National Environmental Research Institute, Roskilde
Monitoring Cruise Report with r/v Gunnar Thorson; Cruise no.: 217 (18 - 22 August 2003)
Area: The Sound, Kattegat, Belt Sea and Arkona Sea
The minimum oxygen concentrations in August this year were significantly higher than at the same time
last year, except in the north-eastern Kattegat. The lowest oxygen concentration observed was
1.1 ml/L (17 % saturation) in the Mecklenburg Bight. In the south-eastern Kattegat and the
Sound the minimum concentrations were 2.3 - 3.0 ml/L (33 - 46 %), and in the
Fehmarn Belt and Arkona Sea 2.6 ml/L (40 - 42 %).
In week 34 (18 - 22 August) the actual area distribution of oxygen deficiency
(<4 mg/L) and severe oxygen deficiency (<2 mg/L) in the Kattegat, the Sound and the Belt
Sea with associated estuaries, but excluding the Arkona Basin, was only about 1/5 of the distribution at the
same time last year. The geographical distribution of oxygen deficiency generally resembled that of 2001, which
can be though to represent an average distribution of oxygen deficiency. The area afflicted by oxygen
deficiency was larger in the first half of August 2003, but strong westerly winds in the middle of the
month improved the oxygen conditions at some localities (e.g. the Limfjord, Aarhus Bight and north of Funen).
A report on oxygen deficiency in July and August 2003 is available at:
(in Danish with an English summary).
At the cruise both the salinity and the temperature generally were higher than normal in
the whole water column due to the warm weather in July and August and the strong wind in mid August.
The wind accomplished an inflow to the Arkona Sea, where the bottom water salinity reached 20.
In the surface layer generally no inorganic nitrogen nutrients were present, while traces of phosphate,
and silicate concentrations of 1 - 7 µmol/L, were present in the surface water in most
areas, except in the north-eastern Kattegat. In the bottom water nitrate concentrations above
6 µmol/L were observed only in the eastern Kattegat. In the Sound and Belt Sea the nitrate
concentration in the bottom water was unusually low. Very high ammonium concentrations
(6.0 - 8.8 µmol/L) were observed in the bottom water in the Great Belt and especially
in the Fehmarn Belt and Mecklenburg Bight. High silicate and phosphate concentrations were observed in the
bottom water in the Fehmarn Belt and Arkona Sea.
The chlorophyll-a concentration was relatively low (<1.5 µg/L) in most of the Kattegat.
However, in the south-western Kattegat and northern Great Belt a pronounced subsurface maximum
(3.4 - 6.8 µg/L) was present in 15 - 17 m depth. In the Fehmarn Belt
the highest concentrations (3.0 - 3.6 µg/L) were observed in the surface layer.
The complete report is to be found at: