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

Abstract

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

Seasonal variation in denitrification and dissolved nitrogen fluxes in intertidal sediments of the Tagus estuary, Portugal

“…Oxygen uptake by the sediment showed a distinct seasonal pattern, clearly induced by temperature, as found for other systems (Nedwell et al 1983, Nedwell & Trimmer 1996. O 2 uptake rates, ranging from 140 to 4800 µmol O 2 m -2 h -1 in the intertidal sediments of the Tagus estuary, were comparable to rates observed in other ecosystems (Nedwell et al 1983, Koop et al 1990, Sundbäck et al 1991, Nedwell & Trimmer 1996.…”

“…O 2 uptake rates, ranging from 140 to 4800 µmol O 2 m -2 h -1 in the intertidal sediments of the Tagus estuary, were comparable to rates observed in other ecosystems (Nedwell et al 1983, Koop et al 1990, Sundbäck et al 1991, Nedwell & Trimmer 1996. High O 2 -uptake values suggested a more intense benthic activity within the sediments during the summer period.…”

“…Sediments may be sources of some nutrients and sinks for others, depending on the biogeochemical transformations occurring at the sediment-water interface. With regard to nitrogen, sediments generally represent an important source of NH 4 + (Boynton & Kemp 1985, Rizzo 1990) and a sink for NO 3 - (Seitzinger et al 1984, Nedwell & Trimmer 1996. Denitrification is known to remove nitrogen by bacterial reduction of NO 3 -to nitrogen gas via NO 2 -in anoxic sediment environments, and may be based on both NO 3 -diffusing from the water column into the sediments and on NO 3 -produced by nitrification (Seitzinger 1988).…”

Control of the diffusive boundary layer on benthic fluxes: a model study

“…The predictions of Archer et al (1989) suggest that oxygen dynamics should be most affected by the DBL in shallow coastal sediments where thin oxic layers (i.e. low OPD, < 0.5 cm) are often observed (Revsbech et al 1980, Lohse et al 1996, Nedwell & Trimmer 1996, Rysgaard et al 2001. However, such predictions contrast with the theoretical study of Jørgensen & Boudreau (2001).…”

“…The molar carbon to nitrogen (C:N) ratio of sedimentary organic matter is highly variable (e.g. 6 to 56, Nedwell & Trimmer 1996, Lohse et al 1998, Rys-gaard et al 2001 and is not necessarily an indicator of degradability (Lohse et al 1998). A value of 15 is set (DBL sensitivity to changes in the value of the C:N ratio in the model were found to be negligible, data not shown).…”