Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Skagerrak, North Sea, Belt Sea and Arkona Sea, 17-29 August 1998. Cruise no. 185.

 

Report: Gunni Ærtebjerg
Cruise leader: Bjarke Rasmussen/Hanne Ferdinand
Participants: Susanne Hemmingsen, Dorete Jensen, Peter Kofoed, Lars Renvald, Christian Ubbesen, Berit Bojsen, Stiig Markager.

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

Summary and conclusions

The Jutland Coastal Current (JCC) was evident along the Danish North Sea coast with lower salinity and higher concentrations of total-N, total-P and chlorophyll-a than further off shore. In the Skagerrak traces of the JCC was observed only just outside Hirtshals. In the surface only traces of nitrate were observed in the Skagerrak and North Sea. Phosphate and silicon were present along the Danish North Sea coast, and silicon also at the western-most stations.

In the Kattegat, Sound and Belt Sea area the surface temperature was lower (0.6-2.3° C) and the bottom water temperature higher (0.4-1.8° C) compared to long term monthly means for August (1931-1960). The surface salinity was general higher than long term mean, while the bottom water salinity was lower than normal in the southern Kattegat. This is due to more intensive mixing and water exchange in the cold and windy summer.

While generally no nitrate was observed, both phosphate and silicon were present in the surface layer in the southern Kattegat, Sound, Belt Sea and western Arkona Sea.

The mean chlorophyll-a concentration in the uppermost 10 m increased from 0.8-1.7 m g/l in the Sound and Kattegat to 2.5 m g/l in the Fehmarn Belt and about 3.5 m g/l in the western Arkona Sea. The chlorophyll was relatively homogenous distributed in the uppermost 15 m, except for a higher concentration (2.5 m g/l ) at 15 m depth in the north-eastern Kattegat and low concentrations at 15 m depth in the southern Belt Sea and western Arkona Sea.

The lowest oxygen concentration of 1.2 ml/l (18% saturation) was observed in the Fehmarn Belt. In the Sound and central Great Belt the minimum concentrations were 2.6-2.7 ml/l (40-42%). In the southern Kattegat, remaining Belt Sea and east of Gedser Rev the minimum concentrations were 3.2-3.7 ml/l (50-54%). In the northern Kattegat the concentrations were 4.0-4.5 ml/l (68-79%). In Denmark oxygen depletion is 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 central Great Belt and serious oxygen depletion in the Fehmarn Belt.

Compared to August last year the minimum oxygen concentrations this year were generally more than 1 ml/l lower. Compared to mean for August in the 1980s the minimum oxygen concentrations this year were lower in the northern Sound and central Great Belt.

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Figure 1. Monitoring cruise with r/v Dana and r/v Tyra 17-29 August 1998 in the Sound, Kattegat, Skagerrak, North Sea, Belt Sea and Arkona Sea. Gunnar Thorson cruise no. 185.

 

General

Due to break down of the CTD/rosette equipment on r/v Gunnar Thorson the cruise was actually conducted with r/v Dana from the Danish Fishery Research Institute (North Sea, Skagerrak and most of Kattegat) and with r/v Tyra from the County of Vejle (southern Kattegat, Sound 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-a.

Meteorology

Characteristics of the weather conditions in the months since the cruise in February are given in table 1. The spring was mild and wet, the summer cold and wet, and the whole period rather windy dominated by westerly wind, except in April and parts of May.

Table 1. Deviations in monthly mean temperature and precipitation March-August 1998 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).

Month

Temperature deviation ° C

Precipitation

% deviation

Mean wind speed

m/s

Dominating wind direction

March

+1.6

+30

5.6

W

April

+0.8

+93

5.0

E

May

+1.0

-42

4.7

NE and W

June

-0.4

+38

4.8

SW

July

-1.2

+39

5.2

W

August

-1.1

-12

5.1

W

 

North Sea and Skagerrak

Hydrography

The surface temperature ranged from 14.1-14.5° C in the central Skagerrak to about 16.5° C at the south-western most station in the German Bight. The surface salinity ranged from 31.1-31.6 at the coast-near stations in the German Bight to about 34.4-34.7 at the western-most stations in the North Sea. Along the coast the salinity increased from 31.1 in the German Bight to 33.1 at Limfjorden, as the German Bight water within the JCC is mixed with North Sea water. In the Skagerrak the salinity was lowest in the central part increasing from 29.9-31.4 to 33.4-33.7 at the Danish coast, except for only 32.6 just outside Hirtshals harbour.

Nutrients

In the surface only traces of nitrate (0-0.2 m mol/l) were found, except for 0.6 m mol/l at the north-western most station in the Skagerrak. Phosphate was observed at the coast near stations (0.2-0.4 m mol/l) and the south-western stations (0.2 m mol/l) in the North Sea. The silicon concentrations were below 1 m mol/l at the 3 coast near stations at the southern-most transect in the German Bight and in a tongue to the north-northwest. At the Danish coast the silicon concentrations were 1.3-4.9 m mol/l and at the western-most North Sea stations 1.6-3.7 m mol/l.

