Report: Gunni Ærtebjerg
Cruise leader: Kjeld Sauerberg
Participants: Lars Renvald, Hanne Ferdinand, Peter Kofoed, Charlotte Andersen (student), Claus Jacobsen (technician trainee), Steffen Sørensen
(trainee)
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Summary
The salinity stratification of the water column was strong in all investigated areas due to outflow of brackish surface water from the Baltic Sea. Generally, the water temperature was lower and the bottom water salinity higher than long-term means for September.
In the surface only traces of nitrate and nitrite were observed, while about 0.5 m
mol/l ammonium was found in most areas. Phosphate and silicate were present in the surface; however, in the north-eastern Kattegat only in low concentrations. Relatively high concentrations of ammonium, phosphate and silicate were present in the bottom water in the areas with low oxygen concentrations; that is the south-eastern Kattegat, southern Belt Sea and Arkona Sea.The mean chlorophyll concentration in the uppermost 10 m varied between 1.2 m g/l in the Sound to 3.7 m g/l in the Fehmarn Belt. The chlorophyll was relatively homogeneously distributed in the uppermost 10 m of the water column. Significant subsurface maximum was only observed close to the bottom in the shallow western Kattegat (3.3 m g/l) and Kiel Bight (5.4 m g/).
Since the cruise in August the minimum oxygen concentration had decreased in all investigated areas. The lowest oxygen concentration of 0.3 ml/l (4% saturation) was observed in the central Arkona Sea. In the Fehmarn Belt, Mecklenburg Bight and Gedser Rev area the minimum oxygen concentration was only 0.5-0.9 ml/l (8-13%), and in Kiel Bight 1.1 ml/l. In the Sound and southern Kattegat 1.2-1.7 ml/l (18-26%) was observed and in the Great Belt the concentration was 1.7-2.2 ml/l (28-35%). Even in the shallow western Kattegat the oxygen concentration at the bottom was only 3.1 ml/l (52%).
Compared to September last year and to mean for September in the 1980’s the minimum oxygen concentrations this year are lower, except in the north-eastern Kattegat. The concentrations in the Great Belt are probably the lowest ever recorded at these stations.
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 Sound, southern Kattegat, Kiel Bight, Fehmarn Belt, Mecklenburg Bight, Gedser Rev area and central Arkona Sea, and oxygen depletion occurred in the south-eastern Kattegat and all Great Belt. In figure 9 is shown the stations visited by Danish counties and NERI within the first three weeks of September 2000, and where oxygen depletion or serious oxygen depletion was observed.