Monitoring cruise with r/v Gunnar Thorson in the Sound, Kattegat, Belt Sea and Arkona Sea, 18-22 August 1997. Cruise no. 179.

 

Report: Gunni Ærtebjerg

Cruise leader: Jan Damgaard

Participants: Susanne Hemmingsen, Hanne Ferdinand, Dorete Jensen, Winnie Martinsen Dan Boding Jensen (NERI).

18 August: Martin Larsen, Gunni Ærtebjerg (NERI).

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

 

 

Summary and conclusions

Due to the calm and warm weather and dominating outflow from the Baltic Proper the surface temperature was 4-5 C higher and the salinity 2-8 psu lower than long term means. The bottom water temperature was lower and the salinity higher than normal, and thus the stratification of the water column was unusually strong.

The nitrate-cline was situated below the primary pycnocline. In the northern and eastern Kattegat this was also true for the phosphate-cline, while phosphate and silicate were found in the surface layer in the Sound, Belt Sea and Arkona Sea. High concentrations of nitrate, phosphate and silicate were found at the bottom in the Fehmarn Belt, Gedser Rev and Arkona Sea area, where also oxygen depletion (below 2.8 ml/l = 4 mg/l) was observed.

The minimum oxygen concentration in the Fehmarn Belt, Gedser Rev and Arkona Sea area was 1.7-2.1 ml/l (25-32%). In the north-western Kattegat and central Sound the minimum concentrations were 3.2-3.5 ml/l (51-59%). In these area (except the Sound) the concentration was lower than long term mean, but in the other open areas it was higher than normal. Due to the weather situation, oxygen depletion was widely spread in the coastal waters. In figure 6 oxygen depletion observed within the first three weeks of August by NERI or Danish counties are shown.

Generally the chlorophyll-a concentration at the surface was low and increasing with depth. However, in areas with cyanobacteria blooms also the surface concentrations were relatively high (2.0-4.3 m g/l). This was observed in the southern part of the Sound, in the Arkona Sea, the Great Belt, and the south-western Kattegat.

In most cases, the surface accumulations formed marine snow in the uppermost 1-2 m of the water column. Besides cyanobacteria (Nodularia spumigena, Aphanizomenon flos-aquae), Ceratium spp., Prorocentrum minimum, P. micans and different diatoms were important constituents of the snow. North of Djursland (south-western Kattegat), N. spumigena formed dense yellow-green blooms at the surface. In general Nodularia was the most common species in the snow, but sometimes Aphanizomenon or Ceratium were dominant. Marine snow was also found in the south-eastern Kattegat; here Ceratium was the prominent genus.

 In all areas subsurface chlorophyll maxima of different sizes were observed, most often between 10 m and 15 m depth, at a few stations between 5 m and 10 m depth. In the south-eastern Kattegat pronounced maxima of 7 to 26 m g/l were found as deep as 24 and 18 m depth, respectively. At the Arkona Sea stations Aphanizomenon flos-aqua and Prorocentrum minimum were common in the peaks, while at all other stations dinoflagellates and diatoms constituted the subsurface phytoplankton community. Particularly, species of Ceratium spp were characteristic.

General

The basic scope of the cruise was to monitor the hydrographic situation and the spatial variation in plankton, primary production, oxygen and nutrients. Special efforts were made in mapping the geographical distribution of minimum oxygen concentrations and cyanobacteria (blue-green algae) blooms, and 14 extra stations were included, besides running observations during sailing. Besides the monitoring and mapping measurements, sampling and measurements in the research project DECO were performed. The three transects of regular monitoring stations used in the following figures are shown in figure 1.

Meteorology

The monthly mean temperature in Denmark was in July 1.8 C above long term mean 1961-1990, and the precipitation was 6% below normal. Weak wind mostly from the northern sector prevailed all month. The warm and calm weather with weak wind from varying directions continued into August and during the cruise (Danish Meteorological Institute).

Hydrography

The surface temperature (1 m depth) had increased 4-6 C since primo July and was unusually high ranging from 21.5-22.1 C at Gedser Rev (St. 954) and Fehmarn Belt (St. 952) to 23.4 C in the western Kattegat (St. 409, 415). The bottom water temperature had increased much less and ranged from 7.2-8.0 C in the Sound and southern Kattegat to above 10 C in the north and western Kattegat and the Arkona Sea (Fig. 2). The temperature difference between surface and bottom was generally 10-15 C, except in the shallow parts of the Arkona Sea and western Kattegat. In the central Arkona Sea (St. 444) a temperature minimum of 12-13 C was observed in an intermediate layer in 32-35 m depth.

The surface salinity had generally decreased since primo July and ranged from 7.3-7.5 in the Arkona Sea (St. 441, 444) to 16.1-17.3 in the northern Kattegat. The bottom water salinity ranged from 9.3-16.3 in the Arkona Sea (St. 441, 444, 449) to 33.1-33.6 in the north-eastern Kattegat (St. 413, 905, 1001, 1007) (Fig. 2). Except in the Arkona Sea the salinity stratification was strong with differences between surface and bottom of 12-19 psu and in the Sound 24 psu. The upper boundary of the pycnocline was situated between 5 m and 10 m depth, except in the southern Belt Sea and Arkona Sea, where it was found a little deeper.

 

Figure 2. Surface (1 m) and bottom near temperature along transect I, and salinity in 1 m, 5 m, 10 m, 15 m, 20 m depth and near bottom along the transects I, II and III (see figure 1).

Compared to long term monthly means (1931-1960) for August the surface temperature during this cruise was 4-5 C higher and the surface salinity significant (2-8 psu) lower than normal. The bottom water temperature was generally 1-4 C lower and the bottom water salinity was higher than long term means.