Oxygen and chlorophyll-a

The oxygen concentrations were about saturation level at all stations in the North Sea and above 80% saturation in the Skagerrak. The mean chlorophyll-a concentrations in the surface layer (0-10 m) were above 5 m g/l within the JCC with a maximum of 9.0-11.2 m g/l at some stations in the German Bight. At the western-most North Sea stations the concentrations were 1.3-3.2 m g/l. In the Skagerrak the concentrations were lowest in the central part (0.5-0.7 m g/l) increasing to 2.3-5.8 m g/l at the Danish coast.

Kattegat, Sound, Belt Sea and Arkona Sea

Hydrography

The surface temperature (1 m depth) ranged from 14.8° C in the western Kattegat (St. 409) to 16-17° C in the Sound and Belt Sea. The bottom water temperature ranged from 8.4-9-8° C in the south-eastern Kattegat (St. 413, 921, 922) to 11.8-13.6° C in the northern Kattegat (Fig. 2).

Figure 2. Surface (1 m) and near bottom temperature, and salinity in 1 m, 5 m, 10 m, 15 m, 20 m depth and near bottom along transect I.

The temperature difference between surface and bottom ranged from 1.8° C in the north-western Kattegat (St. 403, 1009) to 4.7-5.5° C in the Belt Sea and 6.3-7.9° C in the Sound and south-eastern Kattegat (St. 431, 921, 922, 413).

Figure 3. Surface and near bottom concentrations of nitrate, nitrite and ammonium along transect I.

The surface salinity ranged from 7.7 in the Arkona Sea (St. 449) to 28.8-30.0 in the north-western Kattegat (St. 403, 1008, 1009). The bottom water salinity ranged from 22.8 in the Fehmarn Belt (St. 952) to 32.9-34.3 in the northern Kattegat (St. 403, 413, 905, 1001, 1007, 1008, 1009). The salinity stratification was strong in the Sound (21.9 psu), the south-eastern Kattegat (10-19 psu) and the Belt Sea (11-15 psu) (Fig. 2).

Compared to long term monthly means (1931-1960) for August the surface temperature was lower (0.6-2.3° C) and the bottom water temperature higher (0.4-1.8° C) than normal. The surface salinity was general higher than long term mean, while the bottom water salinity was lower than normal in the southern Kattegat. This is due to more intensive mixing and water exchange than long term mean.

Nutrients

Generally no nitrate or nitrite were present in the surface layer (0-10 m), except for 0.7-0.8 m mol/l nitrate in the western Kattegat (St. 409, 415). Above 5 m mol/l nitrate were observed in the bottom water in the Sound and eastern Kattegat (Fig. 3). The nitrite concentrations in the bottom water decreased from 0.5-0.6 m mol/l in the north-eastern Kattegat to about 0.2 m mol/l in Fehmarn Belt. Significant concentrations of ammonium (>1.5 m mol/l) were found in the bottom water in the Great Belt (Fig. 3).

Figure 4. Surface and near bottom concentrations of phosphate and silicon along transect I.

In the surface water the phosphate concentrations increased from close to zero in the north-eastern Kattegat to above 0.2 m mol/l in the Sound, southern Belt Sea and Arkona Sea. Generally also the silicon concentrations increased from north to south (Fig. 4). In the bottom water relatively high concentrations of both phosphate and silicon were observed in the eastern Kattegat and southern Belt Sea (Fig. 4).

Chlorophyll-a

The mean chlorophyll-a concentration in the uppermost 10 m increased from 0.8-1. m g/l in the Sound and Kattegat to 2.5 m g/l in the Fehmarn Belt and about 3.5 m g/l in the western Arkona Sea. The chlorophyll was relatively homogenous distributed in the uppermost 15 m, except for a higher concentration (2.5 m g/l ) at 15 m depth in the north-eastern Kattegat and low concentrations at 15 m depth in the southern Belt Sea and western Arkona Sea (Fig. 5).

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

Oxygen

The lowest oxygen concentration of 1.2 ml/l (18% saturation) was observed in the Fehmarn Belt. In the Sound and central Great Belt the minimum concentrations were 2.6-2.7 ml/l (40-42%). In the southern Kattegat, remaining Belt Sea and east of Gedser Rev the minimum concentrations were 3.2-3.7 ml/l (50-54%). In the northern Kattegat the concentrations were 4.0-4.5 ml/l (68-79%) (Fig. 6).

Compared to August last year the minimum oxygen concentrations this year were generally more than 1 ml/l lower. Compared to mean for August in the 1980s the minimum oxygen concentrations this year were lower in the northern Sound and central Great Belt.

In Denmark oxygen depletion is 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 central Great Belt and serious oxygen depletion in the Fehmarn Belt. In figure 7 are shown the stations visited by either Danish counties or NERI within the first three weeks of August 1998, and where oxygen depletion or serious oxygen depletion were observed.

Figure 6. Minimum oxygen concentrations along transect I.

 

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Figure 7. Stations visited by Danish counties or NERI within the first three weeks of August 1998, and where oxygen depletion (<4 mg/l) and serious oxygen depletion (<2 mg/l) were observed.