Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Skagerrak, North Sea, Belt Sea and Arkona Sea, 9-19 August 1999. Cruise no. 191.

 

Report: Gunni Ærtebjerg

Cruiseleader:
Participants:
9-16/8  : Gunni Ærtebjerg; 16-19/8: Dorete Jensen;
9-19/8  : Dorete Jensen, Peter Kofoed, Hanne Ferdinand;
9-16/8  : Lars Renvald, Jan Damgaard, Kjeld Sauerberg, Gunni Ærtebjerg;
16-19/8: Martin Larsen
9-10/8  : Ole Jensen (NERI), Thyge Dyrnesli (Danish Fishery Research Institute);

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

Summary

Due to easterly wind prior to the cruise the surface water in the Belt Sea, Kattegat and Skagerrak was dominated by outflowing Baltic water, and the Jutland Coastal Current was broad along the Danish North Sea coast. The stratification was strong in the Arkona Sea, Belt Sea and Kattegat. Compared to long term mean the surface temperature was higher and salinity lower than normal, and vice versa for the bottom layer. Also the North Sea was strongly stratified, except along the Wadden Sea.

In the euphotic layer the nutrient concentrations were generally low, except for silicon. Traces were observed along the Danish North Sea and Skagerrak coast, generally with the highest concentrations in the German Bight, and in the north-western Kattegat, where the pycnocline was situated close to the surface.

The mean chlorophyll-a concentration in the upper 10 m was highest along the North Sea coast (about 8 m g/l) and in the western Kattegat (2.8-4.3 mg/l). The lowest concentrations were found at the western most stations in the North Sea, in the central Skagerrak and north-eastern Kattegat. In the Belt Sea and Kattegat the maximum chlorophyll concentrations were generally found in association with the pycnocline.

Due to the stratification of the water column the minimum oxygen concentrations were relatively low along the Skagerrak coast and parts of the North Sea coast (2.4-4.2 ml/l). Further to the west in the North Sea relatively low oxygen concentrations of 3.5-4.2 ml/l were also observed in a belt from north to south. The lowest oxygen concentration of 1.4 ml/l (21%) was observed in the central Arkona Sea. Also in the Fehmarn Belt area the minimum oxygen concentrations were low: 1.7-2.5 ml/l (25-37%). In the north-western Kattegat unusual low concentrations of 2.0-2.5 ml/l (32-41%) were observed. In the Great Belt the minimum oxygen concentrations were 3.3-3.7 ml/l (52-57%), and in the Sound and southern Kattegat 2.9-3.8 ml/l (43-57%).

Compared to August last year the minimum oxygen concentrations this year are lower in the Kattegat and Fehmarn Belt, but higher in the Sound and Great Belt. Compared to mean for August in the 1980’s the minimum oxygen concentrations this year are lower in the north-western Kattegat, at Gedser Rev and in the Arkona Sea, but higher in the Sound and Great Belt.

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 Fehmarn Belt - Arkona Sea area, in the north-western Kattegat and at a coastal station in the North Sea.

General

The main scope of the cruise was to monitor the oxygen situation, but also the hydrography, nutrients and chlorophyll-a. The stations of the cruise are shown in figure 1.

Meteorology

Characteristics of the weather conditions in the months since the last cruise in February are given in table 1. March and April were warm and rather windy, and March was dark and wet. May was about normal but with more sunshine than usual. June was rather cold and very wet. July and August were relatively warm, sunny and calm, and July was rather dry while August was rather wet. The two weeks prior to the cruise were dominated by wind from east. During the cruise the wind was occasionally strong from west.

 

Month

Temperature deviation
°C

Precipitation
% deviation

Mean wind speed
m/s

Dominating wind
direction

March

+1.5

+85

5.0

SE-S

April

+1.9

-5

5.4

SW-W

May

-0.1

-8

4.8

E and S-SW

June

-0.7

+118

4.3

SW-W

July

+1.6

-15

4.0

SW-W

August

+0.8

+31

4.0

W-NW

Table 1. Deviations in monthly mean temperature and precipitation in March to October 1999 in Denmark compared to long term monthly means 1961-90, monthly mean wind speed and dominating wind direction (based on data from the Danish Meteorological Institute).

