Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Skagerrak, North Sea, Belt Sea and Arkona Sea, February 7th-16th, 1995.

Scientist in charge
: Gunni Ărtebjerg
Participants 7th-16th Feb.: Jan Damgaard, Hanne Ferdinand, Dorete Jensen, Kjeld Sauerberg, Mads Aertebjerg (trainee), Max Nitschke (Ornis Consult Ltd.),
7th-13th Feb.: Susanne Hemmingsen, Lasse Gudmundsson.
General
In December 1994 the wind came mainly from SW, and the precipitation in Denmark was 45% above normal (1961-90). In January 1995 the precipitation was 60% above normal. In the first week of 1995 the wind came mainly from S and in the third week from SE. Otherwise prior to and during the cruise rather strong wind from SW and W was dominating (Danish Meteorological Institute). This should favour high fresh water run off and nitrate loading, well mixing of the water column and the presence of the Jutland Coastal Current.

The route, stations and time schedule of the cruise are shown in figure 1. Due to a storm from WSW Sunday February the 12th, six planed stations in the German Bight had to be cancelled.

Besides profile measurements of salinity, temperature, fluorescence, turbidity, oxygen, nutrients, chlorophyll-a and yellow substance at 68 stations, bottom fauna was sampled at three stations in the North Sea, and sediment for chlorophyll measurements were collected at two stations in the Kattegat and Skagerrak, respectively. Bottom fauna were sampled at two North Sea stations for the University of Aarhus. Methane in seawater was sampled at 23 stations in the Skagerrak and North Sea for the University of Aalborg. Suspended matter was sampled at 21 stations in the North Sea for the University of Copenhagen. Surface sediment for intercalibration of radionuclide measurements was sampled at a station in the Kattegat for Ris° Research Center.

Hydrography in the North Sea-Skagerrak area

The Jutland Coastal Current (JCC) was evident along the Danish coast from the German Bight to Skagerrak and into the northernmost Kattegat, with lower salinity and temperature (Fig. 2 and 3) and higher nutrient content (Fig. 4 to 9) than further off shore. In the Skagerrak the JCC was narrow and at the sampling time nearly interrupted at Hanstholm.

The surface temperature ranged from below 3░C near the Jutland coast in the German Bight to more than 5. 5 ░C at the westernmost stations (Fig. 3). In the central Skagerrak deep water temperatures above 7 ░C were found in 37 m to 208 m depth at station 1106.

The salinity in the North Sea ranged from 28 psu near the coast in the German Bight to 34.75-35.0 psu at the westernmost stations (Fig. 2). Along the coast the salinity increased from 28 psu in the German Bight to 32-33 psu at Hirtshals and in the northernmost Kattegat, as the German Bight water within the JCC is mixed with North Sea and Skagerrak water. In the central Skagerrak (St. 1106) salinities above 35 psu were found only in depths above 105 m, while in the western part (St. 1133, 1135) above 35 psu was found already in 25-50 m depth.

Hydrography in the Kattegat-Arkona Sea area

The temperature ranged from 2.5-2.9 ░C in the surface in most areas to 5.3 ░C at the bottom of the northeastern Kattegat (St. 905 and 1001). The temperature stratification was strongest in the deep eastern Kattegat with a difference between surface and bottom of 2-2.9 ░C. In the Belt Sea the temperature stratifica-tion was very weak or absent (Fig. 10).

The salinity ranged from 8.9 psu in the surface of the Arkona Sea to 34.2 psu at the bottom in the northeastern Kattegat (St. 1001). The salinity stratification was unusually weak, except in the Sound and Arkona Sea (Fig. 11 to 13). The salinity at the bottom in the Arkona Sea was 20 psu, indicating a preceding smaller inflow from the Sound and Belt Sea.

Compared to long term monthly means (1931-60) for February the temperature and salinity at this cruise were higher in the surface water and lower in the bottom water, except for higher temperature and salinity in the bottom water of the northern Kattegat.

Oxygen

The minimum oxygen concentrations were in all areas above 90% saturation, except in 300-440 m depth in the central Skagerrak (St. 1106), where 85-89% saturation (5.8-6.2 ml/l) were found (Fig. 14). In the Kattegat and Belt Sea the minimum concentrations were higher than February 1994 and mean for February 1980-89.

Nutrients

In the North Sea the nutrient concentrations varied inversely to the salinity, with the highest concentrations of all nutrients at the lowest salinities in the German Bight, decreasing to the west and north with increasing salinity (Fig. 4 to 9).

At the southernmost transect (St. 1059-1064) in the German Bight a highly significant linear correlation between salinity and the concentrations of all inorganic nutrients were found. The intercepts of the regression lines (Table 1) give the mean concentrations of nutrients in the river water entering the German Bight minus possible sedimentation and/or uptake. The distribution of the nutrients in the rest of the North Sea generally followed the regression lines. Regression on all nitrate samples from the North Sea is shown in figure 15.

The concentrations of nitrate in the surface layer were generally lower than in February 1994, when exceptionally high nitrate concentrations were found in all the investigated areas, (except the Skagerrak and southern Belt Sea). The nitrate washed out by the flood in central Europe at the end of January this year seemed not yet to have reached the Danish part of the North Sea. In the northern and western Kattegat and the Belt Sea the nitrate concentrations were higher than mean for February 1980-89.

The relatively high nitrate concentrations above 10 Ámol/l in the western Kattegat and the Great Belt area (Fig. 4) are due to local loadings (Fig. 16, salinities below 30 psu), while the high nitrate (and nitrite) concentrations in the northern Kattegat are mainly due to the JCC (Fig. 16, salinities above 32 psu). In the northeastern Kattegat the JCC dives below the Baltic water resulting in an intermediate layer with nitrate concentrations above 12 Ámol/l in 10 to 50 m depth at station 1001 and above 10 Ámol/l in 30 to 50 m depth at station 905 (Fig. 17 and 18).

The distribution of ammonium, silicate, phosphate and total phosphorus along a transect from the Skagerrak/Kattegat border through the Belt Sea to the Baltic Proper (Arkona Sea) is shown in figures 19 to 22, and the distribution of nitrate in the western Kattegat and the Sound is shown in figures 23 and 24. The concentrations of phosphate and total-P in the Sound, Kattegat and Belt Sea were lower than means for February 1980-89.

Chlorophyll-a

The chlorophyll-a concentrations were low, as normal for the winter situation. Concentrations above 1 Ág/1 were only found at the most coast near stations in the Skagerrak and North Sea and a few stations in the Kattegat and Fehmarn Belt. The phyto- plankton spring bloom had not yet started in any of the investigated areas.

Turbidity

In the Kattegat, Belt Sea, Arkona Sea and Skagerrak the turbidity of the surface water was generally low, except in the shallow western Kattegat (Aalborg Bight) and in the JCC influenced northernmost Kattegat and coastal Skagerrak. In the North Sea the turbidity was high along the Jutland coast, decreasing to a minimum about 50 nautical miles from the coast and then increasing again further to the west (Fig. 25).

Satelite pictures from the same period (NOAA 14 Ch.l, Danish Meteorological Institute) show, that the observed western area with high turbidity seems to originate from the British coast of the English Channel, and the coastal high turbidity area originates at the continental side of the North Sea, while the low turbidity area in the middle is connected to the open English Channel. At least the western turbid area seems due to resuspension and not outflow from rivers, as the salinity was high (34.75-35.00 psu) and the nutrient concentrations low. Resuspension might also be the reason in the other turbid areas, except close to the rivers Elbe, Weser and Ems in the German Bight. The analyses of suspended matter and yellow substance might clarify this in more details.

Summary and conclusion

The Jutland Coastal Current (JCC) was evident with lower salinity and temperature and higher nutrient content along the Danish coast from the German Bight into the Skagerrak and northernmost Kattegat, where part of the JCC dived below the Baltic water and flowed south into an intermediate layer.

The concentrations of nitrate were generally lower than the exceptionally high concentrations found in February 1994. Nitrate from the flood in central Europe at the end of January this year seemed not yet to have reached the Danish part of the North Sea. Relatively high concentrations of nitrate were found in parts of Kattegat and in the Belt Sea.

The highest concentrations of all nutrients were found in the German Bight at the stations closest to the rivers Elbe and Weser. In the German Bight a highly significant linear correlation between salinity and all inorganic nutrients was observed, and the distribution of the nutrients in the rest of the eastern North Sea generally followed these regression lines.

The phytoplankton spring bloom had not yet started in any of the investigated areas, and all over the oxygen level was high.

 

Gunni Ărtebjerg

 

Table 1. Linear regression analyses between saliniby and concentrations of nutrients at the Sild transect (St. 1059-1064) in the German Bight 11-12 February 1995, and estimated concentrations in central North Sea water (34.5 psu). Nuniber of samples = 39. Unit = Ámol/l.
Substance Slope Intercept 34.5 psu r 2
Nitrate -7.2 260   10 -0.994
Nitrite -0.39     14 1.0 -0.811
Ammonium   -0.54 19 0   -0.975
Silicate -6.2 220 5.2 -0.990
Phosphate -0.09 3.6 0.54 -0.926
Total-P -0.21 8.2 0.84 -0.880

 

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