cm of sediment. Concentrations of N 2 0 in the water column correlated with nitrate concentrations, due to denitnfication and N 2 0 formation in the surface sediment. There was no formation of N,O in the water column. N 2 0 in the estuary water was always supersaturated with respect to N 2 0 in air, and in the low salinity/high nitrate part of the estuary could be up to 50 times air equ.ilibration. Water-air N 2 0 fluxes, calculated from a thin film model, also showed marked gradients up the estuary. The emission fluxes of N,O from the surface of tidally exposed sediments decreased with time after exposure. This was the result of the fast turnover (c40 min) of the sedimentary nitrate pool, and its depletion when it was no longer recharged by transport of nitrate from the water column. The sedunents 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 N 2 0 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.