Nutrients

Generally only traces of nitrate were found in the surface and pycnocline layers, meaning the nitrate-cline was situated below the primary pycnocline. Close to the bottom generally 1-6 m M nitrate was found, with a maximum of 7-10 m M in the Gedser Rev and Fehmarn Belt area (St. 954, 952) (Fig. 3). Likewise, generally no nitrite was found in the surface layer, and significant amounts of nitrite (~1 m M) were only found in the bottom water in the north-eastern Kattegat and Arkona Sea (St. 1001, 444) (Fig. 3). In the surface layer the ammonium concentrations were generally below 0.5 m M. In the bottom water ammonium concentrations of 2-5.6 m M were observed in the northern Kattegat and central Arkona Sea (St. 403, 1009, 1007, 444) (Fig. 4).

 

Figure 3. Surface and bottom near concentrations of nitrate along the transects I, II and III, and surface and bottom near concentrations of nitrite along transect I.

In the surface layer traces of phosphate were found in the Sound, Belt Sea and Arkona Sea, except the southern Great Belt (St. 443, 450). In the Kattegat no phosphate was found in the surface layer, and in the northern and eastern Kattegat phosphate was absent down to 40 m and 25 m depth, respectively. Close to the bottom the phosphate concentrations were higher than 1.0 m M in the Fehmarn Belt, Gedser Rev and Arkona Sea area (Fig. 4). Low silicate concentrations below 1 m M were observed in the surface water in the northern and eastern Kattegat, increasing through the Belt Sea to the Arkona Sea. In the bottom water above 20 m M were observed in the Fehmarn Belt, Gedser Rev and Arkona Sea area (Fig. 4). A subsurface minimum in the silicate concentration was observed at 10-20 m depth in the Arkona Sea, and at 10 m depth in the Great Belt, Aarhus Bight and at some stations in the western and southern Kattegat.

 

Figure 4. Surface and bottom near concentrations of ammonium, phosphate and silicate along the transect I.

Oxygen

Since the cruise in July the minimum oxygen concentration had decreased in all areas. The lowest oxygen concentrations of 1.7-2.1 ml/l (25-32% saturation) were found in the Fehmarn Belt and Gedser Rev area (St. 952, 954) and the central Arkona Sea (St. 444). In the north-western Kattegat (St. 403, 1008, 1009) 3.2-3.5 ml/l (52-59%) was observed close to the bottom, and also in the central Sound the minimum oxygen concentration was 3.5 ml/l (51%). In all other deeper areas the minimum oxygen concentrations were above 4.1 ml/l (60%) (Fig. 5).

Compared to mean for August in the 1980s the minimum oxygen concentrations this year are higher, except in the Gedser Rev - Arkona Sea area and in the north-western Kattegat, where it is lower than normal.

Oxygen depletion is in Denmark defined as below 2.8 ml/l (4 mg/l), and serious oxygen depletion as below 1.4 ml/l (2 mg/l). From these definitions oxygen depletion was during the cruise observed in the Fehmarn Belt, Gedser Rev and Arkona Sea area (St. 952, 954, 449, 444). In figure 6 is shown the stations visited by either the Danish counties or NERI within the first three weeks of August, and where oxygen depletion or serious oxygen depletion was observed.

 

Figure 5. Minimum oxygen concentrations along the transects I, II and III. (see figure 1).

Chlorophyll-a and phytoplankton species composition

Generally the chlorophyll-a concentration at the surface was low (0.5-1.5 m g/l) and increasing with depth. However, in areas with cyanobacteria blooms also the surface concentrations were relatively high, 4.3 m g/l at Stevns in the Arkona Sea (St. 441), and 2.0-3.7 m g/l in the southern Great Belt and Fehmarn Belt (St. 939, 443, 450, 952) (Fig. 7).

 

Figure 7. Chlorophyll-a concentrations in 1 m, 5 m, 10 m and 15 m depths along the transects I, II and III (see figure 1).

During the cruise surface and/or near surface accumulations of cyanobacteria was observed in the southern part of the Sound, in the Arkona Sea, the Great Belt, and the southern Kattegat. No accumulations were observed along the ship-track in the Gedser Rev - Fehmarn Belt area, but the county of Storstrøm reported cyanobacteria in this area as well as in Smålandsfarvandet. Unlike the week before, no accumulation of cyanobacteria were observed in the south-eastern Kattegat. It should, however, be noticed that this area was partly visited during the night.

In most cases, the surface accumulations formed marine snow in the uppermost 1-2 m of the water column. Besides cyanobacteria (Nodularia spumigena, Aphanizomenon flos-aquae), Ceratium spp., Prorocentrum minimum, P. micans and different diatoms were important constituents of the snow. North of Djursland (south-western Kattegat), N. spumigena formed dense yellow-green blooms at the surface. In general Nodularia was the most common, but in other areas sometimes Aphanizomenon or Ceratium were dominant. Marine snow was also found in the south-eastern Kattegat; here Ceratium was the prominent genus.

In all areas subsurface chlorophyll maxima of different sizes were observed, most often between 10 m and 15 m depth, at a few stations between 5 m and 10 m depth (St. 441, 443, 450, 409). In the south-eastern Kattegat (St. 413, 418, 922) pronounced maxima of 7 to 26 m g/l were found as deep as 24 and 18 m depth, respectively. At the Baltic stations 1728 and 449, Aphanizomenon and Prorocentrum minimum were common in the peaks, while at all other stations dinoflagellates and diatoms constituted the subsurface phytoplankton community. Particularly, species of Ceratium spp were characteristic.