Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Skagerrak, North Sea, Belt Sea and Arkona Sea, 8-18 February 1999. Cruise no. 189.

 

Report: Gunni Ærtebjerg
Cruise leader: Gunni Ærtebjerg

Participants: 8-18/2: Dorete Jensen, Peter Kofoed, Kjeld Sauerberg;
8-15/2: Lars Renvald, Jan Damgaard, Hanne Ferdinand;
8-10/2: Martin Larsen (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

January and the first week of February was dominated by strong wind from south-west, which facilitated the Jutland Coastal Current (JCC). Prior to the cruise Norway and Sweden had observed high surface concentrations of nitrate from the JCC in the south-eastern Skagerrak and in the bottom water in the north-eastern Kattegat down to Fladen. During the cruise the JCC with lower salinity and temperature and higher concentrations of nutrients was evident but relatively narrow along the Danish North Sea coast and also observed at the most coast near stations in the Skagerrak. Some of the JCC water was still present in the surface of the northern Kattegat and the inflow as bottom water to the Kattegat and Belt Sea had continued during the stormy period prior to the cruise.

In the North Sea the nutrient concentrations, except nitrite, varied inversely to the salinity. In the Sound, Kattegat, Belt Sea and Arkona Sea the phosphate concentrations were lower than long term mean, while the silicon concentrations were generally higher than long term mean in the Sound, southern Kattegat and Great Belt.

The lowest oxygen saturation of 76% was observed in the Sound. In the southern Kattegat the minimum saturation was 82-89%. In the remaining Kattegat, the Belt Sea and Arkona Sea the minimum saturation was 90-100%. Compared to mean for February in the 1980’s the minimum oxygen concentrations this year were a little higher, except in the Sound. In the North Sea and Skagerrak the minimum oxygen saturation was about saturation level (97-100%), except at the three deepest stations in the Skagerrak, where 78-92% was observed.

The highest chlorophyll concentration (2.3 m g/l) was observed in Kiel Bight. In the rest of the Belt Sea, Arkona Sea, southern and western Kattegat and southern JCC, and at the end of the cruise also in the Sound, the mean chlorophyll concentrations were between 1.0 m g/l and 2.0 m g/l. The phytoplankton spring bloom had not yet started, but seemed just about to start in certain areas of the Kattegat and southern Belt Sea.

General

The main scope of the cruise was to monitor the winter nutrient levels, but also the hydrography and the concentrations of oxygen (in order to check that the high winter level is reached) and chlorophyll-a (in order to check that the spring bloom had not yet started to decrease the nutrient levels). The stations of the cruise are shown in figure 1.

Meteorology

Characteristics of the weather conditions in the months since the last cruise in the beginning of November 1998 are given in table 1. November was cold, dry and relatively calm. The same goes for the first half of December, then it changed to warmer, wet and windy, which continued during January. The first week of February was dominated by strong wind (gales and storm up to 28 m/s) from west and north-west. The second week (equal to the first week of the cruise) there was very little wind, mostly from south-west. The third week (second week of the cruise) strong wind from west was dominating.

Table 1. Deviations in monthly mean temperature and precipitation in November 1998 to February 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).

Month

Temperature deviation ° C

Precipitation

% deviation

Mean wind speed

m/s

Dominating wind direction

Nov. 98

-2.7

-41

4.4

E-SE

Dec. 98

-0.3

-9

5.5

S-SW

Jan. 99

+2,2

+33

5.7

S-SW

Feb. 99

+0.9

+45

5.5

SW-W

North Sea and Skagerrak

Hydrography

The Jutland Coastal Current (JCC) with lower salinity and temperature was evident but relatively narrow along the Danish North Sea coast and also observed at the most coast near stations in the Skagerrak. At the southern-most transect in the German Bight it seems, that river influenced water was present also at the western-most stations, maybe coming from the river Rhine. Otherwise central North Sea water with salinity above 34.4 was found at the western stations in the North Sea and in a tongue into the south-western Skagerrak (figure 2). The surface temperature ranged from 2.6-3.1° C outside the Danish Wadden Sea to above 5.5° C at the western most station in the North Sea and into the south-western Skagerrak (figure 3). Along the coast the salinity increased from about 30 in the German Bight to 32.6 at Limfjorden, 33.5 at Hanstholm and 34.3 at Hirtshals, as the German Bight water within the JCC is mixed with North Sea water. In the Skagerrak the surface salinity was lowest in the central part (32.2-32.8 at the stations 1006, 1104 and 1106), probably due to out-flowing water from the Kattegat.

Nutrients

In the North Sea the nutrient concentrations varied inversely to the salinity. The results of linear regression on measurements of nutrients and salinity from the 36 stations at the 6 transects in the North Sea are shown in table 2. For nitrite and phosphate the significance is low. For nitrite this lack of relation to salinity is often seen and is due to very scattered concentrations at salinities below 33. For phosphate the concentrations are relatively high and scattered at salinities above 34. Deleting these samples with salinities above 34 increases R2 to 0.79.

Table 2. Linear regression analyses of salinity and concentrations of nutrients at the 36 stations in the North Sea 11-13 February 1999. The intercept gives the estimated mean concentrations in the river water entering the eastern North Sea. 34.5 psu gives the estimated concentrations in central North Sea water. N = number of observations. Unit = m mol/l.

Nutrient

Slope

Intercept

34.5 psu

N

R2

Nitrate

-8.36

297

8.7

225

0.95

Nitrite

-0.13

4.70

0.16

230

0.51

Ammonium

-0.51

17.6

0.03

228

0.90

Phosphate

-0.095

3.65

0.36

221

0.48

Silicon

-3.77

134

4.0

230

0.90

Due to the relation to the salinity the nutrient concentrations were highest in the south-eastern German Bight, decreasing to the north and west (figures 4, 5, 6, 7 and 8; please neglect the isolines in the Little Belt area as they are not based on measurements).

Oxygen and chlorophyll-a

The minimum oxygen concentrations were about saturation level (97-100%) at all stations, except at the deepest stations in the Skagerrak, where 78%, 83% and 92% were found at the stations 1006, 1106 and 1135, respectively. The lowest oxygen concentration of 5.4 ml/l (78%) was observed in 620 m depth at the station 1006 (M6). The mean chlorophyll-a concentrations in the surface layer (0-10 m) were 1-2 m g/l within the JCC in the German Bight and up to north of Horns Rev (figure 9), may be due to resuspension of sediments. No signs of spring bloom was present.

Kattegat, Sound, Belt Sea and Arkona Sea

Hydrography

The surface temperature (1 m depth) varied between 1.4° C and 3.1° C, highest in the western Kattegat (figure 3). The bottom water temperature ranged from 1.6-1.8° C at the shallow stations 441 and 954 in the western Arkona Sea to 5.0-6.3° C in the southern Kattegat and the Sound. Thus the bottom water was warmer (0.1-4.2° C) than the surface water.

The surface salinity ranged from 8.0 in the Arkona Sea (St. 444) to 30.2-31.3 in the north-western Kattegat (St. 1008, 1009) (figure 2). The bottom water salinity ranged from 11.0-12.6 in the western Arkona Sea (St. 441, 449) and 17.3 in the central Arkona Sea (St. 444) to 34.2-34.4 in the north-eastern Kattegat (St. 905, 1001) (figure 10). The salinity stratification was strongest in the eastern Kattegat (8.3-13.0 psu) and in the Sound (10.5 psu). In the Great Belt the salinity difference between surface and bottom was 6.1-9.5 psu and in the central Arkona Sea 9.3 psu.

Compared to long term monthly means (Lightship observations 1931-1960) for February the water temperatures during this cruise were generally a little higher (0.2-1.4° C) than normal, except for lower bottom water temperatures in Læsø Rende (St. 403) and south-western Kattegat (St. 925). The surface salinity was lower than long term mean in the eastern Kattegat, Great Belt and at Gedser Rev, but higher than normal in the western Kattegat and Fehmarn Belt, while the bottom water salinity was generally lower than normal, except in the north-eastern Kattegat and the shallow Aalborg Bight (St. 409). The strong wind prior to the cruise has created more intensive mixing and water exchange than long term mean.

Nutrients

Unusually high concentrations of nitrate (12-16 m mol/l) were present in the surface in the northern Kattegat (figure 4), and above 10 m mol/l were observed in the bottom near water in most of the Kattegat and northern Great Belt, and at the end of the cruise also in the Sound (figure 11). This probably stems from an inflow from the Jutland Coastal Current. G.M. Dannevig, Norway, had 20 January observed this nitrate rich water (22 m mol/l) just outside Hirtshals. About one week later Argos, Sweden, observed 15-30 m mol/l nitrate in the surface of the eastern part of the Skagerrak with the highest concentrations close to Skagen, and elevated concentrations of nitrate were traced in the bottom water in the eastern Kattegat down to Fladen (St. 905). Some of this JCC water was at our cruise still present in the northern Kattegat and the inflow as bottom water to the Kattegat and Belt Sea had continued during the stormy period prior to the cruise.

