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