Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 3-7 November 1997. Cruise no. 182.

 

Report: Gunni Ærtebjerg

Cruise leader: Jan Damgaard

Participants: Kjeld Sauerberg, Susanne Hemmingsen, Hanne Ferdinand (NERI).

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

Summary and conclusions

The surface salinity had decreased since the beginning of October, except in the Arkona Sea. The bottom water salinity had since October increased in the Sound and most of the Kattegat due to inflow of saline water from the Skagerrak, but decreased in the Belt Sea and Arkona Sea due to mixing of the water column. The salinity stratification was weak, except in the Sound (21 psu) and Arkona Sea (10 psu), and nearly absent in the southern Great Belt. Generally the halocline was situated deeper than 20 m, except in the Sound where it was found between 10 m and 15 m depth. In the Fehmarn Belt the salinity gradually increased with depth.

Compared to long term monthly means (1931-1960) for November the surface temperature during this cruise was about normal, but the bottom water temperature was generally lower, except in the eastern Kattegat and Fehmarn Belt. The surface salinity was higher, and the bottom water salinity lower than long term means, except for lower surface salinity in the Fehmarn Belt.

Nitrate was still absent in the surface layer in the Fehmarn Belt - Darss Sill - Arkona Sea area as well as some areas of the Kattegat. Phosphate and silicate were present in the whole water column at all stations.

The mean chlorophyll-a concentration in the uppermost 15 m was 1.1-5.8 m g/l. The chlorophyll was rather homogeneously distributed in the uppermost 15 m, except for lower concentrations at 15 m depth in the southern Kattegat and the southern Belt Sea. The lowest mean concentration of 1.1 m g/l was observed in the Sound, and the highest of 5.8 m g/l in the shallow western Kattegat.

Since the cruise in the beginning of October the minimum oxygen concentrations had decreased in the southern Kattegat, the Sound and in the Arkona Sea. The lowest oxygen concentration of 1.8 ml/l (30% saturation) was found in the southern Kattegat. In the central Arkona Sea the minimum oxygen concentration was 2.8 ml/l (44%). At these two stations oxygen depletion (definition: £ 2.8 ml/l) was actually observed. Compared to November last year and mean for November in the 1980s the minimum oxygen concentrations this year were generally lower, except in the Belt Sea area. In the Sound the minimum oxygen concentration was lower than last year but higher than in the 1980s.

Even the oxygen situation in the open Danish waters this year has been relatively good, the autumn gales have not yet been able to transport enough oxygen to the bottom water to raise the oxygen concentration in all areas by inflow of new oxygen rich water and/or mixing of the water column.

  

Figure 1. Monitoring cruise with r/v Gunnar Thorson.
The three lines I, II and III show the transects used in the following figures:
Transect I: Kattegat NE - E - S - Great Belt - Fehmarn Belt - Arkona Sea
Transect II: Kattegat SE - The Sound - Arkona Sea
Transect III: Kattegat W - Aarhus Bight - Great Belt

General

The scope of the cruise was to monitor the hydrographic situation and the spatial variation in nutrients, oxygen, plankton and primary production. The three transects of monitoring stations used in the following figures are shown in figure 1.

Meteorology

The monthly mean temperature in Denmark was in October 1.3° C below long term mean 1961-1990, and the precipitation was 9% above normal. Westerly wind dominated all October. In the beginning of the month the wind was strong, but in the middle of the month rather weak. In the beginning of November the wind turned to south-east (Danish Meteorological Institute).

Hydrography

The surface temperature (1 m depth) had decreased 4.5-6° C since the cruise in the beginning of October, and ranged from 6.6° C in the north-western Kattegat (St. 1009) to 9.1° C in the eastern Kattegat (St. 905).

Figure 2. Surface (1 m) and near bottom temperature along transect I, and salinity in 1 m, 5 m, 10 m, 15 m, 20 m depth
and near bottom along transect I (see figure 1).

Also the bottom water temperature had decreased 0.5-4.8° C, except in the eastern Kattegat (St. 413, 905), and ranged from 8.4-9.3° C in the Great Belt to 13.2° C in the south-eastern Kattegat (St. 418, 921, 922) (Fig. 2). The temperature difference between surface and bottom was 0.2-5.5° C, with the largest difference in the Sound, north-western Kattegat and central Arkona Sea (St. 431, 921; 403, 1009; 444).

The surface salinity had decreased since the beginning of October, except in the Arkona Sea (St. 444), and ranged from 8.3 in the Arkona Sea (St. 444) to 28.1-30.0 in the northern Kattegat (St. 403, 1001, 1007, 1008, 1009). The bottom water salinity ranged from 10.1-15.5 in the western Arkona Sea (St. 441, 449) and 18.7 in the central Arkona Sea to 34.0-35.0 in the northern Kattegat (St. 1009, 905, 1001). The bottom water salinity had since October increased in the Sound and most of the Kattegat due to inflow of saline water from the Skagerrak, but decreased in the Belt Sea and Arkona Sea due to mixing of the water column (Fig. 2 and 3). The salinity stratification was weak, except in the Sound (21 psu) and Arkona Sea (10 psu), and nearly absent in the southern Great Belt. Generally the halocline was situated deeper than 20 m, except in the Sound where it was found between 10 m and 15 m depth. In the Fehmarn Belt the salinity gradually increased with depth.

