Report: Gunni Ærtebjerg
Cruise leader: Jan Damgaard
Participants: Lars Renvald, Hanne Ferdinand, Dorete Jensen, Claus Heinrich (technician trainee).
This report is based on preliminary data, which might later be corrected. Citation permitted only when quoting is evident. This file contains the summary only. To get the full report, click here. This file is in Adobe Acrobat ™ format. If you do not have a a Adobe Acrobat ™ Reader, click here, to download
Summary
The salinity stratification was strong in the Sound, southern Kattegat and northern Great Belt, but weak in the southern Belt Sea due to intense mixing since the cruise in August. Compared to long term monthly means the temperature was 0.2-2.8°C higher than normal, except for lower bottom water temperature east of Anholt. Generally, the salinity was higher than normal, except in Aalborg Bight and in the bottom water of the central and southern Great Belt and southern Belt Sea.
In the surface traces of nitrate were observed only in the Fehmarn Belt - Gedser Rev area. Phosphate was generally not present in the surface water in Kattegat, but only in the Sound, Belt Sea and Arkona Sea. The mean chlorophyll concentration in the uppermost 10 m varied from 1.3-2.1 mg/l in most of Kattegat to 4.6 mg/l in the north-western Kattegat and 7.4 mg/l in Mecklenburg Bight. The chlorophyll was generally homogeneously distributed in the uppermost 15 m.
Since the cruise in August the minimum oxygen concentration had decreased, except in the southern Belt Sea and Gedser Rev area. In the Sound, southern Kattegat and Arkona Sea the decrease was as high as 2.0-2.8 ml/l. The lowest oxygen concen-tration of 0.7 ml/l was found in the central Arkona Sea. In the Sound, southern Kattegat and northern Great Belt 2.2-2.8 ml/l was observed. In the southern Belt Sea and Gedser Rev area the minimum oxygen concentration had increased to 3.3-5.9 ml/l.
The reason for the large decrease in oxygen concentration, especially in the Sound and southern Kattegat, is believed to be the bloom of cyanobacteria in July in the Arkona Sea, southern Belt Sea, Sound and south-eastern Kattegat. In the beginning of August shortly after the bloom chlorophyll concentrations of 6-10 mg/l and 2-6 mg/l were observed in the bottom water in the Sound and south-eastern Kattegat, respectively.
Compared to September last year, when wide spread oxygen depletion occurred, the minimum oxygen concentration this year is higher, except in the northern Kattegat.
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 central Arkona Sea, and oxygen depletion occurred in the Sound and southern 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 September 2001, and where oxygen depletion or serious oxygen depletion was observed.