NITROGEN STABLE-ISOTOPE DYNAMICS IN THE CENTRAL BALTIC SEA - INFLUENCE OF DEEP-WATER RENEWAL ON THE N-CYCLE CHANGES

The vertical profiles of N-3(-), NH4+, O-2, and H2S as well as the iso topic composition of particulate nitrogen and NH4+ were sampled yearly over a 5 yr period in the Gotland Basin to follow biochemical changes in N-cycling resulting from an inflow of saltwater. The water column has a pronounced interface at 80 to 120 m depth which separates warm ( 13 degrees C) brackish surface waters (salinity 7 psu) and the underly ing cold winter water layer from more saline (9 to 11 psu) bottom wate rs originating from irregularly occurring inflow events of oxygenated, nitrate-rich North Sea water masses. Anoxic conditions usually exist in the deep stagnant waters, where nutrients only occur as ammonia, wh ich reaches concentrations of up to 30 mu mol 1(-1). In spring 1993 la rge amounts of nitrate-and oxygen-rich water were transported into the deep waters of the Gotland Basin, thus displacing the stagnant deep w ater body. With the inflow, oxygen and nitrate concentrations rose by 3 mi 1(-1) and more than 10 pmol 1(-1) respectively. During the follow ing years the concentrations of oxygen in the near bottom layer decrea sed again. The isotope signature of the suspended particles in the lay er below 120 m reflects these changes: in 1993 the mean stable nitroge n isotope value in the anoxic water was at 1.1 parts per thousand. We assume bacterial incorporation of ammonia to be the mechanism producin g isotopically light particles. A fractionation factor calculated for ammonia uptake of 11 parts per thousand supports this hypothesis. Duri ng the following years the particles in the oxygenated water column we re around 8 parts per thousand which is characteristic for microbially degraded material. The surface sediment of the central Gotland Sea ha s a low isotope signal of 3 to 4 parts per thousand. These findings mi ght have consequences for the interpretation of sediment delta(15)N da ta where low isotope contents are usually taken as an indicator of hig h nutrient concentrations in surface waters.