NITROGEN CYCLING IN 2 TYPES OF SEDIMENTS OF THE SOUTHERN NORTH-SEA (FRISIAN FRONT, BROAD FOURTEENS) - FIELD DATA AND MESOCOSM RESULTS

Benthic nitrogen cycling was examined at a sandy station (Broad Fourteens, B14) and at a silty station (Frisian Front, FF) in the southern North Sea. Both stations are shallow (B14: 29 m, FF: 39 m) and had a vertically well-mixed water column throughout the period of investigation (January 1989 to April 1990). Both O2 consumption and NH4+ release were maximal during the summer, their rates at B14 were 20 and 0.5 mmol.m-2.d-1, respectively, and at FF, 40 and 1.7 mmol.m-2.d-1, respectively. At B14, NOx (nitrate + nitrite) fluxes were directed into the sediment during the first half of 1989 while an efflux was measured from August onwards. At FF, an efflux of NOx was observed throughout the year, though at very low rates during the period January-June. Maximum denitrification rates were measured during April and June, while minimum rates were observed in August and November. On an annual basis, at B14 approximately 50% and at FF approximately 85% of the ammonium produced by benthic mineralization was regenerated to the water column. Consequently, the N retention in the sediment (denitrification and burial), as a percentage of ammonification, was lower at the silty FF than at the sandy B14. It is argued that this is probably caused by lower O2 availability due to higher mineralization rates and consequently lower nitrification rates at FF as compared to the organic-poor B14. Benthic nitrification supplied more than 85% of the NO3- annually needed for denitrification, thus demonstrating the importance of nitrification-denitrification coupling in these sediments. Parallel to the field measurements, the effect of a single addition of Phaeocystis material to the B14 and FF sediments was studied in mesocosms. Oxygen respiration strongly increased directly after addition of this organic substrate but after approximately 1 month, respiration decreased to values lower than before food addition. This pattern could be described with a simple multi-G model (n=3). Directly after the supply of organic matter to the mesocosms, nitrate concentrations in the pore water and denitrification rates substantially decreased. After approximately 2 months when most of the added material was respired, both nitrate concentrations and denitrification recovered.