North Sea - Skagerrak. Low saline water (29-33 psu) was spread out over the eastern North Sea and southern Skagerrak. In the surface 34 psu was only observed at the westernmost station 1064. The surface temperature ranged from 15.8°C to 18.4°C, and varied inversely to the salinity. A pycnocline was present in certain areas. Nitrate, nitrite and ammonium were observed in the surface, especially west of the Limfjord and in the German Bight. Also phosphate and silicate was present, especially in the German Bight. The chlorophyll-a concentration was unusually high for the season, up to 11 mg/l at the north-western station 1025, and 8.6-8.8 mg/l in the German Bight and north of Horns Rev. The lowest oxygen concentration of 4.2-4.4 ml/l (73-75%) was observed at the stratified north-western stations.

Kattegat, Belt Sea, Arkona Sea. Compared to long term monthly mean for August the bottom water temperature in the southern Kattegat and Belt Sea was 0.4-1.0°C lower than normal, while the surface temperature generally was 0.4-2.0°C higher. The salinity was generally higher than normal, both at the surface and in the bottom water.

In the surface water nitrate and nitrite was very low, while ammonium was generally present. Also phosphate and silicate were present, except in mid and southern Kattegat. At the bottom high phosphate and silicate concentrations were observed in Fehmarn Belt and Arkona Sea, highest at Gedser Rev, that is in the areas with low oxygen concentrations.

The mean chlorophyll concentration in the uppermost 10 m varied from 0.9-1.0 mg/l in the northern Great Belt to 3.0-5.1 mg/l in the north-western Kattegat. Pronounced subsurface maximum was observed in the Sound and at Gedser Rev.

Oxygen Below 3 ml/l was observed in the southern Belt Sea and western Arkona Sea with the lowest oxygen concen­tration of 1.05 ml/l observed at Gedser Rev.

Compared to August last year and to mean for August in the 1980’s the minimum oxygen concentrations this year are higher in all areas, except at Gedser Rev.

According to Danish definitions serious oxygen depletion (<2 mg/l) occurred at Gedser Rev, and oxygen depletion (<4 mg/l) occurred in the Fehmarn Belt, Mecklenburg Bight, and south of Møn. In figure 20 is shown the stations where oxygen depletion and serious oxygen depletion was observed by Danish counties and NERI within the first 3 weeks of August 2000.

The objectives of the cruise were:

• To determine the actual situation in the open Danish waters;
• To trace the influence of land based discharges of nutrients;
• To establish reference data for the local monitoring in coastal areas;
• To continue time series for trend monitoring.

The cruise is part of the Danish nation wide monitoring programme NOVA 2003, the HELCOM monitoring programme for the Baltic Sea area (Arkona Sea, Sound, Belt Sea, Kattegat), and the OSPARCOM monitoring programme for the Greater North Sea (Kattegat, Skagerrak, North Sea). The main scope of the cruise was to monitor the oxygen situation, but also the hydrography and the concentrations of nutrients and chlorophyll-a. The stations of the cruise are shown in figure 1. Yellow substance samples were collected for calibration of fluorescence profiles.

Characteristics of the weather conditions since the last cruise in February are given in table 1. As the winter also the spring (March-May) was generally warm, June was colder than average, while the rest of the summer (July-August) was warm. April, June and August were wet, March, May and July dry. The mean wind was rather low, especially in July.

Table 1. Deviations in monthly mean temperature and precipitation in March to August 2001 in Denmark compared to long term monthly means 1961-90, monthly mean wind speed and dominating wind direction (based on data from the Danish Meteorological Institute).