Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 8-16 November 1998. Cruise no. 188

 

Report: Gunni Ærtebjerg
Cruise leader: Gunni Ærtebjerg/Lars Renvald.
Participants: Kjeld Sauerberg, Peter Kofoed, Jan Damgaard, Hanne Ferdinand, Dorete Jensen, Stine Cortzen.

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 November the temperatures during this cruise were lower in the whole water column in all areas. The surface and bottom water salinities were lower than long term means, except for higher surface salinity in most of the Kattegat. The halocline was generally situated deeper than 15 m and in the Belt Sea deeper than 20 m depth.

Nitrate was present in the surface layer in all areas ranging from 0.8 m mol/l in the eastern Kattegat to 3.1 m mol/l in the Sound. Generally, also nitrite and ammonia were present in the whole water column. Nitrite surface concentrations exceeded the bottom water concentrations, except in the Kattegat and northern Great Belt. Vice versa the ammonium surface concentrations exceeded the bottom water concentrations in the Sound, Kattegat and northern Great Belt. Also phosphate and silicon were present in the whole water column with the lowest surface concentrations in the Kattegat and maximum concentrations in the Sound and Belt Sea of 0.4-0.5 m mol/l for phosphate and 15-17 m mol/l for silicon.

The mean chlorophyll-a concentration in the uppermost 15 m was highest (7.0 m g/l) in Kiel Bight. In the northern Kattegat the mean concentrations ranged from 1.9 m g/l north of Læsø to 4.4 m g/l in Aalborg Bight. In the southern Kattegat and northern Great Belt the mean concentrations were 1.2-1.9 m g/l, in Fehmarn Belt 2.5-2.8 m g/l, and in the Arkona Sea 1.1-1.8 m g/l. The lowest mean concentrations of 0.7-1.0 m g/l were observed in the Sound. The highest chlorophyll concentrations were generally observed within the uppermost 10 m.

Since the cruise in October the minimum oxygen concentrations had increased significantly, except in the south-western Kattegat, where the lowest concentration of 3.6 ml/l was observed. Thus no oxygen depletion was any more present in the areas investigated. The lowest oxygen concentrations of 3.6-4.1 ml/l (56-65% saturation) were observed in the Sound, southern-most Kattegat and northern Great Belt. At the rest of the stations the concentrations were above 72% saturation.

Compared to November last year the minimum oxygen concentrations this year were generally higher, except in the southern Kattegat and Great Belt. Compared to mean for November in the 1980s the minimum oxygen concentrations this year were also generally higher, except in the Great Belt.

kort.gif (18172 bytes)
Figure 1. Monitoring cruise with r/v Dana and r/v Tyra 8-16 November 1998. r/v Gunnar Thorson cruise no. 188. 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 November 2.8° C below long term mean 1961-1990, and the precipitation was 40% below normal. At the beginning of November the wind came from west, but the rest of the month relatively weak wind from easterly directions dominated (Danish Meteorological Institute).

Hydrography

The surface temperature (1 m depth) had decreased 2.8-4.2° C since the cruise in October, and ranged from 7,0° C in the southern Great Belt (St. 450) to 8.4° C in the western Arkona Sea (St. 441, 449) (Fig. 2).

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

The bottom water temperature had also decreased 0.9-4.9° C, except in the north-eastern Kattegat, and ranged from 7.8° C in the southern Great Belt (St.443) to 11.0-11.1° C in the northern Great Belt (St. 925, 935) and northern Kattegat (St. 1007) (Fig. 2). The temperature difference between surface and bottom ranged from 0.4° C at Gedser Rev (St. 954) to 3.0-3.3° C in the north-eastern Kattegat (St. 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 increased since October, except in the Sound and southern Great Belt (St. 431, 921, 443, 450), and ranged from 8.5-8.9 in the Arkona Sea (St. 441, 444) and 9.1 in the Sound (St. 431) to 27.0-29.4 in the north-western Kattegat (St. 403, 1007, 1008, 1009). The bottom water salinity ranged from 12.0-17.1 in the Arkona Sea (St. 441, 444, 449) and 14.2 at Gedser Rev (St. 954) to 33.4-34.6 in the north-eastern Kattegat (St. 413, 905, 1001, 1007, 1008) (Fig. 3). The bottom water salinity had decreased since October, except in the Fehmarn Belt and Gedser Rev area (St. 952, 954, 449). The salinity stratification was still strong (>20 psu) in the Sound, but otherwise weakened much since October, and practically no stratification was present in the shallow western Kattegat. The halocline was generally situated deeper than 15 m and in the Belt Sea deeper than 20 m depth. An exception was the northern Sound (St. 921), where the halocline was found between 10 m and 15 m depth.

Compared to long term monthly mean (1931-1960) for November the temperatures during this cruise were lower in the whole water column in all areas. The surface and bottom water salinity was lower than long term mean, except for higher surface salinity in most of the Kattegat.

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

Nutrients

Nitrate was present in the surface layer in all areas ranging from 0.8 m mol/l in the eastern Kattegat (St. 413) to 3.1 m mol/l in the Sound (St. 431). Close to the bottom the nitrate concentrations were 6-8 m mol/l in the Kattegat and northern Great Belt and 8-9 m mol/l in the Sound (Fig. 4). In the southern Belt Sea no significant differences in nitrate concentrations between surface and bottom were seen.

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

Generally nitrite and ammonia were present in the whole water column in all areas. Concerning nitrite the surface concentrations exceeded the bottom water concentrations, except in the Kattegat and northern Great Belt. Vice versa the ammonium surface concentrations exceeded the bottom water concentrations in the Sound, Kattegat and northern Great Belt (Fig. 5).

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

Also phosphate and silicon were present in the whole water column in all areas with the lowest surface concentrations in the Kattegat and maximum concentrations in the Sound and Belt Sea of 0.4-0.5 m mol/l for phosphate and 15-17 m mol/l for silicon. In the bottom water the phosphate concentrations were highest in the Sound and Kattegat (0,7-0,9 m mol/l), while the silicon concentrations were highest (15.2-17.5 m mol/l) in the Great Belt and 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 concentration in the uppermost 15 m was highest (7.0 m g/l) in Kiel Bight (St. N3). In the northern Kattegat the mean concentrations ranged from 1.9 m g/l north of Læsø (St. 1008) to 4.4 m g/l in Aalborg Bight (St. 409). In the southern Kattegat and northern Great Belt the mean concentrations were 1.2-1.9 m g/l, in Fehmarn Belt 2.5-2.8 m g/l and in the Arkona Sea 1.1-1.8 m g/l. The lowest mean concentrations of 0.7-1.0 m g/l were observed in the Sound (St. 431, 1728). The highest chlorophyll concentrations were generally observed within the uppermost 10 m, except at station 413 in the eastern Kattegat (Fig. 7).

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

Oxygen

Since the cruise in October the minimum oxygen concentrations had increased significantly, except at station 925 in the south-western Kattegat. Thus no oxygen depletion was present in the investigated areas. The lowest oxygen concentrations of 3.6-4.1 ml/l (56-65% saturation) were observed in the Sound (St. 431), southern Kattegat (St. 921, 922, 925) and northern Great Belt (St. 935) with the lowest concentration at station 925. At the rest of the stations the concentrations were above 72% saturation (Fig. 8).

Compared to November last year the minimum oxygen concentrations this year were generally higher, except in the southern Kattegat (St. 418, 921, 922, 925) and parts of the Great Belt (St. 939, 450). Compared to mean for November in the 1980s the minimum oxygen concentration this year were also generally higher, except in the Great Belt.