Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 17-21 March 1997. Cruise no. 175.

Report: Gunni Ærtebjerg
Cruise leader: Jan Damgaard
Participants: Peter Kofoed, Dorete Jensen, Kjeld Sauerberg (NERI). Katrine Worsaae (student).


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


 Summary and conclusions

The strong wind from southwest and west during February and beginning of March had mixed the water masses, and the bottom water salinity had generally decreased since medio January. However, the salinity of 21 psu in the Arkona Sea indicates inflow from the Sound and Belt Sea, and 34.4 psu in the north-eastern Kattegat indicates inflow from the Skagerrak. The surface salinity had decreased in the southern parts due to outflowing Baltic surface water, but increased in the northern parts. The temperature was higher than long term mean, and the bottom water salinity higher than normal, except in the Kattegat. The stratification was weaker than normal for the season, except in the Sound.

The phytoplankton had depleted the nitrate, phosphate and silicate in the surface water in the Aarhus Bight and southern Belt Sea. Nitrate was also depleted in the Arkona Sea and southern Kattegat, while silicate was low in the southern Kattegat and in an intermediate layer in 10-20 m depth in the eastern Kattegat. Nitrate concentrations of 9-10 m M were observed in the inflowing Skagerrak bottom water in the northern Kattegat.

The chlorophyll concentrations were low in the Sound and eastern Kattegat, even nutrients were still available. The phytoplankton spring bloom was still going on in the western Kattegat, the Belt Sea and Arkona Sea with mean chlorophyll concentrations in the uppermost 15 m of 2.4-10.7 m M with the highest concentrations east of Samsoe and in the Arkona Sea.

The minimum oxygen concentrations were generally higher than long-term mean for March, except in the Fehmarn Belt and Arkona Sea. The lowest concentrations of 5.4 ml/l (73%) and 6.1 ml/l (79%) were observed in the southern Kattegat and southern Great Belt, respectively. At the other stations the minimum oxygen saturation's were 82-102%.

General

The scope of the cruise was to determine the hydrographic situation, the progress of the phytoplankton spring bloom and the oxygen and nutrient concentrations. The route, time schedule and stations of the cruise are shown in figure 1. Besides hydrographic, hydrochemical and pelagic biological monitoring measurements, living phytoplankton was sampled in the western Kattegat (St. 409, 403, 1009) and zoobenthos at Kullen (St. 921) for research purposes.

Meteorology

In January the monthly mean temperature was 1.2° C belove long term mean 1961-1990, and the precipitation was only 9% of normal. The wind was weak from southeast to southwest. In February, the monthly mean temperature was 2.9° C above normal and the precipitation about the double of long term mean. Strong wind from southwest and west dominated (Danish Meteorological Institute). This weather type continued in the beginning of March.

Hydrography

The surface temperature ranged from about 3° C in the north-eastern Kattegat (St. 905, 1001) and central Arkona Sea (St. 444) to 4.2-4.4 ° C in the Aarhus Bight (St. 427, 432), and had generally increased 2-3° C since medio January. The bottom water temperature ranged from below 4° C in the eastern Kattegat (St. 413, 905) and Arkona Sea (St. 441, 444, 449, 954) to about 5° C in the Aarhus Bight and 5.7° C in the north-eastern Kattegat (St. 1001) (Fig. 2), and had generally decreased since medio January, except in the Belt Sea. The temperature difference between surface and bottom was generally below 1° C.

The surface salinity ranged from 8.1 psu in the central Arkona Sea (St. 444) to 28.4 psu in the north-western Kattegat (St. 1008, 1009), and had since medio January decreased in the Sound, Belt Sea and Arkona Sea, but increased in the Kattegat and Aarhus Bight. The bottom water salinity ranged from 13.6-13.9 psu in the coastal and 21.1 psu in the central Arkona Sea (St. 441, 449, 444) to 34.4 psu in the north-eastern Kattegat (St. 1001) (Fig. 3, 4 and 5), and had generally decreased since medio January, except in the southern Belt Sea and central Arkona Sea. The salinity stratification was strong in the Sound (St. 431, 921) with differences between surface and bottom of more than 19.5 psu. More than 10 psu difference was also found in the central Arkona Sea (St. 444), southern Belt Sea (St. 443, 450, 952) and north-eastern Kattegat (St. 1001).

Compared to long term monthly means (1931-1960) for March the temperatures during this cruise were generally higher, except in the eastern Kattegat bottom water. The surface salinity was lower than long term means in the southern Belt Sea and north-eastern Kattegat. The bottom water salinity was generally higher than normal, except in the southern and eastern Kattegat. The salinity stratification was weaker than normal, except in the southern Belt Sea.

Nutrients

The nitrate concentrations were low in the whole water column in the south-western Kattegat (St. 415, 925) and Aarhus Bight (St. 427, 432) (Fig. 6) and in the surface water in the southern Kattegat, Arkona Sea and southern Belt Sea (Fig. 7 and 8). Nitrate concentrations of 9-10 m M were observed in the bottom water in the northern Kattegat (St. 403, 1001, 1008, 1009, 905). Nitrite concentrations above 0.5 m M were found in the bottom water at Gedser Rev (St. 449, 954) and in the northern Kattegat (Fig. 9), while ammonium concentrations above 0.5 m M were found in the bottom water at most stations (Fig. 10) and in the Sound also in the surface water. Compared to mean for March in the 1980’s the nitrogen nutrient concentrations were lower this year

Very low phosphate concentrations (<0.05 m M) were observed only in the surface water in the coastal Arkona Sea, southern Belt Sea and Aarhus Bight (Fig. 11). The silicate concentrations were below 2 m M in the surface water in the southern and northern Belt Sea, Aarhus Bight, southern Kattegat and in an intermediate layer in 10-20 m depth in the eastern Kattegat (Fig. 12). Also the phosphate and silicate concentrations were generally lower this year compared to mean for March in the 1980’s.

Chlorophyll-a

The mean chlorophyll-a concentrations in the uppermost 15 m were in the Sound and eastern Kattegat only 0.7-0.9 m g/l; increasing in the western Kattegat from 2.4 m g/l in the north (St. 1008) to 5.4 m g/l in the south (St. 925). In the Belt Sea the mean chlorophyll concentrations ranged from 2.9-3.5 m g/l in the central Great Belt (St. 935, 939) to 6.7 m g/l in Aarhus Bight and southern Great Belt (St. 427, 450) and 10.7 m g/l west of Samsoe (St. 432). In the Arkona Sea the mean chlorophyll concentrations were between 6.7 m g/l (St. 449) and 9.7 m g/l (St. 441). In the Kattegat a tendency to small subsurface chlorophyll, maxima in 10-15 m depth were observed.

Oxygen

Since medio January the bottom water oxygen concentrations had generally increased, except in the Fehmarn Belt. The lowest oxygen concentration of 5.4 ml/l (73%) and 6.1 ml/l (79%) were found in the southern Kattegat (St. 922) and southern Great Belt (St. 450), respectively. In the Sound, eastern Kattegat and Fehmarn Belt the minimum saturation’s were 82-84%, and in the central Arkona Sea 86%. In all other areas the minimum oxygen saturation’s were 92-102% (Fig. 13).

Compared to March last year and mean for March 1980-1989 the minimum oxygen concentrations this year are higher, except in the Fehmarn Belt, Gedser Rev and Arkona Sea area.