Nitrogen stable isotope dynamics in the central Baltic Sea: influence of deep-water renewal on the N-cycle
Montoya, Joseph P.
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The vertical profiles of NO₃-, NH₄+, O₂, and H₂S as well as the isotopic 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°C) brackish surface waters (salinity 7 psu) and the underlying cold winter water layer from more saline (9 to 11 psu) bottom waters 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, which reaches concentrations of up to 30 µmol l-1. In spring 1993 large amounts of nitrate- and oxygen-rich water were transported into the deep waters of the Gotland Basin, thus displacing the stagnant deep water body. With the inflow, oxygen and nitrate concentrations rose by 3 ml l-1 and more than 10 µmol l-1 respectively. During the following years the concentrations of oxygen in the near bottom layer decreased again. The isotope signature of the suspended particles in the layer below 120 m reflects these changes: in 1993 the mean stable nitrogen isotope value in the anoxic water was at 1.1o/oo. We assume bacterial incorporation of ammonia to be the mechanism producing isotopically light particles. A fractionation factor calculated for ammonia uptake of 11 ‰ supports this hypothesis. During the following years the particles in the oxygenated water column were around 8o/oo which is characteristic for microbially degraded material. The surface sediment of the central Gotland Sea has a low isotope signal of 3 to 4o/oo. These findings might have consequences for the interpretation of sediment δ15N data where low isotope contents are usually taken as an indicator of high nutrient concentrations in surface waters.