Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 7-11 July 1997. Cruise no. 178.

 

Report: Gunni Ærtebjerg

Cruise leader: Gunni Ærtebjerg

Participants: Jan Damgaard, Hanne Ferdinand, Kjeld Sauerberg, Dorete Jensen, Arash Ghatei (NERI). Susanne Reimert (ICES)

7-8 July: Britta Pedersen, Jens Jacobsen (NERI).

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

Summary and conclusions

The sunny weather and weak wind from east and north prior to and during the cruise favoured outflow from the Baltic Sea, resulting in lower salinity and higher temperature in the surface than normal. As the bottom water temperature was lover and salinity higher than normal the stratification of the water column was unusually strong. The pycnocline was situated close to 10 m depth.

In the surface layer no nitrate or nitrite was present, and the ammonium concentrations were generally below 0.5 m mol/l. Traces of phosphate were found in the surface layer in most areas, except the southern Kattegat and central Arkona Sea. Silicate was present in the surface water, except in most of the Kattegat. In the bottom water the highest concentrations of nutrients were observed in the Fehmarn Belt - Gedser Rev - Arkona Sea area.

The mean chlorophyll-a concentrations in the uppermost 15 m ranged from 1.1 m g/l in the eastern Kattegat to 2.0 m g/l in the northern Belt 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.

The lowest oxygen concentrations of 3.7-3.9 ml/l (52-54% saturation) were found in the Fehmarn Belt - Gedser Rev area and central Arkona Sea. In the Great Belt 4.03 ml/l (58%) was observed close to the bottom at station 935. In all other areas the minimum oxygen concentrations were above 4.8 ml/l and 70% saturation. Since the cruise in June the minimum oxygen concentration had increased 0.8 ml/l in the central Sound and decreased a little in the Great Belt, Aarhus Bight and western Kattegat. In the rest of the Kattegat the decrease was 0.5-0.9 ml/l, and in the Fehmarn Belt - Gedser Rev area 0.8-1.1 ml/l.

Compared to mean for July in the 1980s the minimum oxygen concentrations this year are 0.7-1.0 ml/l higher, except in the north-eastern Kattegat, where it is about the same as in the 1980s. In the Sound the minimum oxygen concentration this July was 1.5 ml/l higher than in the 1980s.

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. Internal audition of the sampling and laboratory work was performed. Besides the monitoring measurements sampling and measurements in the research project DECO were performed.

Meteorology

The monthly mean temperature in Denmark was in June 0.6 C above long term mean 1961-1990, and the precipitation was 5% above normal. Weak wind from east prevailed in the first week of June, followed by wind from west during the middle of the month, and again turning to east at the end of the month (Danish Meteorological Institute). Weak wind from north dominated during the cruise.

Hydrography

The surface temperature (1 m depth) ranged from 15.8 C at Gedser Rev (St. 954) to above 18 C in most of the Kattegat. The bottom water temperature ranged from 6.5 C in the central Sound (St. 431) and 6.7-7.0 C in the southern Kattegat, Aarhus Bight and Great Belt to above 10 C in the Arkona Sea and north-western Kattegat (Fig. 2). The temperature difference between surface and bottom was generally 5.6-11 C, except in the shallow parts of the Arkona Sea. In the central Arkona Sea (St. 444) a temperature minimum of 7.8-10 C was observed in an intermediate layer in 22-36 m depth.

 

Figure 2. Surface (1 m) and bottom near temperature along transect I, and salinity in 1 m, 5 m, 10 m, 15 m, 20 m depth and near bottom along the transects I, II and III (see figure 1).

The surface salinity ranged from 7.6-7.9 in the Arkona Sea (St. 441, 444, 449) to 18.7-19.4 in the northern Kattegat (St. 403, 1007, 1008, 1009, 905). The bottom water salinity ranged from 9.5-14.1 in the Arkona Sea (St. 441, 444, 449) to 33.7-34.2 in the eastern Kattegat (St. 921, 418, 413, 905, 1007) (Fig. 2). Except in the Arkona Sea the salinity stratification was strong with differences between surface and bottom of 11-18 psu and in the Sound 22 psu. The pycnocline was in the western Kattegat and Great Belt situated in about 10 m depth, in the other areas a little deeper.

Compared to long term monthly means (1931-1960) for July the surface temperature during this cruise was generally higher and the surface salinity lower than normal. The bottom water temperature was generally lower and the bottom water salinity higher than long term means.

Nutrients

No nitrate was left in the surface layer down to the pycnocline. Close to the bottom generally 1-5 m M nitrate was found, with a maximum of 7 m M in the Fehmarn Belt (St. 952) (Fig. 3). Likewise, generally no nitrite was found in the surface layer, and significant amounts of nitrite (0.9-1.3 m M) were only found in the bottom water in the Gedser Rev - Arkona Sea area (St. 954, 449, 444) (Fig. 3). In the surface layer the ammonium concentrations were generally below 0.5 m M. In the bottom water ammonium concentrations of 1.4-3.4 m M were observed in the northern Kattegat and central Arkona Sea (St. 1007, 1008, 905, 413, 444) (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 traces of phosphate were found in most areas, except the southern Kattegat and central Arkona Sea. Close to the bottom the phosphate concentrations were generally 0.4-1.0 m M (Fig. 4). Silicate concentrations of 4-7 m M were present in the surface water in the Sound, Arkona Sea and Belt Sea. In the Kattegat traces of silicate in the surface layer were found only in the western part. In the bottom water above 15 m M were observed in the Fehmarn Belt, Gedser Rev and Arkona Sea area (Fig. 5).

 

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

Oxygen

The lowest oxygen concentrations of 3.7-3.9 ml/l (52-54% saturation) were found in the Fehmarn Belt - Gedser Rev area (St. 952, 954) and central Arkona Sea (St. 444). In the Great Belt 4.03 ml/l (58%) was observed close to the bottom at station 935. In all other areas the minimum oxygen concentrations were above 4.8 ml/l and 70% saturation (Fig. 5). Since the cruise in June the minimum oxygen concentration had increased 0.8 ml/l in the central Sound and decreased a little in the Great Belt, Aarhus Bight and western Kattegat. In the rest of the Kattegat the decrease was 0.5-0.9 ml/l, and in the Fehmarn Belt - Gedser Rev area 0.8-1.1 ml/l.

 

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

Compared to mean for July in the 1980s the minimum oxygen concentrations this year are 0.7-1.0 ml/l higher, except in the north-eastern Kattegat, where it is about the same as in the 1980s. In the Sound the minimum oxygen concentration this July was 1.5 ml/l higher than in the 1980s.

Chlorophyll-a

The mean chlorophyll-a concentrations in the uppermost 15 m ranged from 1.1 m g/l in the eastern Kattegat (St. 413) to 2.0 m g/l in the northern Belt Sea (St. 432). The concentrations were lowest in the uppermost 5 m, increasing to 10 m depth, and the highest concentrations were observed at 15 m depth (Fig. 6).

 

Figure 6. Chlorophyll-a concentrations in 1 m, 5 m, 10 m and 15 m depths along the transects I, II and III (see figure 1).