Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 14-17 August 1995. Cruise no 161.


Cruise leader: Jan Damgaard

Participants: Hanne Ferdinand, Susanne Hemmingsen, Peter Kofoed.

Guests 14-16 August: Torkel Gissel Nielsen, Lene Nielsen, Birgit Sborg, K. Pagou, K. Akepsimaidis, E.D. Christou.


General

The route, time schedule and stations of the cruise are shown in figure 1. Besides the pelagic monitoring measurements, sediment for chlorophyll determination was sampled at station 413.

The monthly mean temperature in July 1995 was in Denmark 1.7 C above the mean for the period 1961-90, and the precipitation was 58% below normal. The wind speed was relatively low (Danish Meteorological Institute). The weather prior to and during the cruise was mostly sunny, hot and calm.

Hydrography

The surface temperature ranged from 18.9 C at Gedser Rev (St. 954) to above 21 C in the southeastern Kattegat, and had increased 2.5-4.5 C since the cruise mid July. The bottom water temperature was between 6.5 C east of Anholt (St. 413) and 11.4 C in the central Arkona Sea, but above 13 C in the shallow western parts of the Kattegat and the Arkona Sea (Fig. 2), and had also increased, except in the northwestern Kattegat and around Gedser Rev. The temperature difference between surface and bottom was generally 9-13 C, except in the areas with high bottom water temperature mentioned above.

The surface salinity ranged from 7.7 psu in the central Arkona Sea to only 18.3-20.6 psu in the northern Kattegat (Fig. 3, 4 and 5). The surface salinity had increased since mid July in northern and eastern Kattegat and southern Great Belt. The bottom water salinity ranged from 10-13 psu in the Arkona Sea to 34.0-34.96 psu in the northeastern Kattegat. The bottom water salinity had generally decreased since mid July, except in the southern Great Belt (St. 450), at Gedser Rev and in the Arkona Sea. Except in the Arkona Sea the salinity stratification was still very strong with differences between surface and bottom of 10.0-23.3 psu, even in the shallow western Kattegat.

Compared to long term monthly means (1931-60) for August the surface temperatures during this cruise were 2-3 C higher and the bottom water temperatures generally lower than normal. The salinities were lower than long term means, except in the northeastern Kattegat bottom water

Oxygen

The minimum oxygen concentrations had decreased much (0.5-3.2 ml/l) since mid July. The lowest concentration of 1.1 ml/l (16% saturation) was observed in the Fehmarn Belt. At Gedser Rev and in the central Arkona Sea the minimum oxygen concentrations were 2.0-2.1 ml/l (29-30%) (Fig. 6). In the Sound 2.2 ml/l (33% sat.) was found (Fig. 7), and 2.3-2.8 ml/l (35-44% sat.) were found in all Great Belt, Aarhus Bight, Aalborg Bight (Fig. 8) and east of Anholt.

Compared to August last year the minimum oxygen concentrations this year are 0.2-1.5 ml/l lower, except in the Arkona Sea and the northeastern Kattegat. Compared to mean for August 1980-89 the minimum concentrations this year are generally 0.4-1.4 ml/l lower, except in the northeastern Kattegat.

Nutrients

Generally the nitrate-cline (0.5 mol/l isoline) was situated within the depth interval 15 to 20 m, except in 10 m depth in the central Sound and in more than 30 m depth in the Arkona Sea (Fig. 9, 10, 11).

In the bottom water high nitrate concentrations (10-14 mol/l) were only found in the southeastern Kattegat (St. 413) and Fehmarn Belt (St. 952) where the bottom water judged from the oxygen concentrations have been rather stagnant (Fig. 12).

Relatively high concentrations of nitrite and ammonium were only found in the northern Kattegat bottom water, but ammonium also in the southern Kattegat (Fig. 13, 14).

In the surface water phosphate concentrations below 0.05 mol/l were found only in the southwestern and western Kattegat (Fig. 15). Phosphate concentrations above 1 mol/l were found in the bottom water at some stations with relatively low oxygen concentrations: East of Anholt (St. 413), in the Sound (St. 431), Fehmarn Belt (St. 952) and the Arkona Sea (St. 444). The silicate concentrations in the surface layer were below 2 mol/l in all Kattegat, above 2 mol/l in the Sound and Belt Sea increasing to more than 12 mol/l in the Arkona Sea. Above 20 mol/l were found in the bottom water in many areas (Fig. 16).

Chlorophyll-a

The mean chlorophyll-a concentrations in the uppermost 15 m ranged from about 1 g/l in the southeastern Kattegat to 2.7-4.2 g/l in the Belt Sea. In most areas subsurface chlorophyll-a maxima were observed associated with the halocline with 3.3-5.0 g/l in the Belt Sea, 5.2 g/l in Ls Rende (St. 403) and about 3.6 g/l close to the bottom in the shallow western Kattegat.

Summary and conclusion

The last month prior to the cruise was mainly hot, sunny and calm with relatively little water exchange and mixing. The surface temperatures were 2-3 oC above long term monthly mean for August (1931-60), and the bottom water temperatures were generally lower than normal. The salinities were lower than long term means, except in the northern Kattegat bottom water.

Nitrate and nitrite were not present in the uppermost 15-20 m of the water column. Phosphate concentrations were low only in the western Kattegat surface layer. The silicate concentrations were below 2 mol/l in the Kattegat surface water, increasing through the Sound and Belt Sea to above 12 mol/l in the Arkona Sea.

The minimum oxygen concentrations had decreased much (0.5-3.2 ml/l) since mid July. The lowest concentration of 1.1 ml/l (16% saturation) was observed in the Fehmarn Belt. At Gedser Rev and in the central Arkona Sea the minimum oxygen concentrations were 2.0-2.1 ml/l (29-30%). In the Sound 2.2 ml/l (33%) was found, and 2.3-2.8 ml/l (35-44%) were found in all Great Belt, Aarhus Bight, Aalborg Bight and east of Anholt.

Compared to August last year the minimum oxygen concentrations this year are 0.2-1.5 ml/l lower, except in the Arkona Sea and the northeastern Kattegat. Compared to mean for August 1980-89 the minimum concentrations this year are generally 0.4-1.4 ml/l lower, except in the northeastern Kattegat.

Oxygen depletion is in Denmark arbitrarily 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 serious oxygen depletion now occurred in the Fehmarn Belt, and oxygen depletion occurred in the Arkona Sea, the Sound, all Great Belt, Aarhus Bight, Aalborg Bight and east of Anholt.

Since 1981 oxygen depletion have often been observed in August in the area Fehmarn Belt - Gedser Rev - Arkona Sea, and approaching depletion level is not unusual for the Sound. However, the oxygen depletion in the Kattegat and especially in the Great Belt area is about 1 month earlier than average. In these areas the culmination of oxygen depletion is often in October.

Based on data from the Danish Counties figure 17 shows the approximate areas, where oxygen depletion has been observed this summer until mid August in Danish marine waters.

Judged from the present oxygen level, and depending on the time of reoxygenation (storms), serious oxygen depletion might develop further this autumn in the deeper stratified areas as the southern Kattegat, Sound, Belt Sea and Arkona Sea.

Gunni rtebjerg


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