ABSTRACT: Sediment-water nutrient exchange, oxygen uptake, denitrification (acetylene blockage) and pore water nutrient concentration profiles were measured at intertidal sediment (predominantly silt/clays) sites in the lower estuary of the river Great Ouse, England. Sediments were consistent sinks for NO₃^- (310 µmol m^-2 h^-1, mean sites 4 to 9) and O₂ (2800 µmol m^-2 h^-1, mean sites 4 to 7), sources of NH₄⁺ (270 µmol m^-2 h^-1, mean sites 4 to 9) but neutral with respect to NO₂^- and urea. Oxygen uptake was significantly correlated (p < 0.05) with seasonal temperature. Nitrate exchange became saturated at NO₃^- concentrations > 400 µM, at a rate of about 400 µmol NO₃^- m^-2 h^-1. Denitrification accounted annually for 46% of the NO₃^- exchanged into the sediments and approached asymptotic rates during spring and summer at NO₃^- concentrations > 400 µM. Of the total N flux through the sediments, NH₄⁺ efflux accounted for 51%, whilst 49% was converted to gases, compared to >90% in the upper estuary. Freshwater flushing times were calculated for a defined area of the estuary and ranged from 20.5 d in June to 3.25 d in November. Attenuation of the riverine total oxidised nitrogen (NO₃^- + NO₂^-) load to the estuary ranged from 1% in the middle of winter to 56% at the height of summer and annually the sediments denitrified 38.4 Mmol N. Recycling of nitrogen in the sediments, via NO₃^- ammonification (calculated by difference) and organic ammonification, generated an annual NH₄⁺ efflux of 128 Mmol N, equivalent to 22% of the primary production N requirement in the Wash.

KEY WORDS: Bottom sediments · Denitrification · Freshwater flushing · Nutrient attenuation

Published in MEPS Vol. 163 (1998) on March 12
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1998