Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 7-11 October 1996. Cruise no. 172.

Report: Gunni Ærtebjerg
Cruise leader: Kjeld Sauerberg
Participants: Peter Kofoed, Dorete Jensen , Susanne Hemmingsen (NERI).


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


Summary and conclusions

In September the dominating wind from north and east caused a large net outflow from the Baltic Sea. At the turn of the month and the first two weeks of October the wind was south-west causing inflow through the Danish straits.

The temperature was lower than long term mean and had decreased since medio September, except in the bottom water of the Sound and the Fehmarn Belt - Arkona Sea area. The thermocline was weak. The salinity was higher than long term mean and had increased since medio September, except in the Fehmarn Belt - Arkona Sea area. The halocline was relatively strong, though weakened since medio September. The bottom water salinity in the central Arkona Sea was as high as 19.3-19.5 psu from 37.5 m depth to the bottom, indicating inflow from the Sound and Belt Sea.

Generally phosphate and silicate were present in the surface water in all areas. In the western Kattegat, the Sound, Great Belt and at Gedser Rev also nitrate was present within the euphotic zone in 5-10 m depth. High concentrations of phosphate (>1 m mol/l), total-P (>2 m mol/l), ammonium (>3 m mol/l) and silicate (>30 m mol/l) were observed in the bottom water with the lowest oxygen concentrations; that is in Fehmarn Belt (St. 952) and in the western Arkona Sea (St. 449).

The mean chlorophyll-a concentrations in the uppermost 15 m ranged from 1.0 m g/l in the Sound to 4.2-4.6 m g/l in the central Arkona Sea (St. 444) and northern Kattegat (St. 403, 1007). A pronounced subsurface maximum was only observed in the Fehmarn Belt area in 10-15 m depth with concentrations up to 5 m g/l.

The minimum oxygen concentrations had increased since medio September, except in the Fehmarn Belt, where now only 0.85 ml/l (13%) was found. In the western Arkona Sea (St.449) 2.5 ml/l was observed (38%). In all other areas the minimum oxygen concentration was above 3.5 ml/l (55%). The spatial coverage, length and strength of oxygen depletion in the open Danish waters in 1996 have been limited compared to earlier years. This even the wind conditions have been unfavourable without gales/storms and with dominating directions from east and south, which limits the bottom water exchange, especially in the southern Belt Sea, where also the longest lasting oxygen depletion periods have been observed.

General

The route, time schedule and stations of the cruise are shown in figure 1. Besides the pelagic monitoring measurements, suspended matter was sampled at all stations in 1 m depth for calibration of satellite images.

Meteorology

In September the mean temperature was 1.2 ° C below long term mean 1961-1990, and the precipitation was 26% below normal. The wind came mainly from north in the first two weeks of the month, then from east in the next two weeks, and ended up in south-west at the turn of the month. The wind stayed in south-west the first two weeks of October, including during the cruise (Danish Meteorological Institute).

The water exchange in September was dominated by a net outflow from the Baltic Sea of about 120 km³ through the Danish straits. The outflow was interrupted for short periods by inflows 1-3 September, 11-12 September and 28-30 September (Royal Danish Administration of Navigation and Hydrography).

Hydrography

The surface temperature ranged from 11.2 ° C in the north-western Kattegat (St. 1008, 1009) to 13.2 ° C in the central Arkona Sea (St. 444), and had decreased 3-4.5 ° C since medio September. The bottom water temperature ranged from 7.3-7.6 ° C in the eastern Kattegat (St. 413, 905) to above 12 ° C in the Gedser Rev - Arkona Sea area (St. 954, 449, 444) (Fig. 2), and had generally decreased since medio September, except in the Sound and Fehmarn Belt - Arkona Sea area (St. 431, 952, 954, 449, 444). The thermocline was weak with a maximum difference between surface and bottom of 4.0-4.5 ° C in the eastern Kattegat (St. 413, 905, 1001).

