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

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.