 

North Sea and Skagerrak

Hydrography

The hydrography mirrored the dominating easterly wind prior to the cruise. The Jutland Coastal Current with lower salinity, higher temperature and concentrations of total-P and total-N was broad along the Danish coast. Central North Sea water with salinity above 34 was found only at the north-western stations in the North Sea. The Skagerrak surface water was dominated by Baltic water flowing out from the Kattegat. A tongue with salinity above 32 went from the North Sea into the south-western Skagerrak (figure 2). The surface temperature ranged from about 16° C in the central Skagerrak to above 19° C along the coast in the Skagerrak and the German Bight (figure 3). A homogenous water column was observed only at 5 stations in the North Sea; that is the coastal stations 1041 and 1042 north-east of Horns Rev and the south-eastern most stations (1080, 1081, 1082) in the German Bight. The stratification was weak at the stations (1052, 1053, 1059, 1060, 1086) close to the Danish Wadden Sea, but relatively stronger further to the west.

Nutrients

In the North Sea surface water rather high concentrations of phosphate (0.1-0.6 mM) were present in the inner German Bight and up to Ringkøbing Fjord (figure 4). Nitrate concentrations were very low. Only in the central Skagerrak and outside the Danish Wadden Sea 0.2-0.3 mM were observed (figure 5). Nitrite concentrations above 0.01 mM were found along the Danish North Sea coast and up to 0.06 mM outside the Wadden Sea (St. 1052) (figure 6). Also ammonium was present along the coast and in the German Bight. The highest concentrations of 0.4-1.3 mM were observed at the entrance to the Limfjord and might be due to outflow from the fjord (figure 7). Silicon above 1 mM was present all over, except in the central Skagerrak and at the north-western stations in the North Sea (figure 8). The highest concentrations of total-P and total-N were found in the German Bight, decreasing to the north and west (figures 9 and 10).

Chlorophyll-a

The mean chlorophyll-a concentration in the upper most 10 m was highest along the coast (about 8 mg/l) in the North Sea and German Bight, except at the stations 1041 and 1042 north-east of Horns Rev. In the central Skagerrak and at the western most stations in the North Sea the mean concentration was below 1 mg/l (figure 11).

Oxygen

Due to the stratification of the water column the minimum oxygen concentrations were relatively low along the coast: 4.2 ml/l at Hirtshals and Hanstholm (St. 1101 and 1019), 3.5 ml/l at Thyborøn (St. 1022) and 2.4 ml/l north of Ringkøbing Fjord (St. 1034). Further to the west in the North Sea relatively low oxygen concentrations of 3.5-4.2 ml/l were also observed in a belt from station 1025 in the north to 1083 and 1084 in the south (figure 12).

 

Kattegat, Sound, Belt Sea and Arkona Sea

Hydrography

The surface temperature (1 m depth) varied from 16.5 °C in the Arkona Sea to above 20° C in the north-eastern Kattegat (figure 3). The bottom water temperature ranged from 7.0 °C east of Anholt (St. 413) to 13.5-13.9 in the shallow western Kattegat (St. 409, 415). The temperature stratification was strong with a temperature difference between surface and bottom of 4.8 °C in the central Arkona Sea, 5.6-9.6 °C in the Belt Sea and western Kattegat and 10.2-13.3 °C in the rest of Kattegat and in the Sound.

The surface salinity ranged from 7.3-8.1 in the Arkona Sea (St. 441, 444, 449, 954) to only 19.1-19.6 in one m depth in the north-western Kattegat (St. 1007, 1008, 1009) (figure 2). The bottom water salinity ranged from 7.8 at Stevns (St. 441) and 19.3 in the central Arkona Sea (St. 444) to 34.6-35.1 in the north-eastern Kattegat (St. 905, 1001, 1007) (figure 13). The salinity stratification was very strong, except at Stevns (St. 441), with a salinity difference between surface and bottom at the other stations ranging from 11 in the shallow western Kattegat (St. 409) to 25 in the Sound (St. 431).

Compared to long term monthly means (Lightship observations 1931-1960) for august the surface temperature during this cruise was generally higher (0.8-3.5 °C) and the bottom water temperature lower than normal, except for higher bottom water temperature in the western Kattegat (St. 409, 925). The surface salinity was lower than long term mean, except in the Great Belt, while the bottom water salinity was generally lower than normal, except in the shallow Aalborg Bight (St. 409).

