Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 1-5 May 1995. Cruise no. 157.


Cruise leader: Jan Damgaard

Participants: Dorete Jensen, Hanne Ferdinand, Peter Kofoed


General

The route, time schedule and stations of the cruise are shown in figure 1. Besides the pelagic monitoring measurements, sediment for chlorophyll determination was sampled at station 413.

The monthly mean temperatures in March and April 1995 were in Denmark 0.8 and 0.4 °C, respectively, above mean for the period 1961-90, and the precipitation was in March 32% above and in April about normal. The wind was in March relatively strong, in April more calm, mostly from southwest and west (Danish Meteorological Institute).

Hydrography

The surface temperature was 7.1-8.9 °C, and had generally increased 4-5 oC since the cruise mid March. The bottom water temperature ranged from 3.7 °C in the Arkona Sea and 4.5 °C in the Sound and southeastern Kattegat to more than 6 °C in the northeastern Kattegat (Fig. 2), and had generally increased, in the southern Belt Sea with more than 2 °C. The temperature stratification was weak with the surface water 2-4 °C warmer than the bottom water.

The surface salinity ranged from 8.05 psu in the Arkona Sea to only 21.3 psu in the northwestern Kattegat (Fig. 3, 4 and 5). The surface salinity had decreased significantly since mid March, especially in the Kattegat. The bottom water salinity ranged from 8.5-17.1 psu in the Arkona Sea to 35.1 psu in the northeastern Kattegat. The bottom water salinity had increased since mid March, except in the Sound and southeastern Kattegat. The salinity stratification was strong with a difference between surface and bottom of 18-21 psu in the Sound, eastern Kattegat and the central Great Belt, and else more than 9 psu, except at shallow depths in the Arkona Sea and Aalborg Bight.

Compared to long term monthly means (1931-60) for May both the surface and bottom water temperatures and salinities during this cruise were generally lower.

Oxygen

The minimum oxygen concentrations had generally decreased 1.5-2.5 ml/l since mid March, but less in the northern Kattegat and Arkona Sea. The lowest concentration was observed in the southwestern Kattegat (St. 925) with 4.1 ml/l (55% sat.). In the Sound (Fig. 6), central Great Belt (Fig. 7) and Aarhus Bight (Fig. 8) 4.2-4.5 ml/l (56-60% sat.) were found.

Compared to mean for May in the 1980's the minimum oxygen concentrations this year were generally the same or a little higher. Compared to the same time last year the minimum concentrations this year are generally lower, except in the southern Belt Sea.

Nutrients

In the surface water nitrate concentrations above 0.5 µmol/l were found only in the western Kattegat (Fig. 9). Generally the nitrate-cline (0.5 µmol/l isoline) was situated between 10 and 15 m depth in the Sound (Fig. 10) and Kattegat, and between 15 and 20 m depth in the Belt Sea, but as deep as 25 m at station 925 in the southwestern Kattegat and even deeper in the Arkona Sea (Fig. 11).

In the bottom water high nitrate concentrations (8-16 µmol/l) were found in the Sound and Kattegat at salinities of 28.9-29.8 psu in the Sound and 30.3-33.0 psu in the Kattegat with maximum concentrations at 31.9-32.5 psu (Fig. 12). A minimum in nitrate concentrations (below 4 µmol/l) was found in the interval 33-34 psu (former Skagerrak surface water), increasing to about 8 µmol/l at 35.1 psu (Skagerrak deep water).

The high nitrate concentrations in the bottom water in the Sound and Kattegat originates partly from the inflow in March from the Jutland Coastal Current, maybe reinforced by mineralisation of the spring bloom.

In the surface water phosphate (above 0.05 µmol/l) was only found in the Sound, Arkona Sea and Gedser Rev area. The phosphate cline shows a tendency to be situated deeper than the nitrate cline, especially in the northeastern Kattegat (Fig. 13).

Relatively high nitrite concentrations were only found in the Kattegat bottom water (Fig. 14). Silicate showed in the Kattegat a minimum in 10-20 m depth (Fig. 15).

Compared to monthly mean for May 1980-89 the nitrate concentrations in the bottom water were in the Sound and Kattegat generally higher and in the Belt Sea lower this year. The phosphate and silicate concentrations show a tendency to generally lower concentrations than in the 1980's.

Chlorophyll-a

The mean chlorophyll-a concentrations in the uppermost 15 m were low ranging from 0.4 µg/l at station 449 east of Gedser Rev to 1.6 µg/l in Aalborg Bight (St. 409). A general tendency to subsurface maximum was observed with up to 3 µg/l at 13.5-15 m depth in Aalborg Bight and the southern Belt Sea.

Summary and conclusion

The hydrography showed strong outflow from the Baltic Sea and a beginning inflow of highly saline bottom water from the Skagerrak, creating a strong salinity stratification.

Phosphate and nitrogen nutrients were generally abscent from the surface layer, except for nitrate in Aalborg Bight and phosphate in the Sound and Arkona Sea. High nitrate concentrations were still found in the bottom water in the Sound and southern and eastern Kattegat, partly originating from the inflow in March of water from the Jutland Coastal Current. In the Belt Sea the bottom water nitrate concentrations had decreased much since mid March, indicating relatively strong vertical mixing.

The strong stratification and low chlorophyll-a concentrations with tendency to subsurface maxima witness relatively calm weather conditions prior to the cruise.

The minimum oxygen concentrations had decreased much after the spring bloom and were generally lower than at the same time last year, except in the southern Belt Sea, but about or a little higher than the mean for May in the 1980's.

Gunni Ćrtebjerg


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