Nitrogen cycling in North Sea sediments: interaction of denitrification and nitrification in offshore and coastal areas

Benthlc nltrogen cycling was studled In different areas of the southeastern North Sea in August 1991 and February 1992 Main attention was focused on the coupling between nltrlficatlon and demtnfication Additionally, extensive measurements of sediment-water exchange of lnorganlc nitrogen compounds and of the penetration of oxygen Into the sediment were made Denltnfication rates ranged from 0 to 8 pm01 N m-' h-', whlle nitnfication rates ranged between 5 and 26 pm01 N m-2 h -' Although the oxic layer of the sedlrnent was t h n n e r in August, nltnfication was hlgher dunng that period, indicating the importance of ammonlum availabihty rather than oxygen penetration Except in the deposltlonal area in the German Blght, denitrlflcation was completely dependent on nitnfication In general, however, the coupllng between the 2 processes was incomplete On average, 3 and 25 % of the produced nitrate was denltr~fied In August and February respectively The bulk of the produced nltrate diffused upward (along the steepest concentratlon gradient) to the nitrate-depleted overlylng water and was unavailable for denitnficatlon, part~cularly in August It 1s postulated that the coupllng efficiency is indirectly influenced by the nitrate concentratlon in the overlylng water '