Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 12-16 June 1995. Cruise no. 159.


Cruise leader:Gunni rtebjerg(12-14/6)/Jan Damgaard

Participants: Kjeld Sauerberg, Hanne Ferdinand, Susanne Hemmingsen; Beth Paludan Carlsen (trainee). Jakob Fog, Jonathan Fog, Anders Hansen, TV3-Denmark (12-13/6)


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 May 1995 was in Denmark 0.9 C below mean for the period 1961-90 and the precipitation was 10% above normal. The wind speed was relatively normal, mostly from western directions (Danish Meteorological Institute).

Hydrography

The surface temperature ranged from 10.5 C at Gedser Rev (St. 954) to above 14 C in the Sound and southern Kattegat, and had generally increased 4.2-6.8 C since the cruise primo May. The bottom water temperature ranged from 4.3 C in the Arkona Sea and 5.4-5.7 C in the Sound, southern Kattegat and Great Belt to more than 8 C in the northwestern Kattegat and at Gedser Rev (Fig. 2), and had also increased. The temperature stratification was stronger than in May with the surface water now 6-8 C warmer than the bottom water, except at shallow depths in the Arkona Sea.

The surface salinity ranged from 7.73 psu in the central Arkona Sea to only 21.35 psu in the northeastern Kattegat (Fig. 3, 4 and 5). The surface salinity had generally decreased further since primo May, especially in the southernmost and northwestern Kattegat. The bottom water salinity ranged from 8.0-13.8 psu in the Arkona Sea to 34.3-34.6 psu in the northeastern Kattegat. The bottom water salinity had generally increased since primo May, except in the northern Kattegat and Arkona Sea. The salinity stratification was very strong with a difference between surface and bottom of 16-24 psu, except for 11-13 psu at the stations 409, 952 and 1001 and below 7 psu in the Arkona Sea.

Compared to long term monthly means (1931-60) for June both the surface and bottom water temperatures and salinities during this cruise were generally lower, except for higher temperature in Ls Rende (St. 403).

Oxygen

Except in the Sound and southern Kattegat, the minimum oxygen concentrations had generally decreased since primo May, most in the southern Belt Sea and Arkona Sea with a decrease of 1.2-3.0 ml/l. Concentrations below 4 ml/l were now found in the southern Belt Sea (St. 443, 450 and 952; 3.7-3.9 ml/l; 51-54% sat.) (Fig. 6) and in Ls Rende (St. 403; 3.9 ml/l; 59%) (Fig. 7). 4.0 ml/l (48-56%) were found in the central Arkona Sea and central Sound (Fig. 8), and 4.1-4.6 (57-64%) were observed in the southeastern Kattegat and northern Belt Sea.

Compared to June last year and to mean for June in the 1980's the minimum oxygen concentrations this year are generally lower, except in the Sound and northern Kattegat.

Nutrients

Nitrogen nutrient were not present in the surface layer. Generally the nitrate-cline (0.5 mol/l isoline) was situated in about 10 m depth, shallower in the Great Belt, deeper in the northern Kattegat, and in more than 40 m depth in the Arkona Sea (Fig. 9, 10, 11).

In the bottom water high nitrate concentrations (9-13 mol/l) were found in the southern Kattegat, the Sound, Aarhus Bight and Great Belt at salinities of 28.9-34.6 psu (Fig. 12). The nitrate rich bottom water observed in the Kattegat primo May had thus flown into the Sound and Belt Sea.

Relatively high concentrations of nitrite and ammonium were only found in the Kattegat bottom water, but ammonium also in the Sound and close to the bottom in the central Arkona Sea (Fig. 13, 14).

In the surface water phosphate (above 0.05 mol/l) was only found in the Sound, central Great Belt and Arkona Sea (Fig. 15). The silicate concentrations in the surface layer were above 2 mol/l, and more than 20 mol/l were found in the bottom water in the Sound and southern Belt Sea (Fig. 16).

Compared to monthly mean for June 1980-89 the nitrate concentrations in the bottom water were in the Sound and Great Belt generally higher and in the Kattegat lower this year. Phosphate shows a tendency to generally lower concentrations than in the 1980's.

Chlorophyll-a

The chlorophyll-a concentrations in the uppermost 5 m were relatively low ranging from 0.6 g/l in the Sound (St. 431) to about 3 g/l in Aalborg Bight (St. 409) and Aarhus Bight (St. 427). In most areas subsurface chlorophyll-a maxima were pronounced with up to 27 g/l at 13 m depth in Aalborg Bight, 17 g/l in 18 m depth at station 413 and in 10 m depth at station 1009; often 3.2-4.4 g/l were observed in 10-15 m depth.

Summary and conclusion

The hydrography showed strong outflow of brackish water from the Baltic Sea and inflow of saline bottom water from the Skagerrak, creating a very strong salinity stratification. Saline bottom water rich in nitrate had since primo May flown from the Kattegat into the Sound and Great Belt.

Nitrogen nutrients were absent from the surface layer, and in the Arkona Sea down to more than 30 m depth. Phosphate was present in the surface of the Sound, the central Great Belt and the Arkona Sea, and silicate was present all over. The nitrate cline was generally found in 10-15 m depth, and higher nitrate concentrations than in the 1980's were found in the bottom water in the Sound and Great Belt.

Pronounced subsurface chlorophyll-a maxima were observed in most areas with up to 17-27 g/l at a few stations in the Kattegat, but most often 3-4.5 g/l in 10-15 m depth, and in the Sound and Arkona Sea below 2 g/l.

Except in the Sound and southern Kattegat, the minimum oxygen concentrations had decreased since primo May, most in the southern Belt Sea and Arkona Sea with a decrease of 1.2-3.0 ml/l. Concentrations below 4 ml/l were now found in the southern Belt Sea (3.7-3.9 ml/l, 51-54% sat.). In the Sound and Arkona Sea 4.0 ml/l (48-56%) were found, and 4.1-4.6 (57-64%) were observed in the southeastern Kattegat and northern Belt Sea.

Compared to June last year and to mean for June in the 1980's the minimum oxygen concentrations this year are generally lower, except in the Sound and northern Kattegat.

Gunni rtebjerg


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