Publication Abstract

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

M. Trimmer, D.B. Nedwell, D.B. Sivyer, and S.J. Malcolm

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 N03^-(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 N03 concentrations > 400 µM, at a rate of about 400 µmol m^-2 h^-1. Denitrification accounted annually for 46% of the N03 exchanged into the sediments and approached asymptotic rates during spring and summer at N0₃ 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 (N0₃^- + N0₂^-) 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 N0₃^- 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.

Reference:

M. Trimmer, D.B. Nedwell, D.B. Sivyer, and S.J. Malcolm, 1998. Nitrogen fluxes through the lower estuary of the river Great Ouse, England: the role of the bottom sediments. Marine Ecology Progress Series, 163: 109-124.