136
W. Schramm
1978; wallentinus 1991). The marine flora of the Baltic in principle
derives from the North Sea flora as already described by Reinke (1889).
Most of the characteristic North Sea forms, however, penetrate only as
far as the Kattegat. With decreasing salinity, the number of marine
macro algae decreases considerably. Of the more than 300 occurring
macroalgal species along the Swedish and Danish coasts of the Kattegat,
only 42 can be found in the innermost Baltic Sea (Bothnian Bay). As is
typical for many brackish or estuarine systems, the number of marine
species in the Baltic does not decrease continuously with the salinity
gradient, but unsteadily. Of the main systematic macro algal groups, the
decline is more pronounced in the red and brown seaweeds compared to
the greens. Of the total number of macro algae the percentage of green
algae even increases due to the higher proportion of euryhaline forms
among the green algae and because of the incoming freshwater species
(Wallentinus 1991; Nielsen et al. 1955; Fig. 5.4; Table 5.1).
A critical boundary for many fully marine organisms can be found in
the Belt Sea with its unstable osmotic conditions due to salinity fluctuations as high as to ± 10 PSU. A further drop in the number of marine
110
90
70
50
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~30
E
<>
T"
10
~
.......
I
II
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~
~ 70
~
150
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l~
~
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B 10
Brown
J
E
"6
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Z
~
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: C : -.
o 10 20 30
0
10
20
30
saUnity (ppI)
saUnity (ppI)
Fig. 5.4. Number and percentage of red, brown and
green macroalgae (> 1 cm)
along the average salinity
gradient in the Baltic Sea.
(After Wallentinus 1991)