AME 12:275-284 (1997)  -  doi:10.3354/ame012275

Denitrification and microphytobenthic NO3- consumption in a Danish lowland stream: diurnal and seasonal variation

Pind A, Risgaard-Petersen N, Revsbech NP

Seasonal and diurnal variation of denitrifying activity was determined in a microphyte-covered lowland stream sediment using an isotope pairing technique based on the stable 15N-isotope. By this approach it was possible to divide total denitrification (Dtot) into denitrification based on NO3- diffusing from the overlying water (Dw) and coupled nitrification-denitrification within the sediment (Dn). Dtot exhibited seasonal variations between 0 and 17 mmol m-2 d-1 with a pronounced minimum during winter. Dw showed the same seasonal pattern as Dtot and accounted for 75 to 90% of the total denitrification activity. In contrast to the pattern previously found in estuarine systems, a minimum in Dw activity was found during the winter period when the NO3- concentration in the water was highest. This was apparently due to a dramatic increase in water discharge during the winter period increasing the erosion of the stream bed, thereby reducing the carbon availability for the denitrifiers and probably also their numbers. Temporary sediment N accumulation was estimated as the difference between rates of total net dissolved inorganic nitrogen uptake (NH4+, NO3-) and total denitrification (Dn +Dw). The rate of temporary sediment N accumulation varied seasonally between 0 and 13 mmol N m-2 d-1 with a pronounced maximum during spring (April-May) coincident with the microphytobentic spring bloom. Most of the N load to the stream occurred during the winter and as a consequence the retention capacity of the system was low on an annual basis. Denitrification and accumulation in the stream were both found to account for less than 1% of the N transported on an annual basis, while approximately 60% was removed by denitrification and sediment accumulation during the summer period (May-August).

Denitrification · N accumulation · N load

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