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Oxygen depletion in Danish waters in 2003

  National Environmental Research Institute, Dänemark
Department of Marine Ecology

(http://www.dmu.dk/)

The development and decline of areas and water volumes affected by oxygen depletion (<4 mg/L and severe oxygen depletion (<2 mg/L) was analysed week by week through the last half of 2003, and compared to 2001 and 2002 (Figure 1). The analysis is based on profiles of oxygen concentrations measured by national and regional authorities in Denmark, Germany and Sweden in the Kattegat, the Sound, the Belt Sea and the western Arkona Sea with associated estuaries and bights. The study area appears from Figure 2. The method used for the analysis is described in HELCOM (2003).

In 2003 oxygen depletion and severe oxygen depletion developed slowly through July (weeks 27 - 31) and August (weeks 32 - 35), comparably to the situation in 2001 (Figure 1). In September 2003 the development accelerated to a maximum coverage in week 38 (15 - 21 September), when oxygen depletion covered an area of ca. 10,000 km², and severe oxygen depletion ca. 2,200 km² (Figure 2). The corresponding affected water volumes were ca. km³ and ca. 6.6 km³, respectively. The following week the area coverage of hypoxia declined rapidly, while the water volume affected by severe oxygen depletion did not decrease until week 41 (6 - 12 October). At the end of October (weeks 43 - 44) the hypoxia had been strongly reduced. However, both oxygen depletion and severe oxygen depletion prevailed in the southern Little Belt area until mid December. The persistence of the oxygen depletion and severe oxygen depletion in 2003 in different areas is shown in Figure 3.

The year 2001 can be considered as an average year with respect to hypoxia in the study area. The area coverage of oxygen depletion and severe oxygen depletion was in 2003 nearly 1.5 times the coverage in 2001, and thus more widespread than average (Figure 1, Table 1 and 2). However, it was far less widespread than in 2002, when the areas suffering from oxygen depletion and severe oxygen depletion covered more than 2 and more than 3 times the areas in 2001, respectively. Besides, in 2002 the hypoxia escalated earlier and persisted at a high level for much longer time than in both 2001 and 2003. The 2002 hypoxia is the most serious recorded in Danish waters.

Regional assessment of areas impacted by hypoxia in 2003 shows that oxygen depletion was more widespread than in 2001 mainly in the southern Belt Sea, the Little Belt, the northern Belt Sea and the southern Kattegat but also in the Great Belt and the central Kattegat (Table 1). Severe oxygen depletion was more widespread mainly in the Little Belt and southern Belt Sea, but also in the Flensborg Fjord, the northern Belt Sea and the southern and central Kattegat (Table 2). In the Sound and the Limfjorden the 2003 hypoxia covered about the same areas as in 2001. In the northern Kattegat hypoxia is only very rarely recorded, but was in 2002.

Conclusion:

The hypoxia in 2003 was nearly 1.5 times more widespread than average (2001), but far less widespread than in 2002. The 2003 hypoxia was more widespread than usual in the Belt Sea and the southern Kattegat, with the southern Little Belt being most seriously affected. In the Sound and the Limfjorden the 2003 hypoxia covered about the same areas as in 2001.

Reference:

HELCOM 2003: The 2002 oxygen depletion event in the Kattegat, Belt Sea and Western Baltic. Baltic Sea Environment Proceedings No. 90, 64 pp.

Area and volume impacted by oxygen depletion and severe oxygen depletion in 2001, 2002 and 2003, GIF-Graphic=25,0 KB

Figure 1: Area and volume impacted by oxygen depletion (<4 mg/L) (left panel) and severe oxygen depletion (<2 mg/L) (right panel) in 2001, 2002 and 2003. Includes the entire study area except the Arkona Basin.

Figure 2, File=21,8 KB

Figure 2: The most widespread hypoxia was in 2003 observed in week 38 (15 - 21 September). Oxygen depletion (<4 mg/L) covered an area of ca. 10,000 km², with severe oxygen depletion covering ca. 2,200 km².

The length in weeks of bottom exposure to oxygen depletion and severe oxygen depletion in 2003, GIF-Graphic=129 KB

Figure 3: The length in weeks of bottom exposure to oxygen depletion (<4 mg/L) (left map) and severe oxygen depletion (<2 mg/L) (right map) in 2003. Intensive colours indicate long exposure to hypoxic conditions.

Table 1: Total area and proportion affected by oxygen depletion (<4 mg/L) for each region and the entire area (excluding the Arkona Sea) in 2001, 2002 and 2003.

Total area and proportion affected by oxygen depletion

   

Oxygen depletion <4 mg l>-1

Region

Total area ( km²)

2001

2002

2003

Northern Kattegat 4405 2 0 % 106 2 % 16 0 %
Limfjorden 1522 385 25 % 377 25 % 416 27 %
Central Kattegat 8491 66 1 % 1688 20 % 674 8 %
Southern Kattegat 9432 2386 25 % 7012 74 % 4123 44 %
The Sound 1049 248 24 % 443 42 % 227 22 %
Northern Belt Sea 4027 1421 35 % 2596 64 % 1827 45 %
Great Belt 4012 653 16 %) 1740 43 % 918 23 %
Little Belt 3019 1103 37 % 1641 54 % 1350 45 %
Flensborg Fjord 293 160 55 % 162 55 % 157 54 %
Southern Belt Sea 7597 2970 39 % 4820 63 % 3993 53 %
Total area 43847 9394 21 % 20586 47 % 13700 31 %

Table 2: Total area and proportion affected by severe oxygen depletion (<2 mg/L) for each region and the entire area (excluding the Arkona Sea) in 2001, 2002 and 2003.

Total area and proportion affected by severe oxygen depletion

   

Severe oxygen depletion <2 mg l>-1

Region

Total area ( km²)

2001

2002

2003

Northern Kattegat 4405 0 0 % 0 0 % 0 0 %
Limfjorden 1522 329 22 % 251 17 % 344 23 %
Central Kattegat 8491 8 0 % 524 6 % 160 2 %
Southern Kattegat 9432 93 1 % 1360 14 % 331 4 %
The Sound 1049 3 0 % 205 20 % 15 1 %
Northern Belt Sea 4027 220 5 % 1208 30 % 298 7 %
Great Belt 4012 37 1 % 965 24 % 60 1 %
Little Belt 3019 608 20 % 1281 42 % 979 32 %
Flensborg Fjord 293 121 41 % 138 47 % 133 45 %
Southern Belt Sea 7597 1491 20 % 3241 43 % 2024 27 %
Total area 43847 2909 7 % 9173 21 % 4343 10 %

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 © 2016 Bundesamt für Seeschifffahrt und Hydrographie Last Update: 19.04.2013 18:03:32  
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