Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 15-19 September 1997. Cruise no. 180.

 

 

Report: Gunni Ærtebjerg

Cruise leader: Kjeld Sauerberg

Participants: Susanne Hemmingsen, Hanne Ferdinand, Peter Kofoed (NERI).

Hanne Grøn Jensen (trainee).

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

Summary and conclusions

The prevailing gales from west prior to and during the cruise had mixed and exchanged water masses. The surface temperature had decreased and the surface salinity increased significantly since mid August, and vice versa in the bottom water. However, in the Arkona Sea the bottom water salinity had increased to 20-21 showing inflow from the Sound and Belt Sea to the Baltic Sea. In the northern Kattegat and southern-most Great Belt the water column was mixed from top to bottom. In the other areas the stratification was rather weak, except in the Sound and Arkona Sea. In the eastern Kattegat the halocline depth increased from about 20 m in the south to about 40 m in the north. Compared to long term monthly means (1931-1960) for September both the surface and bottom water temperature during this cruise were generally higher or normal. The surface salinity was higher, and the bottom water salinity was generally lower than long-term means, except for higher bottom water salinity in the Great Belt and Arkona Sea.

Generally at least traces of the nutrients nitrate, phosphate and silicate were now found at the surface in all areas, except for no nitrate in the southern Kattegat, the Sound and northern Belt Sea. In the bottom water nutrient concentrations were relatively low in the western and southern Kattegat and the Belt Sea.

In spite of the strong wind the minimum oxygen concentrations had decreased since mid August in the eastern and southern Kattegat, the Sound, northern Belt Sea and east of Gedser Rev. The lowest oxygen concentration of 1.7 ml/l (25%) was found at the bottom east of Gedser Rev together with high concentrations of nutrients. In the northeastern Kattegat unusually low concentrations of 3.0-3.5 ml/l (47-57%) were observed. The bottom water here was very turbid, but with low fluorescence, indicating that the turbidity was not due to newly settled fytoplankton blooms. In the Sound the minimum concentration was 3.4 ml/l (52%). In the northern Belt Sea the minimum concentrations were 3.9-4.2 ml/l (63-72%). In all other deeper areas the minimum oxygen concentrations were above 4.4 ml/l (71%). Compared to September last year and mean for September in the 1980s the minimum oxygen concentrations this year are higher, except in the north-eastern Kattegat and east of Gedser Rev, where it was lower than normal.

The mean chlorophyll-a concentration in the uppermost 15 m was low (0.5-2.4 m g/l) in the shallow western part of the Arkona Sea. In the Belt Sea, Kattegat and Sound the mean chlorophyll concentration was relatively high to very high (2.4-14.3 m g/l), with the lowest concentrations in the northeastern Kattegat and the highest in the western Kattegat (Aalborg Bight). Pronounced subsurface maxima (6-22 m g/l) were observed in the southern Kattegat and northern Belt Sea. In the southeastern and western Kattegat high concentrations were also observed at the surface. During the cruise Gyrodinium aureolum was observed in all areas, except the Arkona Sea and northeastern Kattegat. The species was especially numerous in subsurface chlorophyll maxima and especially in the northern Belt Sea, but occurring more moderately in the southern Belt Sea, the Sound and southern Kattegat. Diatoms were frequent in all areas, especially in the western Kattegat and Arkona Sea. Also Ceratium species were frequent at most stations. Protoperidinium species were frequent in the southern Kattegat, the Sound and the Belt Sea, and Prorocentrum micans was frequent in the Sound and Belt Sea.

General

The scope of the cruise was to monitor the hydrographic situation and the spatial variation in plankton, primary production, oxygen and nutrients, with special emphasis on mapping the geographical distribution of minimum oxygen concentrations. The three transects of monitoring stations used in the following figures are shown in figure 1.

Meteorology

The monthly mean temperature in Denmark was in August 20.3° C, which is 4.5° C above long term mean 1961-1990, and august 1996 was the warmest month ever recorded in Denmark. The precipitation was 36% below normal. Weak wind mostly from east and Southeast prevailed all month, and the first days of September. From September 8th and during the cruise gales from west prevailed (Danish Meteorological Institute).

Hydrography

The surface temperature (1 m depth) had decreased 7-8° C since the cruise in mid August, and ranged from 14.7° C in the southern Belt Sea (St. 450, 954) to 16.1-16.3° C in the northern Kattegat (St. 403, 905, 1001, 1008, 1009) and 16.6-16.9° C in the Arkona Sea (St. 441, 444).

 

 

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 transect I (see figure 1).

The bottom water temperature had increased 2-8° C and ranged from 9.0-9.2° C east of Anholt and in the Sound (St. 413, 431) to above 16° C in the northwestern Kattegat (Fig. 2). The temperature difference between surface and bottom was generally low, only in the northeastern and southern Kattegat, The Sound, Great Belt and east of Gedser Rev was the difference above 2° C.

The surface salinity had increased much since mid August and ranged from 8.3-8.4 in the Arkona Sea (St. 441, 444) to 31.3-31.6 in the northern Kattegat. The bottom water salinity ranged from 20.4-21.1 in the Arkona Sea (St. 441, 444, 449) to 33.3-33.4 in the north-eastern Kattegat (St. 413, 905, 1001, 1007), and had generally decreased, except in the Arkona Sea, where inflow of saline water from the Sound and Belt Sea was evident (Fig. 2 and 3). The salinity stratification was weak, except in the Sound and Arkona Sea, which had differences between surface and bottom of 10.5-14.8. In the Great Belt the difference was 6.5-9.5. In the northwestern Kattegat (St. 403, 1007, 1008, 1009) and southernmost Great Belt (St. 450) the water column was nearly homogenous from top to bottom. In the eastern Kattegat the pycnocline depth increased from about 20 m in the south to about 40 m in the north.

