National Environmental Research Institute, Dänemark
DMU (Denmarks Miljoundersogelser)
English summary of the report on the oxygen situation in Kattegat, Sund, Belt Sea und Arkona Sea in October 2003
During October the oxygen conditions in Danish waters have improved significantly. Oxygen deficiency no longer occurs in most of the Arkona Sea, the southern Belt Sea, the southern Great Belt, the sea north of Funen, the Aarhus Bight and the Limfjord. However, oxygen deficiency (<4 mg/L) still persists in the deepest parts of the southern Kattegat, the Sound, the northern Great Belt and the southern Little Belt, even though the conditions in these areas also have improved, except in the southern Kattegat. Severe oxygen depletion (<2 mg/L) is still widespread in the outer Flensborg Fjord, and occurs also in the deepest parts of the inner Flensborg Fjord, the southern Little Belt and the archipelago south of Funen.
The 2003 oxygen deficiency culminated around 1st October. The cooling of the surface has reduced the strength of the stratification and has facilitated wind mixing. Especially, the strong westerly wind on 9 - 11 October mixed and exchanged in many areas the oxygen poor bottom water with oxygen rich surface water.
During October the area coverage of oxygen deficiency and severe oxygen deficiency was reduced to 30 % and 11 %, respectively, of the coverage in mid September. The amount of severe oxygen deficiency now equals the situation at the same time in 2001, but is only 1/5 of the distribution in October 2002.
The 2003 oxygen deficiency has been more serious than average, but much less than last year. The precipitation and washout of nutrients were low in the first quarter of 2003, and although the precipitation deficit was reclaimed during April - June, the nutrient washout to coastal waters has probably been relatively low in 2003. The oxygen deficiency which has been more serious than average might be due to the warm summer and the long periods with light wind from mid July to mid August and again in the first 3 weeks of September.
The strongest effects of the 2003 oxygen deficiency were located to the Limfjord, the western Aarhus Bight and the Mariager Fjord, where the amount of bottom fauna was strongly reduced. However, dead bottom fauna has to a lesser extent also been observed in other areas, and dead fish caught in nets have been reported, e.g. from the archipelago south of Funen and at Omų in the Great Belt.
The strong autumn winds will probably further improve the oxygen situation through the coming weeks, and the deficiency will probably end during November.
The complete report can be found in the internet at:
National Environmental Research Institute, Roskilde
Monitoring Cruise Report with r/v Gunnar Thorson; Cruise no.: 219 (13 - 17 Oktober 2003)
Area: The Sound, Kattegat, Belt Sea and Arkona Sea
Due to reduced temperature stratification and strong westerly wind mid October, the minimum oxygen concentrations had generally increased since September, except in the Sound, parts of the southern Kattegat and in the northern Great Belt. Generally the concentrations were higher than at the same time last year, except at 4 stations in the areas mentioned above. However, oxygen deficiency (<2.8 ml/L) was still observed in the southern Kattegat, the Sound and the northern Great Belt, while severe oxygen deficiency (<1.4 ml/L) was only observed in the Sound (1.1 ml/L). Measurements made by Danish counties showed that oxygen deficiency and severe oxygen deficiency also still occurred in the southern Little Belt and Flensborg Fjord. A report on oxygen deficiency in October 2003 is available at: http://iltrapport.dmu.dk (in Danish with an English summary).
The stratification of the water column was unusually weak in the southern Great Belt and southern Belt Sea due to vertical mixing. Also in other areas the stratification was relatively weak for the season, except in the Sound and the Arkona Sea. Warmer water masses (14.5 - 15.4 °C) were observed as intermediate layers both in the eastern Kattegat and in the Arkona Sea.
Compared to long-term monthly means (Lightship observations 1931 - 1960) for October, the temperature was 0.2 - 1.5 °C higher than normal in the whole water column. During the present cruise the salinity was also generally higher than normal (0.2 - 5.8 psu), except for a little lower than normal in the south-western Kattegat bottom water.
In the surface layer nitrate was now present (0.1 - 1.1 µmol/L) in the Sound, the Great Belt and the Fehmarn Belt with the highest concentrations in the central Great Belt. Phosphate (0.06 - 0.4 µmol/L) and silicate (1 - 10 µmol/L) were also present in the surface water in all areas. In the bottom water the highest concentrations of nitrate, phosphate, nitrite and silicate were observed in the Sound, the southern Kattegat and the northern Great Belt, that is where oxygen deficiency occurred. High ammonium concentrations (2.2 - 2.3 µmol/L) were only observed in the bottom water in the central Arkona Sea.
A strong bloom of Proboscia alata and Ceratium spp. occurred in the Mecklenburg Bight with a chlorophyll-a concentration of 6.1 - 6.7 µg/L in the upper 15 m, and at 10 m depth in the Kiel Bight. In most other areas the vertical chlorophyll-a distribution was rather
homogenous within the upper 15 - 20 m and ranged between 1 and 3 µg/L with the highest concentrations in the surface of the northern Great Belt.
The complete report is to be found at: