Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Skagerrak, North Sea, Belt Sea and Arkona Sea, 9-19 February 1998. Cruise no. 183.

 

Report: Gunni Ærtebjerg

Cruise leader: Gunni Ærtebjerg

Participants: 9-19 February: Kjeld Sauerberg, Jan Damgaard

9-16 February: Jørgen Nørrevang, Hanne Ferdinand, Peter Kofoed, Dorete Jensen

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

Summary and conclusions

The wind was rather weak and mostly coming from east and southeast in November to January, while February was dominated by strong wind from south-west and west. The precipitation was below normal July-December 1997, and the run off prior to the cruise probably relatively low.

The Jutland Coastal Current (JCC) was evident along the Danish coast from the German Bight to Skagerrak with lower salinity and temperature and higher nutrient concentrations than further off shore. In the Skagerrak the JCC was narrow and did not enter the northern Kattegat. In the German Bight the nutrient concentrations varied inversely to the salinity, except for nitrite which did not show any relation to salinity. The nutrient concentrations, except nitrite, were highest at the coastal stations in the German Bight and decreased along the coast from south to north and with the distance form the coast. In the North Sea and Skagerrak no sign of spring bloom was observed, and the water was about saturated with oxygen.

In the Kattegat and Belt Sea both the temperature and salinity were generally higher in the whole water column than long term means (1931-1960) for February. The stratification of the water column was strongest in the Sound and southern Kattegat. A very early spring bloom was observed in the south-eastern Kattegat with chlorophyll concentrations up to 10 m g/l (mean 0-10 m), decreasing to 3-4 m g/l in the north-western Kattegat. In the Sound and Kiel Bight 3.9-4.1 m g/l was observed. Due to the unusual phytoplankton bloom the surface concentrations of nitrate, phosphate and silicon were low in the southern Kattegat, and only in the Sound, Belt Sea and Arkona Sea the concentrations were about normal for the season. However, in agreement with a relatively low run off prior to the cruise the nutrient concentrations were generally lower than mean for February in the 1980’s, except for rather high nitrite concentrations in the Fehmarn Belt.

Compared to mean for February in the 1980’s the minimum oxygen concentrations this year were about the same or higher, except for lower concentrations in the Sound and Great Belt.

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Figure 1. Monitoring cruise with r/v Gunnar Thorson 9-19 February 1998 in
the Sound, Kattegat, Skagerrak, North Sea, Belt Sea and Arkona Sea. Cruise no. 183.

General

The scope of the cruise was to monitor the hydrographic situation and the spatial variations in the concentrations of winter nutrients, oxygen and chlorophyll-a. Bottom fauna was sampled at three stations in the Great Belt, Fehmarn Belt and Arkona Sea, respectively.

Meteorology

November 1997 was a little colder than long term mean (1961-90) and the precipitation was only about half of normal. December 1997 was nearly 1 degree warmer than normal and the precipitation about mean. January 1998 was 2.3° C warmer than long term mean and the precipitation was 36% above normal. February was nearly 5° C warmer than long term mean and the precipitation was 32% above normal. The wind was rather weak in the whole period November to January and mostly coming from east and south, with periods of southwestern wind at the end of December and in January. In February strong wind from southwest and west dominated (Danish Meteorological Institute).

 

North Sea and Skagerrak

Hydrography

The Jutland Coastal Current (JCC) was evident along the Danish coast from the German Bight to Skagerrak with lower salinity and temperature and higher nutrient concentrations than further off shore. In the Skagerrak the JCC was narrow and did not enter the northern Kattegat.

The surface temperature ranged from about 3.8° C along the coast to about 6.5° C at the western-most stations. The surface salinity ranged from 29.1-30.7 at the coastnear stations in the German Bight to about 34.7 at the western-most stations in the North Sea. Along the coast the salinity increased from 29.1 in the German Bight to 32.9 at Hansthom and 33.1 at Hirtshals, as the German Bight water within the JCC is mixed with North Sea and Skagerrak water (table 2). In the Skagerrak the salinity rapidly increase to 33.9-34.6 with the distance from the coast and then decreased to 32.7-33.2 in the central Skagerrak.

