Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 18-22 March 1996. Cruise no. 167.

Report: Gunni Ærtebjerg

Cruise leader: Jan Damgaard

Participants: Susanne Hemmingsen, Dorete Jensen, Pia Davidsen (18-22/3). Anka Hansen (DTU), Charlotte Verdier (18-19/3). Jørgen Nørrevang, Thomas Andersen (19-22/3).


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


Summary and conclusions

After the cold winter with a mean air temperature December-February of -2.4°C (3°C below long term mean) the temperature of the surface water during the cruise was about zero degrees, and the whole water column was generally colder than normal. The dominating easterly winds caused outflow of brackish water from the Baltic Sea, and the salinity stratification was unusually strong for the time of the year.

The phytoplankton spring bloom was in progress. In the Kattegat the bloom had reduced the concentrations of nitrate, phosphate and silicate to very low levels in the surface, and subsurface chlorophyll maxima of 7-14 mg/l were found in 10-15 m depth. Also in the northern Great Belt the nitrate and silicate were used up in the surface, and subsurface chlorophyll maxima of up to 20 mg/l were found in 5-10 m depth. In the rest of the Belt Sea all nutrients were still present in the surface layer, and chlorophyll concentrations of 10-14 mg/l were homogenously distributed in the uppermost 10 m. In the Arkona Sea the spring bloom was just started with relatively high nutrient concentrations and chlorophyll concentrations of 3-4 mg/l.

The minimum oxygen concentrations were relatively high, except in the Sound and southern Kattegat, where concentrations of 4.8-5.2 ml/l (68-75% saturation) were a little lower than long term mean. In the same area the strongest stratification of the water column was found, and in the bottom water the highest concentrations of nitrate, phosphate and silicate were observed. This indicate rather stagnant bottom water in the Sound and southern Kattegat.

General

The route, time schedule and stations of the cruise are shown in . Besides the pelagic monitoring measurements, sediments for chlorophyll determination (St. 413) and bottom fauna spatial distribution were sampled (St. 409 and 413), and suspended matter was measured at all stations in 1 m depth for calibration of satellite images.

Meteorology

The monthly mean temperature in February was in Denmark about 3°C below normal (mean 1961-1990), and the precipitation (mostly snow) was about 85% of normal. In the first half of the month the wind came mostly from east and in the second half strong wind from west and north dominated (Danish Meteorological Institute). In the beginning of March the wind returned to east and stayed there also during the cruise.

Hydrography

The surface temperature was -0.4 to +1.0°C and had generally increased since the cruise in mid February, in the northern Kattegat with up to 1.5°C. The bottom water temperature ranged from below zero in shallow areas of the Arkona Sea to above 6°C in the Sound and southern Kattegat (, and had generally decreased since mid February. The temperature difference between surface and bottom ranged from less than 0.5°C in the Arkona Sea to more than 6°C in the Sound and southern Kattegat.

The surface salinity ranged from 7.8-8.2 psu in the Arkona Sea and the Sound to 21.4-24.5 psu in the northern Kattegat, indicating prevailing outflow from the Baltic Sea, and had generally decreased since mid February. The bottom water salinity ranged from 18 psu in the Arkona Sea to 34.7 psu in the northern Kattegat. Above 34 psu was found also in the southern Kattegat (.The bottom water salinity had since mid February decreased in the Sound, southern and eastern Kattegat and increased in the other areas. Except for shallow areas, the salinity stratification was strong with a difference between surface and bottom of more than 10 psu, in the Sound more than 20 psu.

Compared to long term monthly means (1931-1960) for March the temperatures during this cruise were lower than normal, except in the Kattegat bottom water. Generally, the salinity stratification was stronger with lower surface water salinities and higher bottom water salinities than long term means.

Oxygen

The lowest oxygen concentrations of 4.8-5.2 ml/l (68-75% saturation) were found in the Sound (St. 431, 921 ) and southern Kattegat (St. 413, 418, 922, 925) (. Compared to March last year the minimum oxygen concentrations this year are generally lower, except in the southern Belt Sea and Arkona Sea. Compared to monthly mean for March 1980-1989 the concentrations this year are generally higher, except in the Sound and southern Kattegat.

Nutrients

The phytoplankton spring bloom had removed the nitrate from the surface layer, except in the Sound, southern Belt Sea and Arkona Sea (,and . In the surface of Kattegat and northern Great Belt also the silicate concentrations and in the Kattegat the phosphate concentrations were low ( and.

The highest concentrations of both nitrate, phosphate and silicate were found in the bottom water in the southern Kattegat. The highest concentrations and nitrite and ammonium were found in the surface and bottom water of the southern Belt Sea, respectively ( and).

Chlorophyll-a

The mean chlorophyll-a concentrations in the uppermost 15 m ranged from 3.2-5.9 µg/l in the northern Kattegat to 10.3-12.8 mg/l in the Belt Sea. In the Kattegat a subsurface maxima of 7-14 µg/l were observed in 10 to 15 m depth.