Aarhus University Seal / Aarhus Universitets segl

Iltsvind i de danske farvande i juli-august 2008

Udarbejdet af Peter Henriksen & Ole Manscher, DMU

29. august 2008


Hele rapporten i PDF-format (812 kb)

Sammenfatning

English summary 


Klik på figurnummeret for at få en større udgave.
Click on the figure number to enlarge the picture.

 

Figur 1 juli 2008/Figur 1 august 2008    Kortene viser de stationer, hvor iltforholdene (svensk: syreforholdene) er undersøgt af danske og svenske institutioner, og hvor der er observeret iltsvind (syrebrist) (<4 mg/l) eller kraftigt iltsvind (<2 mg/l) i hhv. juli måned og i perioden 1.-21. august 2008.

Figure 1    The maps show stations visited by Danish and Swedish authorities in July and in the period 1-21 August 2008, and where oxygen deficiency (<4 mg/l) and severe oxygen deficiency (<2 mg/l) was observed.

Figur 2     Aktuel udbredelse af iltsvind modelleret ud fra målinger i uge 34, 18.-21. august 2008. Blå farve indikerer iltsvind (<4 mg/l) og rød farve kraftigt iltsvind (<2 mg/l).

Figure 2    Present distribution mid August 2007 of oxygen deficiency (<4 mg/l, blue) and severe oxygen deficiency (<2 mg/l, red) modelled from the latest measurements in the period 18-21 August 2008.

Dansk

Svensk

English

Deutsch

Ilt

Syre

Oxygen

Sauerstoff

Iltsvind

Syrebrist

Oxygen deficiency

Sauerstoffmangel

Sammenfatning

Udbredelsen af iltsvind midt i august 2008 var halvt så stor som på samme tidspunkt i 2007 og knap 2/3 af middelværdien for perioden 2003-2006. Forskellen fra 2007 skyldes specielt, at der i august 2008 ikke blev registreret iltsvind i Kattegat og Storebælt og kun lille udbredelse i det nordlige Lillebælt samt området syd for Langeland og Ærø.

Nedbørsmængden i perioden juli 2007 til og med juni 2008 svarede til langtidsnormalen. Dette, kombineret med mange perioder med kraftigere vind end normalt, bevirkede at udbredelsen af iltsvind i august 2008 var mindre end middelværdien for perioden 2003-2006.

Mens udbredelsen af iltsvind i de åbne farvande var begrænset, udvikledes der i juli-august iltsvind eller kraftigt iltsvind i flere af de lavvandede områder og fjorde som fx Limfjorden, det Sydfynske Øhav og Flensborg Fjord. I Limfjorden har iltsvindet i korte perioder været afløst af bedre iltforhold som følge af blæst.

Midt i august dækkede iltsvindet 2.700 km2 (Figur 3), hvoraf 509 km2 var ramt af kraftigt iltsvind. Hovedudbredelsen af kraftigt iltsvind var det sydlige Lillebælt og tilstødende fjorde, hvilket er områder, der tilbagevendende rammes af iltsvind.

 

Figur 3   Areal dækket af iltsvind (<4 mg/l) uge for uge i sidste halvdel af 2002 og i middel for årene 2003-2006, samt midt i august i 2008 og 2007.

Figure 3   <font color="#000000">Area covered by oxygen deficiency (<4 mg/l) per week in the last half of 2002 and in average for the years 2003-2006, as well as mid August 2008 and 2007.</font>

English summary

The area covered by oxygen depletion in mid-August 2008 was half of the area covered at the same time in 2007 and approximately 2/3 of the mean area covered during 2003-2006. By mid-August 2008 oxygen depletion was not recorded in the Kattegat and the Great Belt and only small areas were affected in the northern Little Belt and in the western Baltic Sea south of the islands Langeland and Ærø.

The precipitation during July 2007 – June 2008 was equivalent to the long term average, and January till mid-August 2008 were characterised by frequent periods of winds stronger than average. Most likely, this combination resulted in a smaller area than average for 2003-2006 covered by oxygen depletion.

While oxygen depletion was not observed in the open waters of the Kattegat and the Great Belt, severe oxygen deficiency developed during July-August in several fjords and shallow areas like the Limfjorden, Flensborg Fjord and the area south of Funen. In the Limfjorden oxygen conditions have temporarily improved during periods of strong winds.

In mid-August oxygen depletion (<4 mg O2/l) covered a total area of 2,700 km2 (Figure 3), out of which 509 km2 had severe oxygen depletion (<2 mg O2/l). The main areas affected by severe oxygen depletion were the southern Little Belt and adjacent fjords. Severe oxygen depletion is a recurring phenomenon in these areas.