Nutrients

In the German Bight the nutrient concentrations varied inversely to the salinity, except for nitrite which did not show any relation to salinity (Fig. 2). The results of linear regression on measurements of nutrients and salinity from the 18 stations at the 3 southernmost transects (St. 1052-1086) are shown in table 1.

Table 1. Linear regression analyses between salinity and concentrations of inorganic nutrients at the 3 transects in the German Bight (Station no 1052-1086) 13-15 February 1998. The intercept gives the estimated mean concentrations in the river water entering the German Bight. 34.5 psu gives the estimated concentrations in central North Sea water. Unit = m mol/l. Number of observations = 114.

Substance

Slope

Intercept

34.5 psu

R2

Nitrate

-18.9

657

4.95

0.93

Nitrite

-0.06

2.50

0.43

0.24

Ammonium

-0.72

25.0

0.16

0.75

Phosphate

-0.17

6.29

0.43

0.72

Silicon

-4.75

167

3.13

0.89

The nutrient concentrations, except nitrite, were highest at the coastal stations in the German Bight and decreased along the coast from south to north and with the distance form the coast (table 2).

Table 2. Mean temperature, salinity and concentrations of inorganic nutrients and chlorophyll-a in the surface layer (0-10 m) at stations along the Jutland west coast from north to south; St. 1013 = Hirtshals, St. 1019 = Hanstholm, St. 1022 = Harboöre, St. 1041 = Horns Rev north, St. 1080 = German Bight, and at the western-most station in the North Sea (St. 1046) and northern-most station in the Skagerrak (St. 1104).

Station

Temp.

° C

Salinity

psu

Nitrate

m mol/l

Nitrite

m mol/l

Ammon.

m mol/l

Phosph.

m mol/l

Silicon

m mol/l

Chloro.

m g/l

1013

3.8

33.1

10.0

0.71

1.3

0.72

7.1

1.6

1019

3.8

32.9

10.2

0.49

1.1

0.81

13.1

2.0

1022

3.4

32.4

17.2

0.33

1.2

1.20

16.0

2.1

1041

3.7

31.8

53.4

0.16

2.0

1.18

17.1

2.2

1080

3.2

29.1

115.9

0.64

5.0

1.15

32.6

2.1

1046

6.5

34.7

5.3

0.27

0

0.58

5.7

0.6

1104

4.7

32.7

5.2

0.29

0

0.36

5.4

2.0

Oxygen and chlorophyll-a

The oxygen concentrations were about saturation level at all stations in the North Sea and Skagerrak. The mean chlorophyll-a concentrations in the surface layer (0-10 m) varied between 0.6 m g/l and 3.2 m g/l. The concentrations were highest at the stations nearest to the coast and in the central Skagerrak (St. 1104, 2.0 m g/l), and lowest at the western-most stations in the North Sea. The higher concentrations along the coast might be due to resuspension, as the chlorophyll measurements are not corrected for degradation products of chlorophylls. No sign of spring bloom was observed in the North Sea and Skagerrak.

 

Figure 2. Nitrite concentrations as function of the salinity from all samples taken at the 36 stations in the North Sea.

Kattegat, Sound, Belt Sea and Arkona Sea

Hydrography

The surface temperature (1 m depth) ranged from 1.7-2.2° C in the south-western Kattegat (St. 921, 922, 418, 413) to 3.9-4.5° C in the north-eastern Kattegat (St. 1007, 1001). The bottom water temperature ranged from 3.0-3.3° C in the Arkona Sea (St. 441, 444, 449) to 7.5° C in the south-eastern Kattegat (St. 921, 922) (Fig. 3). The temperature difference between surface and bottom was negligible in the southern Belt Sea and more than 5.5° C in the south-eastern Kattegat (St. 921, 922).

