Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 10-13 July 1995. Cruise no. 160.


Cruise leader: Jan Damgaard

Participants: Kjeld Sauerberg, Hanne Ferdinand, Peter Kofoed


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 and precipitation in June 1995 was in Denmark very close to the means for the period 1961-90. Also the wind speed was relatively normal, mostly from western directions (Danish Meteorological Institute). The first days of the cruise the wind came from northeast 10-17 m/s.

Hydrography

The surface temperature ranged from 14.1 C in the central Arkona Sea (St. 444) to above 17 C in the Sound, eastern and northern Kattegat, and had generally increased 2.5-4.5 C since the cruise mid June. The bottom water temperature was between 6 and 8 C, except for higher temperatures in the northwestern Kattegat, southern Great Belt (St. 450) and at Gedser Rev and shallow parts of the Arkona Sea (Fig. 2), and had also increased. The temperature difference between surface and bottom was generally 9-11 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 19.0-19.3 psu in the northwestern Kattegat (Fig. 3, 4 and 5). The surface salinity had generally increased since mid June, especially in the Great Belt and southern Kattegat. The bottom water salinity ranged from 8.7-12.4 psu in the Arkona Sea to 34.0-34.95 psu in the northeastern Kattegat. The bottom water salinity had decreased in the Great Belt and Aarhus Bight since mid June, but increased in the Fehmarn Belt. Except in the Arkona Sea the salinity stratification was very strong with differences between surface and bottom of 11.2-22.6 psu, even in the shallow western Kattegat.

Compared to long term monthly means (1931-60) for July the bottom water temperatures during this cruise were generally lower, except for higher temperature in Ls Rende and at Gedser Rev (St. 403). The surface salinities and in the Belt Sea also the bottom water salinities were lower than long term means.

Oxygen

Except at 4 stations (St. 905, 403, 450, 444) the minimum oxygen concentrations had decreased since mid June. The lowest concentration of 2.3 ml/l (32% saturation) was observed in the Fehmarn Belt (Fig. 6). In the Sound 3 ml/l (43% sat.) was found in 25-30 m depth (Fig. 7), and 3.5-4.0 ml/l (50-57% sat.) were found in the southern Kattegat, Aarhus Bight (Fig. 8) and Great Belt.

Compared to July last year and to mean for July in the 1980's the minimum oxygen concentrations this year are generally lower, except in the southern Great Belt (St. 450) and in the Arkona Sea.

Nutrients

In the central Great Belt nitrogen nutrient were present in the surface layer (Fig. 9), and generally the nitrate-cline (0.5 mol/l isoline) was situated within the photic zone in less than 15-20 m depth, except in the southern Belt Sea and the Arkona Sea (Fig. 10, 11).

In the bottom water high nitrate concentrations (9-14 mol/l) were still found in the southeastern Kattegat, the Sound, and central Great Belt at salinities of 28.7-34.1 psu (Fig. 12) and in the Fehmarn Belt close to bottom at 24.8 psu.

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

In the surface water phosphate (above 0.05 mol/l) was present in the Sound, central Great Belt, Fehmarn 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 Fehmarn Belt (Fig. 16).

Chlorophyll-a

The chlorophyll-a concentrations in the uppermost 5 m were relatively low ranging from about 1 g/l in the southeastern Kattegat to 2.8 g/l in the central Arkona Sea (St. 444) and 3.9 g/l in northern Great Belt (St. 935). In most areas subsurface chlorophyll-a maxima were pronounced with up to 4.6-8.5 g/l in 17-19 m depth in the southeastern Kattegat and 4.0-6.7 g/l in 10-15 m depth in the northern Belt Sea and western Kattegat.

Summary and conclusion

The surface salinity was still dominated by outflowing Baltic water, creating a very strong salinity stratification (11-22 psu) reinforced by a temperature stratification of generally 9-11 oC. The bottom water temperature was relatively low compared to long term means. The surface salinities and in the Belt Sea also the bottom water salinities were lower than long term means. However, the surface salinity had increased and in the Great Belt the bottom water salinity had decreased since mid June indicating periods of dynamic hydrography due to stronger wind.

Nitrogen nutrients were present at the surface in the central Great Belt due to upwelling, and generally nutrients (N, P and Si) were found within the photic zone, except for nitrogen in the southern Belt Sea and Arkona Sea.

The chlorophyll-a concentrations in the uppermost 5 m were higher than in mid June and the subsurface chlorophyll-a maxima lower. However, pronounced subsurface chlorophyll-a maxima were still observed in most areas with up to 4.6-8.5 g/l in the southeastern Kattegat and 4.0-6.7 g/l in the northern Belt Sea and western Kattegat.

Generally the minimum oxygen concentrations had decreased since mid June. The lowest concentration of 2.3 ml/l (32% saturation) was observed in the Fehmarn Belt (St. 952). In the Sound 3 ml/l (43% sat.) was found, and 3.5-4.0 ml/l (50-57% sat.) were found in the southern Kattegat, Aarhus Bight and Great Belt.

Compared to July last year and to mean for July in the 1980's the minimum oxygen concentrations this year are generally lower, except in the Arkona Sea, where the concentration is relatively high for the season.

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 oxygen depletion now occurs in the Fehmarn Belt, and in the Sound the minimum concentration is approaching the depletion level.

Judged from the nitrogen load this year and the present oxygen level, and depending on the time of reoxygenation (storms), serious oxygen depletion might develop this autumn in the Sound, Belt sea and southern Kattegat.

Gunni rtebjerg


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