Report: Gunni Ærtebjerg
Cruise leader: Jan Damgaard
Participants: Hanne Ferdinand, Peter Kofoed, Dorete Jensen, Malene Skude (NERI).
Lars Lund-Hansen, Ole Mikkelsen (University of Århus).
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Summary
Since the last cruise in mid October, the surface salinity had increased, the bottom water salinity decreased and the salinity stratification declined in all areas, except in the central Sound, showing that a significant mixing and exchange of water masses had taken place. The halocline was situated unusually deep (>20 m depth) in the southern Belt Sea due to a gale from southeast. Compared to long term monthly mean (Lightship observations 1931-1960) for November the temperature was in the whole water column 0.8-2.5° C higher than normal, and the salinity was higher than normal in the whole water column in the Belt Sea, but lower than normal in the eastern Kattegat.
Nutrients had been mixed into the surface layer, and a phytoplankton autumn bloom was present in most areas. Due to the bloom the surface nitrate concentration was still low (<0.2 m mol/l) in the eastern Kattegat, Fehmarn Belt and the central Arkona Sea, as also the silicate concentration (0.3-0.6 m mol/l) in the eastern Kattegat.
The mean chlorophyll-a concentration in the uppermost 10 m varied between 1.2-1.8 m g/l in the Sound to 5.1-5.4 m g/l in Kiel Bight and Fehmarn Belt. The chlorophyll was either rather homogeneously distributed in the uppermost 10-15 m of the water column or highest at the surface, except for a subsurface maximum of 4.3 m g/l in 13.5 m depth at the northern entrance to the Sound.
Since the cruise in October the minimum oxygen concentration had increased in all areas, except in the south-eastern Kattegat. The lowest oxygen concentration of 2.4 ml/l (39% saturation) was observed in the central Sound, where the oxygen concentration was 2.4-2.5 ml/l in the whole water column from 20 m depth to the bottom in 51 m depth. In the Great Belt the minimum concentration had increased 1.6-2.8 ml/l to 4.2-4.8 ml/l (68-76%). In the south-eastern Kattegat the minimum concentration was 3.5-4.4 ml/l (58-74%).
Compared to November last year the minimum oxygen concentrations this year are now higher, except in the northern Kattegat. Compared to mean for November in the 1980s, the minimum oxygen concentrations this year are lower in the Sound, south-eastern Kattegat and from the central Great Belt to Fehmarn Belt.
In Denmark oxygen depletion is defined as minimum oxygen concentrations below 2.8 ml/l (4 mg/l). From this definition oxygen depletion still occurred on 9 November in a 30 m high water column in the Sound.