Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 4-8 November 1996. Cruise no. 173.

Report: Gunni Ærtebjerg
Cruise leader: Susanne Hemmingsen
Participants: Peter Kofoed, Dorete Jensen , Hanne Ferdinand (NERI).
4-6 November 1996: Jens Jacobsen and Thomas Andersen (NERI).


This report is based on preliminary data which might later be corrected. Citation permitted only when quoting is evident.


Summary and conclusions

The strong south-westerly and westerly winds at the end of October and beginning of November had strongly influenced the hydrographic situation observed during the cruise. Except in the Arkona Sea, the salinity in the surface water had increased and in the bottom water decreased since primo October indicating vertical mixing. The halocline was relatively weak, and a thermocline was practically none-existing. The Kattegat-Skagerrak front was pressed into Kattegat and was situated just south of Læsø, and the water column was homogenous at the stations in the northern Kattegat north of the front.

Inflow of saline water to the Baltic Sea was observed in the Arkona Sea. From 37.5 m depth to the bottom the salinity was 20.1-20.8 psu and the temperature 10.1·C. This water probably originates from the Sound. Above this bottom layer was found a layer at 30-37 m depth with salinity's of 12-18 psu and temperatures around 12 ·C probably originating from the Fehmarn Belt.

Both phosphate and nitrate were present in the surface water in all areas, and nutrient limitation of the primary production was no longer likely.

The mean chlorophyll concentration in the upper 15 m was generally below 2 mg/l, except for 3.0 mg/l in the central Arkona Sea, 4.2 mg/l at Gedser Rev and 5.4 mg/l in Fehmarn Belt. Relatively high chlorophyll concentrations in the Fehmarn Belt - Arkona Sea area are often seen in late autumn following dynamic periods with strong wind and increased water exchange and mixing.

The minimum oxygen concentrations had increased since primo October, and oxygen deficiency (<2.8 ml/l) was no longer observed. Compared to mean for November 1980-89 the concentrations this year are higher, except in the central Great Belt and southern Belt Sea.

General

The route, time schedule and stations of the cruise are shown in figure 1. Besides the pelagic monitoring measurements, common whelk Buccinum undatum was sampled in the Great Belt for TBT analyses, and suspended matter was sampled at all stations in 1 m depth for calibration of satellite images.

Meteorology

In October the mean temperature was ½ ·C above long term mean 1961-1990, and the precipitation was about normal. The wind came mainly from south-west, except for a period in the middle of the month with easterly wind. At the end of October and beginning of November strong wind from south-west and west dominated, and a storm past the area 4-5 November during the cruise (Danish Meteorological Institute).

Hydrography

The surface temperature ranged from 9.8 ·C in the southern Kattegat (St. 921, 925) to above 11 ·C in the north-eastern Kattegat (St. 905, 1001, 1007), and had decreased 0.5-2.5 ·C since primo October. The bottom water temperature ranged from 8.4 ·C in the Sound (St. 431) to above 11.2 ·C in the northern Kattegat (St. 403, 1001, 1007, 1008, 1009) (Fig. 2), and had generally decreased since primo October, except in the northern and eastern Kattegat. Only in the Sound (St. 431, 921) was the temperature difference between surface and bottom more than 1 ·C. In the Arkona Sea a warmer intermediate layer with temperatures around 12 ·C was observed in 30-36 m depth, while the temperatures at the surface and at the bottom were 10.7 ·C and 10.1 ·C, respectively.

The surface salinity ranged from only 7.6 psu in the central Arkona Sea (St. 444) to 32.7 psu in the north-eastern Kattegat (St. 1001, 1007), and had, except in the Arkona Sea, increased much (2-6.4 psu) since primo October. The bottom water salinity ranged from 18.1 psu at Gedser Rev (St. 449, 954) to 34.0 psu in the Sound (St. 431, 921) (Fig. 3 and 4), and had generally decreased since primo October, except in the Sound and Arkona Sea. The bottom water salinity in the central Arkona Sea was as high as 20.0-20.8 psu from 37.5 m depth to the bottom, and 12-18 psu in 30-37 m depth, indicating inflow from the Sound and Belt Sea, respectively. Except for the Arkona Sea, the salinity stratification was rather weak with differences between surface and bottom of less than 10 psu, and in the northern Kattegat less than 1 psu.

Compared to long term monthly means (1931-60) for November the temperatures during this cruise were higher than normal, except in the bottom water of the southern Kattegat, (St. 413, 925). Also the salinity's were generally higher than long term means, except in the bottom water of the northern and eastern Kattegat.

Oxygen

Since primo October the bottom water oxygen concentrations had increased. The lowest oxygen concentrations of 3.3-3.6 ml/l (50-55% saturation) were now found in the Sound (St. 431, 921 ). In the central Great Belt (St. 935, 939) the minimum concentrations were 3.7-3.8 ml/l (58-60%). In all other areas the oxygen saturation was 66-95% (Fig. 5 and 6).

Compared to November last year the minimum oxygen concentrations this year are lower in the Great Belt and south-western Kattegat. Compared to mean for November 1980-89 the concentrations this year are higher, except in the central Great Belt and southern Belt Sea.

Nutrients

Only in the Fehmarn Belt (St. 952, 954) and central Arkona Sea (St. 444) the nitrate concentrations in the surface layer were as low as 0.3-0.5 mmol/l. In the northern Kattegat (St. 1001, 1007, 1008, 1009, 403) 0.8-1 mmol/l were found in the whole water column. In the bottom water nitrate concentrations above 5 mmol/l were observed in the Sound, southern Kattegat and Great Belt at salinity's above 31 psu (Fig. 7, 8, 9).

The highest nitrite concentrations were found in the bottom water. In the eastern Kattegat above 0.5 mmol/l was observed (Fig. 10). Ammonium concentrations above 1 mmol/l were found in the whole water column in the northern Kattegat and east of Falster (St. 449), and in the Arkona Sea bottom water, while no ammonium was found in the bottom water in the Sound, southern Kattegat and Great Belt (Fig. 11).

The lowest phosphate concentrations of about 0.13 mmol/l were observed in the Arkona Sea surface water, and the highest concentrations above 0.8 mmol/l were found in the bottom water in the Sound and Great Belt (Fig. 12). The silicate concentrations were below 5 mmol/l in the northern Kattegat and the surface of the eastern Kattegat. High silicate concentrations (>10 mmol/l) were found in the bottom water of the Sound, Belt Sea and western Arkona Sea (Fig. 13).

Chlorophyll-a

The mean chlorophyll-a concentrations in the uppermost 15 m ranged from 0.6-1.0 mg/l in the Sound and northern Kattegat to 3.0-5.4 mg/l in the central Arkona Sea, at Gedser Rev and in Fehmarn Belt. In the other areas the mean chlorophyll concentration was generally below 2 mg/l and uniformly distributed in the upper part of the water column.