Distribution of N 2O in the Baltic Sea during transition from anoxic to oxic conditions
In January 2003, a major inflow of cold and oxygen-rich North Sea Water terminated an ongoing stagnation period in parts of the central Baltic Sea. In order to investigate the role of North Sea Water inflow in the production of nitrous oxide (N
2O), we measured dissolved and atmospheric N<
2O at 26 stations in the southern and central Baltic Sea in October 2003.
At the time of our cruise, water renewal had proceeded to the eastern Gotland Basin, whereas the western Gotland Basin was still unaffected by the inflow. The deep water renewal was detectable in the distributions of temperature, salinity, and oxygen concentrations as well as in the distribution of the N 2O concentrations: Shallow stations in the Kiel Bight and Pomeranian Bight were well-ventilated with uniform N 2O concentrations near equilibrium throughout the water column. In contrast, stations in the deep basins, such as the Bornholm and the Gotland Deep, showed a clear stratification with deep water affected by North Sea Water. Inflowing North Sea Water led to changed environmental conditions, especially enhanced oxygen (O 2) or declining hydrogen sulphide (H 2S) concentrations, thus, affecting the conditions for the production of N 2O. Pattern of N 2O profiles and correlations with parameters like oxygen and nitrate differed between the basins. Because of the positive correlation between ΔN 2O and AOU in oxic waters the dominant production pathway seems to be nitrification rather than denitrification.
Advection of N 2O by North Sea Water was found to be of minor importance. A rough budget revealed a significant surplus of in situ produced N 2O after the inflow. However, due to the permanent halocline, it can be assumed that the N 2O produced does not reach the atmosphere. Hydrographic aspects therefore are decisive factors determining the final release of N 2O produced to the atmosphere.