The surface salinity ranged from 7.5 psu in the central Arkona Sea (St. 444) to 28.0-30.5 psu in the northern Kattegat, and had increased much since medio September. The bottom water salinity ranged from 19.5-20.4 psu in the Gedser Rev - Arkona Sea area (St. 954, 449, 444, 441) to 34.1-34.75 psu in the eastern and northern Kattegat (St. 921, 418, 413, 905, 1001, 1007, 1008) (Fig. 3 and 4), and had generally increased since medio September, except in the Fehmarn Belt and Arkona Sea. The bottom water salinity in the central Arkona Sea was as high as 19.3-19.5 psu from 37.5 m depth to the bottom, indicating inflow from the Sound and Belt Sea. The salinity stratification had weakened since medio September, but still differences between surface and bottom of 10-14 psu were found in the Sound, southern Kattegat, Great Belt and Arkona Sea.

Compared to long term monthly means (1931-60) for October the temperatures during this cruise were in the surface about the same, but in the bottom water lower than normal. The salinity's were generally higher than long term means, except in the Fehmarn belt surface water..

Nutrients

In the central Arkona Sea (St. 444), Fehmarn Belt (St. 952) and north-eastern Kattegat (St.905, 1001, 1007) no nitrate was present in the uppermost 20-30 m. In the southern Kattegat (St. 413, 418, 922) nitrate was found in 15 m depth. In the other areas nitrate was found within the euphotic zone in 5-10 m depth. In the bottom water nitrate concentrations above 5 m mol/l and up to 11 m mol/l were found only in the Sound and Kattegat (Fig. 5, 6 and 7). Nitrite was only found in the bottom water with a maximum of 0.6 m mol/l in Læsø Rende (St. 403) and 0.4 m mol/l in central Great Belt (St. 935, 939) (Fig. 8).

In the surface layer phosphate concentrations above 0.1 m mol/l were found, except in the Arkona Sea (St. 441, 444) and Southern Kattegat (St. 418, 922) (Fig. 9). The highest concentrations of phosphate (>1 m mol/l), total-P (>2 m mol/l), ammonium (>3 m mol/l) and silicate (>30 m mol/l) were observed in the bottom water with the lowest oxygen concentrations, that is in Fehmarn Belt (St. 952) and east of Falster (St. 449) in the western Arkona Sea (Fig. 10 and 11).

Chlorophyll-a

The mean chlorophyll-a concentrations in the uppermost 15 m ranged from 1.0 m g/l in the Sound to 4.2-4.6 m g/l in the central Arkona Sea (St. 444) and northern Kattegat (St. 403, 1007). In the southern Belt Sea (St. 443, 450, 952) and Aarhus Bigt (St. 427) mean values of 3.6-3.8 m g/l were found. A pronounced subsurface maximum was only observed in the Fehmarn Belt area in 10-15 m depth with concentrations up to 5 m g/l.

Oxygen

Since medio September the bottom water oxygen concentrations had generally increased, except in the Fehmarn Belt (St. 952) and south-eastern Kattegat (St. 921, 922). The lowest oxygen concentration of 0.85 ml/l (13% saturation) was measured in the Fehmarn Belt. In the western Arkona Sea (St. 449) the minimum oxygen concentration was 2.5 ml/l (38%). In the Sound and southern Kattegat (St. 431, 922) 3.5 ml/l (55%) and in the central Great Belt (St. 935, 939) 3.6 ml/l (57%) were observed. In all other areas the oxygen saturation was 57-91% (Fig. 12 and 13).

Compared to October last year and to mean for October 1980-89 the minimum oxygen concentrations this year are higher, except in the Fehmarn Belt - Gedser Rev area.

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 serious oxygen depletion was during the cruise observed in Fehmarn Belt, and oxygen depletion was present in the western Arkona Sea. In figure 14 is shown the stations, where either the Danish counties or NERI have observed oxygen depletion within the first three weeks of October 1996.

In the open Danish waters oxygen depletion have in 1996 been present in the Fehmarn Belt and western Arkona Sea for more than 2 month, at Gedser Rev in more than one month, in the central Arkona Sea in less than a month and in the Sound in about a week. The spatial coverage, length and strength of oxygen depletion in the open Danish waters in 1996 have been limited compared to earlier years. This even the meteorological conditions have been unfavourable without storms and with dominating wind directions from east and south, which limits the bottom water exchange, especially in the southern Belt Sea, where also the longest lasting oxygen depletion periods have been observed.