| Report: | Gunni Ærtebjerg | ||
| Cruise leader: | Jan Damgaard | Participants: |
Kjeld Sauerberg, Peter Kofoed, Dorete Jensen, Jesper Rasmussen
(technician trainee), Tore Hansen (student).
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Summary
Strong south-western winds at the beginning of October caused an inflow of cold, high saline and relatively oxygen rich deep water from Skagerrak to the Kattegat bottom water. Salinity of 34-35 was observed in all eastern Kattegat as far south as the entrance to the Sound. This caused higher than normal bottom water salinity in all areas investigated, and lower than normal bottom water temperature in the eastern Kattegat. At the same time the surface salinity was generally lower than normal, resulting in for the season unusually strong stratification.
The inflow lifted up the original oxygen poor Kattegat bottom water and pressed it into the Sound and Belt Sea. In the south-eastern Kattegat and the Sound the lowest oxygen concentrations were now observed in 20-30 m depth, not at the bottom. The dynamics had also brought nutrients to the surface and triggered an autumn diatom bloom in most areas, but in Mecklenburg Bight dinoflagellates (Ceratim sp. and Prorocentrum micans) dominated. However, nitrate was still not present in the surface water, while phosphate was present in all areas. Silicate was low in the Kattegat surface water, but present in the other areas.
Since the cruise in September the minimum oxygen concentration had increased 0.7-1.9 ml/l, in all Kattagat, but decreased 0.9-4.1 in the Belt Sea. In the area Fehmarn Belt, Mecklenburg Bight, Gedser Rev to east of Falster the minimum oxygen concentration was 1.0-2.8 ml/l (16-43%), lowest in the Fehmarn Belt. In the Sound 2.0 ml/l (30%) was observed, and in the Great Belt the oxygen concentrations were 1.9-3.0 ml/l (30-49%), lowest in the north and highest in the south.
In Denmark oxygen depletion is defined as minimum oxygen concentrations below 2.8 ml/l (4 mg/l), and serious oxygen depletion as below 1.4 ml/l (2 mg/l). From these definitions serious oxygen depletion occurred in the Fehmarn Belt, and oxygen depletion occurred in the Sound, Great Belt and the Mecklenburg Bight - Gedser Rev area. Oxygen depletion was no longer observed in the Kattegat. In figure 9 is shown the stations visited by Danish counties, NERI, SMHI and Landesamt für Natur und Umwelt, Schleswig-Holstein, within the first three weeks of October 2001, and where oxygen depletion or serious oxygen depletion was observed.