Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 11-15 October 1999. Cruise no. 193.

 

Report: Gunni Ærtebjerg
Cruise leader: Jan Damgaard.
Participants: Dorete Jensen, Hanne Ferdinand, Peter Kofoed, Mette Petersen (trainee), Gry Christensen (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

After an extremely warm and rather calm September strong wind from south to west in the first half of October significantly changed the hydrographic situation. Compared to long term monthly mean (1931-1960) for October both the temperature and salinity in the whole water column was higher, due to inflow of highly saline water from the Skagerrak to the Kattegat and mixing of the water column.

Nutrients were mixed from the bottom layer to the surface, and even nitrate was present in the surface water in the Sound and Belt Sea. This supported an autumn phytoplankton bloom in most areas. The highest mean chlorophyll concentrations (7.8-10.8 m g/l) in the upper 15 m were found in the southern Belt Sea.

The oxygen situation had improved significantly, except in the Sound and south-western Kattegat. The lowest oxygen concentrations of 1.9-2.1 ml/l (30-32%) were observed in the Sound. In the southern Kattegat the minimum concentrations were 2.7-3.2 ml/l (45-56%), and in the Kiel Bight and Mecklenburg Bight 2.6-3.2 ml/l. In the Great Belt the minimum concentration increased from 2.9 ml/l (48%) in the north to 5.7 ml/l (91%) in the south.

Compared to October last year the minimum oxygen concentrations this year were lower in the southern and eastern Kattegat, northern Great Belt and central Arkona Sea. Compared to mean for October in the 1980s, when oxygen depletion often occurred, the minimum oxygen concentrations this year were lower in the Sound, north-eastern Kattegat and in the northern to central Great Belt.

Oxygen depletion is in Denmark defined as below 2.8 ml/l (4 mg/l), and serious oxygen depletion as below 1.4 ml/l (2 mg/l). From these definitions oxygen depletion was during the cruise observed in the Sound, south-western Kattegat and Kiel Bight. Serious oxygen depletion was no longer observed in the areas investigated.

In figure 9 are shown the stations visited by Danish counties and NERI within the first three weeks of October 1999, and where oxygen depletion or serious oxygen depletion was observed.