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

Seasonal and diurnal variation of denitrifying activity was determined in a microphytecovered lowland stream sediment using a n isotope pairing technique based on the stable 15N-isotope. By this approach it was possible to divide total denitrification (D,,,) into denitrification based on NO,diffusing from the overlying water (D,,) and coupled nitrification-denitrification within the sediment (D,). D,,, exhibited seasonal variations between 0 and If mm01 m-' d-' with a pronounced minimum during winter D,. showed the same seasonal pattern as D,,,, and accounted for 75 to go'%, of the total denitnfication activity. In contrast to the pattern previously found in estuarine systems, a minimum in D,, activity was found during the winter period when the NO3concentration in the water was highest. This was apparently due to a dramatic increase in water discharge during the winter penod increasing the erosion of the stream bed, thereby reducing the carbon ava~lability for the denitrifiers and probably also their numbers. Temporary sediment N accumulation was estimated a s the difference between rates of total net dissolved inorganic nitrogen uptake (NH,', NO,-) and total denitrification (D, +D,,) The rate of temporary sediment N accumulation varied seasonally between 0 and 13 mm01 N m-2 d ' with a pronounced maximum during spring (April-May) coincident with the microphytobentic spnng bloom. Most of the N load to the stream occurred dunng the \ a n t e r 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 a n annual basis, while approximately 60'% was removed by denitriflcation and sediment accumulation during the summer period (May-August)