Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 13-17 January 1997. Cruise no. 174.

 

 

Report: Gunni Ærtebjerg

Cruise leader: Jan Damgaard

Participants: Peter Kofoed, Dorete Jensen , Kjeld Sauerberg (NERI). Signe Sandbech Clausen (trainee).


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


Summary and conclusions

The cold, calm and sunny weather during December 1996 and beginning of January 1997 had triggered a very unusual phytoplankton bloom in the Kattegat with mean chlorophyll-a concentrations in the uppermost 10 m ranging from 2.8 T g/l in the western Kattegat to >10 T g/l in the southern part.

The phytoplankton bloom had lowered the concentrations of nitrate, phosphate and silicate in the Kattegat surface layer. Therefore the measurements of inorganic nutrients from the Kattegat surface layer from this cruise can not be used for trend analyses of the development of winter nutrient concentrations.

In the north-eastern Kattegat, the central and southern Sound, the Belt Sea and Arkona Sea no phytoplankton bloom was observed. Even then the nutrient concentrations were relatively low compared to means for January 1980-1989. This might be due to low fresh water run off and nutrient load, as the precipitation in Denmark in the period since July 1995 have been very low.

The temperature and salinity stratification was relatively strong for the season. Compared to long term monthly means (1931-1960) for January the temperatures during this cruise were generally lower, except in the Kattegat bottom water. Also the surface salinity’s were generally lower than long term means, but the bottom water salinity’s were higher than normal, except in the Great Belt. Due to upwelling of deep water in the north-eastern Kattegat (St. 1001) the surface salinity and temperature at this station were higher than normal, the nutrient concentrations relatively high and the chlorophyll-a concentration low.

Since last cruise primo November the bottom water oxygen concentrations had increased. The lowest oxygen concentration of 4.4 ml/l (67%) was found in the Sound. Compared to January last year the minimum oxygen concentrations this year are lower, except in the northern Kattegat. Compared to mean for January 1980-1989 the concentrations this year are lower, except in the Fehmarn Belt. The relatively low oxygen concentrations might be due to relatively low water exchange and mixing.

General

The scope of the cruise was to determine the winter nutrient concentrations. The route, time schedule and stations of the cruise are shown in figure 1. Only hydrographic and hydrochemical measurements including chlorophyll were performed.

Meteorology

In November and December 1996 the monthly mean temperatures were 0.1 ° C and 2.2 ° C belove long term mean 1961-1990, respectively, and at the end of December ice was formed in the fjords and coastal areas. The precipitation was in November 28% above and in December 44% belove normal. In the beginning of November strong wind from south-west and west dominated, changing to north-east at the end of the month. December was dominated by rather weak winds from south and north-east, and the number of sun-hours was 44% above normal (Danish Meteorological Institute). The high pressure weather type with weak wind continued in the beginning of January and during the cruise.

Hydrography

The surface temperature ranged from -0.1 ° C in the Sound (St. 431) to 5.5 ° C in the north-eastern Kattegat (St. 1001), and had decreased 7-10 ° C since primo November. The bottom water temperature ranged from 2.8 ° C in the Fehmarn Belt (St. 450, 952) to above 8 ° C in the Sound and southern Kattegat (St. 431, 921, 922, 418, 415, 925) (Fig. 2), and had only decreased a little in the last mentioned area, but up to 7 ° C in the Fehmarn Belt since primo November. The temperature difference between surface and bottom ranged from 1.6 ° C in the southern Great Belt (St. 450) to 9 ° C in the Sound (St. 431).

The surface salinity ranged from 8.2 psu in the central Arkona Sea (St. 444) to 32.5 psu in the north-eastern Kattegat (St. 1001), and had, except in the Arkona Sea, decreased much (3-8 psu) since primo November. The bottom water salinity ranged from 15.5 psu in the central Arkona Sea (St. 444) to 34.4-34.8 psu in the northern Kattegat (St. 403, 905, 1001,1007, 1008, 1009) (Fig. 3, 4 and 5), and had generally decreased since primo November, except in the Kattegat. The salinity stratification was weak in the north-eastern Kattegat (St. 1001) due to upwelling of deep water (Fig. 3), but rather strong for the season in the other areas with differences between surface and bottom of more than 10 psu in the Sound, eastern and southern Kattegat.

Compared to long term monthly means (1931-1960) for January the temperatures during this cruise were generally lower, except in the Kattegat bottom water. Also the surface salinity’s were generally lower than long term means, but the bottom water salinity’s were higher than normal, except in the Great Belt. Due to upwelling of deep water in the north-eastern Kattegat (St. 1001) the surface salinity and temperature at this station were higher than normal.

Chlorophyll-a

Very unusual for the season a phytoplankton bloom was going on in most of the Kattegat with mean chlorophyll-a concentrations in the uppermost 10 m ranging from 2.8 m g/l in the western part (St. 409) to above 10 m g/l in the southern part (St. 418, 922). In the other areas the concentrations were low as normal for the season. In the upwelling area in the north-eastern Kattegat (St. 1001) only 0.9 m g/l was found. In the Sound, Fehmarn Belt and Arkona Sea the mean chlorophyll-a concentrations were 1.0-1.5 m g/l, and in the Great Belt area, including Aarhus Bight, 0.5-0.8 m g/l.

Nutrients

The concentrations of nitrate, phosphate and silicate were generally low for the season compared to mean for January in the 1980’s. In the Kattegat surface water this was partly due to the unusual phytoplankton bloom going on.

The nitrate concentrations in the surface were 3 m M or less in the south-eastern Kattegat (St. 418, 921, 922). In the bottom water nitrate concentrations above 9 m M were observed only in the Sound (Fig. 6, 7 and 8). The highest nitrite (>1 m M) and ammonium (>1.6 m M) concentrations were found in the bottom water in the Fehmarn Belt and Gedser Rev area and for ammonium also in the surface of the Great Belt (Fig. 9 and 10).

Phosphate concentrations of 0.3-0.4 m M were observed not only in the Kattegat surface water, but also in the Sound, Arkona Sea and Fehmarn Belt (Fig. 11). The silicate concentrations were below 6 m M in the surface in parts of the Kattegat, and above 20 m M in the Arkona Sea bottom water (Fig. 12).

Oxygen

Since primo November the bottom water oxygen concentrations had increased. The lowest oxygen concentration of 4.4 ml/l (67% saturation) was found in the Sound (St. 431). In the southern Kattegat and northern Belt Sea (St. 415, 418, 427, 921, 922, 925, 935) the minimum concentrations were 4.7-5.3 ml/l (71-80%). In all other areas the oxygen saturation was 81-92% (Fig. 13 and 14).

Compared to January last year the minimum oxygen concentrations this year are lower, except in the northern Kattegat. Compared to mean for January 1980-1989 the concentrations this year are lower, except in the Fehmarn Belt.