National Environmental Research Institute, Roskilde
Monitoring Cruise Report with r/v Gunnar Thorson; Cruise no.: 214 (10 - 19 February 2003)
Area: The Sound, Kattegat, Belt Sea and Arkona Sea
The Jutland Coastal Current (JCC) with lower salinity and temperature and high
nutrient concentrations, especially nitrate, was evident along all the Danish North Sea and Skagerrak coasts. The JCC also influenced the northern Kattegat north of Lęsų with nitrate concentrations up to 13 - 14 µmol/L in an intermediate layer at 20 - 40 m depth. In the Kattegat, the Sound and the Belt Sea the hydrography was dominated by outflow from the Baltic Sea, less vertical mixing and stronger stratification than normal for the season.
In the North Sea the nutrient concentrations, except nitrite, as usual varied inversely to the salinity. Due to this the nutrient concentrations were generally highest in the southeastern German Bight (nitrate up to 82 µmol/L), decreasing to the north and west. The nitrate concentrations in the Skagerrak waters and Baltic waters mixing in the Kattegat and Belt Sea were relatively high and about 1 µmol/L higher than in February last year. Therefore, the nitrate concentration in the bottom water of the Kattegat, the Sound and Great Belt was relatively high (9 - 10 µmol/L). Contrary, the nitrate concentration in the surface water in the Kattegat and the Belt Sea was normal to low, probably due to low runoff during winter, and because the phytoplankton spring bloom had started in the Belt Sea. Thus, nitrate concentrations in the Belt Sea surface waters do not represent the winter maximum values.
The DIN/DIP ratio varied from 50 60 in the German Bight and above 30 in the JCC to 8 - 15 at the northwestern North Sea stations and central Skagerrak stations. In the Kattegat and the Belt Sea the DIN/DIP ratio was rather close to the Redfield ratio of 16 for phytoplankton uptake, as it varied from about 8 in the Arkona Sea to 16 in the northern Kattegat.
The phytoplankton spring bloom had not started yet in the North Sea, the Skagerrak, the Kattegat, the Sound and the Arkona Sea, and the chlorophyll concentrations were generally below 2 µg/l. However, the bloom had started in the Belt Sea, especially in Kiel Bight and Fehmarn Belt, where a chlorophyll maximum of 17 µg/L was observed in 10 - 15 m depths below the outflowing Baltic water. The minimum oxygen concentrations were about saturation level at all stations in the North Sea. At the deepest station in the central Skagerrak the lowest oxygen concentration of 5.2 ml/L was observed in 600 m depth. In the Kattegat, the Sound and the Belt Sea the lowest oxygen concentration of 4.8 ml/L was found in the Sound. Compared to February last year the minimum oxygen concentrations this year were generally lower, probably due to less water exchange and mixing.