Nutrients

Only traces of phosphate were present in the uppermost 10 m (figure 4). Small amounts of nitrate (max. 0.5 mM) and nitrite (max. 0.04 mM) were observed in the western Kattegat (figures 5 and 6), while ammonium was generally found in concentrations of 0.1-0.7 mM (figure 7). Silicon concentrations above 1 mM were observed in all areas, except the north-eastern Kattegat (figure 8). In the bottom water high concentrations of nitrate (>10 mM) were found in the Læsø Rende (St. 403), southern Kattegat (St. 413, 418), the Sound (St. 431) and Fehmarn Belt (St. 952). High concentrations of phosphate (>1 mM ) and silicon (>35 mM) were observed at the bottom in Fehmarn Belt (St. 952) and central Arkona Sea (St. 444). At the later station also very high ammonium concentrations (>5 mM) were found.

Chlorophyll-a

The mean chlorophyll concentration in the uppermost 10 m was highest (2.8-4.3 mg/l) in the western Kattegat (St. 409, 415) and lowest (1.0-1.2 m g/l) in the north-eastern Kattegat (St. 905, 1001, 1004, 1007). In the Arkona Sea the mean concentration was 2.6 mg/l (figure 11). Generally the chlorophyll maximum was situated in 10 to 15 m depth, except in the southern Great Belt (figure 14).

Oxygen

The lowest oxygen concentration of 1.4 ml/l (21% saturation) was observed in the central Arkona Sea (St. 444). Also in the Fehmarn Belt area (St. 449, 952, 954, M2) the minimum oxygen concentrations were low: 1.7-2.5 ml/l (25-37%) (figure 15). In the north-western Kattegat (St. 403, 409, 1009) unusual low concentrations of 2.0-2.5 ml/l (32-41%) were observed. In the Great Belt the minimum oxygen concentrations were 3.3-3.7 ml/l (52-57%), and in the Sound and southern Kattegat the minimum concentrations were 2.9-3.8 ml/l (43-57%).

Compared to August last year the minimum oxygen concentrations this year are lower in the Kattegat and Fehmarn Belt, but higher in the Sound and Great Belt. Compared to mean for August in the 1980’s the minimum oxygen concentrations this year are lower in the north-western Kattegat, at Gedser Rev and in the Arkona Sea, but higher in the Sound and Great Belt.

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 Fehmarn Belt - Arkona Sea area (St. 444, 449, 952, 954, M2), and in the north-western Kattegat (St. 403, 409, 1009). In the central Arkona Sea (St. 444) the concentration was close to serious oxygen depletion. In figure 16 are shown the stations visited by either Danish counties or NERI within the first three weeks of August 1999, and where oxygen depletion or serious oxygen depletion were observed.

 

Figures

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Figure 1. Stations of the monitoring cruise with r/v Gunnar Thorson 9-19 August 1999 in the Sound, Kattegat, Skagerrak, North Sea, Belt Sea and Arkona Sea. Gunnar Thorson cruise no. 191.

 

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Figure 2. Interpolated distribution of surface salinity (mean 1, 5 and 10 m depth).

 

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Figure 3. Interpolated distribution of surface temperature (mean 1, 5 and 10 m depth).

 

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Figure 4. Interpolated distribution of surface phosphate concentrations (mean 1, 5 and 10 m depth).

 

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Figure 5. Interpolated distribution of surface nitrate concentrations (mean 1, 5 and 10 m depth).

 

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Figure 6. Interpolated distribution of surface nitrite concentrations (mean 1, 5 and 10 m depth).

 

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Figure 7. Interpolated distribution of surface ammonium concentrations (mean 1, 5 and 10 m depth).

 

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Figure 8. Interpolated distribution of surface silicon concentrations (mean 1, 5 and 10 m depth).

 

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Figure 9. Interpolated distribution of surface total-P concentrations (mean 1, 5 and 10 m depth).

 

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Figure 10. Interpolated distribution of surface total-N concentrations (mean 1, 5 and 10 m depth).

 

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Figure 11. Interpolated distribution of surface chlorophyll-a concentrations (mean 1, 5 and 10 m depth).

 

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Figure 12. Interpolated distribution of minimum oxygen concentrations independent on the water depth of the stations.

 

Figure 13. Salinity in 1 m, 5 m, 10 m, 15 m, 20 m depth and near bottom along transect I from the north-eastern Kattegat through the Great Belt and Fehmarn Belt to the Arkona Sea.

 

Figure 14. Chlorophyll-a concentrations in 1 m, 5 m, 10 m and 15 m depths along transect I.

 

Figure 15. Minimum oxygen concentrations along transect I.

 

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Figure 16. Stations visited by Danish counties or NERI within the first three weeks of August 1999, and where oxygen depletion (<4 mg/l) and serious oxygen depletion (<2 mg/l) were observed.