English summary

The exceptionally strong, long lasting and widespread oxygen deficiency in the Kattegat, Sound, Belt Sea and western Baltic Sea during summer and autumn 2002 is now greatly reduced. This is due to more windy and colder weather during October and November causing mixing and exchange of the oxygen depleted bottom water masses. However, widespread severe oxygen depletion is still present in the southern Little Belt, Flensborg Fjord and the Ærø Basin in the archipelago south of Funen, and poisonous hydrogensulphide is still present in the bottom waters north of the island of Als and in the Ærø Basin. However oxygen conditions in these waters have progressively improved during the last month. In the southern Kattegat, the Sound and northern Belt Sea the oxygen concentration in the bottom water is still low for the season, eventhough oxygen depletion is no longer apparent.

The mixing of nutrient rich bottom water into the surface water has created an unusually large autumn bloom of phytoplankton in the Kattegat and Belt Sea, which after sedimentation might delay the re-oxygenation of the bottom water.

In July and August the oxygen depletion was strongest in the estuaries and fjords, e.g. Limfjorden and Mariager Fjord, while in the Kattegat, Sound, Belt Sea and Arkona Sea the coverage and strength of the oxygen deficiency culminated in September. The extent of the area coverage of oxygen deficiency (<4 mg/l) and serious oxygen depletion (<2 mg/l) in the estuaries, fjords, Kattegat, Sound, Belt Sea and Arkona Sea, excluding bordering Swedish and German coastal waters, is in round figures given in table 1. The area suffering from oxygen depletion was in August and September about 13.000-14.000 km2, which during October was reduced considerably to about 800 km2 in mid November. The area suffering from serious oxygen depletion culminated in September covering about 5.000 km2, decreasing through October to less than 300 km2 in mid November. Especially parts of the southern Little Belt, Flensborg Fjord, Åbenrå Fjord, the Ærø Basin, the sea north of Funen, Århus Bight, south-eastern Kattegat, northern and central Sound, Arkona Sea south of the island of Møn and Fehmarn Belt suffered from long lasting periods of serious oxygen depletion.

Week	Date	Oxygen <4 mg/l	Oxygen <2 mg/l
no.		km2	km2
34-35	19.-30. August	13,000	3,400
36-37	2.- 13. September	13,200	5,000
38-39	16.-27. September	13,800	4,700
41-42	7.-18. October	6,000	2,200
43-44	21. Oct. - 1. Nov.	3,000	870
46-47	11.-22. November	810	280

Tabel 1. The coverage of oxygen deficiency in round figures in the estuaries, fjords, Kattegat, Sound, Belt Sea and Arkona Sea, excluding bordering Swedish and German coastal waters during late summer and autumn 2002. Note the weeks 40 and 45 are not included due to insufficient data. Maps showing the coverage of oxygen deficiency and serious oxygen deficiency within the 6 time periods can be seen at the interactive "iltsvindskort".


The oxygen depletion in the Danish waters in 2002 will be remembered for the dramatic events with dead fish and bottom fauna washed ashore in Ålborg Bight, Vejle inner estuary, Kalø Inlet, Ebeltoft Inlet and Hjelm Deep in the beginning of October. However, the oxygen deficiency leaves behind other serious trails not visible from the surface in the form of reduced or extinct bottom fauna communities in large areas, which will take several years without serious oxygen depletion to re-establish. The feeding resources for dermersal fish and wintering diving ducks is reduced in these areas, and the function of the benthic ecosystem is changed.

The Danish counties and NERI are intensively working at mapping the severity and extent of the damage caused by oxygen deficiency to the benthic fauna. Preliminary results from a few investigated areas show 0-100% reduction in the abundance of the bottom fauna in different areas. Large reductions are seen in areas exposed to severe and long lasting oxygen deficiency, as seen in the southern Little Belt and Flensborg Fjord, where the fauna is extinct at depths greater than 17-20 m. Contrary, in areas only shortly exposed to less severe oxygen deficiency no visible damages to the bottom fauna is observed (e.g. along the coasts of the North Zealand).
In the short term the oxygen conditions are expected to slowly improve, although the weather conditions are still dominated by weak winds from eastern and southern directions. The bottom waters will be re-oxygenated during winter as usual. In the somewhat longer term, work is underway in order to get an overview of the extent of the areas with bottom fauna affected by the oxygen depletion, the degree and nature of the effects and to assess the cause - effect relationships. In the long term, it will be a challenging excersie to follow the development of the bottom fauna in the affected areas in the coming years.