Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 11-16 October 1998. Cruise no. 187.

 

Report: Gunni Ærtebjerg
Cruise leader: Kjeld Sauerberg /Lars Renvald
Participants: Peter Kofoed, Anja Fløjtrup, Birgit Groth, Stiig Markager,.

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

Summary

Compared to long term monthly mean (1931-1960) for October the temperature during this cruise was lower, except in the bottom water in the Great Belt. 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 and at Gedser Rev. The salinity stratification was strong (>10 psu), except in the Fehmarn Belt and Gedser Rev area, where the water column was well mixed.

Only traces of nitrate and nitrite were present in the surface layer in the Kattegat, but up to 1.4-1.8 m mol/l nitrate were observed in the Sound, southern Great Belt and Fehmarn Belt. In the surface nitrite concentrations increased through the Great Belt to the Fehmarn Belt (up to 0.15 m mol/l), and ammonium generally increased from the northern Kattegat to the Fehmarn Belt (up to 2.3 m mol/l). Phosphate and silicon were found in the surface layer in all areas with the lowest concentrations in the Kattegat increasing through the Sound and Great Belt to a maximum in the Fehmarn Belt of 0.6 m mol/l for phosphate and 19.8 m mol/l for silicon. In the bottom water relatively high concentrations of phosphate (1.3-1.5 m mol/l), silicon (26.4-31.1 m mol/l), nitrite (0.3-0.5 m mol/l) and ammonium (0.9-3.4 m mol/l) were observed in the Sound, Great Belt and western Arkona Sea.

The mean chlorophyll-a concentrations in the uppermost 15 m ranged from 1.3-1.7 m g/l in the Sound and southern Belt Sea to 3.0-4.2 m g/l in the western Kattegat.

Since the cruise in September the minimum oxygen concentrations had increased, except in the Great Belt. The lowest oxygen concentrations of 1.8-2.8 ml/l (29-44%) were observed in the Great Belt and the Sound. 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 and Great Belt, while the depletion in September in Fehmarn Belt no longer was present.

Compared to October last year the minimum oxygen concentrations this year were significantly lower (4.1-5.2 ml/l) in the Great Belt. Also in the sound and most of the Kattegat the minimum concentrations were lower than last year. Compared to mean for October in the 1980s the minimum oxygen concentration this year were 1.5 ml/l lower in the Great Belt and 0.2 ml/l lower in the Sound.

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Figure 1. Monitoring cruise with r/v Dana and r/v Tyra 11-16 October 1998. r/v Gunnar Thorson cruise no. 187. 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 (most of the 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 October 0.6° C below long term mean 1961-1990, and the precipitation was 125% above normal. At the beginning of October the wind came from east, but the rest of the month strong wind from south-west and west was dominating (Danish Meteorological Institute).

Hydrography

The surface temperature (1 m depth) had decreased up to 2.4-4.4° C since the cruise in September, and ranged from 10.8° C at Gedser Rev (St. 954) to 11.3-11.6° C in the north-eastern Kattegat and the Great Belt (St. 1001, 1007, 935, 939, 443, 450) (Fig. 2).

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

The bottom water temperature had also decreased, except in Læsø Rende (St. 403, 1009), the Sound (St. 431, 921) and northern Great Belt (St. 925, 935, 939), and ranged from 7.5-8.9° C in the north-eastern Kattegat (St. 905, 1001, 1007, 1008) to 12.9° C in the northern Great Belt (St. 925, 935, 939) (Fig. 2). The temperature difference between surface and bottom ranged from 0.1° C at Gedser Rev (St. 954) to 3.2-3.8° C in the north-eastern Kattegat (St. 905, 1001, 1007).

