Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 10-14 June 1996. Cruise no. 169.

Report: Gunni Ærtebjerg


Cruise leader: Gunni Ærtebjerg (10-12/7), Jan Damgaard (12-14/7)

Participants: Susanne Hemmingsen, Peter Kofoed, Dorete Jensen (NERI); Christina Sørensen (trainee).

County of Vejle: Torben Vang, Erik Pedersen (10-11/7), Else Marie Pedersen, Mona Nielsen (12-14/7).

Journalist Ulla Lund (Det Fri Aktuelt) and photographer (10/7)

Nine Advisers of the Plant Growing Society, Danish Agriculture, (12/7)


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

Summary and conclusions

A very strong stratification was present in the Kattegat, Sound and Belt Sea with higher temperatures and lower salinities than long term means in the surface and vice versa in the bottom water, showing prevailing outflow from the Baltic Sea in the surface and a preceding inflow of highly saline bottom water from the Skagerrak to the eastern and southern Kattegat.

Except for the northwestern Kattegat and Arkona Sea, nutrients (nitrate, phosphate, silicate) were generally present at least in the lower part of the euphotic zone. Silicate was low only in the northeastern Kattegat and showed a subsurface minimum in most Kattegat in 10-20 m depth.

The mean chlorophyll concentrations in the upper 15 m were relatively low with 1-2 µg/l, generally with a subsurface maximum in 10-15 m depth of 2-4 µg/l.

The minimum oxygen concentrations in the bottom water were relatively high compared to June last year and mean for the 1980s, indicating that the spring bloom and oxygen demand this year has been relatively low. This was to be expected due to the low nutrient load conditional on the exceptional low precipitation and run off this winter and spring.

General

The route, time schedule and stations of the cruise are shown in figure 1. Besides the pelagic monitoring measurements, bottom fauna were sampled at the stations 952 and 444, and suspended matter was measured at all stations in 1 m dept for calibration of satellite images.

Meteorology

The extreme low precipitation during winter (Dec.-Feb. only 32% of normal 1961-1990) continued during March and April (22.5% of normal), but in May the precipitation was 23% above normal. The monthly mean temperatures in March and May were 2.2 °C and 2.1 °C below normal, respectively, while April was 0.8 °C warmer than normal. In March the wind came mainly from east and in May from west, while in April unusually weak winds came from different directions. The first week of June had wind mainly from south, and during the cruise rather strong wind came from west (Danish Meteorological Institute).

Hydrography

The surface temperature ranged from 9.4°C in the Arkona Sea to above 16°C in the southern Kattegat (St. 413, 415, 922). The bottom water temperature ranged from 5.3-5.6°C in the eastern Kattegat (more than one degree lower than in March) to above 7°C in the Fehmarn Belt and Arkona Sea (Fig. 2). The temperature difference between surface and bottom was highest in the Kattegat, Sound and Great Belt (7.3-11.0°C) and lowest in the Arkona Sea (1.8°C).

The surface salinity ranged from only 7.1 psu in the Arkona Sea to 22.2 psu in the northern Kattegat indicating prevailing outflow from the Baltic Sea (Fig. 3). The bottom water salinity ranged from 10-17 psu in the Arkona Sea to 34.3 psu in the eastern and southern Kattegat. Above 34 psu was found in all southern Kattegat, and above 33 psu even in the central Great Belt (St. 939) showing an earlier inflow of highly saline water from the Skagerrak. The bottom water salinity in the northern Kattegat was now below 34 psu. Except for the Arkona Sea, the salinity stratification was strong with a difference between surface and bottom of more than 10 psu even at the shallow stations in the western Kattegat, and up to 25 psu in the Sound.

Compared to long term monthly means (1931-60) for June the temperatures during this cruise were higher than normal in the surface and lower than normal in the bottom water, except for higher bottom water temperatures in the southwestern Kattegat (St. 925). Generally, the surface water salinities were lower and the bottom water salinities were higher than long term means, except for higher surface salinity in Ålborg Bight (St. 409) and lower bottom water salinity at Gedser Rev (St. 954).

Oxygen

The lowest oxygen concentrations of 4.7-5.0 ml/l (67-70% saturation) were found in the Sound (St. 431, 921 ), Fehmarn Belt (St. 952) and in the central Arkona Sea (St. 444) (Fig. 4). In the shallow (13-15 m deep) western Kattegat (St. 409, 415) supersaturation (106%, 7.3 ml/l) was observed even close to the bottom.

Compared to June last year and to mean for June 1980-89 the minimum oxygen concentrations this year are higher.

Nutrients

In the northern (St. 1007, 1008, 1009, 403) and western Kattegat (St. 409, 415) and the Arkona Sea (St. 441, 444, 449, 954) nitrate was practically absent from the whole water column. In the other areas nitrate was generally found already in 5 to 15 m depth. In the bottom water nitrate above 7 µmol/l was observed in the Sound, southern Kattegat and Great Belt at salinities above 32 psu (Fig. 5, 6, 7).

Ammonium concentrations above 1 µmol/l were found in the bottom water in the northern Kattegat, Fehmarn Belt and Arkona Sea, while the highest concentrations of nitrite were observed in the bottom water in the Sound and Great Belt (Fig. 8, 9).

In the surface water also phosphate was very low in the Kattegat, Fehmarn Belt and Arkona Sea, and the highest concentrations (>1 µmol/l) were found in the bottom water in the Great Belt (Fig. 10). Silicate concentrations were low in the northeastern Kattegat and showed a subsurface minimum within 10 to 20 m depth in most Kattegat. High silicate concentrations (>10 µmol/l) were found in the bottom water of the Sound and southern Belt Sea (Fig. 11).

Chlorophyll-a

The mean chlorophyll-a concentrations in the uppermost 15 m were between 1 and 2 µg/l. Generally a subsurface maximum of 2-4 µg/l was observed in 10 to 15 m depth, except in the Fehmarn Belt (St. 450, 952) and central Arkona Sea (St. 444).