No abstract

Denitrification rates in continental shelf sediments of the North Sea were measured by the simultaneous application of the acetylene inhibition technique (AIT) and the isotope pairing method (IPM). Nitrification was the predominant source of nitrate in these sediments. Both techniques were able to detect coupled nitrification-denitrification activity. Denitrification rates measured by the AIT were only measurable during the first 60 min of the incubation. Rates obtained by the AIT in this period ranged between 5.4 and 7.3 pm01 N m" h-' Continued incubation of acetylene amended sediment cores resulted in decreasing denitrification rates, suggesting that accumulated nitrous oxide was consumed despite the presence of acetylene. Denitrification rates obtained by the application of the IPM ranged between 9.8 and 13.2 pm01 N m-' h-' Constant production of single-labelled ('"N1'N) relative to double-labelled (lSN''N) dinitrogen indicated a homogeneous mixlng of both isotopic nitrate species through the denitnfication zone. The present results suggest that previously published data severely underestimated denitnfication rates in North Sea continental shelf sediments. Concomitant measured fluxes of nitrate and ammonium across the sediment-water interface reveal that in these sediments most of the deposited nitrogen is returned to the water column as ammonium and nitrate.

No abstract

Oxygen pore water profiles in North Sea s e d~m e n t s were measured with microelectrodes during 2 contrasting seasons. The measurements were conducted In a wide variety of sediments, including non-depositional areas on the southern shelf as well as depositional areas in the Skagerrak.All measurements were performed within minutes on board at In sltll temperature. The curvature of oxygen profiles in sandy sediments on the southern shelf Indicated the presence of a surface layer characterised by enhanced diffusion. The occurrence of enhanced diffusion was related to sedimentological and seasonal differences. Quantitative evaluat~on of the pore water profiles by a diffusion-reaction model indicated that the effective diffusion coeff~c~ents In a 0.2 to 16 mm subsurface layer were 1.5 to > l 0 0 times higher than the ~nolecular diffusion coefflcient. Highest effective diffusion coefficients were reported for non-depositional sediments characterised by low diffusive oxygen fluxes. Oxygen profiles in sediments of the depositional area of the Skagerrak indicated constant diffusivity throughout the sediment column. Diffusive fluxes cdlculated from profiles ranged from 5.2 to 8.9 ~nlnol m-2 d-' in August 1991, and were between 0.8 and 6.2 mm01 nl ' d-' in February 1992. Seasonal differences in sediments locatt!d In the Skagerrak area were minor. It is proposed that near-bottom tidal currents induce enhanced diffus~on transport processes in the upper millimetres of the sandy sediments of the southern North Sea, while less energetic hydrodynamical conditions in the depos~tional area of the Skagerrak favour sediment-water exchange based on molecular diffusion only. Biogeochemical implications of the enhanced diffusivity close to the sediment-water interface are discussed.

Sediment-water fluxes of ammonium and nitrate, as well as nitrification and denitrification were measured in different types of sediments in the North Sea in August 1991 and February 1992. Stations were located along the main transport route of organic matter and included depositional areas located on the southern shelf(German Bight) and on the north-eastern shelf-slope transition in the Skagerrak. Based on sedimentary organic carbon content and grain size distribution, the stations were divided into three clusters. I: depositional areas with median grain size < 50 #Lm; II: transition zones with grain sizes between 80 and 180#Lm, and III: coarse sediments with median grain size > 280 #Lm. Porewater profiles of nitrate and ammonium corresponded well to this clustering.