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 September in the Kattegat and Fehmarn Belt, but increased in the Sound and Great Belt (St. 431, 921, 925, 935, 443, 450), and ranged from 7.9-8.6 in the Arkona Sea (St. 441, 449) to 20.1-20.9 in the north-eastern Kattegat (St. 905, 1001). The bottom water salinity ranged from 10.4-14.0 at Gedser Rev and in Fehmarn Belt (St. 449, 954, 952) to 35.0-35.2 in the north-eastern Kattegat (St. 905, 1001, 1007, 1008) (Fig. 3). The bottom water salinity had increased since September, except in the Belt Sea (St. 935, 939, 443, 952, 954, 449). The salinity stratification was strong (>10 psu), except in the Fehmarn Belt and Gedser Rev area. The halocline was generally found between 10 m and 15 m depth.

Compared to long term monthly mean (1931-1960) for October the temperature during this cruise was lower, except in the bottom water in Læsø Rende and the Great Belt. 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 and at Gedser Rev.

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

Nutrients

Only traces of nitrate and nitrite were present in the surface layer in the Kattegat, but up to 1.4-1.8 m mol/l nitrate were observed in the Sound, southern Great Belt and Fehmarn Belt. Close to the bottom the nitrate concentrations were 6-10 m mol/l in the Kattegat, Sound and Great Belt (Fig. 4).

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

In the surface nitrite concentrations increased through the Great Belt to the Fehmarn Belt (up to 0.15 m mol/l), and ammonium generally increased from the northern Kattegat to the Fehmarn Belt (up to 2.3 m mol/l). In the bottom water relatively high concentrations of both nitrite (0.3-0.5 m mol/l) and ammonium (0.9-3.4 m mol/l) were observed in the Sound, Belt Sea and western Arkona Sea (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 with the lowest concentrations in the Kattegat increasing through the Sound and Great Belt to a maximum in the Fehmarn Belt of 0.6 m mol/l for phosphate and 19.8 m mol/l for silicon. In the bottom water high phosphate (1.3-1.5 m mol/l) and silicon (26.4-31.1 m mol/l) concentrations were observed in the Sound and Great Belt (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 concentration in the uppermost 15 m was highest (4.2 m g/l) in Læsø Rende (St. 403). In the rest of the Kattegat and in Great Belt the mean concentrations were 1.8-3.0 m g/l, lowest in the north-western Kattegat (St. 1008, 1009) and highest in Aalborg Bight (St. 409). In the Sound and southern Belt Sea the mean concentrations were 1.3-1.7 m g/l. The highest chlorophyll concentrations were observed within the uppermost 10 m, except in the Fehmarn Belt and east of Falster (St. 952, 449), where the highest concentrations were observed at 10-15 m depth (Fig. 7).

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

Oxygen

Since the cruise in September the minimum oxygen concentrations had increased, except in the Great Belt (St. 935, 939, 450) and parts of the northern Kattegat (St. 905, 1001, 1009). The lowest oxygen concentrations of 1.8-2.8 ml/l (29-44% saturation) were observed in the Sound and Great Belt (St. 431, 921, 935, 939, 443, 450) with the lowest concentration at station 935 in the Great Belt. At the shallow station 409 in the western Kattegat the minimum oxygen concentration was only 2.85 ml/l, due to an unusually strong stratification. In the Fehmarn Belt, where practical no oxygen was present at the sediment surface the month before, the minimum oxygen concentration now was close to saturation (6,2 ml/l; 89%). At the rest of the stations the concentrations were above 61% saturation (Fig. 8).

Compared to October last year the minimum oxygen concentrations this year were significantly lower (4.1-5.2 ml/l) in the Great Belt. Also in the sound and most of the Kattegat the minimum concentrations were lower than last year. Compared to mean for October in the 1980s the minimum oxygen concentration this year were 1.5 ml/l lower in the Great Belt and 0.2 ml/l lower in the Sound.

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, 921) and Great Belt (St. 935, 939, 443, 450), while the depletion in September in Fehmarn Belt no longer was present. In figure 9 are shown the stations visited by either Danish counties or NERI within the first three weeks of October 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 October 1998, and where oxygen depletion (<4 mg/l) and serious oxygen depletion (<2 mg/l) were observed.