Aarhus University Seal / Aarhus Universitets segl

Iltsvind i de danske farvande i august - september 2008

Udarbejdet af Jørgen L.S. Hansen & Ole Manscher, DMU

26. september 2008


Hele rapporten i PDF-format (969 kb)

Sammenfatning

English summary 


 

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

 

Figur 1 august 2008 / Figur 1 september 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. august måned og i perioden 22. august-18. september 2008.

Figure 1   The maps show stations visited by Danish and Swedish authorities in August and in the period 22 August-18 September 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 38, 15.-18. september 2008. Blå farve indikerer iltsvind (<4 mg/l) og rød farve kraftigt iltsvind (<2 mg/l).

Figure 2    Present distribution mid September 2008 of oxygen deficiency (<4 mg/l, blue) and severe oxygen deficiency (<2 mg/l, red) modelled from the latest measurements in the period 15-18 September 2008.

 

 Dansk

 Svensk

 English

 Deutsch

 Ilt

 Syre

 Oxygen

 Sauerstoff

 Iltsvind

 Syrebrist

 Oxygen deficiency

 Sauerstoffmangel

Sammenfatning

Den samlede udbredelse af iltsvind i midten af september 2008 var lidt over middelværdien for perioden 2003-2006, men større end på samme tidspunkt i 2007. Siden august, hvor iltforholdene var bedre end normalt, er der således sket et kraftigt fald i iltkoncentrationen. Udbredelsen af iltsvindet er især øget i den sydlige del af Kattegat og i de dybe dele af Bælthavet. I det sydlige Lillebælt samt i Femer Bælt, hvor der var iltsvind i august, er iltkoncentrationen faldet yderligere. Der er endvidere observeret iltfrie forhold samt forekomst af giftigt svovlbrinte i bundvandet i det sydlige Lillebælt, i det Sydfynske Øhav samt i Aabenraa Fjord.

I det foregående år har nedbørsmængden svaret til langtidsnormalen, og bundvandets temperatur har ligget på samme niveau som de seneste 5 år. På den baggrund vurderes det, at iltforbruget i vandsøjlen og bunden har været på et normalt niveau. Perioder med mere vind end normalt har medført, at udbredelsen af iltsvind indtil midten af august 2008 var mindre end normalt.

Den seneste måneds ret kraftige fald i iltkoncentrationen i bundvandet i de åbne farvande er sammenfaldende med et generelt vejskifte. Indtil slutningen af august var vejret præget af lavtrykspassager med vind fra vestlige retninger samt periodevis kraftig vind. Fra slutningen af august har vejret været præget af perioder med højtryk og vinde fra østlige retninger, som sidst i perioden har været ret svage.

Vejrskiftet har betydet, at vandudskiftningen i de dybere dele af de åbne farvande har været begrænset, og iltkoncentrationen er faldet gradvist i specielt den vestlige Østersø og det sydlige Bælthav. Derimod er iltforholdene blevet bedre i Limfjorden på grund af kraftig vind i september. I dele af det nordlige Lillebælt, i Smålandsfarvandet samt i Hjelm Bugt er der ligeledes sket en bedring på vanddybder over 20 m, fordi springlaget kom til at ligge dybere i forbindelse med skiftet til østenvind.

I midten af september dækkede iltsvindet et areal på 6.966 km2 (figur 1, 2, 3), hvoraf 1.996 km2 var ramt af kraftigt iltsvind. Hovedudbredelsen af kraftigt iltsvind var i Femer Bælt, det sydlige Lillebælt og de tilstødende fjorde. Disse områder rammes af tilbagevendende iltsvind. Iltsvindet for¬ventes at brede sig yderligere i de dybere dele af de åbne farvande i nærmeste fremtid, så længe det nuværende højtryksprægede vejr fortsætter.

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 september 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 September 2008 and 2007.</font>

 

English summary

The area covered by oxygen depletion in mid-September was slightly larger than the average for the same month during the period 2003–2006. However, the size of the area was nearly twice as big as in 2008. Since mid-August 2008, where the oxygen condition was better than normal, the oxygen concentrations have declined rapidly. In particular the distribution of oxygen depletions has increased in the southern Kattegat and the deeper part of the Belt Sea. In the southern Little Belt and the Femer Belt, where the oxygen depletion started in August, severe oxygen depletion had developed during the last month. Toxic hydrogen sulphide has been observed in the bottom water of the Aabenraa Fjord, the Archipelago of Southern Fyn and the southern Little Belt.

The precipitation during July 2007 - September 2008 was at the same level as the long-term average, and the temperature of the bottom water was close to the average for the last five years. Therefore it is assumed that the oxygen consumption in the bottom during 2008 was at a normal level. Periods of strong winds during most of the summer 2008 until the end of August resulted in a relatively small area covered by oxygen depletion in August.

The relatively fast decline in oxygen concentrations during the last month was caused by a general change in the weather in the same period. Until the end of August the weather was characterised by westerly winds and periods of strong winds. At the end of August the weather changed and became dominated by periods of high pressures and easterly winds.

The change in the weather conditions reduced the ventilation of the bottom water and consequently, the oxygen concentration declined, particularly in the western Baltic Sea and in the Belt Sea. In the Limfjorden the oxygen conditions have improved during September due to strong easterly wind. In the northern Little Belt, the Smålandsfarvandet and in Hjelm Bugt the oxygen conditions have also improved at water depths less than 20 m because of a deepening of the mixed layer.

In mid-September the area covered by oxygen depletion was 6,966 km2 (Figure 1, 2 and 3) and the area covered with severe oxygen depletion was 1,996 km2. The main areas affected by oxygen depletion were in the Femer Belt, the southern Little Belt and adjacent fjords. Severe oxygen depletion events are frequently occurring in these areas. It is expected that the oxygen concentration will continue to decline in these areas as long as the present weather conditions with high pressure and easterly wind are prevailing.