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 Ou se estuary, England, was examined over the course of a year. The sedim ents were consistent sinks for NO3- from the overlying river water, an d were weak sources of NH4+. Simultaneous measurements of oxygen uptak e, nutrient exchange and sulphate reduction, when combined with the me asured C:N ratios of the sediment organic matter, permitted calculatio n of the amount of N released within the sediment by organic matter mi neralisation. 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 o rganic matter was not matched by measured exports of N from the sedime nts. 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 mu st have been converted to gases by coupled nitrification-denitrificati on 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 surfa ce area of sediment to freshwater flow.