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

28. november 2008

Hele rapporten i PDF-format  (471 kb)

 
Figur 1 oktober 2008 / Figur 1 november 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. oktober måned og i perioden 1. - 20. november 2008.

Figure 1   The maps show stations visited by Danish and Swedish authorities in October and in the period 1 - 20 November 2008, and where oxygen deficiency (<4 mg/l) and severe oxygen deficiency (<2 mg/l) was observed.

Sammenfatning
Iltsvindet startede generelt lidt senere i de indre danske farvande i 2008 end i de foregående år på grund af megen blæst hen over sommeren. Frem til begyndelsen af september var udbredelsen af iltsvind i de danske farvande mindre end middelværdien for perioden 2003 - 2007. Efter en længere periode med stille vejr i september bredte iltsvindet sig hurtigt i de indre danske farvande. Den største udbredelse blev observeret omkring 1. oktober, hvorefter iltforholdene gradvist blev bedre. I begyndelsen af november skete der en indstrømning af nyt iltet bundvand til de centrale dybe dele af det sydlige Lillebælt, som afsluttede iltsvindet i dette område. Ved de seneste målinger i midten af november var der kun iltsvind i afgrænsede områder i Øresund, Mariager Fjord samt i Bornholmsdybet. Dermed forventes iltsvindet i 2008 stort set at være afsluttet, og iltforholdene i 2008 har således været lidt bedre end gennemsnittet for perioden 2003 til 2007 på grund af de jævnlige hændelser med kraftig vind. De områder, der i 2008 særligt blev berørt af iltsvind, var det Sydfynske Øhav, det sydlige Lillebælt, Aabenraa Fjord og Flensborg Fjord. I disse områder var der vedvarende kritisk lave iltkoncentrationer i omkring 3 måneder samt forekomst af giftig svovlbrinte i bundvandet, og i disse områder kan der følgelig forventes skader på bundens dyreliv.

I det forløbne år har den samlede nedbørsmængde svaret til langtidsnormalen. Det forventes derfor, at udvaskningen af næringsstoffer til havmiljøet, og dermed produktionen af organisk materiale i vandsøjlen, har været på samme niveau som i de seneste år. Temperaturen i bundvandet i de indre danske farvande har i efteråret 2008, i lighed med de seneste 5 år, været 1 - 1,5 grader højere end langtidsmidlen. På den baggrund antages det, at iltforbruget i vandsøjlen og havbunden har været på nogenlunde det samme niveau som i de seneste 5 år.

Det blæsende vejr igennem det meste af sommeren frem til slutningen af august og igen i oktober og november sikrede en god ventilation af bundvandet i de indre danske farvande. Derfor startede udviklingen af iltsvind lidt senere i 2008 end i de foregående år og udbredelsen af iltsvind var frem til begyndelsen af september mindre en middelværdien for de seneste 5 år. I forbindelse med vejrskifte i september, hvor der opstod en længere periode præget af højtryk og svage østlige vinde, øgedes udbredelsen af iltsvind hurtigt og nåede den største udbredelse i begyndelsen af oktober. Herefter skiftede vejrtypen igen og har siden været præget af jævnlige hændelser med kraftig vind, som har medført, at iltforholdene gradvist er blevet bedre. I begyndelsen af november skete der en indstrømning af nyt og iltet bundvand til de centrale dele af det sydlige Lillebælt. Denne indstrømning skete tidligere end normalt, og betød at iltsvindet ophørte i dette område. Ved de seneste målinger i midten af november var iltsvindet stort set ophørt i alle dele af de indre danske farvande, og der forventes ikke flere kritiske iltsvind i 2008.

Iltsvindene i 2008 har således haft en lidt kortere varighed og mindre udbredelse end i perioden 2003 - 2007 på grund af især de jævnlige hændelser med kraftig vind. Men med en varighed på omkring 3 måneder med kritisk lave iltkoncentrationer i nogle af de berørte område, kan der forventes skader på bundfaunaen.

English summary
The oxygen depletion events occurred generally a little later in the season in 2008 than seen during the previous years due to relatively strong winds during the summer. Until the beginning of September the area covered by hypoxia was smaller than the average of the period 2003 - 2007. During a relatively long period in September, where the weather was dominated by high pressure and weak wind, the oxygen concentration declined fast and hypoxia developed in many areas and reached the maximum extension around the first of October. This trend was reversed at the beginning of October where the weather again became dominated by westerly winds, and since then the oxygen conditions have gradually improved in the inner Danish coastal waters. The latest monitoring of the oxygen conditions showed that, with a very few exceptions, there is no longer hypoxia in the Danish waters. Therefore, it is expected that the hypoxic season in 2008 is over. Assuming that the hypoxic season is over, the oxygen conditions in 2008 were slightly better than seen during the previous years due to frequent events with strong winds. The areas severely affected in 2008 were the Archipelago of Funen, the southern Little Belt, Aabenraa Fjord and Flensborg Fjord. In these areas severe hypoxic conditions prevailed for about three months and hydrogen sulphide was frequently observed in the bottom water and damages of the benthic fauna can be expected.

The total precipitation during 2007 - 2008 corresponded to the long-term average. Therefore, it is expected that the runoff of nutrients from land to the marine environment, and thereby the primary production in the plankton, corresponds to the average of recent years. During the last five years, the temperature of the bottom water has generally been 1 - 1.5 degrees above the long-term average in the inner Danish coastal waters. Therefore, it is assumed that the oxygen consumption in the bottom water in 2008 was at the same level as during the last 5 years.

The winds were generally strong during most of the summer until the end of August and again during October and November. This ensured efficient ventilation of the bottom water and therefore the hypoxic season started slightly later and the total extension of the hypoxic areas was smaller than seen in the last years. However, a longer period with low winds during September resulted in a rapid decline in the oxygen concentrations and the oxygen depletion gradually progressed and peaked around the first of October. Hereafter the weather changed and became dominated by passages of low pressures and frequent incidents of strong westerly winds. Since then the oxygen conditions have gradually improved. At the beginning of November there was an inflow of new and well-oxygenated bottom water to the central parts of the southern Little Belt, which ended the oxygen depletion event here. The latest surveillance in mid November showed that with a very few exceptions, normoxic conditions were prevailing in the inner Danish coastal waters.

It is expected that the hypoxic season in 2008 is over now and thus the oxygen conditions in 2008 have been slightly better than the average of the period 2003 - 2007 because of frequent incidents of strong winds during the summer and most of the autumn. Nevertheless, critical hypoxia, anoxic events and incident of toxic hydrogen sulphide have prevailed for about 3 months in certain areas and severe damages are expected in the bottom fauna in these areas.