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

The fate of nitrogen (N) in the bottom sediments of the upper Great Ouse estuary, England, was examined over the course of a year. The sediments were consistent sinks for NO3- from the overlying river water, and were weak sources of NH4+. Simultaneous measurements of oxygen uptake, nutrient exchange and sulphate reduction, when combined with the measured C:N ratios of the sediment organic matter, permitted calculation of the amount of N released within the sediment by organic matter mineralisation. With the exception of a site with thixotropic sediment, at all other sites the amount of inorganic N entering the sediment by transport from the overlying water and by internal ammonification of organic matter was not matched by measured exports of N from the sediments. We calculate that >90% of the flux of N through the sediment was lost as gases, and that 50% of the N ammonified from organic matter must have been converted to gases by coupled nitrification-denitrification within the sediments. When compared to the total flux of N through the entire estuary, any N loss by denitrification in the sediments of the upper estuary was minor (similar to 1%) because of the small surface area of sediment to freshwater flow.