Nitrogen fluxes through the upper estuary of the Great Ouse, England:The role of the bottom sediments

Abstract

The fate of nitrogen (N) in the bottom sediments of the upper Great Ouseestuary, England, was examined over the course of a year. The sediments wereconsistent sinks for NO3 from the overlying river water, and were weeksources of NH4+. Simultaneous measurements of oxygen uptake, nutrientexchange and sulphate reduction, when combined with the measured C:N ratiosof the sediment organic matter, permitted calculation of the amount ofreleased within the sediment by organic matter mineralisation. With theexception of a site with thixotropic sediment, at all other sites the amountof inorganic N entering the sediment by transport from the overlying waterand by internal ammonification of organic matter was not matched by measuredexports of N from the sediments. We calculate that >90% of the flux of Nthrough the sediment was lost as gases, and that 50% of the N ammonified fromorganic matter most have been converted to gases by coupled nitrification-denitrification within the sediments. When compared to the total flux of Nthrough the entire estuary, any N loss by denitrification in the sediments ofthe upper estuary was minor (-1%) because of small surface area of sedimentto freshwater flow.