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 Ouse estuary, England, was examined over the course of a year. The sedirnents were consistent sinks for NO3-from the overlying river water, and were weak sources of NH,'. 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 w ~t h thixotropic sediment, at all other sites the amount of inorganic N entering the sedlment by transport from the overlying water and by internal ammonification of organic matter was not matched by measured exports of N from the sedirnents. We calculate that >90% of the flux of N through the sediment was lost as gases, and that 50'L 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 (-1 X ) because of the small surface area of sediment to freshwater flow