Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 13-18 September 1998. Cruise no. 186.

 

Report: Gunni Ærtebjerg
Cruise leader: Stiig Markager/Lars Renvald
Participants: Kjeld Sauerberg, Hanne Ferdinand, Dorete Jensen, Peter Kofoed, Anja Fløjtrup,.

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

Summary

Generally the surface temperature and salinity had decreased and the bottom water temperature and salinity increased since the cruise in August. Compared to long term monthly mean (1931-1960) for September the temperature during this cruise was lower, except in the Belt Sea bottom water. The surface salinity was lower, and the bottom water salinity higher than long term mean, except for lower bottom water salinity in the Fehmarn Belt.

Only traces of nitrogen nutrients were present in the surface layer. Close to the bottom the nitrate concentrations were above 10 m mol/l in the Sound and above 8 m mol/l in the eastern Kattegat. Phosphate and silicon were found in the surface layer in all areas, except for phosphate in the Arkona Sea. Significant concentrations of ammonium, phosphate and silicon were observed in the bottom water in the Fehmarn Belt and Arkona Sea at the stations with lowest oxygen concentrations.

Since the cruise in August the minimum oxygen concentrations had decreased 0.3-0.8 ml/l, except in the northern Kattegat. The lowest oxygen concentrations of 0.2-0.9 ml/l (3-13% saturation) were observed in the Arkona Sea, Mecklenburg Bight and Fehmarn Belt. In the Sound, southern Kattegat and Great Belt the minimum oxygen concentrations were 2.0-2.8 ml/l (30-46%). At the rest of the stations the concentrations were 3.3-5.1 ml/l (51-84%).

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 Sound, southern-most Kattegat and Great Belt. Serious oxygen depletion was observed in the central Arkona Sea, Mecklenburg Bight and Fehmarn Belt.

Compared to September last year the minimum oxygen concentrations this year were significantly lower (0.5-4.8 ml/l), except in the eastern Kattegat. Compared to mean for September in the 1980s the minimum oxygen concentration this year were also lower, except in the northern Kattegat.

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Figure 1. Monitoring cruise with r/v Dana and r/v Tyra 13-18 September 1998. r/v Gunnar Thorson cruise no. 186.
The 2 lines I and II 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.

General

Due to break down of the CTD system on r/v Gunnar Thorson the cruise was actually conducted with r/v Dana from the Danish Fishery Research Institute (Kattegat) and r/v Tyra from the County of Vejle (southern-most Kattegat, Sound, Arkona Sea and Belt Sea) (Fig. 1). The scope of the cruise was to monitor the hydrographic situation and the spatial variations in the concentrations of oxygen, nutrients and chlorophyll. The two transects of monitoring stations used in the following figures are shown in figure 1.

Meteorology

The monthly mean temperature in Denmark was in September 0.7° C above long term mean 1961-1990, and the precipitation was 21% below normal. At the beginning of September strong wind from east dominated, turning to south and west in the middle of the month, and ending up with weak wind from east at the end of the month (Danish Meteorological Institute).

Hydrography

The surface temperature (1 m depth) had decreased up to 2.2° C since the cruise in August, and ranged from 13.5° C at Gedser Rev (St. 954, 449) to 15.2-15.3° C in the southern Kattegat and northern Great Belt (St.921, 922, 925, 935) (Fig. 2).

Figure 2. Surface (1 m) and near bottom temperature along transect I (see figure 1).

The bottom water temperature had increased, except in the northern Kattegat, and ranged from 8.5-8.9° C in the north-eastern Kattegat to 14.1-14.5° C at Gedser Rev (St.449, 954) (Fig. 2). The temperature difference between surface and bottom was 0.6-6.6° C, with the largest difference in the north-eastern Kattegat (St. 905, 1001).

