Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 6-10 October 1997. Cruise no. 181.

 

 

Report: Gunni Ærtebjerg
Cruise leader: Susanne Hemmingsen
Participants: Dorete Jensen, Hanne Ferdinand, Peter Kofoed (NERI). 6-7/10: Ole Lund Jensen. 7-10/10: Jørgen Nørrevang, Thomas Andersen (NERI). Vanessa (trainee).

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

Summary and conclusions

The windy periods in mid September and beginning of October had mixed and exchanged the water masses. The bottom water salinity had increased in most Kattegat due to inflow of high saline water (34.2-34.4 psu) from the Skagerrak. In the Arkona Sea inflow to the Baltic Proper from the Sound and Belt Sea was evident with bottom water salinities of 19-23 psu. The stratification of the water column was generally weak, except in the Arkona Sea and eastern Kattegat. Compared to long term monthly means (1931-1960) for October both the surface and bottom water temperatures during this cruise were 1-2°C higher than normal. The surface salinity was higher, and the bottom water salinity generally lower than long term means, except for higher bottom water salinity in the northern and western Kattegat and the Arkona Sea.

 

In the surface layer nitrate was found in the Sound and most of the Kattegat, and phosphate and silicate were present in all areas. In the bottom water the nutrient concentrations generally were low, except in the eastern Kattegat and Arkona Sea. High concentrations of all nutrients were found in a thin stagnating bottom layer with low oxygen concentration (2.5 ml/l; 40%) at a station in the Great Belt.

 

The minimum oxygen concentrations in the north-eastern Kattegat were relatively low, and the lowest concentration of 1.8 ml/l (29%) was observed east of Anholt. In the Sound the minimum oxygen concentration was 3.4-3.5 ml/l (57-58%). At the other deeper stations the concentrations were 4.0-5.9 ml/l (65-92%). Compared to October last year and mean for October in the 1980s the minimum oxygen concentrations this year are higher, except in the eastern Kattegat and at a station in the Great Belt.

 

High chlorophyll concentrations (8.7-18.6 m g/l) in the western Kattegat was in the Aalborg Bight due to a heavy diatom bloom, but in the north-western part due to a heavy bloom of Gyrodinium aureolum, which only was observed in the northern-most Kattegat. Otherwise, diatoms, Ceratium sp. and Protoperidinium sp. were frequent in the phytoplankton.

A heavy bloom of Gyrodinium aureolum occurred in September in the northern Belt Sea and western Kattegat. According to information from the local counties the bloom collapsed and fall to the bottom at the beginning of October, and effects as dead fishes and bottom fauna and decreasing oxygen concentrations were reported. During the cruise effects of the bloom were investigated at a transect north of Funen and at another transect in the Aalborg Bight, western Kattegat. Preliminary results indicate at both transects that the bottom fauna abundance was low in areas with high chlorophyll concentrations in the near bottom water, independent on the water depth. At the transect north of Funen many Arctica islandica were lying on the bottom, 10-20% of which were dead, and dead polychaetes were observed. Oxygen depletion seems not the cause to the dead animals.

Figure 1. Monitoring cruise with r/v Gunnar Thorson.
The three lines I, II and III show the transects used in the following figures:
Transect I: Kattegat NE - E - S - Great Belt - Fehmarn Belt - Arkona Sea
Transect II: Kattegat SE - The Sound - Arkona Sea
Transect III: Kattegat W - Aarhus Bight - Great Belt

General

The scope of the cruise was to monitor the hydrographic situation and the spatial variation in plankton, primary production, oxygen and nutrients. Special investigations of the effects of the Gyrodinium aureolum bloom on the bottom fauna in the northern Belt Sea and western Kattegat were performed. 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 September 0.5° C above long term mean 1961-1990, and the precipitation was 41% below normal. Weak wind prevailed in the first and the last week of the month, while gales from west and north-west dominated the two weeks in between, and in the beginning of October (Danish Meteorological Institute).

 

Hydrography

The surface temperature (1 m depth) had decreased 1-3° C since the cruise in mid September, and ranged from 13° C in the western Kattegat (St. 409, 1009) to 14° C in the north-eastern Kattegat and Arkona Sea (St. 1001, 441).

 

 

Figure 2. Surface (1 m) and near bottom 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).

 

Also the bottom water temperature had generally decreased 1-3° C, except in the south-eastern Kattegat, the Sound and Great Belt, and ranged from 10,6° C in the north-eastern Kattegat (St. 905) to 14.6° C in the south-eastern Kattegat (St. 418, 922) and 15.0° C in the Arkona Sea (St. 444) (Fig. 2). The temperature difference between surface and bottom was generally below 1° C, except for a difference of 2.4-3.3° C in the eastern Kattegat (St. 413, 905, 1001).

 

The surface salinity had decreased since mid September, except in the Sound and Great Belt, and ranged from 8.0 in the Arkona Sea (St. 444) to 31.2 in the north-western Kattegat (St. 403, 1007, 1008, 1009). The bottom water salinity ranged from 17.2 at Gedser Rev (St. 954) and 19.0-23.0 in the Arkona Sea (St. 441, 444, 449) to 34.2-34.4 in the north-eastern Kattegat (St. 905, 1001), and had generally decreased, except in most of the Kattegat due to inflow of saline water from the Skagerrak. In the Arkona Sea inflow of saline water from the Sound and Belt Sea was evident (Fig. 2 and 3). The salinity stratification was weak, except in the Arkona Sea which had differences between surface and bottom of 10.2-13.8. Generally the salinity gradually increased with depth without a pronounced halocline, except in the Arkona Sea.

