SEDIMENT-WATER FLUXES OF INORGANIC NITROGEN-COMPOUNDS ALONG THE TRANSPORT ROUTE OF ORGANIC-MATTER IN THE NORTH-SEA

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 Eight) 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 mu m; II: transition zones with grain sizes between 80 and 180 mu m, and III: coarse sediments with median grain size > 280 mu m. Porewater profiles of nitrate and ammonium corresponded well to this clustering. The very high mineralisation in the German Eight in August demonstrates the importance of deposition of locally produced organic matter in this area. Total ammonium production in the Skagerrak in August and February indicates that here N-cycling is dominated by the input of refractory organic matter from the southern shelf. Consequently, the shelf-slope transition in the Skagerrak does not represent an area of intensified nitrogen mineralisation.