Report: Gunni Ærtebjerg
Cruise leader: Kjeld Sauerberg
Participants: Hanne Ferdinand, Peter Kofoed, Dorete Jensen, Malene Skude.
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Summary
The salinity stratification of the water column was unusually strong due to brackish Baltic Sea water at the surface, and inflow of salt bottom water from the north, even into the Arkona Basin. The water temperature was about 12-13° C in all water masses and 0.4-1.6° C higher than normal for the season. The surface salinity was lower and the bottom water salinity higher than long-term means for October.
Phosphate and silicate were present all over in the surface water as also at least traces of nitrate in most areas. High concentrations of ammonium, phosphate and silicate were observed in the bottom water in the area east of Falster with low oxygen concentrations.
The mean chlorophyll concentration in the uppermost 10 m varied between 1.7 m g/l in the south-eastern Kattegat to 3.7-4.8 m g/l in Kiel Bight and Fehmarn Belt. The chlorophyll was highest and homogeneously distributed in the uppermost 5 m of the water column, and very low in the water below the halocline.
Since the cruise in September, the minimum oxygen concentration had increased in all areas, except east of Falster. The lowest oxygen concentration of 0.4-0.7 ml/l (6-11% saturation) was observed in that area. In the central Sound, Great Belt and Fehmarn Belt the minimum oxygen concentrations were 2.0-2.6 ml/l (33-43%), and in Kiel Bight 2.9 ml/l (45%).
Compared to October last year the minimum oxygen concentrations this year are lower in the central Sound, Great Belt and Arkona Sea. Compared to mean for October in the 1980s, the concentrations this year are also lower in the northern Kattegat and southern Belt Sea.
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 east of Falster, and oxygen depletion still occurred in the central Sound, all Great Belt and Fehmarn Belt.
Figure 9 shows the stations visited by Danish counties and NERI within the first three weeks of October 2000, and where oxygen depletion or serious oxygen depletion was observed. The severity and area coverage of oxygen depletion this autumn in the Sound, southern Kattegat and Belt Sea is the worst case seen during the last 10 years. The minimum oxygen concentrations observed in the deeper parts of the Great Belt at the end of September and beginning of October are the lowest recorded from this area. Hydrogen sulphide, death of bottom fauna and demersal fish, has been recorded in a number of areas, although systematic investigations of such effects have not been performed.