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

The surface salinity had decreased since August, except in the Sound and southern Belt Sea (St. 431, 921, 952, 449), and ranged from 7.3 in the Arkona Sea (St. 444) to 23.1-23.9 in the north-western Kattegat (St. 403, 1008, 1009). The bottom water salinity ranged from 15.2-15.4 in the Arkona Sea (St. 444, 449) to 34.0-35.0 in the northern Kattegat (St. 403, 905, 1001, 1007, 1008, 1009) (Fig. 3). The bottom water salinity had increased since August, except in the south-eastern Kattegat (St. 921, 922) and southern Belt Sea (St. 450, 952). The salinity stratification was strong (>10 psu), except in the southern Belt Sea and Arkona Sea.

Compared to long term monthly mean (1931-1960) for September the temperature during this cruise was lower, except in the Belt Sea bottom water. The surface salinity was lower, and the bottom water salinity higher than long term mean, except for lower bottom water salinity in the Fehmarn Belt.

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

Nutrients

Only traces of nitrogen nutrients (nitrate, nitrite, ammonium) were present in the surface layer. Close to the bottom the nitrate concentrations were above 10 m mol/l in the Sound (St. 431, 921) and above 8 m mol/l in the eastern Kattegat (Fig. 4).

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

Relatively high nitrite concentrations (>0.4 m mol/l) were present in the bottom water in the southern Kattegat and the Belt Sea (Fig. 5). Significant concentrations of ammonium (1.5-4.6 m mol/l) were found at the bottom in the Fehmarn Belt and Arkona Sea (St. 450, 952, 444) (Fig. 5).

 

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

Phosphate and silicon were found in the surface layer in all areas, except for phosphate in the Arkona Sea, with the highest concentrations in the southern Belt Sea. In the bottom water high phosphate and silicon concentrations were observed in the Fehmarn Belt and the Arkona Sea (Fig. 6).

 

Figure 7. Chlorophyll-a concentrations in 1 m, 5 m, 10 m and 15 m depths along the transects I and II (see figure 1).

Chlorophyll-a

The mean chlorophyll-a concentrations in the uppermost 15 m were 1.0-1.8 m g/l in the Kattegat and Fehmarn Belt, except for 2.4-3.1 m g/l in the north-western Kattegat (St. 1008, 1009) with a maximum of 6.9 m g/l in 5 m depth at station 1009. In the Sound, Arkona Sea, Great Belt, Kiel Bight and Mecklenburg Bight the mean chlorophyll concentrations were 2.1-3.0 m g/l. The chlorophyll was relatively homogeneously distributed in the uppermost 15 m, except for lower concentrations at 15 m depth in the Belt Sea and in 10 and 15 m depth in the Sound (Fig. 7).

Figure 8. Minimum oxygen concentrations along the transects I, II and III. (see figure 1).

Oxygen

Since the cruise in August the minimum oxygen concentrations had decreased 0.3-0.8 ml/l, except in the northern Kattegat. The lowest oxygen concentrations of 0.2-0.9 ml/l (3-13% saturation) were observed in the Arkona Sea and Fehmarn Belt (St. 444, 952). In the Sound (St. 431), southern Kattegat (St. 921, 922, 925) and Great Belt (St. 935, 939, 443) the minimum oxygen concentrations were 2.0-2.8 ml/l (30-46%). At the rest of the stations the concentrations were 3.3-5.1 ml/l (51-84%) (Fig. 8).

Compared to September last year the minimum oxygen concentrations this year were significantly lower (0.5-4.8 ml/l), except in the eastern Kattegat. Compared to mean for September in the 1980s the minimum oxygen concentration this year were also lower, except in the northern Kattegat.

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 Sound (St. 431), southern-most Kattegat (St. 921, 922, 925) and Great Belt (St. 935, 939, 443). Serious oxygen depletion was observed in the central Arkona Sea (St. 444) and Fehmarn Belt (St. 952, M2). In figure 9 are shown the stations visited by either Danish counties or NERI within the first three weeks of September 1998, and where oxygen depletion or serious oxygen depletion were observed.

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Figure 9. Stations visited by Danish counties or NERI within the first three weeks of September 1998, and where oxygen depletion (<4 mg/l) and serious oxygen depletion (<2 mg/l) were observed.