Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 9-13 June 1997. Cruise no. 177.

 

Report: Gunni Ærtebjerg

Cruise leader: Jan Damgaard

Participants: Peter Kofoed, Hanne Ferdinand, Susanne Hemmingsen, Hanne Kaas (NERI). Else Juhl Nielsen (ICES), Ane Mollerup (trainee).

 

 

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

 

Summary and conclusions

The warm and sunny weather just prior to and during the cruise had warmed up the sea surface to a little above long term mean for June, but the bottom water temperature was generally lower than normal. The prevailing weak wind from east in May and beginning of June had favoured outflow from the Baltic Sea, and the salinity was lower than normal, except at the bottom in the north-eastern Kattegat. The pycnocline was generally situated in about 10 m depth, and the stratification of the water column was unusually strong.

In the surface layer small amounts of nitrate were found only in Aarhus Bight and the northern Belt Sea, and phosphate was observed in the Sound, Arkona Sea and southern Belt Sea. Silicate was present in the surface layer in all areas. Generally about normal concentrations of all nutrients were found in the bottom water.

The mean chlorophyll-a concentration in the uppermost 15 m was low ranging from 0.5 m g/l in the Sound to 2.0 m g/l in the Arkona Sea. The concentrations were lowest in the uppermost 5 m, increasing to a subsurface maximum of 2.5-3.5 m g/l (15 m depth), especially in the Kattegat and Great Belt, but not in the Arkona Sea.

The lowest oxygen concentrations of 3.8 ml/l (47%) and 4,0 ml/l (56%) were found in the Arkona Sea and the Sound, respectively. Minimum concentrations of 4.6-4.9 ml/l (61-65%) were observed in the Fehmarn Belt and south-eastern Kattegat. In all other areas the minimum oxygen concentrations were above 5 ml/l and 70% saturation.

Compared to June last year the minimum oxygen concentrations this year are generally lower, except in the southern Kattegat and Great Belt. Compared to mean for June in the 1980s the minimum oxygen concentrations this year are generally higher, except in the central Arkona Sea.

General

The scope of the cruise was to determine the hydrographic situation, the spatial variation in the phyto- and zooplankton species composition and biomass, the phytoplankton primary production, and the oxygen and nutrient concentrations. The stations of the cruise and the three transects used in the following figures are shown in figure 1. Besides the monitoring measurements, sampling and measurements in the research project DECO were performed.

Meteorology

The monthly mean temperature was in March 1.6 C above long term mean 1961-1990, in April about normal, and in May 1.2 C below mean. The precipitation was in March and April 35% and 7% lower than normal, respectively, and in May 42% above mean.. The wind came in March and April mainly from west, in May and beginning of June weak wind from east prevailed (Danish Meteorological Institute).

Hydrography

The surface temperature ranged from 12-13 C in the Arkona Sea (St. 441, 444, 449, 954) to 16.8-18 C in the central Kattegat (St. 922, 409, 413, 418). The bottom water temperature ranged from 5.1 C in the central Sound (St. 431) and below 6 C in the southern Kattegat (St. 921, 922, 925), northern Belt Sea (St. 427, 935, 939) and central Arkona Sea (St. 444) to 7-8 C in the northern Kattegat (Fig. 2). The temperature difference between surface and bottom was generally 6-11 C, except in the shallow parts of the Arkona Sea.

 

Figure 2. Surface (1 m) and bottom near temperature and salinity along transect I, and surface and bottom near salinity along the transects II and III.

The surface salinity ranged from 7.4 in the central Arkona Sea (St. 444) to 18-21.3 in the north-western Kattegat (St. 403, 1009). The bottom water salinity ranged from 11-15.6 in the Arkona Sea (St. 444, 449, 954) to above 34 in the north-eastern Kattegat (St. 905, 1007, 1008) (Fig. 2). Except in the Arkona Sea the salinity stratification was strong with differences between surface and bottom of 12-20 psu and in the Sound 24 psu. The pycnocline was generally situated in about 10 m depth.

Compared to long term monthly means (1931-1960) for June the surface temperature during this cruise was generally higher and the bottom water temperature lower than normal. The salinity was lower than long term mean, except in the bottom water in the north and eastern Kattegat. The salinity stratification was stronger than long term mean.

Nutrients

Practically no nitrate was left in the surface layer, except in the Aarhus Bight and northern Belt Sea (St. 427, 432, 935) with 0.6-1.4 m M (Fig. 3). In the bottom water 5-7 m M were found in the Belt Sea and about 10.3 m M in the Sound (St. 431). Significant amounts of nitrite were only found in the bottom water at Gedser Rev (1.3 m M, St. 954) (Fig. 3), while ammonium concentrations above 1.5 m M were observed in the bottom water in the north-eastern Kattegat, southern Belt Sea and central Arkona Sea (Fig. 4).

 

Figure 3. Surface and bottom near concentrations of nitrate along the transects I, II and III, and surface and bottom near concentrations of nitrite along transect I.

 In the surface layer also the phosphate concentrations were low, above 0.05 m M was only found in the Sound (St. 431, 1728), in the shallow parts of the Arkona Sea (St. 441, 449), in the Fehmarn Belt - Gedser Rev area (St. 450, 952, 954, 449) and at Fornaes (St. 415) (Fig. 4). Silicate concentrations of 2-8 m M were present in all the surface water, and above 15 m M were observed in the bottom water in the Belt Sea and Arkona Sea (Fig. 5).

Figure 4. Surface and bottom near concentrations of ammonium and phosphate along the transect I.

 Oxygen

The lowest oxygen concentrations of 3.8 ml/l (47%) and 4,0 ml/l (56%) were found in the Arkona Sea (St. 444) and the Sound (St. 431), respectively. Minimum concentrations of 4.6-4.9 ml/l (61-65%) were observed in the Fehmarn Belt (St. 450, 952) and south-eastern Kattegat (St. 921). In all other areas the minimum oxygen concentration was above 5 ml/l and 70% saturation (Fig. 5).

Figure 5. Surface and bottom near concentrations of silicate along transect I, and minimum oxygen concentrations along transect I.

Compared to June last year the minimum oxygen concentrations this year are generally lower, except in the southern Kattegat and Great Belt. Compared to mean for June in the 1980s the minimum oxygen concentrations this year are generally higher, except in the central Arkona Sea.

Chlorophyll-a

The mean chlorophyll-a concentration in the uppermost 15 m was low ranging from 0.5 m g/l in the central Sound to 2.0 m g/l in the central Arkona Sea. The concentrations were lowest in the uppermost 5 m, increasing to 10 m depth, and the highest concentrations were observed at 15 m depth, except in the southern Great Belt (St. 443) and central Arkona Sea (St. 444) (Fig. 9).

Figure 6. Chlorophyll-a concentrations in 1 m, 5 m, 10 m and 15 m depths along the transect I from the north-eastern Kattegat through Great Belt and Fehmarn Belt to the Arkona Sea.