Compared to mean for February in the 1980’s the nitrate concentrations during this cruise were higher in the Kattegat and Great Belt, except for lower concentrations in the deepest water in the north-eastern Kattegat. In the southern Belt Sea and Arkona Sea the concentrations were equal to or lower than in the 1980’s.

Rather high concentrations of nitrite (figure 5) and ammonium (figure 6) were found in the Belt Sea (figure 11). Relatively high concentrations of phosphate were observed in the southern Kattegat and Great Belt (figures 7 and 12), but the concentrations were all over lower than mean for February in the 1980’s. Except for low concentrations in the northern Kattegat the silicon concentrations were relatively high and constant in the whole water column from the southern Kattegat through the Belt Sea to the Arkona Sea (figures 8 and 12). The silicon concentrations were generally higher than mean for February in the 1980’s in the Sound, southern Kattegat and Great Belt.

Chlorophyll-a

The mean chlorophyll concentration in the uppermost 10 m was highest (2.3 m g/l) in the Kiel Bight (St. N3). In the rest of the Belt Sea, Arkona Sea, southern and western Kattegat, and at the end of the cruise also in the Sound the mean chlorophyll concentrations were between 1.0 m g/l and 2.0 m g/l, but in the northern Kattegat only 0.7-0.9 m g/l (figure 9). The chlorophyll was homogeneously distributed in the uppermost 10 to 15 m of the water column (figure 13). The phytoplankton spring bloom seemed just about to start in certain areas of the Kattegat and southern Belt Sea.

Oxygen

The lowest oxygen concentration of 5.4 ml/l (76% saturation) was observed in the Sound. In the southern Kattegat the minimum concentrations were 5.9-6.7 ml/l (82-89%). In the remaining Kattegat, the Belt Sea and Arkona Sea the minimum saturation was 90-100%.

Compared to mean for February in the 1980’s the minimum oxygen concentrations this year were a little higher, except in the Sound.

 

FIGUR01.gif (14326 bytes)
Figure 1. Monitoring cruise with r/v Gunnar Thorson 8-18 February 1999 in the Sound, Kattegat, Skagerrak, North Sea, Belt Sea and Arkona Sea. Gunnar Thorson cruise no. 189.  



FIGUR02.gif (14762 bytes)
Figure 2. Interpolated distribution of surface salinity (mean 1, 5 and 10 m depth).

 



FIGUR03.gif (14808 bytes)
Figure 3. Interpolated distribution of surface temperature (mean 1, 5 and 10 m depth).



FIGUR04.gif (19302 bytes)
Figure 4. Interpolated distribution of surface nitrate concentrations (mean 1, 5 and 10 m depth).



FIGUR05.GIF (14729 bytes)
Figure 5. Interpolated distribution of surface nitrite concentrations (mean 1, 5 and 10 m depth).



FIGUR06.GIF (17691 bytes)
Figure 6. Interpolated distribution of surface ammonium concentrations (mean 1, 5 and 10 m depth).



FIGUR07.gif (12915 bytes)
Figure 7. Interpolated distribution of surface phosphate concentrations (mean 1, 5 and 10 m depth).



FIGUR08.gif (17215 bytes)
Figure 8. Interpolated distribution of surface silicon concentrations (mean 1, 5 and 10 m depth)



FIGUR09.gif (16129 bytes)
Figure 9. Interpolated distribution of surface chlorophyll-a concentrations (mean 1, 5 and 10 m depth).

 

FIGUR10.GIF (23606 bytes)
Figure 10. 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.



FIGUR11A.gif (10632 bytes)
FIGUR11B.gif (10391 bytes)
FIGUR11C.gif (10937 bytes)
Figure 11. Surface and near bottom concentrations of nitrate, nitrite and ammonium
along transect I.



FIGUR12A.gif (9707 bytes)
FIGUR12B.gif (10296 bytes)
Figure 12. Surface and near bottom concentrations of phosphate and silicon along transect I.



FIGUR13.gif (12838 bytes)
Figure 13. Chlorophyll-a concentrations in 1 m, 5 m, 10 m and 15 m depths along transect I.