 

Figure 3. Salinity in 1 m, 5 m, 10 m, 15 m, 20 m depth and near bottom along the transects II and III (see figure 1).

Compared to long term monthly means (1931-1960) for September both the surface and bottom water temperature during this cruise was generally higher or normal. The surface salinity was higher, and the bottom water salinity was generally lower than long-term means, except for higher bottom water salinity in the Great Belt and Arkona Sea.

 

 

Figure 4. Surface and bottom near concentrations of nitrate along the transects I, II and III.

Nutrients

Traces of nitrate were found in the surface layer in the northern Kattegat, Great Belt, Fehmarn Belt and south-western Arkona Sea, but not in the rest of Kattegat, the Sound and northern Belt Sea. Close to the bottom the nitrate concentrations were low (< 2 m M), except in the north-eastern Kattegat (St. 1001, 905, 413), the Sound (St. 921, 431) and east of Gedser Rev (St. 449) (Fig. 4). The nitrite concentrations at the surface were generally low, except at the well mixed stations in the northern Kattegat. In the bottom water 0.2-0.7 m M were found, except for lower concentrations in the Fehmarn Belt (Fig. 5). In the surface layer the ammonium concentrations were generally below 0.5 m M, except in the northern Kattegat. In the bottom water ammonium concentrations of 1-3 m M were generally observed (Fig. 5).

 

Figure 5. Surface and bottom near concentrations of nitrite and ammonium along transect I.

Phosphate was found in the surface layer at most stations. In the bottom water relatively high phosphate concentrations were observed in the northeastern Kattegat and the Sound (Fig. 6). Silicate was observed in the surface water at all stations. In the bottom water above 15 m M were observed in the northeastern Kattegat and the Sound. In the Arkona Sea high concentrations of silicate (~ 24 m M) were found in intermediate layers and east of Gedser Rev (St. 449) at the bottom (Fig. 6). This indicates that these water layers originate from previous stagnating oxygen poor bottom water.

 

Figure 6. Surface and bottom near concentrations of phosphate and silicate along transect I.

 

Oxygen

Since the cruise in August the minimum oxygen concentration had decreased in the eastern and southern Kattegat, the Sound, the northern Belt Sea and east of Gedser Rev (St. 449). In the northern Kattegat the minimum concentration had increased 1.6-2.1 ml/l, and in the Fehmarn Belt 1.4-4.2 ml/l. In these areas the oxygen concentration now was close to saturation (93-100%) in the whole water column. Also in the central Arkona Sea (St. 444) the minimum oxygen concentration had increased 3.1 ml/l to 5.2 ml/l (84%).

The lowest oxygen concentration of 1.7 ml/l (25%) was found east of Gedser Rev (St. 449). In the Sound (St. 431) the minimum concentration was 3.4 ml/l (52%). In the north-eastern Kattegat (St. 1001, 905, 413) unusually low concentrations of 3.0-3.5 ml/l (47-57%) were observed. The bottom water here was very turbid, but with low fluorescence, indicating that the turbidity was not due to newly settled fytoplankton. In the northern Belt Sea (St. 427, 432, 935, 939) the minimum concentrations were 3.9-4.2 ml/l (63-72%). In all other deeper areas the minimum oxygen concentrations were above 4.4 ml/l (71%) (Fig. 7).

 

Figure 7. Minimum oxygen concentrations along the transects I, II and III. (see figure 1).

Compared to September last year and mean for September in the 1980s the minimum oxygen concentrations this year are higher, except in the north-eastern Kattegat and east of Gedser Rev, where it was lower than normal.

Oxygen depletion is in Denmark 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 was during the cruise only observed east of Gedser Rev (St. 449). In figure 8 is shown the stations visited by either the Danish counties or NERI between the 8th and 22nd of September, and where oxygen depletion or serious oxygen depletion was observed.

 

 

 

Figure 8. Stations visited by Danish counties or NERI between the 8th and 22nd of September 1997, and where oxygen depletion (>4 mg/l) and serious oxygen depletion (>2 mg/l) was observed.

Chlorophyll-a

The mean chlorophyll-a concentration in the uppermost 15 m was low (0.5-2.4 m g/l) in the shallow western part of the Arkona Sea (St. 441, 449, 954, 1728). In the Belt Sea, Kattegat

 

 

Figure 9. Chlorophyll-a concentrations in 1 m, 5 m, 10 m and 15 m depths along the transects I, II and III (see figure 1).

and Sound the mean chlorophyll concentration was relatively high to very high (2.4-14.3 m g/l), with the lowest concentrations in the north-eastern Kattegat and the highest in the western Kattegat (Aalborg Bight) (St. 409, 415). Pronounced subsurface maxima (6-22 m g/l) were observed in the southern Kattegat and northern Belt Sea. In the south-eastern and western Kattegat high concentrations were also observed at the surface (Fig. 9).

During the cruise Gyrodinium aureolum was observed in all areas, except the Arkona Sea and north-eastern Kattegat. The species was especially numerous in subsurface chlorophyll maxima and especially in the northern Belt Sea (St. 432, 427), but occurring more moderately in the southern Belt Sea, the Sound and southern Kattegat. Diatoms were frequent in all areas, especially in the western Kattegat and Arkona Sea. Also Ceratium species were frequent at most stations. Protoperidinium species were frequent in the southern Kattegat, the Sound and the Belt Sea, and Prorocentrum micans was frequent in the Sound and Belt Sea.