NITROGEN CYCLING IN NORTH-SEA SEDIMENTS - INTERACTION OF DENITRIFICATION AND NITRIFICATION IN OFFSHORE AND COASTAL AREAS

Benthic nitrogen cycling was studied in different areas of the southea stern North Sea in August 1991 and February 1992. Main attention was f ocused on the coupling between nitrification and denitrification. Addi tionally, extensive measurements of sediment-water exchange of inorgan ic nitrogen compounds and of the penetration of oxygen into the sedime nt were made. Denitrification rates ranged from 0 to 8 mumol N m-2 h-1 , while nitrification rates ranged between 5 and 26 mumol N M-2 h-1. A lthough the oxic layer of the sediment was thinner in August, nitrific ation was higher during that period, indicating the importance of ammo nium availability rather than oxygen penetration. Except in the deposi tional area in the German Bight, denitrification was completely depend ent on nitrification. In general, however, the coupling between the 2 processes was incomplete. On average, 3 and 25 % of the produced nitra te was denitrified in August and February respectively. The bulk of th e produced nitrate diffused upward (along the steepest concentration g radient) to the nitrate-depleted overlying water and was unavailable f or denitrification, particularly in August. It is postulated that the coupling efficiency is indirectly influenced by the nitrate concentrat ion in the overlying water.