Figure 3. Salinity in 1 m, 5 m, 10 m, 15 m, 20 m depth and near bottom along the transects II and III (see figure 1).

Compared to long term monthly means (1931-1960) for November the surface temperature during this cruise was about the same, but the bottom water temperature was generally lower, except in the eastern Kattegat and Fehmarn Belt. The surface salinity was higher, and the bottom water salinity lower than long term means, except for lower surface salinity in the Fehmarn Belt.

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Figure 4. Surface and near bottom concentrations of nitrate along the transects I, II and III.

Nutrients

Generally nitrate was found in the surface layer, except in the Fehmarn Belt - Darss Sill - Arkona Sea area, as well as the north-western and eastern Kattegat (St. 403, 1009; 413, 418). Close to the bottom the nitrate concentrations were in the Belt Sea relatively low (<2 m M), but in the Sound and deeper parts of the Kattegat and Arkona Sea above 6 m M (Fig. 4). Nitrite was present in the whole water column in concentrations relatively normal for the season (Fig. 5). Significant concentrations of ammonium (1-3 m M) were only found in the north-western Kattegat (St. 303, 1009) and the Belt Sea (Fig. 5).

Figure 5. Surface and near bottom concentrations of nitrite and ammonium along transect I.

 

Phosphate and silicate were found in the surface layer in all areas, with the highest concentrations in the Belt Sea, the Sound and western Kattegat, and for silicate also in the Arkona Sea. In the bottom water high phosphate and silicate concentrations were observed in the southern Kattegat, the Sound and the Arkona Sea (Fig. 6).

Figure 6. Surface and near bottom concentrations of phosphate and silicate along transect I.

 

Oxygen

Since the cruise in the beginning of October the minimum oxygen concentrations had decreased 1.6-2.5 ml/l in the southern Kattegat (St. 418, 922, 925), 0.3-0.4 ml/l in the Sound (St. 431, 921) and 1.4 ml/l in the Arkona Sea (St. 444). At the rest of the deeper stations the oxygen concentration had increased.

The lowest oxygen concentration of 1.8 ml/l (30% saturation) was found in the southern Kattegat (St. 922). At the other stations in the southern Kattegat (St. 413, 418, 925) the concentration was 3.4 ml/l (54-56%). In the central Arkona Sea (St. 444) the minimum oxygen concentration was 2.8 ml/l (44%). In the Sound (St. 431, 921) the minimum concentrations were 3.1-3.2 ml/l (51-52%). At all other deeper stations the minimum oxygen concentrations were 3.7-7.0 ml/l (61-101%) (Fig. 7).

Figure 7. Minimum oxygen concentrations along the transects I, II and III. (see figure 1).

 

Compared to November last year and mean for November in the 1980s the minimum oxygen concentrations this year are generally lower, except in the Belt Sea area and shallow western Kattegat. In the Sound (St. 431) the minimum oxygen concentration was lower than last year but higher than in the 1980s.

Oxygen depletion is in Denmark defined as below 2.8 ml/l (4 mg/l), and serious oxygen depletion as below 1.4 ml/l (2 mg/l). From these definitions oxygen depletion was during the cruise observed in the southern Kattegat (St. 922) and in the central Arkona Sea (St. 444). In the Sound oxygen depletion with 2.1-2.6 ml/l (34-42%) was observed outside Copenhagen mid November by the county of Copenhagen.

Figure 8. Chlorophyll-a concentrations in 1 m, 5 m, 10 m and 15 m depths along the transects I, II and III (see figure 1).

Chlorophyll-a

The mean chlorophyll-a concentration in the uppermost 15 m was 1.1-5.8 m g/l. The chlorophyll was rather homogeneously distributed in the uppermost 15 m, except for lower concentrations at 15 m depth in the southern Kattegat (St. 418, 922) and the southern Belt Sea (St. 450, 952, 954) (Fig. 8). The lowest mean concentration of 1.1 m g/l was observed in the Sound (St. 921, 431, 1728), and the highest of 5.8 m g/l in the western Kattegat (St. 409). At the other Kattegat stations the mean concentrations were 1.8-3.6 m g/l. In the Belt Sea the mean concentrations varied from 1.2-1.4 m g/l in mid Great Belt to 3.7-4.5 m g/l in the northern part and 2.4-4.4 m g/l in the Fehmarn Belt. In the Arkona Sea the mean concentrations were 2.9-3.4 m g/l.

The lowest chlorophyll concentrations occurred in the areas with the strongest (the Sound) and weakest (Great Belt) stratification. The highest concentration occurred in the shallow western Kattegat with no stratification of the water column.