 

 

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 October both the surface and bottom water temperatures during this cruise were 1-2°C higher than normal. The surface salinity was higher, and the bottom water salinity generally lower than long term means, except for higher bottom water salinity in the northern and western Kattegat and the Arkona Sea.

 

 

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

Nutrients

Traces of nitrate were found in the surface layer in the Sound and most of the Kattegat. Close to the bottom the nitrate concentrations were low (< 1 m M), except in the eastern Kattegat (St. 1001, 905, 413), the Sound (St. 921, 431), the Arkona Sea (St. 449, 444) and at station 935 in the Great Belt (Fig. 4). The nitrite concentrations at the surface were generally low (< 0.1 m M), except in the northern Kattegat. In the bottom water 0.1-0.7 m M were found, except for lower concentrations in the Great Belt (Fig. 5). In the surface layer the ammonium concentrations were generally below 0.5 m M, except in the Sound, Great Belt and south-western Kattegat. In the bottom water ammonium concentrations of 1-3 m M were generally observed, except for higher concentrations in the Sound and northern Belt Sea (Fig. 5).

 

 

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

 

Phosphate and silicate were found in the surface layer in all areas, with the highest phosphate concentrations in the Sound and Belt Sea and silicate concentrations in the Arkona Sea. In the bottom water relatively high phosphate and silicate concentrations (> 1 m M) were observed in the north-eastern Kattegat, the Arkona Sea and at station 935 in the Great Belt (Fig. 6).

 

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

Oxygen

Since the cruise in mid September the minimum oxygen concentrations had increased 0.2-1.4 ml/l in the Sound, south-western Kattegat and most of the Belt Sea, but decreased 0.2-1.2 ml/l in the rest of the Kattegat and the central Arkona Sea, and 1.4 ml/l at station 935 in the Great Belt.

The lowest oxygen concentration of 1.8 ml/l (29%) was found in the Kattegat east of Anholt (St. 413). At station 935 in the Great Belt the minimum concentration was 2.5 ml/l (40%) in a 4 m thin bottom layer. In the north-eastern Kattegat (St. 905) 3.2 ml/l (51%) was observed. In the Sound (St. 431, 921) the minimum oxygen concentration was 3.4-3.5 ml/l (57-58%) At all other deeper stations the minimum oxygen concentrations were 4.0-5.9 ml/l (65-92%) (Fig. 7).

 

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

Compared to October last year and mean for October in the 1980s the minimum oxygen concentrations this year are higher, except in the eastern Kattegat and at station 935 in the Great Belt, where it was 0.8-2.6 ml/l lower than normal. In the central Arkona Sea the minimum oxygen concentration was lower than last year but higher than in the 1980s.

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 observed in the Kattegat east of Anholt (St. 413) and in the Great Belt at station 935.

 

 

Figure 8. Chlorophyll-a concentrations in 1 m, 5 m, 10 m and 15 m depths along the transects I, II and III (see figure 1). Note the different scales.

Chlorophyll-a and phytoplankton

The mean chlorophyll-a concentration in the uppermost 15 m was 1.3-3.8 m g/l, except in the western Kattegat (St. 403, 409, 1009), where mean concentrations of 8.5-11.5 m g/l were observed. The chlorophyll was rather homogeneously distributed in the uppermost 15 m, except for lower concentrations at 15 m depth in the north-western Kattegat (St. 403, 1009) and the Fehmarn Belt - Gedser Rev area (St. 450, 952, 954, 449) (Fig. 8).

 

The high chlorophyll concentration in the western Kattegat was in the Aalborg Bight (St. 403, 409) due to a heavy diatom bloom, but in the north-western part (St. 1009) due to a heavy bloom of Gyrodinium aureolum, which in the surface layer only was observed in the northern-most Kattegat. Otherwise, diatoms, Ceratium sp. and Protoperidinium sp. were frequent in the phytoplankton. A few Prorocentrum micans were observed in the eastern Kattegat, and a few Dinophysis sp. in the south-eastern Kattegat.

Effects of Gyrodinium aureolum

A heavy bloom of Gyrodinium aureolum occurred in September in the northern Belt Sea and western Kattegat. According to information from the local counties the bloom collapsed and fall to the bottom at the beginning of October and effects as dead fishes and bottom fauna and decreasing oxygen concentrations were reported. During the cruise an east-west transect north of the island of Funen and another transect in the Aalborg Bight going south-west from station 409 were investigated. At 6 stations at each transect bottom fauna samples were taken together with phytoplankton, chlorophyll and oxygen samples from the near bottom water. At some stations a trawl was used.

 

At the transect north of Funen the near bottom oxygen concentrations were 2.5-2.7 ml/l (40-44%) at the three deepest stations (17-21 m) and 3.1-5.2 ml/l (48-83%) at the other stations. The chlorophyll concentrations ranged from 2.2 m g/l to 7.0 m g/l. At the Aalborg Bight transect the near bottom oxygen concentrations were 5.3-5.5 ml/l (87-90%), and the chlorophyll concentrations ranged from 5.8 m g/l to 34.7 m g/l.

 

Preliminary results indicate at both transects that the bottom fauna abundance was low in areas with high chlorophyll concentrations in the near bottom water, independent on the water depth. At the transect north of Funen many Arctica islandica were lying on the bottom, 10-20% of which were dead. Also dead polychaetes were seen on video films. Arctica islandica is extremely tolerant to oxygen depletion, and the presence of living crustaceans and blue mussels, which are much more sensitive to low oxygen concentrations, shows that oxygen depletion is probably not the cause to the dead animals. The final results of the investigation will be published later.