The surface salinity ranged from 7.7 in the central Arkona Sea (St. 444) to 30.2-31.9 in the northern Kattegat (St. 403, 1001, 1007, 1008, 1009). The bottom water salinity ranged from 15.5-18.6 in the Arkona Sea (St. 441, 449, 954, 444) to 34.0-34.5 in the northern Kattegat (St. 403, 905, 1001, 1007, 1008, 1009). The bottom water salinity had increased since November due to inflow of saline water from the Skagerrak. The salinity stratification was relatively weak, except in the Sound (21 psu), southern Kattegat (11.5-14) and Arkona Sea (11 psu).

Figure 3. 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.

 

 

Figure 4. Salinity in 1 m, 5 m, 10 m, 15 m, 20 m depth and near bottom along the transects II and III.

Compared to long term monthly means (1931-1960) for February both the temperature and the salinity during this cruise were generally higher, both at the surface and in the bottom water.

.

Figure 5. Surface and near bottom concentrations of nitrate along the transects I, II and III.

Nutrients

Generally the nitrate concentrations were highest in the Sound and Belt Sea decreasing towards the Baltic Sea and the Kattegat. Low nitrate concentrations (2-3.5 m M) were observed in the surface water in the southern Kattegat due to an unusually early phytoplankton spring bloom (Fig. 5). The nitrite concentrations were relatively high (>1.5 m M) in the whole water column in the southern Belt Sea and in the bottom water in the north-eastern Kattegat (Fig. 6). Significant concentrations of ammonium (>0.5 m M) were also found in the southern Belt Sea and in the bottom water in the Arkona Sea (Fig. 6).

 

Figure 6. Surface and near bottom concentrations of nitrite and ammonium along transect I.

 

In the surface water a minimum in the phosphate and silicate concentrations were found in the southern Kattegat due to the phytoplankton bloom. In the bottom water the phosphate concentration generally decreased from north to south through the Kattegat and Belt Sea to the Arkona Sea (Fig. 7). The silicate concentrations increased through the Kattegat to the Belt Sea, and relatively low concentrations (<5 m M) were found in the Kattegat surface water, except in the western part (Fig. 7).

Compared to mean for February in the 1980s the nutrient concentrations this year were generally lower, except for nitrite in the southern Belt Sea.

Figure 7. Surface and near bottom concentrations of phosphate and silicate along transect I.

 

Oxygen

Since the cruise in the beginning of November the minimum oxygen concentrations had increased significantly, except in the northern Great Belt (St. 935, 939).

The lowest oxygen concentrations of 5.0-5.7 ml/l (72-83% saturation) were found in the areas with the strongest stratification of the water column, that means the Sound, southern Kattegat and northern Belt Sea. In the northern Kattegat, southern Belt sea and Arkona Sea the whole water column was about saturated with oxygen (Fig. 8).

Figure 8. Minimum oxygen concentrations along the transects I, II and III.

 

Compared to mean for February in the 1980s the minimum oxygen concentrations this year were about the same or higher, except for lower concentrations in the Sound and Great Belt.

Figure 9. Chlorophyll-a concentrations in 1 m, 5 m, 10 m and 15 m depths along the transects I, II and III.

Chlorophyll-a

The mean chlorophyll-a concentration in the uppermost 10 m was very high (7.8-10.9 m g/l) in the south-eastern Kattegat, decreasing to 2.1-4.0 m g/l in the northern Kattegat. In the Sound 4.1 m g/l and 2.1 m g/l were found at the first and the last day of the cruise, respectively. Also in Kiel Bight relatively high mean chlorophyll concentrations of 3.3-3.9 m g/l were observed. In the rest of the Belt Sea the mean concentrations were 0.8 m g/l in the Fehmarn Belt to 2.2 m g/l in the northern Great Belt.